U.S. patent number 3,610,800 [Application Number 04/872,599] was granted by the patent office on 1971-10-05 for digital electronic keyboard instrument with automatic transposition.
This patent grant is currently assigned to North American Rockwell Corporation. Invention is credited to Ralph Deutsch.
United States Patent |
3,610,800 |
Deutsch |
October 5, 1971 |
DIGITAL ELECTRONIC KEYBOARD INSTRUMENT WITH AUTOMATIC
TRANSPOSITION
Abstract
In an electronic organ, the actuation of keys in accordance with
corresponding, audible tones to be reproduced effects the gating of
pulses into time slots of a time decision multiplexed signal, the
time slots of the multiplexed signal being structured in accordance
with a desired assignment sequence to correspond to the keys and to
be representative thereof for identifying each note capable of
being generated by the organ. A set of note, or tone, generators
with availability assignment control means for capturing a pulse in
the multiplexed signal are each rendered responsive to a given
captured pulse for generating the tone represented by that pulse.
Automatic transposition of notes, by a specified number of half
steps higher or lower than the note played, is selectively effected
by a time shift of pulses in the multiplexed signal by one time
slot per half note to be transposed. In this manner, when an
organist plays a musical selection in an original musical key, the
organ produces the audible musical output in the selected,
transposed musical key.
Inventors: |
Deutsch; Ralph (Sherman Oaks,
CA) |
Assignee: |
North American Rockwell
Corporation (N/A)
|
Family
ID: |
27582831 |
Appl.
No.: |
04/872,599 |
Filed: |
October 30, 1969 |
Current U.S.
Class: |
84/619; 84/657;
984/323; 984/332; 984/392; 984/338 |
Current CPC
Class: |
G10H
1/0575 (20130101); G10H 1/20 (20130101); G06F
1/0328 (20130101); G10H 1/182 (20130101); G06F
1/02 (20130101); G10H 7/04 (20130101) |
Current International
Class: |
G10H
1/20 (20060101); G10H 1/057 (20060101); G10H
1/18 (20060101); G06F 1/02 (20060101); G10H
7/04 (20060101); G06F 1/03 (20060101); G10H
7/02 (20060101); G01h 001/02 () |
Field of
Search: |
;84/1.01,1.03,1.04,1.11,1.13,1.17-1.23,1.24,1.26,DIG.8,10,11,12,16,23,29 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hirshfield; Milton O.
Assistant Examiner: Witkowski; Stanley J.
Claims
What is claimed is:
1. An electronic musical instrument, comprising:
a plurality of keys individually actuable to cause the production
of corresponding notes of the musical scale,
means for sequentially and repetitively scanning said keys to
generate a digital signal containing note assignments as developed
by respective actuated keys, the note assignments in said digital
signal identifying the notes to be called forth in accordance with
the positions of said assignments in said signal,
means for selecting a desired amount of transposition of the
identified notes to a different pitch,
means responsive to the note assignments in the digital signal for
producing the notes identified thereby and
means responsive to said transposition selection means for
controlling said note producing means to produce notes at pitches
different from those of the notes identified by the assignments in
the digital signal, by the selected amount of transposition.
2. The electronic musical instrument according to claim 1 wherein
successive note assignment positions of the digital signal
correspond to notes of successive, different pitches, and wherein
said transposing controlling means selectively shifts the note
assignments corresponding to the actuated keys through a desired
number of positions in said signal in accordance with the amount of
transposition desired.
3. The electronic musical instrument according to claim 2 wherein
said transposing controlling means selectively shifts all of said
note assignments an equal number of positions in said signal for
any selected amount of transposition.
4. The electronic musical instrument according to claim 3 wherein
said transposing controlling means effects an appropriate delay of
said digital signal relative to a predetermined reference time with
which said signal is normally synchronized to introduce the desired
transposition shift.
5. The electronic musical instrument according to claim 2 wherein
there is further provided means for selectively introducing
assignments of notes into said digital signal in an octave
differing from the octave for the notes actually called for by
actuation of said keys, to produce those notes otherwise outside
the range of note generation of said instrument as a result of the
amount of transposition selected, in that different octave and thus
within the said range.
6. The electronic musical instrument according to claim 5 wherein
said means for selectively introducing assignments of notes into a
different octave produces a displacement of only one octave from
the octave for the corresponding notes actually called for by
actuation of said keys.
7. The electronic musical instrument according to claim 6 wherein
said one octave displacement is below the octave for the notes
actually called for by actuation of said keys.
8. The electronic musical instrument according to claim 5 wherein
said means for selectively introducing assignments of notes into a
different octave comprises switch means for transferring signals
representative of actuation of said keys which due to the
transposition selected call for the generation of notes at a pitch
exceeding the range of note generation of said instrument, to
positions normally occupied by signals representative of the keying
of the corresponding notes in said different octave lying within
the range of note generation, to be detected during said scanning
of said keys.
9. The electronic musical instrument according to claim 1 wherein
said transposition controlling means is responsive to said
transposition selection means for weighting the note assignments to
which said note generating means is responsive by a factor
calculated to introduce the amount of transposition desired in the
generated note.
10. The electronic musical instrument according to claim 9 wherein
said weighting factor is the common ratio of adjacent notes in a
musical scale of equal temperament, said factor being varied in
accordance with the number of semitones of transposition desired
within said musical scale.
11. The electronic musical instrument according to claim 9 wherein
there is further provided means for selectively introducing
assignments of notes into said digital signal in an octave
differing from the octave for the corresponding notes actually
called for by actuation of said keys, to produce those notes
otherwise outside the range of note generation of said instrument
as a result of the amount of transposition selected in that
different octave and thus within the said range.
12. The electronic musical instrument according to claim 11 wherein
said means for selectively introducing assignments of notes into a
different octave produces a displacement of only one octave from
the octave actually called for by actuation of said keys.
13. The electronic musical instrument according to claim 12 wherein
said means for selectively introducing assignments of notes into a
different octave comprises switch means for transferring signals
representative of actuation of said keys to positions normally
occupied by signals representative of the keying of corresponding
notes in said different octave, to be detected during said scanning
of said keys.
14. An electronic musical instrument, comprising:
a plurality of switches selectively operable to develop signals
identifying respectively associated notes to be produced by said
instrument,
means responsive to operation of switches among said plurality of
switches for processing the respectively developed signals in a
digital multiplexed waveform to identify the notes to be produced
from said instrument in accordance with the time position of the
signals in the digital multiplexed waveform,
means responsive to the signals in the digital multiplexed waveform
for producing the notes identified by the time positions of the
signals in that waveform,
means for selecting a desired amount of transposition of the
identified notes to a pitch differing from the normal pitch
thereof, and
means responsive to said transposition selection means for
controlling said note producing means to produce notes at pitches
different from the normal pitches of the identified notes in
accordance with the selected amount of transposition.
15. The invention defined by claim 14 wherein said instrument is a
keyboard instrument.
16. The invention defined by claim 15 wherein said instrument is an
organ.
17. The electronic musical instrument according to claim 14 wherein
said transposition controlling means comprises means for
selectively shifting the positions of said signals in said
multiplexed waveform by a number of positions based on the amount
of transposition desired.
18. The electronic musical instrument according to claim 14
wherein
said transposition control means conditions said note generating
means to shift the pitch of the note generated thereby by an amount
corresponding to the amount of transposition selected.
19. An automatic transposition system for an electrical musical
instrument for automatically and selectively transposing the notes
selected by actuation of keys of that instrument to higher and
lower pitches than those normally corresponding to the respectively
actuated keys, comprising:
means for generating a digital multiplex waveform having a
plurality of time slot positions preassigned to respectively
corresponding ones of said plurality of keys,
means for sequentially and repetitively scanning said keys to
produce a signal in response to each actuated key in the
respectively corresponding preassigned time slot positions,
means responsive to said digital multiplex waveform for generating
notes at pitches corresponding to the time slot positions of
signals in said digital multiplex waveform,
means for selecting a desired amount of transposition of the notes
to be generated in response to key actuation from the normal pitch
thereof to another pitch, and
means responsive to said transposition selection means for
controlling said note generally means to generate notes at pitches
different from the normal pitches corresponding to the actuated
keys, by the selected amount of transposition.
20. An automatic transposition system as recited in claim 19
wherein said transposition control means comprises:
a shift register for receiving said digital multiplex waveform,
said shift register having a plurality of outputs for selectively
deriving the received multiplex waveform therefrom as an output
multiplex waveform shifted in time at the successive outputs by
successive time slot positions, and
means for selecting the output multiplex waveform from the shift
register output affording a shift of time slot positions
corresponding to the selected amount of transposition.
21. An automatic transposition system as recited in claim 19
wherein there is further provided:
means for selectively introducing signals into time slots of said
multiplex waveform in a next adjacent octave to the octave for the
notes actually called for by actuation of said keys, to produce
those notes otherwise outside the range of note generation of said
instrument as a result of the amount of transposition selected as
the corresponding notes in that next adjacent octave and thus
within the said range.
22. An automatic transposition system as recited in claim 19
wherein said note producing means includes:
means for storing a plurality of digital sample values of a
waveform to be reproduced for the generation of notes,
means for addressing said storage means for repetitively deriving
therefrom a succession of digital sample value words defining the
waveform and at a rate corresponding to the frequency of the note
to be produced, and
said transposition control means controls said addressing means to
address said storing means at a rate different from the normal rate
by an amount corresponding to the amount of transposition selected
for deriving the digital sample value words therefrom at a rate
corresponding to the frequency of the transposed note.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention resides broadly in the field of electronic musical
instruments and is particularly adaptable for use in an electronic
organ as a digital selection system for calling forth desired tones
from those available to be produced by the organ. The term "organ"
is used throughout the specification and claims in a generic sense
(as well as in a specific sense) to include any electronic musical
instrument having a keyboard such as electronic organs, electric
pianos and accordions, and the principles of the present invention
are, in fact, applicable to any musical instrument in which musical
sounds are generated in response to the actuation of key switches,
regardless of whether those switches are actuated directly, i.e.,
by the musician's fingers, or indirectly, e.g., by the plucking of
strings. The term "key" is also used in a generic sense, to include
depressible levers, actuable on-off switches, touch- or
proximity-responsive (e.g., capacitance- or inductance-operated)
devices, closable apertures (e.g., a hole in a "keyboard" of holes
which when covered by the musician's finger closes or opens a
fluidic circuit to produce a tonal response), and so forth.
2. Description of the Prior Art
The function of an electronic organ is to faithfully reproduce, or
to simulate by electronic means, the sounds or tones developed by a
true pipe organ in response to playing of the organ by the organist
in the selection of notes, and voices and other characteristics of
those notes. It is frequently desirable to transpose the musical
selection being played, whether on an electronic organ, an
electronic piano, or any other electronic musical keyboard
instrument, from one musical key to another, and for a skilled
musician this may not present any great problem. The typical home
organist, however, may play for his own personal enjoyment and
usually does not possess the skill required to freely and
comfortably transpose keys. Accordingly, it is desirable to provide
the instrument with means for automatically transposing between
musical keys, as preselected by the musician, while permitting the
musician to play the musical piece by keying the instrument as
though the piece were being played in the the original key.
Even the professional musician can be substantially assisted by an
automatic transposition capability. Frequently, a singer will ask
the accompanying musician to play a full tone higher or possibly
one and one-half tones lower so that the range of a particular song
can be adapted to the singer's range. The same transposition
requirement may exist when a solo instrument such as a B-flat
trumpet or clarinet is to be played with an organ or piano
accompaniment, in cases where the selected musical piece has not
been transcribed to sheet music to meet the requirements of the
solo instrument range and is scored for a concert key instrument.
One possible solution is for the soloist to play each note a full
tone higher than it appears in the sheet music. A solution
preferred by most soloists is for the organist or pianist to play
each note a full tone lower. Clearly, the provision of automatic
transposition in a keyboard instrument can solve such problems in a
quick and efficient manner. The terminology "automatic
transpostion" is used in a generic sense to denote a method by
which the instrument itself can be preset to cause each key on the
keyboard to sound a note frequency other than that normally
associated with that key when the key is depressed. For example,
the instrument may be implemented to cause each actuated key to
sound a semitone higher, e.g., depressing the key for C.sub.4
actually causes C.sub.-4 to sound.
While the concept of automatic transposition for keyboard
instruments dates back many years, the problems of cast and
complexity of implementing ans servicing the prior art
configurations for producing automatic transposition have
discouraged any common usage of the concept. In one prior art
arrangement a single manual organ having no pedal board was
provided with keys resting on pegs which in turn controlled the
airflow to the organ reeds. A lever was attached to the key
mechanism in such a manner that shifting the lever to the right or
left caused the keys to shift to the right or left respectively by
a corresponding amount. Thus, by shifting the lever the distance of
one tone on the keyboard, the entire keyboard was shifted to
operate the pegs controlling tones one halftone higher or lower
than was the case in the normal position of the keys. This
arrangement, of course, was limited as to the amount of
transposition available, and usually provided the organ with the
capability to transpose up to 12 halftones. This device has not
survived to the extent of use in modern instruments. One
significant disadvantage was its lack of adaptability to the
requirements of large organs or pianos. For example, there is
apparently no simple method of expanding the scheme to
simultaneously transpose keys throughout several keyboards of a
single instrument, e.g., the manuals and pedal boards of an organ.
Moreover, the arrangement requiring a physical sliding of keys from
a normal position is an undesirable feature.
Another more recent automatic transposition scheme for larger
organs requires individual switching of each of the key switches on
each manual. Since a typical manual has 61 keys, a 61 pole,
12-position switch would be required for each manual. In addition,
each of these manual switches must be ganged together for common
operation, so that all manuals are transposed simultaneously. Here
again, cost and complexity of the arrangement has resulted in
infrequent use of the system.
In the copending application of G. A. Watson entitled "multiplexing
System For Selection Of Notes And Voices In An Electronic Musical
Instrument" filed on even date herewith, and assigned to the same
assignee as the present invention, there is described an electronic
organ in which every key of every keyboard is scanned in cyclic
sequence. The actuation of a key or keys on any keyboard is entered
as information in a parallel digital format developed by the
scanning of the keyboards, the information indicating the order and
combination, as well as each individual one, of the keys that have
been actuated and deactuated. THis parallel format is continuously
converted to a serial format to provide information regarding key
actuation in the form of pulses in appropriate time slots of the
time division multiplexed signal which is supplied to the tone
generating section of the organ. Each time slot of the multiplexed
signal is representative of a specific key to permit identification
of the notes associated with the respective keys and thereby to
result in the appropriate sounds being generated in response to the
playing of the keys.
The tone generators which respond to the incoming multiplexed
signal to bring forth the appropriate tones corresponding to those
keys that have been actuated, in the order and combination of such
actuation, produce digital amplitude samples of a waveform of the
desired sound at a frequency corresponding to the desired note
frequency.
Such an arrangement permits reduction of complexity that is usually
found in electronic organs, and in particular permits elimination
of a substantial number of wires and cables that are usually
required between the keyboards and the tome generators.
Furthermore, since the digital electronic organ of the
aforementioned Watson application provides simple and efficient
assignment of a small number of tone generators, relative to the
number of keys available, to the keys which have been actuated,
there is a further reduction in complexity of mapping the subset of
depressed keys into the available tone generators, over
conventional requirements. Still further, the digital electronic
organ overcomes such difficulties as may occur when a key switch
has faulty or dirty contacts, a situation that would ordinarily
lead to intermittent electrical contact and discontinuity of tone.
By use of a multiplexed signal, the presence of a pulse in a
particular time slot of a repeating signal is sufficient to
represent the actuation of the corresponding key, and less than
perfect contact is required to produce that pulse.
Each of the limited number of tone generators provided in the
digital electronic organ of the aforementioned Watson application
is associated with generator assignment logic constructed and
arranged to assign an available tone generator to an incoming pulse
in the multiplexed signal which has not yet captured to tone
generator. Each tone generator includes a memory means storing
digital representations of amplitudes of the wave shape to be
synthesized at a large number of sample points. When the tone
generator is captured by a pulse, the memory means associated with
that tone generator is accessed to read out amplitude samples in
accordance with the frequency of the tone to be generated.
Effectively, each tone generator constitutes a master oscillator
from which all of the musical frequencies encompassed by the organ
are obtained (in digital format) by frequency synthesis. The
present invention takes advantage of this fact to provide a
relatively simple technique for implementing an automatic
transposition system within the digital electronic organ, or other
keyboard instrument, this being the primary object of the
invention.
SUMMARY OF THE INVENTION
Briefly, in accordance with the present invention, musical keys are
automatically transposed in an electronic digital keyboard
instrument in which key switch operation (note selection)
information is entered as respective pulses into preassigned time
slots of a multiplexed signal, by shifting (or effectively
shifting) the pulses by one time slot per semitone of desired
transposition.
In one specific form of implementation, the original keyboard
multiplexed signal or pulse train is subjected to delay to the
extent necessary to provide the desired time shift and hence the
desired transposition. For example, the multiplexed information may
be applied serially to a 12-bit shift register which acts
effectively as a 12-bit tapped delay line, each successive tap of
the shift register constituting an additional one-bit delay so that
a delay up to 12 bits in length (one octave) may be selected
according to the tap from which the output is taken. Of course,
only one tap, or output terminal, serves to provide an output for
any given transposition, the entire multiplexed waveform emanating
from that tap with the desired time shift.
The direction of shift, upward or downward in key, depends on the
direction in which the keyboards are scanned to provide the
multiplexed waveform. That is to say, the direction of shift
depends on whether the scanning is from the lowest to the highest
frequency for each keyboard or from the highest to the lowest
frequency. It is immaterial to the present invention in which
direction the keyboards are scanned, but if it is from the highest
to the lowest frequency then clearly a 12-bit maximum delay will
permit only a downward shift in key; that is, a transposition to
notes of lower frequency. In a similar manner, scanning in the
opposite direction will permit only an upward shift in key, for a
12-bit maximum delay. To obtain an upward shift in the highest to
lowest frequency scanning arrangement, or what is effectively an
upward shift, the delay may span almost, but not quite, a complete
cycle of the multiplexed waveform. Specifically, the delay should
extend to that time slot preceding, by a desired time shift, the
time slot normally occupied by the pulse in question.
In a second specific implementation an approach is taken which is
actually merely a substitution of operation in the frequency domain
for operation in the time domain, by exercising the desired shift
at the tone generation end of the organ electronics, rather than
producing a shift of the pulses in the time division multiplexed
waveform.
BRIEF DESCRIPTION OF THE DRAWINGS
In describing the present invention, reference will be made to the
accompanying Figures of drawings in which:
FIG. 1 is a simplified block diagram of a system for producing a
time division multiplexed signal containing a recycling sequence of
time slots each associated with a particular key of the organ and
in which each time slot containing a pulse is indicative of the
actuation of the associated key;
FIG. 2 is a circuit diagram of an exemplary decoder for use in the
system of FIG. 1;
FIG. 3 is a more detailed circuit diagram of the switching array
and encoder used in the system of FIG. 1;
FIG. 3A is a circuit diagram of an alternative encoder to that
shown in FIG. 3, for use in the system of FIG. 1;
FIG. 4 is a circuit diagram of the input-output bus connecting
means at each intersection of the switching array of FIG. 3;
FIG. 5 is illustrative of a multiplex waveform developed by the
system of FIG. 1 is response to actuation of selected keys;
FIG. 6 is a simplified block diagram of generator assignment and
tone generating apparatus for processing the multiplexed signal
produced by the system of FIG. 1 to develop the desired tones as an
audible output of the organ;
FIG. 7A and 7B together constitute a circuit diagram of one
embodiment of the tone generator assignment logic for the system of
FIG. 6;
FIG. 8 is a block diagram of tone generator suitable for
synthesizing the frequency of every note capable of being played in
the organ, for use with the assignment logic of FIGS. 7A and 7B in
the system of FIG. 6;
FIG. 9 is illustrative of a complex waveshape of the type produced
by a pipe organ, and of the sample points at which amplitude values
are taken, for simulation at selected note frequencies;
FIG. 10 is a block diagram of an embodiment of an attack and decay
control unit for use in an electronic digital musical instrument of
the type shown and described with reference to the preceding
figures of drawing;
FIG. 11 is a representation of the multiplexed signal showing the
appearance of three consecutive note assignments in the repetitive
signal;
FIG. 12 is a simplified block diagram of one embodiment of the
present invention;
FIG. 13 is a more detailed circuit diagram of the embodiment of
FIG. 12;
FIG. 14 is a circuit diagram of a system for providing octave
folding during transposition; and
FIG. 15 is a block diagram of another embodiment of the invention
for use in the tone generating apparatus of FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the keyboard multiplexing system or note
selection system includes a keyboard counter 1 which is implemented
to provide a specified count for each key of each keyboard
(including manuals and pedal divisions) of the organ. If, for
example, the electronic organ in which the multiplexing system is
used has four keyboards, such as three manuals and a pedal board,
each encompassing up to eight octaves, then keyboard counter 1
should have the capability of generating 4.times.8.times.12=384
separate counts (digital words). It is essential that the counter
be capable of developing a count representative of every key on
every keyboard of the organ; however, it may be desirable to
provide a counter that can produce a count greater than the number
of available keys in order to have available certain redundant
counts not associated with any keys. Such redundancy is readily
provided by simply utilizing a counter of greater capacity than the
minimum required count.
Keyboard counter 1 is divided into three separate sections (or
separate counters) designated 2, 3 and 4. The first section
(designated 2) is constructed to count modulo 12 so as to designate
each of the 12 keys associated with the 12 notes in any octave. The
second section (designated 3) is adapted to count modulo 8, to
specify each of the eight octaves encompassed by any of the four
keyboards. The last section (designated 4) is designed to count
modulo 4 to specify each keyboard of the organ. Therefore, the
overall keyboard counter is arranged to count modulo 384, in that
at the conclusion of every 384 counts, the entire set of keyboards
have been covered (scanned) and the count repeats itself. To that
end, each counter section may be composed of a separate
conventional ring counter, the three counters being connected in
the typical cascaded configuration such that when section 2 reacts
its maximum count it advances the count of counter section 3 by
one, and will automatically initiate a repetition of its own count.
Similarly, attainment of its maximum count by counter section 3 is
accompanied by advancement of the count of section 4 by one.
Advancement of the count of counter 2 is accomplished by
application of clock pluses thereto from a master clock source 5
which delivers clock pluses at a sufficiently rapid repetition rate
(frequency) to ensure resolution of depression (actuation) and
release (deactuation) of any key on any keyboard, i.e., to supply a
pulse at the instant of either of these events. Scanning of all
keyboards of the organ at a rate of 200 or more times a second is
deemed quite adequate to obtain this desirable resolution. For the
exemplary keyboard counter set forth above, this is equivalent to a
minimum of 200.times.384=76,800 counts per second, so that a master
clock delivering clock pulses at a rate of 100 kc./s. is quite
suitable.
A total of four lines emanate from counter 4, one line connected to
each ring counter stage, to permit sensing of the specific keyboard
which is presently being scanned. Similarly, stages, respectively
of octave counter 3 to detect the octave presently being scanned.
Thus, a total of 12 lines extend from counters 3 and 4, and these
twelve lines can carry signals indicative of 32 (8.times.4)
possible states of the keyboard counter. The specific one of the 32
states, representative of a particular octave on a particular
keyboard, which is presently being scanned is determined by use of
a decoder circuit 7 composed of 32 AND gates designated 8-1, 8-2,
8-3, ..., 8-32 (FIG. 2), each with two input terminals and an
output terminal. The gates are arranged in four groups of eight
each, with every gate of a particular group having one of its two
input terminals (ports) connected to one of the four lines of
counter 4. Distinct and different ones of the eight lines from
counter 3 are connected to the other input terminal of respective
ones of the eight AND gates of that group. A corresponding
situation exists for each group of AND gates, with the only
difference being that each group is associated with a different
output line of counter section 4. Using this arrangement, the
decoder logic designates every octave of keys in the organ by a
respective driver pulse when a count corresponding to that octave
is presently contained in the counter.
The output pluses deriving from the AND gates (or drivers) of
decoder circuit 7 are supplied on respective ones of 32 bus bars
(or simply, buses), generally designated by reference numeral 10,
to a keyboard switching array 11. From the preceding description,
then, it will be clear that array 11 has one input bus 10 for every
octave of keys in the organ (including every octave on every
keyboard), and that a drive pulse will appear on each input bus
approximately 200 times per second, the exemplary rate of scan of
the keyboards, as noted above, for obtaining adequate resolution of
operation of the keys. Switching array 11 also has 12 output buses,
generally designated by reference number 12, each to be associated
with a respective on of the 12 notes (and hence, the 12 keys) in
any given octave.
Array 11 is basically a diode switching matrix, in which spaced
input buses 10 and spaced output buses 12 are orthogonally arranged
so that an intersection or crossing occurs between each input bus
and each output bus (see FIG. 3), for a total of 384 intersections,
one for each count of the keyboard counter 1. As is typical in this
type of matrix, the crossed lines or buses are not directly
interconnected. Instead, a "jump" diode, such as that designated by
reference number 13 in FIG. 4, is connected between the input bus
10 and the output bus 12 at each intersection, the diode poled for
forward conduction (anode-to-cathode) in the direction from an
input bus 10 to an output bus 12. Wired in series circuit or series
connection with each diode 13 is a respective switch 14 which is
normally open circuited and is associated with a distinct
respective one of the keys of the organ, such that depression of
the associated key produces closure (close circuiting) of the
switch 14 whereas release of the associated key results in return
of the switch to its open state. Alternatively, each of switches 14
may itself constitute a respective key of the various keyboards of
the organ.
While switch 14 is shown schematically as being of mechanical
single pole, single throw (SPST) structure, it will be understood
that any form of switch, electronic, electromechanical,
electromagnetic, and so forth, may be utilized, the exact nature of
the switch depending primarily upon the nature of the energization
produced upon operation of the associated key. Switch 14, then, is
adapted to respond to the particular form of energization or
actuation produced upon operation of a key on any keyboard (or, as
observed above, may itself constitute the key), to complete the
circuit connecting associated diode 13 between a respective input
bus 10 and a respective output bus 12 at the intersection of those
buses, when the key is depressed, and to open the circuit
connecting the diode between respective input and output buses at
that intersection when the key is released. Positive pulses
occurring at the rate of approximately 200 per second, for example,
according to the timing established by master clock 5, are
transferred from input bus 10 to output bus 12 via the respective
diode 13 and closed switch 14 when the associated key is depressed.
While a switch alone (i.e., without the series connected diode)
would serve the basic purpose of transferring a signal between the
input and output lines of array 11, the diode provides a greater
degree of isolation form sources of possible interference (noise)
and acts to prevent feedback from output to input lines.
In FIG. 3, the output buses 12 from switching array 11 are
connected to an encoder circuit 15 to which are also connected the
12 output lines, generally designated by reference number 16, from
keyboard counter section 2. To produce an orderly arrangement in
which each key of the organ is assigned a distinct and different
time slot in a time-division multiplex waveform, the switches 14
associated with the respective keys are conveniently arranged in a
specific sequence in the switching array 11. Assume, for example,
that a specific output bus 17 of the switching array is to be
associated with note A of any octave, a second output bus 18 is to
be associated with note B of any octave, and so forth. Then
switches 14 in the row corresponding to output bus 17 in array or
matrix 11 are associated with the keys corresponding to the note A
in each octave of keys in the organ. The column position of each
switch 14 in matrix 11 corresponds to a specific octave of keys in
the organ, and hence, to a specific octave encompassed by a
specific keyboard of the organ.
Each of the output buses 12, including 17, 18, and so forth, is
connected to one of the two input ports or terminals of a
respective AND gate of the 12 AND gates 20-1, 20-2, 20-3, ...,
20-12, of encoder circuit 15. An output lead 16 of counter section
2 associated with the ring counter stage designating the count for
a particular note (key) in a given octave is connected to the
remaining port of an encoder circuit AND gate having as its other
input a pulse on the output bus 12 associated with that same note.
A similar arrangement is provided for each of the remaining 11
output lines 16 of counter section 2 with respect to the AND gates
20 and the output buses 12. Thus, for example, if output bus 17
(associated with the row of switches 14 in matrix 11 for note A) is
connected to one input terminal of AND gate 20-1, then output line
22 from the stage of counter 2 designating the count associated
with note A is connected to the remaining input terminal of gate
20-1. The output terminal of each of AND gates 20 is connected to a
respective input terminal of OR gate 23, the output of the OR gate
constituting the output signal of the encoder circuit. By virtue of
its structure, encoder circuit 15 is effective to convert the
parallel output of array 11 to a serial output signal in accordance
with the scanning of output buses 12 as provided by the advancing
and repeating count sensed in the form of pulses (at a rate of
about 200 per second) appearing on output lines 16. The end result
of this circuitry is the production of a time-division multiplex
(TDM) signal on a single conductor 25 emanating from encoder
15.
As an alternative to the specific logic construction shown for
encoder 15 in FIG. 3, the encoder may have the circuit
configuration exemplified by FIG. 3A. Referring to the latter
Figure, the encoder includes a shift register 80 having 12 cascaded
stages designated SR1, SR2, SR3, ..., SR12, each connected to a
respective output bus 12 of switching matrix 11 to receive a
respective output pulse appearing thereon. The shift register
stages are loaded in parallel with the data read from switching
array 11 on output buses 12, in response to each of the pulses
appearing (i.e., each time a pulse appears) on one of the 12 output
leads 16 of note counter 2. That one output of the note counter
which is to supply the load command for all 12 stages of shift
register 80 is selected to permit the maximum amount of settling
time to elapse between each advance of octave counter 3 and
keyboard counter 4 and the loading of the shift register. In other
words, it is extremely desirable that the data to be entered into
the shift register from the switching array be stabilized to the
greatest possible extent, and this is achieved by allowing the
counters whose scanning develops this data, to settle at least
immediately prior to loading. Thus, the first note counter stage,
or one of the early stages, is selected to provide "load" pulses to
shift register 80.
"Shift" pulses are supplied to the shift register by master clock
5, which also supplies note counter 2, to shift the contents of
each shift register stage to the next succeeding stage except
during those bit times when the shift pulse is preempted by a load
pulse from the note counter. Accordingly, shift register 80 is
parallel loaded, and the data contents of the register are then
shifted out of the register in serial format on encoder output line
25 until a one-bit pause occurs when another set of data is
parallel loaded into the shift register, followed again by serial
readout on line 25. This serial pulse train constitutes the time
division multiplexed output signal of encoder 15 just as in the
embodiment of FIG. 3, except that with the FIG. 3A configuration,
decoder 7 (and the counters 3 and 4 supplying pulses thereto)
undergo a greater amount of settling time.
It will be observed that this operation constitutes a
parallel-to-serial conversion of the information on output buses 12
to a time-division multiplexed waveform on the output line 25 of
encoder 15.
In the TDM signal, each key has a designated time slot in the 384
time slots constituting one complete scan of every keyboard of the
organ. In the specific example of the time base provided by master
clock 5, the TDM waveform (shown by way of example in FIG. 5) is
initiated about 200 times per second. The waveform contains all of
the note selection information, in serial digital form on a single
output line, that had heretofore required complex wiring
arrangements. This waveform development will be more clearly
understood from an example of the operation of the circuitry thus
far discussed. It should be observed first, however, that all of
the counter and logic circuitry described up to this point can be
accommodated within a very small volume of space by fabrication in
integrated circuit form using conventional microelectronic
manufacturing techniques.
When the main power switch for the electronic organ is turned on,
all components are energized to an operational state, the master
clock delivering pulses to keyboard counter 1 at the aforementioned
rate. Upon depression of a key on any keyboard of the organ,
including the manuals and pedal divisions, a respective switch 14
associated in series connection with a diode 13 at the intersection
between the appropriate input bus 10 and output bus 12 of the
switching array 11 is closed, thereby connecting the two buses to
supply pulses appearing on a given bus 10 from decoder 7, to the
appropriately connected output bus 12 for application to encoder
15. If, for example, the key that was depressed is associated with
note C in the second octave, C.sub.2 appears in the appropriate
time slot of the multiplexed signal emanating from encoder 15 and
will repetitively appear in that time slot in each scan of the
keyboards of the organ as long as that key is depressed. That is to
say, a pulse appears on output line 10 of decoder 7 associated with
the second octave in the manual being played, in accordance with
the scan provided by master clock 5, as the counter stage
associated with that octave is energized in keyboard counter octave
section 3 and the counter stage associated with that manual is
energized in section 4 of the keyboard counter. The connection
between the appropriate input bus 10 and output bus 12 of switching
array 11 for the particular octave and keyboard under consideration
is effected by the depression and continued operation of the key
associated with the switch 14 for that intersection in the array.
Since, as previously stated, each switch is associated with a
particular note (key) and is positioned in a specific row of the
switching array, a signal level is thereby supplied to the
appropriate output bus 12 of the switching array arranged to be
associated with that note. Each time the specified note, here the
note C, is scanned in the sequence of count in the note section 2
of the keyboard counter, a second input is provided to the AND gate
20 receiving the signal level on output bus 12, and a pulse is
delivered to OR gate 23. By virtue of this operation, the pulse
which appears at the output of OR gate 23 always appears in the
identical specified time slot in the multiplexed signal for a
specific note associated with a particular key on a particular
keyboard of the organ.
If more than one key is depressed, regardless of whether one or
more keyboards is involved, operation corresponding to that
described above for a single depressed key is effected for every
operated key. Thus, for example, assume that the key associated
with note C.sub.2 is played on one manual, the note B.sub.4 is
played on a second manual, and the notes D.sub.5, E.sub.5, and
G.sub.5 are played on a third manual, the associated keys being
depressed substantially simultaneously to produce desired
simultaneous reproduction of all notes as the audio output of the
organ. Under these conditions, the associated switches 14 in the
switching array 11 are closed to provide through connections
between the respective input buses 10 and output buses 12 for the
specific octaves and manuals involved. As the appropriate AND gates
20 in encoder 15 are supplied with gating signals from the
sequentially energized counter stages of note section 2, during the
scanning operation provided by that keyboard counter section, pulse
levels appearing on output buses 12 for which switches 14 have been
closed are gated in the appropriate time slots of the multiplex
signal on the output lead 25 from OR gate 23 of encoder 15, for the
specific notes involved.
An example of the multiplex signal waveform thus generated is shown
in FIG. 5. While the pulses appearing in the time slots associated
with the specific notes mentioned above are in a serial format or
sequential order, their appearance is repetitive during the
interval in which the respective keys are actuated. Hence, the
effect is to produce a simultaneous reproduction of the notes as an
audio output of the organ, as will be explained in more detail in
connection with the description of operation of the tone generation
section.
Referring now to FIG. 6, the multiplexed signal arriving from
encoder 15 is supplied to generator assignment logic network 26
which functions to assign a tone generator 28 to a depressed key
(and hence, to generate a particular note) when the associated
pulse first appears in its respective time slot in the multiplexed
signal supplied to the assignment logic. If only 12 tone generators
28 are available in the particular organ under consideration, for
example, the assignments are to be effected in sequence (order of
availability), and once particular pulses have been directed to all
of the available generators (i.e., all available tone generators
have been "captured" by respective note assignments), the organ is
in a state of saturation. Thereafter, no further assignments can be
made until one of more of the tone generators is released. The
availability of 12 (or more) tone generators, however, renders it
extremely unlikely that the organ would ever reach a state of
saturation since it is quite improbable that more than 12 keys
would be depressed in any given instant of time during performance
of a musical selection. The output waveforms from the captured tone
generators at the proper frequencies for the notes being played,
are supplied as outputs to appropriate waveshaping and
amplification networks and thence to the acoustical output speakers
of the organ. If the tone generators 28 supply a digital
representation of the desired waveform, as is the case in one
embodiment to be described, then the digital format is supplied to
an appropriate digital-to-analog converter, which is turn supplies
an output to the waveshaping network.
At any given instant of time, each tone generator 28 may be in only
one of three possible states, although the concurrent states of the
tone generators may differ from one tone generator to the next.
These three states are as follows:
(1) a particular note represented by a specific pulse in the
multiplexed signal has captured (i.e., claimed) the tone
generator;
(2) the tone generator is presently uncaptured (i.e., unclaimed or
available), but will be captured by the next incoming pulse in the
multiplexed signal associated with a note which is not presently a
tone generator captor; and
(3) the tone generator is presently available, and will not be
captured by the next incoming pulse. It should be apparent from
this delineation of possible states that any number of the tone
generators provided (12, is this particular example) may be in one
or the other of the states designated (1) and (3), above, but that
only one of the tone generators can be in state (2) during a given
instant of time. That is, one and only one generator can is the
next generator to be claimed. When the specific tone generator in
state (2) is claimed by an incoming pulse, the next incoming pulse
which is not presently claiming a tone generator is to be assigned
to the generator that has now assumed state (2). For example, if
the third tone generator (-3) of the 12 generators is captured by
an incoming pulse (note representation) and the fourth generator
(-4) was and still is captured by a previous note selection, then
tone generator -4 is unavailable to the next incoming pulse, and
the privilege of capture must pass to the next tone generator which
is not presently is a state of capture. If all of the tone
generators are captured, that is, all are in state (1) as described
above, then the organ is saturated and not further notes can be
played until at least on of the tone generators is released. As
previously observed, however, the saturation of an organ having 12
(or more) tone generators is highly unlikely.
Generator assignment system 26 is utilized to implement the logic
leading to the desired assignment of the tone generators 28, and
thus to the three states of operation described above. An exemplary
embodiment of the generator assignment logic is shown in FIGS. 7A
and 7B. Referring to FIG. 7A, a ring counter 30, or a 12-bit
recirculating shift register in which one and only one bit position
is a logical "1" at any one time, is used to introduce a claim
selection, i.e., to initiate the capture, of the next available
tone generator in the set of tone generators 28 provided in the
organ. A shift signal appearing on line 32 advances the "1" bit
from one register or counter stage to the next, i.e., shifts the
"1" to the next bit position. Each bit position is associated with
and corresponds to a particular tone generator, so that the
presence of the logical "1" in a particular bit position indicates
selection of the tone generator to be claimed next, provided that
it is not already claimed.
Each time the logical "1" appears in a stage of shift register 30,
a "claim select" signal appears on the respective output line 34
associated with the stage. This "claim select" signal is supplied
in parallel to one input of a respective one of AND gates 35, on
line 36, and to further logic circuitry (to be described presently
with reference to FIG. 7B), on line 37. The output line of each of
AND gates 35 is connected to a separate and distinct input line of
an OR gate 40 which, in turn, supplies an input to an AND gate 42
whose other input constitutes pulses from the master clock 5.
In operation of the portion of the generator assignment logic shown
in FIG. 7A, assume that shift register stage -2 contains the
logical "1." That stage therefore supplies "claim select 2" signal
to the respectively associated AND gate 35 and, as well, to further
logic circuitry on line 37. If this further logic circuitry
determines that the associated note generator may be claimed, a
"claimed" signal is applied as the second input to the respectively
associated AND gate 35. Since both inputs of that AND gate are now
"true", an output pulse is furnished via OR gate 40 to the
synchronization gate 42. The latter gate produces a "shift" pulse
on line 32 upon simultaneous occurrence of the output pulse from OR
gate 40 and a clock pulse from master clock 5. Accordingly, the
logical "1" is advanced one-bit position, from stage -2 to stage -3
of shift register 30, in preparation for the claiming of the next
tone generator.
Suppose, however, that the tone generator 28 corresponding to stage
-3 is already claimed by a previous note pulse in the multiplexed
signal. In that event a "claimed" signal appears as one input to
the associated AND gate 35, and with the "claim select" signal
appearing as the other input to that gate by virtue of stage -3
containing the single logical "1," another shift pulse is
immediately generated on line 32 to advance the logical "1" to
stage -4 of the shift register. Similar advancement of bit position
of the "1" continues until an unclaimed tone generator is selected.
If it should happen that no note is presently being selected on a
keyboard of the organ at the time when an unclaimed tone generator
is selected, the "1" bit remains in the shift register stage
associated with the selected tone generator until such time as a
"claimed" signal is concurrently applied to the respective AND gate
35, i.e., until the selected tone generator is claimed, because
until that time no further shift signals can occur.
Referring now to FIG. 7B, each tone generator also has associated
therewith a respective portion of the generator assignment logic as
shown in the figure, In other words, the circuitry of FIG. 7B, with
minor exceptions to be noted in the ensuring description, is
associated with the i'th tone generator (where i=1, 2, 3, ..., 12),
and since each of these portions of the assignment logic is
identical, a single showing and description will suffice for all.
An AND gate 50 has three inputs, one of which is the multiplexed
signal deriving from encoder 15 (this being supplied in parallel to
the AND gates 50 of the remaining identical portions of the
assignment logic for the other tone generators, as well), a second
of which is the "claim select" signal appearing on line 37
associated with the i'th stage of shift register 30 (FIG. 7A), and
the third of which is a signal, on line 52, indicating that the
pulse in the multiplexed signal has not captured any tone generator
as yet. Of course, these signals are not present unless the
respective events which produce them are actually occurring, but if
all three signals are simultaneously presented as inputs to AND
gate 50, a "set" signal is applied to a claim flip-flop 53 to
switch that flip-flop to the "claimed" state and simultaneously
therewith to supply a "claimed" signal to the AND gate 35
associated with the i'th stage of shift register 30 and to the
respectively associated tone generator 28.
A modulo 384 counter 55 is employed to permit recognition by the
respective portion of the generator assignment logic of the
continued existence in the multiplexed signal of the pulse (time
slot) which resulted in the capture of the associated tone
generator. To that end, counter 55 is synchronized with keyboard
counter 1 (also a modulo 384 counter) by simultaneous application
thereto of clock pluses from master clock 5. The count of each
counter 55 associated with an uncaptured tone generator is
maintained in synchronism with the count of keyboard counter 1 by
application of a reset signal to an AND gate 58 each time the
keyboard counter assumes a zero count; i.e., each time the count of
the keyboard counter repeats. However, that reset signal is
effective to reset counter 55 only if the associated tone generator
is uncaptured. The latter information is provided by the state of
flip-flop 53, i.e., a "not claimed" signal is supplied as a second
input to AND gate 58 whenever flip-flop 53 is in the "unclaimed"
state.
When the flip-flop (and hence, the associated tone generator) is
claimed, however, it is desirable to indicate the time slot
occupied by the pulse which effected the capture, and for that
reason a "reset" signal is applied to counter 55 at any time that
an output signal is derived from AND gate 50. Thus, in the captured
state, the zero count of counter 55 occurs with each repetition of
the "capturing" pulse in the TDM waveform. Such information is
valuable for a variety of reasons; for example, to prevent capture
of an already captured tone generator when the zero count continues
to appear simultaneously with a pulse in the TDM waveform, and to
provide a "key released" indication when the zero count is no
longer accompanied by a pulse in the TDM waveform Capture
prevention is effected by feeding a signal representative of zero
count from counter 55 to the appropriate input terminal of an OR
gate 60 associated with all of the tone generators and their
respective generator assignment logic. The logical "1" supplied to
OR gate 60 is inverted so that simultaneous identical logical
inputs cannot be presented to AND gate 50. On the other hand, when
the zero count is merely synchronized with the zero count of the
keyboard counter and is not the result of capture of the associated
tone generator id does not interfere with subsequent capture of
that tone generator since it does not occur simultaneously with a
pulse in the TDM signal. A "key release" indication is obtained by
supplying the "zero count" signal to an AND gate 62 to which is
also supplied any signal deriving from an inverter 63 connected to
receive inputs from the TDM signal. If the zero count coincides
with a pulse in the multiplexed signal, the inversion of the latter
pulse prevents an output from AND gate 62, and this is proper
because the coincidence of the zero count and the TDM pulse is
indicative of continuing depression of the key which has captured
the generator. Lack of coincidence is indicative that the key has
been released, and results in the "key release" signal. Scanning of
the keyboards is sufficiently rapid that any delay which might
exist between actual key release and initiation of the "key
release" signal is negligible, and in any event is undetectable by
the human senses. Furthermore, the generation of a false "key
release" signal when the tone generator is presently unclaimed, as
a result of the occurrence of a zero count from counter 55
synchronized with the zero count of the keyboard counter and the
simultaneous absence of a pulse in the TDM signal, can have no
effect on the audio output of the organ since the associated tone
generator is not captured and is therefore not generating any tone.
In any case, the "key release" signal deriving from AND gate 62 is
supplied at attack/delay logic of the tone generator to initiate
the decay of the generated tone.
The "set claim" signal output of AND gate 50 that occurs with the
simultaneous appearance of the three input signals to that gate is
utilized to provide a "key depressed" indication to the
attack/decay circuitry of the tone generator (and to percussive
controls, if desired), as well as to provide its previously recited
functions of "setting" flip-flop 53 and "resetting" counter 55.
The assignment logic embodiment of FIGS. 7A and 7B may be
associated with only a small number of tone generators (12, in the
example previously given), the exact number being selected in view
of the cost limitations and the likely maximum number of keys that
normally may be actuated simultaneously. In that case, each tone
generator must supply every desired frequency corresponding to
every not in every octave that may be played on the electronic
organ. To that end, a digital tone generator of the exemplary
configuration shown in block diagrammatic form in FIG. 8 is
employed.
Before describing the cooperative structural and functional
relationships between the elements of the tone generator shown in
FIG. 8, it is instructive to consider some of the available
alternatives in the construction and operation of digital tone
generators for ultimately generating a desired audio frequency for
a note corresponding to an actuated key. When a key is depressed on
any keyboard of the digital electronic organ, a waveform is to be
generated with a periodicity corresponding to the desired note
frequency in the audible range. The waveform is computed in digital
format consisting of a series of numbers (digital words) which
represent the magnitude of the waveform at a series, or sequence,
of uniformly spaced sample points. The digital sample point values
thus generated are subsequently converted to analog form.
The sample points are preferably uniformly spaced because such a
format permits the most direct analysis, and therefore the most
direct synthesis, of the desired waveform. If desired, the uniform
spacing of sample points may be such that there is provided an
integral number of samples per cycle for each note frequency to be
generated. Such a technique requires a sampling rate that varies
directly with the frequency. Alternatively, the samples may be
spaced uniformly in time, in which case the phase angle between
samples points varies with the frequency of the note to be
generated. Although the synthesis of a multiplicity of note
frequencies can be implemented for either technique, using a single
clock frequency, the preferred frequency synthesis technique is
that in which the phase angle between the sample points varies with
frequency, i.e., in which the sampling rate is fixed for all note
frequencies to be generated, and the various generated note
frequencies are produced as a result of the different phase
angles.
FIG. 8 shows, in block diagram form, a specific exemplary structure
of a tone generator for generating the required note frequencies of
the organ from a memory containing amplitude samples of the desired
waveform obtained at uniformly spaced points in time. The sample
points are accessed at a fixed, single clock frequency for all note
frequencies to be generated and the phase angle between the sample
points thereby varies with the frequency of the note to be
generated. The tone generator includes, as basic components, a
phase angle register 101, a sample point address resister 102, a
read-only memory 103, an address decoder 103d, an accumulator 104,
a sampling clock 105, and a comparator 107. As will be apparent
hereafter, the phase angle calculator 100 and the read-only memory
103 may be shared by all of the tone generators 28. In addition,
each tone generator is addressed or accessed individually and in
sequence and thus once in each cycle of addressing all tone
generators. For that reason, the sampling clock 105 may comprise a
clock rate provided by a master sampling clock, successive clock
pulses of which are directed to the series of tone generators. The
sampling clock addressed to a given tone generator is thus at a
rate comprising the pulse repetition rate of the master sampling
clock divided by the number of tone generators provided in the
system. Further, since the same read-only memory may be addressed
by all tone generators, the accumulator 104 may be a composite
structure associated with appropriate gating circuitry related to
each tone generator for accumulating the information read from the
memory 103 in response to accessing thereof by a given tone
generator.
When a claim flip-flop of the tone generator assignment logic, such
as flip-flop 53 (FIG. 7B), is switched to the claimed state in
accordance with capturing of a pulse in the incoming multiplexed
waveform by a given tone generator 28, the phase angle calculator
100 is instructed to determined the appropriate phase angle for the
frequency of the note to be reproduced as identified by the
captured pulse. A determination of the value of the phase angle
constant, and hence, of the particular not corresponding to the key
that has been actuated, is initiated by supplying both the count
from the main keyboard counter 1 and the count of the modulo 384
counter 55 (e.g., of FIG. 7B) associated with the captured
flip-flop, and which is reset to zero upon that capture, to a count
comparator 107. Comparator 107 subtracts the count of counter 55
from the count of the keyboard counter 1 and supplies a number
representative of the difference, and hence, representative of the
time slot position corresponding to a particular note (i.e., that
note which captured the flip-flop), to phase angle calculator 100.
The difference computed by comparator 107 will always be positive,
or zero, because the computation is elicited from the comparator
only when the associated flip-flop 53 is captured and at that
moment counter 55 is reset to zero, whereas the keyboard counter
probably has some greater count or contains a least count, i.e.,
zero.
On the basis of the difference count supplied by comparator 107,
calculator 100 is informed as to the note for which the phase angle
calculation is to be performed, i.e., the note and thus the
frequency to be produced by the tone generator. The calculator 100
may compute the phase angle as a function of the frequency of the
note to be reproduced and of the number of memory sampling points
of the waveform in storage and thus as approximately equal to the
phase angle of the fundamental between adjacent memory sampling
points for the frequency to be produced. An alternative embodiment
of the phase angle calculator 100 is a conventional storage unit
with look-up capabilities, or simply a memory from which the
correct phase angle is extracted when the memory is suitably
addressed with the identification of the count of the captured
pulse. Alternatively, a combination of a memory with look-up
capabilities and of a calculator capable of computation for
determination of the phase angles may be employed. The synthesis of
note frequencies in accordance with the digitally stored waveform
sample points may be arbitrarily as accurate as desired and, in
effect, provides a true equally tempered scale of the synthesized
not frequencies wherein the notes within the scale differ by the
power of 2.sup. 1/12. The degree of accuracy in a practical system,
however, must be realized within a finite maximum information
content and thus the stored phase angles are quantized and rounded
off.
The phase angle thus developed is supplied to and stored in the
phase angle register 101. Thus, upon capture of a given tone
generator, a command control means such as flip-flop 53 which
establishes the captured state of the tone generator controls the
operation of the comparator 107 and, in turn, the phase angle
determination function of the phase angle calculator 100 for the
given note frequency to be generated, for supply of that phase
angle to the register 101. Since this operation must preceded the
addressing function, a delay may be provided (as by use of a delay
multivibrator 106) to actuate a switch 108 for passage of pulses
from the sampling clock source 105 (which may be an appropriately
gated pulse from a master sampling clock source) to registers 101
and 102.
If desired, the sample point address register 102 may be cleared
when claim flip-flop 53 reverts to a noncaptured state, so that it
is prepared for entry of information from the phase angle register
101 upon each calculation. However, it is important to note that
during accessing of the memory it is the rate at which the value of
register 102 increases and not the absolute value thereof which is
significant in the control of the rate of read out of the memory
103 and thus the cyclic frequency of read out of the memory and,
ultimately, the frequency of the note reproduced by the give tone
generator.
Once each sampling clock time as determined by the sampling clock
source 105, the phase angle value stored in phase angle register
101 is added to the previously stored value of the sample point
address register 102. An address decoder 103a decodes preselected
bit positions of the count established in register 102 to effect
accessing, or addressing, of the memory, 103. The transfer from the
register 101 to the register 102 is a nondestructive transfer such
that the phase angle value is maintained in the register 101 as
long as that tone generator is captured by a given pulse.
Thus, once each clock time, the phase angle register value,
comprising a digital binary word, is added to the sample point
address register value and correspondingly, for each such clock
time, the memory location corresponding to the sample point address
then existing is the register 102 is accessed. As a practical
matter, only a relatively small, finite set of amplitudes can be
stored in the memory 103, because of practical limitations on its
capacity, and thus only a finite number of addresses are available.
Furthermore, the registers such as 101 and 102 must be of a finite,
practical length. In particular, the length of the phase angle
register 101 is determined by the accuracy with which the frequency
of the note is to be generated. The frequency actually produced
will be exactly the value of the phase angle in register 101 times
the memory sampling rate. The sample point address register 102, on
the other hand, must be sufficiently long to accept data from the
phase angle register 101. The register 102, however, preferably
includes additional bit positions which are not used, or not used
at all times, for accessing the memory. In this respect, it will be
apparent that one-bit position in the register 102 is scaled at one
cycle of the fundamental of the frequency of the note to be
generated. A set of next successive less significant bits may
therefore specify the sample point address in accordance with the
function of the decoder 103a. The more significant bits of the
register 102 may be used to count numbers of cycles of the waveform
for various control functions not here pertinent. In addition, by
selecting appropriate bit positions by means of decoder 103a, the
frequency of the note reproduced may be readily adjusted to
different octaves. That is, a one-bit positional shift constitutes
division or multiplication by two, depending upon direction of
shift. For example, if the most significant bit is numbered 1 and
thus bit positions 2 through 6 comprise the sample point address
bits normally used for an 8 foot voice, then a 16 foot voice can be
obtained by using bits 1 through 5 as the sample point address
source. Correspondingly, a 4 foot voice can be obtained by using
bits 3 through 7 as the sample point address bits.
The read-only memory 103 contains digital amplitude values of a
single cycle of the complex periodic waveform to be reproduced for
all note frequencies. That is to say, the same complex periodic
waveform is to be reproduced for each note played, the only
difference being the frequency at which the complex waveform is
reproduced.
Referring to FIG. 9, illustrating a typical complex waveshape 110
of the type that may be produced by a pipe organ, the wave may be
sampled at a multiplicity of points, shown as vertical lines in the
FIG., to provide the amplitude data for storage in memory 103. If
absolute amplitude data is stored in memory 103, then the data
accessed is the actual amplitude of the output waveform at the
respective sample points (i.e., with respect to a "zero" level at
time axis 111). In that event, the digital amplitude data
successively read from the memory may be applied directly to an
appropriate digital-to-analog conversion system. On the other hand,
if incremental amplitude information (i.e., simply the difference
in amplitude between the present sample and the immediately
preceding sample) is stored in memory 103, then the data accessed
must be added to an accumulator (e.g., 104 in FIG. 8) to provide
the absolute amplitude information at each sample point prior to
digital-to-analog conversion. Each of the sample points of the
memory 103 may comprise a digital word of approximately seven or
eight bits.
The digital words thus read out from the memory 103 are supplied to
the accumulator 104 which provides a digital representation of the
waveform at selected sample points over a cycle of the waveform and
at a frequency corresponding to the note to be reproduced. As above
described, this digital waveform representation may itself be
operated upon for waveshape control, e.g., attack and decay, and
subsequently is supplied to a digital-to-analog converter for
producing an analog signal suitable for driving the acoustical
output means, such as audio speakers, of the organ.
Memory 103 may be a microminiature diode array of the type
disclosed by R. M. Ashby et al. in U.S. Pat. No. 3,377,513, issued
Apr. 9, 1968, and assigned to the same assignee as is the present
invention. The array may, for example, contain an amplitude
representation of the desired waveform in the form of an eight-bit
binary ward at each of 48 or more ample points. Such a capacity
permits the storage of up to 128 amplitude levels in addition to a
polarity (algebraic sign) bit. In any event, the capacity of memory
103 should be sufficient to allow faithful reproduction of note
frequencies.
If whole values of amplitude levels at the sample points of the
waveform are read from memory 103 in the embodiment of FIG. 8, the
same sample point may be addressed several times in succession.
This is the result of the requirement that the memory be accessed
at a fixed rate for every note frequency, a requirement which
implies that for decreasing note frequencies an increasing number
of sample points must be read out during each cycle; and since the
number of sample points is fixed and no sample points can be
skipped regardless of note frequency, this simply means repetition
of the same sample point possibly several times in succession. This
does not undesirably affect the ultimate waveform generated,
however, because there is consistent plural sampling of each point
of the stored waveform.
On the other hand, if incremental values of the waveform have been
stored in memory 103, each increment can be read out only once
during each cycle of the waveform. THis is because an accumulation
of incremental values is required, and repetition will produce a
significant error in the accumulation and the ultimate waveform to
be generated, regardless of the note frequency. Since the same
sample point may be read out of memory 103 several times in
succession depending upon the note frequency to be produced, just
as in the whole value sample point case noted above, for
incremental values all but one readout for each sample point must
be inhibited to prevent repetitive application to accumulator 104.
To that end, a gate 103b (shown dotted in FIG. 8) is positioned in
the output line of memory 103 preceding accumulator 104 if
incremental values are utilized. Gate 103b is preferably enabled to
pass the sample value being read from the memory only when the
least significant bit in address register 102 changes. Since such
change occurs upon a "carry" into that position, indicating
advancement to the next memory address, a bit change sensor 102a
many be used to detect the change and to enable gate 103b at each
advancement to a new address. The same sample point may still be
accessed several times in succession, but only one such value will
be "read out" (i.e., will be passed by the gate since it is
disabled at all other times).
The phase angle calculations should be such that the highest note
playable is that note for which a sample point value is read out
each time the memory is addressed. Since the ratio between adjacent
notes on the equally tempered musical scale is an irrational
number, it is preferable that the largest number is the phase angle
register be slightly smaller then the least significant bit in the
address register. If the phase angle number were larger, it would
be necessary to occasionally skip a sample point and this would
lead to inconsistency in the note frequency, whereas if the phase
angle number were equal to the least significant bit in the address
register the note frequency would be slightly higher (i.e., about
one-half of a halftone higher) than the highest note that can be
played. By requiring the phase angle number to be slightly smaller,
the highest note capability of the instrument will not be
exceeded.
The same read-only memory 103 may be shared by all of the tone
generators 28 if the data words (amplitude values of sample points)
read therefrom are gated to respective wave shapers in synchronism
with the addressing of the memory for the respective notes being
played. In other words, simultaneous or concurrent play to two or
more notes requires that these be distinguished as separate sets of
sample points, if a single memory is to be shared for all tone
generators.
In the present embodiment, however, it is assumed that each tone
generator has its own memory (and, incidentally, memories composed
of microminiature diode arrays of the type disclosed in the
aforementioned Ashby et al. patent are readily fabricated with more
than 5,000 diode elements per square inch), which supplies its
digital output to a respectively associated attack and decay
control unit. The binary-valued amplitude samples are applied
directly to the attack and decay circuitry if each sample is a
whole value, or may be applied via an accumulator 104 if each
sample is an incremental value. Alternatively, accumulation of
incremental values may be preformed after shaping, if desired.
Referring the FIG. 10, an embodiment of the attack and decay unit
associated with each tone generator included a multiplier 120 to
which the sample values from memory 103 are applied for
multiplication by an appropriate scale factor or control the
leading the trailing portions of the note waveform envelope. As is
well know, the faithful simulation of true pipe organ sounds by an
electronic organ requires that the latter be provided with the
capability to shape each tone envelope to produce other than an
abrupt rise and fall. Without special attack and decay control, the
note waveform produced by an electronic organ normally rises
sharply to full intensity immediately upon depression of the
respective key, and ceases abruptly when that key is released. At
times, this may be a desirable effect to maintain during the play
of a musical selection. In those cases, the attack and decay
controls may be avoided entirely, or the scale factor supplied to
multiplier 120, and with which the amplitude samples are to be
multiplied, may be set at unity. More often, however, attack and/or
decay are desirable for or in conjunction with special effects such
as percussion, sustain, and so forth.
The multiplying scale factor is varied as a function of time to
correspondingly vary the magnitude of the digital samples, with
which it is multiplied, on a progressive basis to simulate attack
and/or decay. In the embodiment of FIG. 10, the total time duration
and the time constant(s) for the attack or decay are controlled by
a counter 122 which may be selectively supplied with uniformly
timed pulses that are independent of the specific note frequency
under consideration, such as pulses obtained or derived from the
master clock, or with pulses having a repetition rate
representative of or proportional to the note frequency. In this
respect, the counter 122 may be considered as determining the
abscissa of a graph of envelope amplitude versus time and
representative of the attack or decay. The ordinate or amplitude
scale of the graph is represented by the series of scale factors
stored in a read-only memory 125 to be accessed by the counter
itself, or by an address decoder 126 which addresses the memory for
readout of scale factors on the basis of each count (or timed,
separated counts) of counter 122.
The counter may be of the reversible, up-down (forward-backward)
type in which it is responsive to incoming pulses to count upwardly
when its "up" (here, attack) terminal is activated, and to count
downwardly when its "down" (here, decay) terminal is activated. The
attack mode of the overall control unit is entered when the
associated tone generator is captured by a hitherto unclaimed note
pulse in the multiplexed signal. The capture of a tone generator is
accompanied by a signal indicative of a key having been depressed
(see FIG. 7B), form the assignment logic, and it is this signal
which initiates the attack count of counter 122. In particular the
first "key depressed" signal (and possibly the only one) that
occurs upon capture of a tone generator 28 is effective to produce
a count in the first stage of ring counter 128, thereby supplying a
trigger signal form that stage to a monostable delay multivibrator
130 which is set to have an ON time (delay time) of sufficient
duration to ensure that the attack is completed despite release of
the key prior to the normal end of the attack interval. It has been
found that a delay time equal to or greater than approximately the
time occupied by seven cycles (i.e., seven periods) of the lowest
frequency note is quite adequate for multivibrator 130 to ensure
this positive attack. During that interval, the "up" control of
counter 122 is activated by the quasi-stable state of multivibrator
130 and the counter continues to count incoming pulses until the
multivibrator spontaneously returns to its stable state, or until
the note envelope reaches the full desired intensity (magnitude),
if earlier. This full intensity value may be preset into the
attack/decay control logic or it may be determined by logic
circuitry responsive to such factors as the force with which the
respective key is struck (i.e., to velocity-responsive or
touch-responsive device outputs). In the embodiment shown in FIG.
10, the former arrangement is utilized in which a maximum desired
count is set into a fixed counter 131 for continuous comparison in
comparator 133 with the present count of up-down counter 122. If
the latter exceeds the former, a "disable" command is applied to
the counter to terminate the attack.
Pulses to be counted by counter 122 may be obtained at a rate which
is a function of note frequency, as by supplying the output of
phase angle calculator 100 to a phase-to-frequency converter 135,
or at a rate based on the master clock rate, whichever is desired.
Selection of either rate is accomplished by appropriately setting a
switch 136 coupled to an associated switch or key on or adjacent to
one of the keyboards.
In operation of the attack/decay control unit of FIG. 10, after
switch 136 has been set at the desired position, the pulses to be
counted appear at the input of counter 122 but no count is
initiated until a key is depressed and the associated pulse in the
multiplexed signal from the keyboard results in the capture of a
tone generator 28. The "key depress" signal from the generator
assignment logic initiates a count in ring counter 128, which bad
been reset by completion of decay the immediately preceding time
the attack/decay control unit had been used. Preferably, the latter
reset signal is obtained upon switching of the claim flip-flop 53
in the assignment logic 26 to the "not claimed" (delay complete)
state. The up count of counter 122 is thereby enabled and continues
through completion of attack regardless of whether or not the key
remains depressed. The duration of attack depends on whether the
note frequency mode or the fixed time mode is employed.
With each count of counter 122 (or less frequently, by use of
suitably timed "enabling" commands), address decoder 126 develops a
related address code for accessing a digital scale factor stored in
the appropriate address of read-only memory unit 125, to be
combined as a product in multiplier 120 with the amplitude samples
being read from tone generator 28 of FIG. 8. By presetting memory
125 such that the scale factors stored therein are logarithmically
increasing (up to the equivalent of unity) with addresses decoded
according to progressively increasing count in counter 122 (up to
the maximum desired count, representing full note intensity), a
logarithmic attack is provided in the note being played.
When the key is released, a "key release" signal is applied from
AND gate 62 of assignment logic 26 (FIG. 7B) to a flip-flop 138 to
initiate the decay mode of the attack/decay control unit by
enabling the "decay" (down) count of counter 122. Accordingly,
incoming pulses to the counter are counted downwardly from the
count representative of full intensity, until a zero count is
obtained unless decay is terminated earlier. As in the case of the
attack mode, the count in counter 122 is periodically decoded
(e.g., once each count) by unit 126 for addressing of memory 125,
thereby supplying logarithmically decreasing scale factors, from
unity to zero, for multiplication with amplitude samples form the
tone generator in multiplier 120. This produces the desired fall in
note intensity at the trailing portion of the note waveform.
If during decay the same note pulse should reappear in the
multiplexed keyboard signal, a second "key depress" signal is
applied to ring counter 128 thus increasing the count therein to
the second stage and switching flip-flop 138 from the decay state
to its other state, which reintroduces the attack mode. Since decay
is incomplete in this particular instance, the count of counter 122
now proceeds upward from the minimum count which had been attained
when decay was interrupted. If, however, the key is again released,
prior to completion of attack, positive attack is no longer in
effect and the flip-flop 138 reverts immediately to the decay state
by virtue of application of the "key release" signal thereto.
To prevent flip-flop 138 from being in the "decay" state when the
initial attack condition is established in counter 122 (by the
quasi-stable state of delay MV 130 ), flip-flop 138 may be switched
to its "attack" state upon full completion of decay, by the "not
claimed" signal of associated flip-flop 53.
Upon completion of decay of a note whose representative pulse in
the keyboard multiplexed signal resulted in capture of a tone
generator, a "decay complete" signal is applied to the claim
flip-flop 53 (FIG. 7B) of the respective assignment logic unit to
cause that flip-flop to return to its "not claimed" state, and
thereby to release the tone generator for claiming by another
note.
With reference now to FIG. 11, the digital electronic organ thus
far described is peculiarly suited to a simple automatic
transposition technique by virtue of its keyboard multiplexing
scheme. Each position, or time slot, in the multiplexed signal is
assigned to a particular key (and the note associated therewith) on
each keyboard. As was indicated earlier, the multiplexed signal is
structured such that adjacent time slots therein correspond to
adjacent semitones in the equally tempered musical scale. For
example, pulses associated with notes C.sub.4, C.sub.-4, D.sub.4,
will appear in successive positions in the order recited in the
multiplexed signal, as shown for the single cycle of the
multiplexed signal in FIG. 11, whenever the key switches associated
with those notes are concurrently depressed.
The basic scheme of automatic transposition contemplated by the
present invention is the shifting of note frequencies by a selected
amount during play of the keys on each keyboard in the normal
manner. One technique of accomplishing this objective is shown in
simplified form in FIG. 12, where the multiplexed signal from
encoder 15 (FIG. 1) is applied to a pulse delay device 150 which is
preset, or which is adjustable, to introduce a delay into the
multiplexed signal by a number of time slots, or pulse positions,
equal to the number of halftone transpositions desired, prior to
entry of the multiplexed signal into the tone generator assignment
logic (FIG. 6). Thus, for example, if scanning of the keyboards is
from low frequencies to high frequencies, original time slots 83
and 85 could be subjected to a one-pulse delay (a single halftone
transposition) to accomplish a shift of those slots to time slots
84 and 86, respectively, in the delayed multiplexed signal leaving
delay device 150.
A more detailed circuit diagram of the system of FIG. 12 is shown
in FIG. 13. The multiplexed signal appearing on line 25 from
encoder 15 is supplied, via a normally open gate 151 (i.e., a
completed circuit path for passing signal), to a switch 152 having
a switch arm 153 and a pair of contacts 154 and 155. Depending upon
which of the contacts the switch arm is in electrical contact with,
the system of FIG. 13 is capable of producing an upward shift in
frequency or a downward shift in frequency. For the sake of
example, it is assumed that the scanning of the keyboards is in a
direction from the highest frequency to the lowest frequency.
Again, this is immaterial to the present invention, but if the
scanning is instead from the lowest frequency to the highest
frequency of each keyboard, then a slight modification of the
arrangement shown in FIG. 13 is required, a modification which will
be perfectly obvious to one of ordinary skill in the art from a
consideration of the implementation of the circuit of FIG. 13.
If transposition to a lower frequency is desired, any pulses
occuring in the multiplexed signal are inserted into a 12-bit shift
register 155 in their respective time slots, by positioning switch
arm 153 against contact 154 as shown. Shift register 155 is
effectively a 12-bit delay line with an output tap at each stage.
If no transposition is desired, the output is taken from the first
stage of the shift register since at that point no delay has been
introduced into the multiplexed signal. Shifting of pulses through
the register is effected by pulses from the master clock. A
selector switch 157 may have a knob (not shown) positioned on or
near the keyboards or in any position conveniently accessible to
the organist for presetting the amount of delay, and thus the
extent of transposition, into the system. To that end, switch 157
has a rotatable arm 158 connected to output line 159 of the
transposition system and selectively movable against each contact
associated with the output taps of the twelve stages of shift
register 155. If, for example switch arm 158 is positioned against
the contact associated with line 160 of the 12-bit shift register,
as shown, then a one time slot delay is introduced into the
multiplexed signal. Similarly, positioning of arm 158 against the
contact associated with line 161 connected to the third stage of
shift register 155 will introduce a two-time slot delay into the
multiplexed signal, and so forth for each contact associated with
each succeeding stage of the sift register.
Thus, the organist may select any desired delay up to and including
one complete octave (i.e., 12 semitones) of the organ and thereby
any note is audibly produced by the organ a respective number of
halftones lower than the note with which the actuated key is
normally associated. Of course, the organist selects the desired
amount of transposition prior to playing the musical selection.
When transposition from one key to another is desired, the
transposition selection switch 157 is set to the appropriate stage
of delay unit 155 that will produce the desired time delay, and
thus the desired shift in frequency (downward, in the case of
highest frequency to lowest frequency scanning) for each note
played. Suppose, for example, that the organist desires to play a
musical piece written in the key of C natural in the transposed key
of F in the next lower octave. This requires a time shift of seven
time slots in the multiplexed waveform since the notes B, A.sub.-,
A, G.sub.-, G, and F.sub.- lie between C natural and F natural in
the next lower octave. Accordingly, the arm 158 of transposition
switch 157 is set at the contact connected to the eighth stage of
12-bit shift register 155. Thereafter each note played by the
organist in the key of C natural is transposed automatically to the
key of F natural, the transposed pulse train (i.e., delayed
multiplexed signal) being supplied to input terminal 30 of the
generator assignment logic circuitry 31 (FIG. 6).
If an increase in frequency is desired for the transposition under
the stated conditions of scanning (multiplexing) from high to low
notes, one suitable mechanization is to utilize a delay of N--n,
where
N = total number of time slots in a complete cycle of the multiplex
signal; and
n = number of halftones by which the musical selection is to be
transposed above the nominal frequencies.
That is, if the organist desires to transpose from one musical key
to another in the next higher octave, and continuing with the
assumption that the keyboards are scanned from the highest
frequency to the lowest frequency of the organ, then the time shift
of pulses in the multiplexed waveform by one time slot per halftone
to be transposed will require a sufficient delay to obtain access
to time slots in the next higher octave in the next complete
repetition of the cyclically repeating multiplexed signal. Thus,
the selection for transposition to a key in the next higher octave
will require a time shift of almost, but not quite, a complete
number of time slots in the multiplexed signal. To that end, in the
embodiment of FIG. 13 switch 152 may be set in the higher frequency
mode in which arm 153 is placed against contact 155, so that the
incoming multiplexed signal is supplied to a 372-bit delay line 165
(for the previous example of a multiplexed signal containing 384
time slots). In this manner, sufficient delay has been introduced
prior to introduction of the multiplexed signal into shift register
155, to permit adjustment of the transposition selector switch 157
to transpose from the musical key in which the piece is to be
played to the desired key in the next higher octave. In this
instance, however the organist must place the switch arm 158 of the
transposition selector switch 157 to the contact connected to an
appropriate stage of shift register 155 that will provide automatic
transposition according to a reversal of the previous computation
process. For example, if the organist desires to play a selected
musical piece in the note F natural by transposition from C natural
in the same octave, then the shift must be five semitones, but in a
backward or reverse direction, i.e., a delay of 372+ (12-5)= 372+
7= 379 time slots. Therefore, the organist will set switch arm 158
against the contact connected to the fifth from the last stage of
the 12-bit shift register 155.
When a particular musical piece is transposed in key by appropriate
selection using transposition selector switch 157, it may happen
that certain notes to be selected in the musical piece will be out
of the range of the organ. That is, if the transposition is upward
in frequency, the highest notes in the musical piece to be played
may no longer be available to be sounded in the particular organ in
which the digital multiplexing system is employed. This will
depend, of course on the frequency range encompassed by the tone
generators of the organ. If some redundancy has been provided in
the time division multiplexed waveform, as was previously
discussed, then this presents no particular problem. Thus, if say
10 to 12 time slots at the beginning and/or the end of the
multiplexed signal are not associated with any keys of the organ,
then the upward shift will place the unavailable notes into time
slots in which provision has been made to prevent capture of any
tone generator. Accordingly, no note is sounded when the keys
representative of out of range notes are depressed. On the other
hand, if no redundancy is provided in the TDM signal then some
means should be employed to remove (blank) the pulses for notes
which are out of the range of the organ, or to mute the tones
produced as a result of those pulses. This is because these pulses
will have been shifted to time slots which are representative of
notes in the range of the organ but at the completely opposite end
of the musical scale. The notes may lie in an octave several
octaves below that in which the notes are to be sounded.
To prevent the improper notes from sounding, blanking pulses may be
applied to the inhibit terminal of gate 151 in synchronism with the
undesired pulses in the multiplexed signal. Synchronization of
blanking pulses may be achieved by supplying a count representative
of the extent of delay selected by transposition switch 157 to a
pulse generator (not shown) which also receives pulses from the
master clock and which is effective to generate the blanking pulses
at the master clock rate upon attainment of a desired count.
A typical organ, for example, is exhausted of tone generators above
note C.sub.7, and acceptance of such an upper limitation implies
that, in turn, each of the keys in the upper octave sound blanks as
the transposition is increased in steps to higher frequencies. A
preferred alternative to this blanking of notes which has been
described above, however, is what may be referred to as "octave
folding." In the "octave folding" technique, the notes that would
exceed the uppermost note (C.sub.7, in this example) during
transposition are folded, or shifted, such that they sound in the
next lower octave. An exemplary folding scheme is illustrated in
the following table. ##SPC1## The top line in the above table
illustrates the nominal tones of the sixth octave of the organ,
assuming for the sake of example that this is the highest keyed
octave on any organ manual. If a halftone increase is effected by
transposition in the previously described manner, depressing of the
key associated with note C.sub.6 will cause note C.sub.-6 to sound,
and so forth. Similarly, depressing the key associated with note
B.sub.6 will bring forth note C.sub.7, and at this point the
uppermost note in the organ has been sounded. If the musical
selection as written ranges upward to note C.sub.7, when the
organist depresses the respective key the transposition would
ordinarily demand that C.sub.-7 be sounded. The latter note,
however, is unavailable. According to the "octave folding" feature
of the present invention, when the key associated with note C.sub.7
is depressed the note in the next lower octave corresponding to the
transposed note is the one that is sounded. In this instance, then,
note C.sub.-6 rather than note C.sub.-7 (which is unavailable) is
called forth. This, of course, is appropriate to the note
requested, and is much more pleasing to the listener than would be
the total inhibition (blanking) of a note during play of the
piece.
The situation becomes more critical with increase in degree of
transposition. Thus, referring again to the above table, a one and
one-half tone transposition to higher frequencies will cause
selection of note A.sub.6 to call forth note C.sub.7, the highest
available note. This leaves the next three keys, for notes
A.sub.-6, B.sub.6, and C.sub.7, subject to blanking were it not for
the octave folding technique of the present invention. As a
consequence of the latter, however, the notes C.sub.-6, D.sub.6,
and D.sub.-6, respectively, will be sounded. It will be observed
that each half-note increase in transposition requires that an
additional note in the upper octave be brought into the octave
folding scheme.
FIG. 14 shows one simple arrangement for implementing octave
folding. Each key switch, such as 175, 176, 177, 190, 191, 192, on
each manual of the organ is connected to a source of voltage so
that upon depression of a key switch a keying signal is supplied to
call forth the associated tone. In the keyboard multiplexer
embodiment which has been described, the keying signals produced
upon depression of key switches are supplied to switching array 11
(FIG. 1) to operate respective switches 14 (FIG. 4). In the
uppermost octave, and any desired lower octave for that matter, the
leads via which the keying signals are supplied to the overall
multiplexed signal are wired through respective single pole, double
throw (SPDT) octave folding switches. Lead 193 connects key switch
192 to octave folding switch 194, lead 195 connects key switch 191
to octave folding switch 196, lead 197 connects key switch 190 to
octave folding switch 198, and so forth. As previously observed,
each halftone increase in transposition brings an additional note
into the octave folding scheme, in successive downward steps from
the highest available note. Thus, the technique is readily extended
beyond the three note involvement shown in FIG. 14, in the event
that four or more halftone transpositions and related octave
folding capabilities are desired to be made available.
In FIG. 14, when the musical instrument is operated without
transposition, or the transposition is to the lower frequencies,
each octave folding switch is maintained in what may be marked a
"normal" position. Thus, each of switches 194, 196, and 198 has its
switch arm resting against respective contact N when no octave
folding is to be provided. If a halftone transposition in an upward
frequency direction is introduced, the organist merely throws
switch 194, conveniently located on one of the manuals and ganged
with similar switches for the same note on other manuals, to the
one-half tone contact. Thereafter, when key switch 192 (for note
C.sub.7) is struck, the keying signal thereby developed is supplied
along lead 193, through switch 194, and along lead 200, the latter
lead normally associated with the key switch for note C.sub.6.
Signal indicative of note C.sub.6 therefore appears in the
multiplexed signal when note C.sub.7 is called for. However, a
one-half tone transposition is in effect and thus note C.sub.-6
will be sounded. This is precisely the desired operation as will be
observed by further reference to the table above.
As successively greater transposition is selectively instituted,
successively more of the octave folding switches are to be thrown
according to the amount of transposition in effect. The preceding
octave folding switches are also operated to the "folding"
position. For example, if a one and one-half tone transposition to
higher frequencies is selected, switch 198 is thrown to the 11/2
tone position, and all preceding octave folding switches, here 196
and 194, are also operated to the octave folding position. If
desired, the octave folding switches may be ganged to the
transposition selection switch for introduction of the proper
amount of octave folding simultaneously with selection of
transposition.
As previously observed in connection with the description of FIG.
8, and as is generally well known in the art, the ratio of two
adjacent note frequencies in the equally tempered musical scale is
2.sup.1/12. This fact may be utilized to advantage to provide a
further embodiment of an automatic transposition scheme other than
that using a shift of the note assignments in the multiplexed
waveform. While the effect is that of shifting pulses representing
note assignments by one time slot per semitone of desired
transposition, the means for achieving the transposition is
different. In essence, the shift operation is performed in the
frequency domain, rather than in the time domain as was described
earlier, by acting on the selection of waveform sample addresses,
and the rate of accessing, of the waveform memory unit in the tone
generator.
The phase angle computed by calculator 100 of tone generator 28 in
FIG. 8, on the basis of information furnished by comparator 107, is
effectively a number representative of a particular ratio which has
been assigned to each note of the scale and which is used to
increment, or step, the address of wave shape memory 103 at a rate
determined by the note being played. To obtain the desired
transposition, instead of loading the ratio number calculated by
calculator 100 into register 101, this nominal ratio number is
appropriately varied by multiplying it by the transposition
ratio
R.sub.T = 2.sup.n/12,
where n is the number of semitones of transposition desired, and n
is a positive number if the shift is to be to a higher frequency,
and a negative number if the shift is to be to a lower frequency.
This arrangement is shown in FIG. 15, where a multiplier 210 is
inserted into the line between calculator 100 and register 101, and
is supplied with a second input (i.e., the ratio number furnished
by calculator 100 constituting the other input) from a
transposition ratio selector 212. The latter unit may simply
comprise a set of recycling values of R.sub.T, any one of which may
be selected according to desired transposition and which is read
out in synchronization with the output generated by calculators 100
(e.g., using the master clock rate), to supply the value including
the incremental frequency shift to register 101. In this manner,
the rate of addressing of memory 103 is appropriately varied for
accessing the digital amplitude samples of the wave shape stored in
the memory at a rate consonant with the selected transposition.
The octave folding technique previously described in conjunction
with the apparatus of FIG. 14 is usable with any embodiment of the
invention introducing the desired transposition. That is, it may be
employed with the exemplary embodiment of FIG. 13 or with the
exemplary embodiment of FIG. 15.
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