U.S. patent number 3,708,631 [Application Number 05/044,196] was granted by the patent office on 1973-01-02 for quadraphonic reproducing system with gain control.
Invention is credited to Benjamin B. Bauer, Daniel W. Gravereaux.
United States Patent |
3,708,631 |
Bauer , et al. |
January 2, 1973 |
QUADRAPHONIC REPRODUCING SYSTEM WITH GAIN CONTROL
Abstract
Method and apparatus for reproducing information recorded or
transmitted as four separate channels on a medium having only two
independent tracks and presenting it on four loudspeakers so as to
give the listener the illusion of sound coming from a corresponding
number of separate sources of sound. The two tracks may be provided
by any one of several available two-track systems, such as
two-track tape, the stereomultiplex system of broadcasting, or a
stereophonic disc record, on which two of the four channels are
applied as usual, with the third and fourth channels superimposed
thereon by respectively applying equal portions of them in-phase
and out-of-phase to the two tracks in accordance with known
practice. The reproducing apparatus includes transducer means for
recovering the composite signals from the two tracks, circuit means
for deriving the four channels by appropriately adding and
subtracting components of the composite signals, four separate
loudspeakers, and control circuitry which recognizes the channel or
channels having the dominant signal and which controls the
instantaneous amplitudes of signals delivered to the four
loudspeakers in a manner to give a substantially perfect illusion
of four separate independent sources of sound.
Inventors: |
Bauer; Benjamin B. (Stamford,
CT), Gravereaux; Daniel W. (Wilton, CT) |
Family
ID: |
21931011 |
Appl.
No.: |
05/044,196 |
Filed: |
June 8, 1970 |
Current U.S.
Class: |
369/89;
381/23 |
Current CPC
Class: |
H04S
3/02 (20130101) |
Current International
Class: |
H04S
3/00 (20060101); H04S 3/02 (20060101); G11b
003/74 (); H04h 005/00 (); H03g 003/24 () |
Field of
Search: |
;179/1.4ST,1.1TD,1G,1GA,1GP,15BT
;330/124,127,129,131,134,135,138,140,141 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cardillo, Jr.; Raymond F.
Claims
We claim:
1. In apparatus for reproducing on four separate loudspeakers
adapted to be arranged around a listener four individual audio
information signals contained in first and second composite signals
each of which includes a combination of at least three of said
audio information signals with preselected phase and amplitude
relationships, the combination comprising:
first and second input terminals to which said first and second
composite signals are respectively applied,
a first signal combining network connected to said input terminals
and operative to derive by combining the signals in said composite
signals with preselected amplitude and phase relationships four
output signals predominantly containing signals representing the
information in respective ones of said four individual signals each
accompanied by lower level signals representing information
contained in others of said individual signals,
signal coupling means including first, second, third and fourth
variable gain amplifiers for separately coupling one of said output
signals to a respective one of said four loudspeakers, and
gain control means for varying the gain of said variable gain
amplifiers so as to selectively amplify the output signals or
signals instantaneously predominant relative to the remaining
output signals, said gain control means including
control circuit means connected to said first and second input
terminals and operative to derive from said first and second
composite signals four auxiliary signals containing audio
information substantially corresponding to the audio information
contained in corresponding ones of said four output signals,
and
means for continuously comparing the instantaneous amplitudes of
said auxiliary signals.
2. A method of reproducing four individual audio information
signals which are contained in first and second composite signals
each of which includes a combination of at least three of said
audio information signals with preselected phase and amplitude
relationships, comprising the steps of
deriving said four output signals from said first and second
composite signals by
deriving one pair of output signals by combining said first and
second composite signals with preselected phase and amplitude
relationships wherein different phase relationships are used to
derive each signal of said pair of signals, and
deriving the remaining output signals from respective ones of said
first and second composite signals, whereby a different desired one
of said four audio information signals is predominant in each of
said four output signals,
deriving from said first and second composite signals four
auxiliary signals containing audio information substantially
corresponding to the audio information contained in corresponding
ones of said four output signals,
comparing said auxiliary signals to produce control signals,
and
controlling the gain of at least some of said four output signals
depending on a preselected comparison of said four auxiliary
signals.
3. In apparatus for reproducing on at least three sound
reproduction devices an equal number of individual audio
information signals contained in first and second composite signals
each of which includes a combination of at least three of said
audio information signals with preselected amplitude and phase
relationships, the combination comprising,
signal combining means to which said first and second composite
signals are applied and operative to produce output signals equal
in number to the number of individual audio information signals
contained in said composite signals and each predominantly
containing signals representing the information in a respective one
of said individual audio information signals, and
signal coupling means for coupling said output signals to
respective ones of said sound reproduction devices, said signal
coupling means including
control signal generating means connected to receive said first and
second composite signals and operative to derive therefrom
auxiliary signals of substantially constant amplitude regardless of
the amplitudes of said composite signals,
means for combining said auxiliary signals to produce control
signals corresponding to said audio information signals and at any
given instant to produce one or more control signals corresponding
to the predominant audio information signal or signals then present
in said composite signals, and
amplitude-modifying means connected to receive said control signals
and operative in response thereto to enhance the amplitude of the
output signal or signals which instantaneously contain predominant
audio information signals relative to the other output signals.
4. Apparatus according to claim 3 wherein said amplitude modifying
means provides a signal transmission function which remains at a
normal level when substantially all of the audio information
signals contained in said composite signals have approximately the
same amplitude.
5. In apparatus for reproducing on four separate loudspeakers
adapted to be arranged around a listener four separate channels of
program information contained in first and second composite signals
respectively containing first and second component signals
representing the program information in first and second ones of
said channels, and each containing component signals representing
the program information in third and fourth ones of said channels,
the combination comprising:
first and second input terminals for receiving said first and
second composite signals, respectively,
a first signal combining network connected to said input terminals
and operative to derive from said composite signals four output
signals predominantly containing signals representing the
information in respective ones of said four channels each
accompanied by lower level signals representing information
contained in others of said channels,
signal coupling means including first, second, third and fourth
variable gain amplifier means for receiving and separately coupling
a respective one of said four output signals to a respective one of
said four loudspeakers, and
control circuit means having a pair of input terminals connected to
respective ones of said first and second input terminals and four
output terminals connected to respective ones of said variable gain
amplifier means, said control circuit means being operative in
response to said first and second composite signals to produce four
auxiliary signals containing program information substantially
corresponding to the program information contained in respective
ones of said four output signals and to detect the predominant
signal or signals instantaneously present in said auxiliary signals
and to produce control signals for varying the gain of said
variable gain amplifier means so as to increase separation between
adjacent loudspeakers.
6. Apparatus according to claim 5 wherein said control circuit
means further comprises,
means for continuously comparing the relative amplitudes of said
four auxiliary signals and operative to produce control signals for
varying the gain of the variable gain amplifier means carrying the
output signals instantaneously containing said predominant signal
or signals.
7. Apparatus according to claim 6 wherein said control circuit
means includes means for applying said control signals to the
variable gain amplifier means and operable to maintain
substantially equal the gains of the variable gain amplifier means
associated with the first and the fourth of said loudspeakers and
to maintain substantially equal the gains of the variable gain
amplifier means associated with the second and the third of said
loudspeakers.
8. Apparatus according to claim 6 wherein said comparing means
includes means for separately rectifying said four auxiliary
signals, and means operative in response to said rectified
auxiliary signals to maintain the average value of the sum of said
rectified auxiliary signals substantially constant.
9. Apparatus according to claim 8 wherein said comparing means
further includes at least one signal weighting network to which all
of said rectified auxiliary signals are applied for producing a
signal indicative of which of said output signals contains said
instantaneously predominant signal or signals.
10. Apparatus according to claim 8 wherein said comparing means
further includes a single signal weighting network to which all of
said rectified auxiliary signals are applied, said signal weighting
network being operative to combine two of said rectified auxiliary
signals modified by a given weighting factor with the other two of
said rectified auxiliary signals modified by a different weighting
factor to produce a control signal indicative of which of said two
sets of rectified auxiliary signals contains said instantaneously
predominant signals or signals, means for applying said control
signal in one phase to the control terminals of said first and
fourth variable gain amplifier mean, and means for applying said
control signal with opposite phase to the control terminals of said
second and third variable gain amplifier means.
11. In apparatus for reproducing on four separate loudspeakers four
separate channels of program information contained in first and
second composite signals respectively containing first and second
component signals representing the program information in first and
second ones of said channels, and each containing in-phase portions
of a signal representing the program information in a third one of
said channels and phase-displaced portions of a signal representing
the program information in a fourth one of said channels, the
combination comprising:
first and second input terminals to which said first and second
composite signals are respectively applied,
first signal transfer means including a pair of summing networks
connected to said input terminals and operative to add and subtract
selected components of said composite signals to produce four
output signals predominantly containing signals representing the
program information in respective ones of said four channels each
accompanied by signals representing information contained in its
adjacent channels,
first, second, third and fourth signal coupling means, each
including variable gain amplifier means, for coupling said four
output signals to respective ones of four loudspeakers adapted to
be arranged around a listener,
control circuit means connected to respective ones of said first
and second input terminals and including second signal transfer
means operative in response to said first and second composite
signals to produce four auxiliary signals containing program
information substantially corresponding to the program information
contained in respective ones of said four output signals,
signal comparing means operative to continuously compare the
relative amplitudes of said four auxiliary signals and to produce
control signals indicative of which signal or signals is
instantaneously predominant among said four output signals, and
means for coupling said control signals to said variable gain
amplifier means and for varying the gain of said variable gain
amplifier means so as to increase separation between adjacent
loudspeakers.
12. Apparatus according to claim 11 wherein said control signal
coupling means includes
a logic circuit operative to momentarily apply a gain-varying
control signal to the variable gain amplifier means carrying the
output signals corresponding to the instantaneously predominant
signal or signals.
13. Apparatus according to claim 12 wherein said control circuit
means further includes first and second signal shaping networks
connected between said first and second input terminals and said
second signal transfer means, said first and second signal shaping
networks having like transmission characteristics approximating the
equal loudness contour of the human ear at moderate loudness level
and over the audio range of interest and operative to modify said
first and second composite signals, respectively, according to said
transmission characteristics.
14. Apparatus according to claim 12 wherein said second signal
transfer means includes first and second gain control amplifiers,
each having an input terminal to which said modified first and
second composite signals are respectively applied, a control
terminal and an output terminal, means including summing networks
connected to the output terminals of said first and second gain
control amplifiers for adding and subtracting selected components
of said modified composite signals to produce and apply to said
signal comparing means said four auxiliary signals, and wherein
said signal comparing means includes means for separately
rectifying the four auxiliary signals and means for applying a
control signal to the control terminal of both said first and
second gain control amplifiers of a magnitude to so control their
gain as to maintain the average value of the sum of said rectified
four auxiliary signals substantially constant.
15. Apparatus according to claim 14 wherein said logic circuit
includes at least one signal weighting network to which said
rectified four auxiliary signals are applied.
16. Apparatus according to claim 14 wherein said logic circuit
includes first, second, third and fourth signal weighting networks
to each of which all of said rectified four auxiliary signals are
applied, each of said weighting networks being operative to combine
a respective one of said rectified auxiliary signals modified by a
given weighting factor with the other three rectified auxiliary
signals modified by a different weighting factor.
17. Apparatus according to claim 16 wherein said control signal
coupling means includes first, second, third and fourth
wave-shaping networks respectively connected between said first,
second, third and fourth signal weighting networks and respective
ones of said variable gain amplifier means.
Description
BACKGROUND OF THE INVENTION
There is an increasing interest in multiple-channel recording and
reproduction because of the variety of sounds and music forms that
can be achieved thereby due to the well known phenomenon that the
quality of music reproduction is enhanced when the number of
reproduction channels increases. In the early days of the
phonograph, only single channel or monaural recording was used, and
as early as 40 to 50 years ago, investigators realized the value of
recording and transmitting two separate channels of information,
which in modern parlance is known as binaural or stereophonic
sound. However, even two channels of information are not considered
sufficient for full illusion of reality. For example, when a
listener is placed in front of a symphony orchestra he hears sounds
arriving from many different directions and from a variety of
instruments, as well as reflections from the walls and ceiling,
which gives him an accustomed illusion of space perspective.
However, when reproduction is accomplished by utilizing only two
channels it is difficult, if not impossible, to produce true
reality with respect to spatial perspective. Early experiments have
demonstrated that a minimum of three independent channels are
needed to convey a satisfactory illusion of reality in the
reproduction of orchestral music.
The modern stereophonic phonograph is capable of recording, or
encoding, modulation along two separate channels, which
geometrically are at 90.degree. to each other and at 45.degree. to
the disc surface. It is usual practice to include a third channel
by "matrixing" or adding it as a phantom channel to the other two,
which causes it to be recorded as lateral modulation parallel to
the record surface. Oftentimes, to obtain special effects, some of
the channels are applied to the tracks in phase opposition, in a
manner exemplified by test records Models STR 110, 111 and 120
produced and distributed by CBS Laboratories, a Division of the
Assignee of this invention. Upon reproduction, the third (or
central) channel appears on the two loudspeakers of the
stereophonic phonograph, and an observer placed centrally between
the loudspeakers perceives the illusion of the third channel being
located between the other two. The fourth, or vertical, channel
when reproduced on a conventional two-loudspeaker stereophonic
phonograph gives the illusion of "spread" sound. Although there
have been attempts to reproduce the third or center channel on a
separate loudspeaker, the results have not been entirely
satisfactory, and most stereophonic systems, even though many
stereo records carry a "center" channel, employ only two
loudspeakers.
In the copending application of William S. Bachman, Ser. No. 40510
filed May 26, 1970, now abandoned in favor of continuation-in-part
application, Ser. No. 164,675 filed July 21, 1971, and assigned to
the assignee of the present invention, there is described a system
for providing third and fourth playback channels to otherwise
two-channel systems by feeding third and fourth loudspeakers with
signals respectively representing the sum and difference between
the left and right channel signals. The left and right loudspeakers
may be located, for example, on opposite sides of a listening area,
with the loudspeakers for the two virtual channels positioned at
opposite ends of the listening area. Each loudspeaker displays the
particular information fed to its channel accompanied by half-power
signals from its adjacent channels. This system provides a
pseudo-four-channel effect, but does not give complete illusion of
each channel appearing independently on its corresponding
loudspeaker.
If a record as described above is played on a monophonic
phonograph, the vertically recorded channel will not be reproduced.
It is desirable, of course, that such "four-channel" records be
compatible with the older monophonic and stereophonic phonographs,
because of the large numbers in current use. In other words, it is
desirable that when the new medium is played on a monophonic or
stereophonic phonograph, all channels recorded on the
multi-channeled disc be heard with the loudspeaker system of the
old phonograph.
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide a method
and apparatus for reproducing and separately presenting on
independent loudspeakers four channels of information recorded as
described above on an otherwise two-track record medium, such as a
stereophonic disc record, a two-track tape system having separate
recording and reproducing heads for each track, or the
stereo-multiplex broadcasting system which provides for
transmission of two independent channels or "tracks" of
information, such that the listener experiences the illusion of
listening to a corresponding number of separate sources of
sound.
Another object of the invention is to provide a more realistic
illusion of four separate channels than is afforded by the system
described in the aforementioned copending application.
The invention is applicable to any of the aforementioned presently
available two-track systems of recording and/or transmission on
which two of the four separate channels of information are applied
in the usual manner, with the other two channels superimposed on
the two tracks by applying equal portions of them in phase and
relatively shifted in phase, respectively. As applied to a
45.degree.-45.degree. stereophonic disc record, two of the channels
are recorded on the two separate tracks provided by the walls of
the groove, a third channel is recorded by applying equal portions
of the signal in phase to the "left" and "right" channels of the
stereophonic cutter to produce lateral modulation of the groove,
and the fourth channel is recorded by applying it in equal amounts,
but displaced in phase, to the "left" and "right" terminals of the
cutter to produce vertical modulation of the record groove. In
order that the vertical modulation have a horizontal component to
which the older monophonic and stereophonic phonographs will be
sensitive, the fourth signal, rather than being split into two
equal signals which are applied 180.degree. out-of-phase, may be
applied through a phase-shift network which produces two signals
displaced in phase from each other to cause the cutter stylus to
execute an elliptical motion rather than the purely up and down
motion produced by a difference signal.
The information recorded or transmitted on the medium is reproduced
by an appropriate transducer to produce two composite signals, a
"left" signal which contains, in addition to the left channel
signal, a fraction of the third channel and a similar fraction of
the fourth channel, and a "right" signal containing the right
channel signal, a fraction of the third signal, and a similar
fraction of the fourth signal, the latter, however, being in the
negative sense. Four independent signals, in which the original
four channels are predominant, but each also containing to a lesser
degree portions of two other channels, are derived from the
composite signals by appropriately adding and subtracting
components of the composite signals. An important aspect of the
playback apparatus of the invention is that the instantaneous
amplitudes of the four independent signals delivered to four
corresponding loudspeakers are automatically controlled in response
to the signals then present on the four channels so as to give the
listener a substantially perfect illusion of four separate
independent sources of sound.
BRIEF DESCRIPTION OF THE DRAWING
An understanding of the foregoing and additional aspects of this
invention may be gained from a consideration of the following
detailed description, taken in conjunction with the accompanying
drawings, in which:
FIG. 1 is a schematic diagram of a system for recording four
channels of information on a stereophonic record;
FIG. 2 is a vector diagram useful in explaining the motion of the
cutter stylus in response to application of left, right, center and
difference signals;
FIG. 3 is a cross sectional view of a fragmentary portion of a
record showing four record grooves on a greatly enlarged scale, to
illustrate the motion of the cutter in response to various
signals;
FIG. 4 is a schematic diagram of a prior art stereophonic playback
system for providing the illusion of a third channel;
FIG. 5 is a schematic diagram of a system according to the
invention for recording four channels on a two-track stereophonic
record;
FIG. 6 is a greatly enlarged illustration of a record groove
illustrating the effect of applying the "difference" signal to the
left and right channels through a phase shift network;
FIG. 7 is a schematic diagram of one form of playback apparatus
embodying the invention;
FIG. 7A is a circuit diagram of a transmission network forming part
of the system of FIG. 7;
FIG. 8 is a curve showing the transfer characteristic of the logic
circuitry of FIG. 7, useful in explaining the operation of the
system;
FIG. 9 is a block diagram of a preferred embodiment of the playback
apparatus;
FIG. 10 is a curve showing the transfer characteristics of the
logic circuitry of FIG. 9; and
FIG. 11 is a block diagram of still another alternative embodiment
of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although, as noted above, the invention is applicable to any of a
number of known two-track systems, it will be described in the
environment of a 45.degree.-45.degree. stereophonic disc record. By
way of background, the current method of recording stereophonic
signals including a third or center channel, and a method of
reproducing these signals over a stereophonic two-loudspeaker
system will be described with reference to FIGS. 1-4. The currently
provided left (L), right (R) and center (C) signals are applied to
the two terminals of a stereophonic cutter 10 having a cutting
stylus 12 which is adapted to cut a groove in the lacquer of a
master disc 14, revolving on a recording turntable (not shown). The
C signals is applied through a matrix or signal divider 16 of known
configuration resulting in application of portions thereof,
equivalent to 0.707C, to each of the L and R lines in an additive
manner. As is well known in the groove cutting art, the tip of the
cutter is capable of motions contained within a surface generally
perpendicular to the disc in the manner portrayed by the vector
diagram of FIG. 2. When a left signal L is applied, the stylus
executes motions along the arrow L, which is at an angle of
45.degree. to the horizontal, and when an R signal is applied, the
stylus motion is along the arrow R, at an angle of -45.degree. to
the horizontal. Application of 0.707 parts of C to each of the L
and R lines in an additive manner causes motion of the stylus along
the arrow C, equal in magnitude to 0.707 (L + R), which is of the
same magnitude as either L or R, but directed horizontally. It will
be appreciated that instead of applying the L, R and C signals
directly to the cutter, as shown in FIG. 1, they may, in keeping
with common practice, be first recorded on a two-track master tape
recorder and the output of the tape reproducer used to drive the
record cutter. Discussion of the difference signal D illustrated in
FIGS. 1 and 2 will be deferred until later.
The type of groove modulation resulting from the just-described
procedure is shown in FIG. 3. When only the left signal L is
applied, the groove is modulated in accordance with the arrow L,
which is essentially confined to one wall of the groove. Similarly,
when the R signal is applied, the modulation is in the opposite
wall of the groove in the direction of the arrow R, which, it will
be noted, is perpendicular to the arrow L. Application of equal
amounts of the center signal C to the L and R lines causes both
walls of the groove to be simultaneously and equally modulated in
the directions indicated by the arrows L = 0.707C and R = 0.707C,
resulting in horizontal or side to side translation indicated by
arrow C.
Apparatus for reproducing a stereophonic record carrying L, R and C
signals recorded in this manner, schematically illustrated in FIG.
4, includes a stereophonic pickup having a cartridge 18 and a
stylus 20 which enters the groove in the record and is actuated by
the groove modulation to deliver output voltages on the L and R
terminals. If only L signal modulation is present in the groove, an
output signal appears only at the L terminal and is amplified by a
suitable power amplifier 22 and reproduced by a loudspeaker 24.
Similarly, when only R signal modulation is present in the groove,
an output voltage appears at only the R terminal of the pickup,
which is amplified by power amplifier 26 and applied to its
respective loudspeaker 28. When the groove has lateral modulation
consisting of the presence of equal amounts of left and right
signal, then equal signals, namely, 0.707C, appear on both the left
and right loudspeakers, resulting in the appearance of a phantom
source C (shown surrounded by a dashed line circle) midway between
loudspeakers 24 and 28. However, this illusion is preceptible only
to the centrally located observer 30; when he moves to either side,
the C signal is heard over the nearest loudspeaker unless special
precautions are made to adjust the directional characteristics of
the loudspeakers with respect to the position of the observer.
It will be noted that the described three-channel record is
"compatible" because the L, R and C signals all have a horizontal
component and thus will be heard when played on a monophonic
player, which is sensitive only to lateral modulation, albeit their
relative intensities will not be in the balance initially intended
by the recording director. In reality, in spite of the introduction
of a third channel, the above-described system reproduces only two
independent channels of information. The third channel, C, is
contained in both the left and right channels and the listener
will, therefore, usually hear it reproduced from the loudspeaker
nearest to him. This "center" channel may be presented on a
separate loudspeaker system as shown in dotted lines in FIG. 4, and
amplifiers are commercially available for this purpose. This
permits the observer to perceive the "center" information without
having to locate himself equidistant from the left and right
speakers.
Reverting to FIGS. 1-3, a fourth channel D, may be introduced to
the two-channel stereophonic system by dividing it into equal parts
by a matrix or signal divider 32 and applying them in phase
opposition to the left and right channels. As shown in FIG. 2,
application of the D signal in this manner causes motion of the
stylus in the vertical direction, along the arrow D, to an extent
specified as 0.707 times the amount of D contained in the left and
right channels subtracted from each other; i.e., 0.707 (L - R). As
seen in FIG. 3, this causes the left and right motions of the
stylus to be out-of-phase relative to each other, resulting in up
and down motion. When vertical modulation is reproduced by the
system of FIG. 4, the loudspeaker cones are driven in opposite
directions, resulting in out-of-phase sound pressures applied to
the ears of the listener, and since this condition of pressure on
the ears does not correspond to any known normal listening
condition, the observer is unable to localize the sound. The
difference signal D appears at some indefinite point in space,
shown as D in a dashed circle, and the listener is unable to locate
its whereabouts. Furthermore, some listeners of such out-of-phase
sound have complained of a peculiar "pressure in the ears"
sensation. This is in part overcome, however, by the system
described in the aforementioned Bachman application, Ser. No.
164,675 wherein the difference signal, as well as the "center"
signal, are reproduced on separate loudspeakers.
To afford better compatibility with monophonic and conventional
stereophonic players, while the same time improving the illusion of
four separate channels during playback, the difference signal D is
preferably applied in the manner suggested in applicant Bauer's
article entitled "Some Techniques Toward Better Stereophonic
Perspective," IEEE TRANSACTIONS ON AUDIO, Vol. AU- 11, No. 3,
May-June, 1963. In keeping therewith, and as is illustrated in FIG.
5, instead of applying the difference signal equally and oppositely
to the left and right channels as in the circuit of FIG. 1, the D
signal is applied through an acoustical phase shift network 32
which splits the incoming signal into two equal amplitude signals
D.sub.1 and D.sub.2, each containing all of the frequencies of the
D signal, but displaced in phase with respect to each other.
Relative phase displacements in the range of 110.degree. to
170.degree. have been successfully used, with an angle of
135.degree. being particularly suitable. It can be readily
demonstrated that when the two signals are thus displaced relative
to each other, the tip of the stylus instead of undergoing a purely
up and down motion as shown in FIG. 3, executes the elliptical
motion illustrated in FIG. 6. The limits of stylus motion are shown
by the dashed lines and the direction of motion of the ellipse
depends on whether D.sub.1 leads D.sup.2 , or vice versa. The
important consideration is that the groove has a horizontal
component defined by the horizontal width of the ellipse, whereby
both monophonic and stereophonic phonographs will reproduce all
four signals; that is, the record with four separate channels will
be fully compatible with the older playback systems, albeit with
monophonic systems the signal D is attenuated by about 8 db.
Although the description thus far has been concerned primarily with
recording four separate channels of information on a two-track
stereophonic record, it will be recognized by ones skilled in the
art that similar techniques may be employed to record similar
signals on a two-track tape system, for example, or to transmit
comparable signals over the known stereo-multiplex system of
broadcasting. The description to follow will be directed to
apparatus for reproducing the signals in a manner such that the
four channels may be separately and independently presented on four
different loudspeakers. Except for the transducer means required to
derive the signals from the record medium, the playback apparatus
of the invention is applicable to all of such known two-track
systems.
Referring now to FIG. 7, and considering the playback apparatus as
applied to a disc recorded as described above, the recorded signals
are derived by a stereophonic pickup of the type illustrated in
FIG. 4, and may be applied to the system directly or through a pair
of suitable amplifiers 40 and 42. It will be evident from what has
been said earlier that the left signal, labeled L.sub.T, is a
composite signal including in addition to the L signal, 0.707 of
the signal C and 0.707 of the signal D. Similarly, the right
signal, R.sub.T, contains the right channel signal R, 0.707 of the
signal C, and 0.707 of the signal D, the latter being out-of-phase
in the two cases, It is evident that if the outputs of amplifiers
40 and 42 were connected to two respective loudspeakers, the
reproduction would be equivalent to that of a conventional
two-channel stereophonic system. In the present system, however,
the L.sub.T and R.sub.T signals are also applied to a pair of
signal dividing circuits 44 and 46 of known configuration which, by
appropriate addition and subtraction of components of the composite
signals, produce four signals L', C', D and R', respectively
containing the four predominant channels L, C, D and R, with each
also containing to a lesser degree portions of two other channels.
Therefore, the signals delivered to the respective loudspeakers L,
C, R and D, after amplification by suitable power amplifiers 48,
50, 52 and 54, respectively, are not composed of the pure
information of the corresponding L, C, D and R channels, but rather
are diluted with portions of the information from the adjacent
channels.
In accordance with the invention, the instantaneous amplitudes of
the signals delivered to the four loudspeakers are controlled by
logic circuitry contained within the dashed line enclosure 56 in
such a manner that a listener is given a substantially perfect
illusion of four separate independent sources of sound. This
objective is achieved by reason of the character of the music
normally reproduced on a phonograph record, aided by a phenomenon
known in acoustical science as the "precedence effect." In most
musical selections, the individual performers do not play
continuously, but rather, produce a constantly varying pattern of
attacks, decays, percussion sounds, etc., which do not occur
simultaneously but are interleaved with each other. For example,
first the sound of a drum may appear on channel L, followed by the
sound of a cymbal on channel R, followed by the voice of a soloist
pronouncing various syllables on channel C, etc. If one is able to
switch the loudspeaker system in such a way that a loudspeaker is
turned "on," or the signal preferentially amplified, each time a
particular impulsive or percussive sound is started in its channel,
while the remaining loudspeakers are correspondingly turned "off"
or attenuated, the listener will have fixed his attention upon the
particular sound coming from that loudspeaker, and even if the
sound is transferred to another loudspeaker, he will have an
illusion of the sound proceeding from the first loudspeaker. This
results from the well known "precedence effect" which is based on
the observation that when a sound originates from a given
loudspeaker, and then gradually is switched into another
loudspeaker, the listener continues to hear the sound coming from
the given loudspeaker long after it has ceased to be the true
source.
Accordingly, the function to be performed by the control logic
portion 56 of the playback apparatus is to identify which channel
has the strongest signal at any instant in time and to turn that
channel "on," or to preferentially increase its gain, while
attenuating or turning "off" the remaining channels. As the sound
diminishes in the channel first identified and another sound
appears on a different channel, the logic circuit rapidly
attenuates the gain in the first channel and increases the gain in
a different channel. It is useful to think of the action of this
logic in terms of the following truth table:
If there are sound signals in the system it logically follows
that:
If L signal = 0, only R signal is present
If R signal = 0, only L signal is present
If C signal = 0, only D signal is present
If D signal = 0, only C signal is present. It will be observed that
this is a form of negative logic; i.e., the lack of sound in
channel L, for example, or the diminution thereof, serves to turn
on or to enhance the gain of the R channel, etc., in accordance
with the truth table.
Turning now to a discussion of the logic system, the outputs from
amplifiers 40 and 42 (if provided) are respectively applied to gain
control amplifiers 58 and 60, the gains of which are controlled in
unison as indicated by the connection 62 therebetween.
For reasons which will be better understood after considering the
logic circuit, it is desirable to apply the L.sub.T and R.sub.T
signals to amplifiers 58 and 60 through respective signal modifying
networks 59 and 61. These two networks are identical and exhibit
transmission characteristics which resemble the equal loudness
contour of the human ear at moderate loudness level and over the
audio range of interest. A set of equal loudness contours are
illustrated and described in an article by applicant Bauer and Emil
Torick entitled "Researches in Loudness Measurement," IEEE
TRANSACTIONS ON AUDIO AND ELECTROACOUSTICS, Vol. AU-14, No. 3, pp.
141-151, 1966. The 70 phon equal loudness contour developed in the
study described in this article is shown in blocks 59 and 61 in the
inverted, or sensitivity, form. It will be noted that there is a
peak in the transmission characteristic at the higher frequencies
(at approximately 4 KHz), which may be of the order of 8 db, is
essentially constant from approximately 2 KHz down to about 50 Hz
at which it exhibits a drop of approximately 5 db. The function of
the networks 59 and 61 is to so shape the signals delivered by the
transducer to the gain control amplifiers 58 and 60 so that the
signal switching logic (the operation of which is about to be
described) will place the respective L', C', D' and R' signals in
their proper channels on the basis of their relative loudness,
rather than their energy content. For example, the weighting curves
of networks 59 and 61 would preclude the low frequency, but high
energy, signal produced by a drum from incorrectly switching the
higher frequency, lower energy, signal produced by a piccolo, for
example.
While the transmission characteristic exhibited by circuits 59 and
61 may be obtained in a number of ways, a preferred embodiment is
shown in FIG. 7A, consisting of a high resistance R1, in parallel
with a series branch containing a lower valued resistor R2, a
capacitor C1 and an inductor L1, followed by a small resistor R3 to
ground and a series resistor to amplifier 58 (or amplifier 60). The
values of the components in the series branch are selected so that
in cooperation with the parallel resistor R1, the circuit produces
the peak in the transmission characteristic centered at about 4
KHz. Typical values of the components used in the circuit of FIG.
7A are:
R1 = 5.1 kohms
R2 = 1.6 kohms
R3 = 91 ohms
R4 = 1.0 kohms
C1 = 0.01 .mu.f.
L1 = 300 mh.
It will be understood that the exact shape of the characteristic
may be modified by experiment to provide the best results with any
particular decoder, or it may be desirable to provide an adjustment
to permit the user to adjust the decoder for any particular type of
music, or any particular type of listening environment.
Returning now to the control circuit itself, the outputs of
amplifiers 58 and 60 are separated by separating circuits 64 and 66
into four separate outputs L", C", D" and R" in the same manner as
the outputs L', C', D' and R' are produced by circuits 44 and 46.
These two sets of four outputs thus resemble each other in musical
content, but the former set is held at a uniform level, despite
variations in the dynamic range of the record (as modified by
circuits 59 and 61), by the action of gain control amplifiers 58
and 60. To achieve this constant output level, the L", C", D" and
R" signals are rectified by rectifiers 68, 70, 72 and 74,
respectively, and summed by isolating resistors 76, 78, 80 and 82
to develop a sum signal across a common resistor 84. The voltage
developed across resistor 84 is applied over conductor 86 to the
gain control lead 62 of amplifiers 58 and 60, which are operative
in response thereto to keep the average voltage across resistor 84
substantially constant. The gain control action is enhanced by
connecting four capacitors 88, 90, 92 and 94 across resistors 76,
78, 80 and 82, respectively, whereby the rectified voltage
represents the envelope of the wave rather than its instantaneous
value. The automatic gain control action maintains the sum of the
voltages across the resistors 76, 78, 80 and 82 constant because
the voltage across the relatively smaller resistor 84 is the sum of
the four rectified voltages.
The action of the gain control circuit can best be understood by
consideration of several illustrative examples. Let it be assumed
that the system is playing a record which contains a single signal,
say a left signal L, the amplitude of which is arbitrarily assigned
a value of unity. It follows from the vector diagram of FIG. 2 that
the voltages e.sub.1, e.sub.c, e.sub.d and e.sub.r respectively
developed across resistor 76, 78, 80 and 82 would then have
relative values of 1, 0.707, 0.707, and 0. The sum of these
voltages is 2.414, and the gain control amplifiers 58 and 60 are
designed to maintain the voltage across resistor 84 at this value
regardless of the sound level in channel L. This represents one
condition of circuit operation and will be considered later in
further detail.
Now let it be assumed that only the C signal is present, in which
case the voltages e.sub.1, e.sub.c, e.sub.d and e.sub.r will be
0.707, 1, 0 and 0.707, respectively. It will be noted that the sum
of these voltages is also 2.414.
Now, if incoherent signals are simultaneously present on channels L
and C (say, signals resembling white noise; that is, signals
emanating from two different sources), the voltages corresponding
to the sum of these channels will be equal to the square root of
the sum of the squares of both voltages. Therefore, the unadjusted
sum of the signals L and C will be as shown in the following Table
I, namely, e.sub.1 =1.223, e.sub.c =1.223, e.sub.d =0.707 and
e.sub.r =0.707. The sum of these four voltages being 3.860, the
automatic gain control amplifiers instantaneously adjust the
component voltages so as to total 2.414. Thus, each of these
voltages is proportionally reduced by the fraction 2.414/3.860 to
give adjusted values of e.sub.1 =0.760, e.sub.c =0.760, e.sub.d
=0.440 and e.sub.r =0.440.
TABLE I
Signal e.sub.1 e.sub.c e.sub.d e.sub.r
__________________________________________________________________________
L 1.000 .707 .707 0 C .707 1.000 0 .707 (L+C) unadjusted 1.223
1.223 .707 .707 (L+C) adjusted .760 .760 .440 .440 R 0 .707 .707
1.000 (L+C+R) unadjusted 1.223 1.414 1.223 1.223 (L+C+R) adjusted
.557 .667 .557 .557 D .707 0 1.000 .707 (L+C+R+D) unadjusted 1.414
1.414 1.414 1.414 (L+C+R+D) adjusted 0.600 0.600 0.600 0.600
__________________________________________________________________________
Consider now the presence of a third channel, say, R. The voltages
corresponding to this channel acting alone are e.sub.1 =0, e.sub.c
=0.707, e.sub.d =0.707 and e.sub.r =1. Again, assuming that all
three signal are white-noise-like incoherent signals, the resulting
unadjusted values of e.sub.1, e.sub.c, e.sub.d and e.sub.r for the
(L+C+R) condition are obtained by taking the square root of the sum
of the squares of the individual voltages for these three channels,
and are shown in Table I as being 1.223, 1.414, 1.223 and 1.223,
respectively. These four voltages add up to a total of 5.083,
whereby the automatic gain control amplifiers cause these component
voltages to immediately reduce by the factor 2.414/5.083, resulting
in the adjusted voltages shown in Table I.
If a fourth voltage corresponding to the D channel is now added, it
turns out that each of the four summed voltages has a value of
1.414, which, when acted upon by the automatic gain control
amplifiers are reduced to a value to total 2.414, with the result
that each of the component voltages has a value of 0.6.
Returning for a moment to the right-hand portion of FIG. 7, the
four signals L', C', D' and R' are applied to respective gain
control amplifiers 96, 98, 100 and 102, and then to four
loudspeakers L, C, D, and R, respectively, with intermediate
amplification, if necessary, provided by amplifiers 48, 50, 52 and
54. The gain control amplifiers are key elements of the invention
in that they control the gain of the signals applied to the
respective loudspeakers in accordance with the logic described
earlier.
If the aforementioned signals are incoherent, but exhibit definite
frequency-like character, then the added peak value in the logic
will approximate the sum of the signals, rather than the
root-mean-square value. This, however, does not alter the analysis
made hereinabove.
The requisite control is accomplished by applying the component
voltages e.sub.1, e.sub.c, e.sub.d and e.sub.r to the gain control
amplifiers as follows: e.sub.1 is applied to amplifier 102 to
thereby control the R' signal; e.sub.r is applied to amplifier 96
to control the L' signal; e.sub.c is applied to amplifier 100 to
control the D' signal; and e.sub.d is applied to amplifier 98 to
control the C' signal. For convenience in implementing the
above-discussed truth table, the rectifiers 68, 70, 72 and 74 are
connected in a negatively conducting fashion so that voltages
e.sub.1, e.sub.c, e.sub.d and e.sub.r and the various combinations
thereof shown in Table I are negative voltages. Additionally, the
control circuits of gain control amplifiers 96, 98, 100 and 102 are
positively biased with a relative voltage of 0.6 volts by means of
batteries 104, 106, 108 and 110, respectively.
The gain control amplifiers have the control characteristic shown
in FIG. 8, which indicates that when a voltage of +0.6 volt is
applied to the gain control terminal of the amplifier, its gain is
maximum, at a value designated as unity. When the applied voltage
is reduced to zero, the gain of the amplifier is decreased to 0.707
of maximum; that is, the gain is down 3 db. The characteristic then
falls rapidly such that when the gain control voltage is -0.67
volts, the gain is reduced to zero and the amplifier is turned
"off." While the appropriate control characteristic may be obtained
in a number of ways, it is conveniently obtained by using an
integrated DC amplifier Model CA3000, available from RCA, in the
circuit configuration described on page 6 of "RCA Integrated
Circuits Application Note " ICAN-5030 printed in Sept. 1967 and
then available to the industry.
The action of the logic circuit 56 for the four conditions of L
signal only, two signals, such as L + C, three signals, such as L +
C + R in combination, and four random incoherent signals L + C + R
+ D, will now be examined. It will be observed from Table I that
when only the L signal is present, e.sub.r =0 whereby the +0.6 volt
bias turns amplifier 96 (which controls the L' signal) fully "on,"
whereas negative voltages of 1.0, 0.707 and 0.707 are respectively
applied to the amplifiers controlling the R', D' and C' signals.
Consequently, amplifiers 102, 100 and 98 are biased beyond cutoff
so as to have zero gain, and the L signal, which otherwise would
also appear in loudspeakers C and D, appears only in loudspeaker L.
By similar analysis it may be shown that any one signal appearing
in any one individual channel will turn on only the gain control
amplifier appropriate to that signal.
Considering now the condition when L and C signals are both
present, it will be seen from Table I and from the notations in
FIG. 8 that the -0.76 volt applied to amplifiers 100 and 102 exceed
the +0.6 volt bias thereby causing loudspeakers D and R to be
turned "off;" however, the -0.440 volt applied to amplifiers 96 and
98 cuases them both to be turned "on," with only a slight reduction
from unity gain, whereby the L and C signals appear in their
respective loudspeakers L and C.
Similarly, when L, C and R signals are simultaneously present, it
will be seen from Table I and the notations on FIG. 8 that e.sub.c
is sufficiently negative to cause amplifier 100 (which controls the
signal applied to loudspeaker D) to be turned "off" whereas the
-0.577 volts applied to the other three gain control amplifiers
causes them to remain on, but with their gains reduced by
approximately 2.5 db, so that the L', C' and D' signals are
reproduced on their respective loudspeakers.
Finally, when L, C, R and D signals are all present in equal
amounts, the respective gain control voltages e.sub.1, e.sub.c,
e.sub.d and e.sub.r are all 0.6 volts which allows all four
amplifiers 96, 98, 100 and 102 to be turned "on" and to apply the
signals to their respective loudspeakers. However, it will be seen
from the control characteristic that the gain of the amplifiers is
reduced by approximately 3 db when all four signals are present.
Thus, the total sound energy reproduced by the loudspeakers remains
essentially constant regardless of the number of signals
present.
It is evident from the foregoing that the logic system is operative
to turn on those loudspeakers which correspond to the predominant
sounds instantaneously present in the system, thereby accomplishing
an important object of the invention. In actual practice, all
signals seldom occur simultaneously, but rather there is a constant
interplay of the various instruments which turns the loudspeakers
on and off in a manner to give a completely natural illusion of
four separate sources of sound being present and reproduced over
the four loudspeakers.
For best operation, it is preferable that the time constants of the
rectifier circuits 68-74 have a very rapid attack time of the order
of 0.1 milliseconds and relatively slow decay time of approximately
10 milliseconds. Likewise, the attack time of the gain control
amplifiers 96-102 should be extremely rapid--of the order of 0.1
millisecond--while a decay time of the order of 0.4 second has been
found satisfactory. It is to be understood, however, that these
attack and decay times may be adjusted between relatively wide
limits without seriously impairing the performance of the
circuit.
While the system of FIG. 7 is sound in principle, it suffers the
disadvantage that gain control amplifiers 58 and 60 must maintain
extremely close control of the voltages developed across resistors
76-82 because of the narrow range of discrimination between the
"off" and "on" conditions protrayed by the control characteristic
of FIG. 8. For this reason, it has been found advantageous to
employ a differential discriminator circuit, shown in FIG. 9. The
right-hand portion of this circuit is identical to the
corresponding portion of the FIG. 7 circuit, and like parts are
identified with like reference numerals. The logic portion 56'
differs in that the rectifiers 68, 70, 72 and 74 are reversed in
polarity so that the e.sub.1, e.sub.c, e.sub.d and e.sub.r voltages
developed across resistors 76, 78, 80 and 82, respectively, are
positive. Additionally, rather than directly applying these
voltages to the gain control amplifiers, they are differentially
added in adding elements 112, 114, 116 and 118 which are designed
to weight the predominant voltage negatively by a factor of three
while the remaining three voltages are added positively with a
factor of one. For example, to the element 112 are applied the
predominant e.sub.r signal developed scross resistor 82, which is
added negatively by a factor of three, along with the e.sub.1,
e.sub.c and e.sub.d voltages, each of which are added with a
weighting factor of +1. The predominant voltages applied to adding
elements 114, 116, 118 are e.sub.d, e.sub.c and e.sub.1,
respectively. The adding elements may be of the form described on
page 42 of "Applications Manual for Operational Amplifiers"
published in 1968 by Nimrod Press, Boston, for Philbrick/Nexus
Research, a Teldyne Company, Dedham, Massachusetts. The adding
elements provide respective output voltages e'.sub.1, e'.sub.c,
e'.sub.d and e'.sub.r, the values of which for the various
combinations of signals discussed earlier appear in Table II below.
These values are, of course, derived by applying the above
weighting factors to the relative voltage values presented in Table
I.
TABLE II
Signal e.sub.1 ' e.sub.c ' e.sub.d ' e.sub.r '
__________________________________________________________________________
L +2.414 -0.414 -0.414 -1.586 (L+C) +0.640 +0.640 -0.640 -0.640
(L+C+R) +0.414 -0.414 0 (L+C+R+D) 0 0 0
__________________________________________________________________________
it is readily apparent from Table II that the discrimination range
has been greatly increased by the differential adding elements, and
make it possible to utilize the gain control curve illustrated by
FIG. 10. In this case, the amplifiers, which may, for example, take
the form of Motorola's monolithic four-quadrant multiplier Model
MC1495L, the characteristics of which are described in the
specification sheet therefor published in March 1969, have unity
gain when the control signal has a value of +2.4 volts, and a gain
of 0.707 (3 db down) when the control voltage is equal to zero. The
charactersitic then falls off rapidly such that the amplifiers are
cut off when the control voltage has a value of -0.414 volt.
Analysis of the kind applied to the gain control characteristic of
FIG. 8 will show that the correct amplifiers will be turned "on" in
response to the predominant sounds present in the system at any
given time. The gain control charactersitic of FIG. 10 being
considerably less critical with respect to cutoff than the gain
control curve of FIG. 8, the system of FIG. 9 is considerably more
stable. Moreover, since the voltages e'.sub.1, e'.sub.c, e'.sub.d
and e'.sub.r are differential voltages, slight deviations from
perfect in the gain control characteristics of amplifiers 58 and 60
are less critical to the performance of the system than in the
circuit of FIG. 7.
The control signals developed by the adding elements 112-118 are
each applied through suitable wave shaping circuitry to the gain
control element of a respective gain control amplifier 96-102.
Circuitry for establishing the gain control characteristic of FIG.
10 can be obtained in a variety of ways, but in an embodiment which
has been successfully operated, this circuitry consists of a
clipper 120 having the illustrated characteristic, with clipping
action at -0.4 volt and +0.4 volt, followed by a time constant
circuit 122, which, in turn, is followed by a wave shaper 124
having a transfer characteristic as shown. This transfer
characteristic may be provided with a conventional limiter circuit,
or by an arbitrary function fitting circuit of the type described
on page 52 of the aforementioned "Applications Manual for
Operational Amplifiers." These circuits co-act to provide an
amplification control charactersitic as illustrated in FIG. 10 for
controlling the gain of amplifiers 94-102. The circuit is designed
to have a rapid attack, of the order of 0.1 millisecond, and a
slower decay time in the order of 0.4 second. It is to be
understood, however, that these are only typical values, and both
are subject to a range of values without departing from the spirit
of the invention.
It will be evident from the foregoing that in the circuits of FIGS.
7 and 9 all four channels operated independently. That is, at any
given instant only one loudspeaker might be on, with the remainder
off or at reduced gain. By the relatively simple modification shown
in FIG. 11, it is possible to switch two of the channels together,
in response to a predominant signal in one of them; similarly, the
remaining two channels may be switched in unison, again in response
to the presence of a predominant signal in either. For example, the
gain control amplifiers 96 and 102 respectively controlling the
gain of the signals applied to the L and R loudspeakers may be
subjected to the same control signal, and amplifiers 98 and 100
controlling loudspeakers C and D, respectively, similarly
controlled by the same signal. The L and R loudspeakers may be
placed in diagonally opposite corners of a listening area and the C
and D loudspeakers placed in the other opposite corners of the area
whereby the speakers at opposite corners go up and down
together.
The control signals are derived in the same manner as in the
circuits of FIGS. 7 and 9 (and a description thereof will not here
be repeated) except that only one summing element 130, which is
similar in form and function to summing elements 112-118 except
that it has different weighting factors, and a phase inverter 132
are needed to develop the signals for application to gain control
amplifiers 96-102. The e.sub.1 and e.sub.r voltages derived from
the rectifiers are applied to the summing element with a weighting
factor of -1, and the e.sub.c and e.sub.d voltages are applied with
a weighting factor of +1. It will be evident that if either the R
or L channels are alone present, it will produce a control signal,
which may be modified by a suitable time constant circuit 134, for
application to amplifiers 96 and 102 to turn "on" both the L and R
loudspeakers. But, since only one of these channels is "on," sound
will come out of the corresponding loudspeaker. Similarly, if both
the L and R channels are simultaneously present, with no signal in
the C and D channels, the L and R loudspeakers will again be turned
on, each reproducing its respective signal, and thus simulating a
pair of independent channels.
Similarly, the presence of either of channels C and D only produces
a control signal, which by reason of the +1 weighting, must be
inverted to give the proper polarity to turn amplifiers 98 and 100
on. At the same time, the non-inverted signal applied to amplifiers
96 and 102 turns them off or substantially lowers their gains. It
follows that if both channels C and D are present, the same two
loudspeakers, namely, C and D, will be turned on and the other two
turned off. Since each of loudspeakers C or D reproduces only its
appropriate channel, the result is a good simulation of two
additional channels.
Should signals be present simultaneously in any two adjacent
channels, the control signal turns out to be zero, and since the
phase inversion in this case has no effect, all four amplifiers are
turned on, but at reduced gain; i.e., 3 db down.
It will be evident from the foregoing that applicants have provided
systems for reproducing and presenting on four independent
loudspeakers four channels of information carried as two composite
signals on a two-track system. An important aspect of the
reproduction apparatus which gives it its capability of creating a
substantially perfect illusion of sound proceeding from four
separate sources is the concept of sensing which channel has the
preponderant signal and switching to that channel, while at the
same time attenuating the signals in the other channels, in
response to the transients of musical sounds, to give the illusion
of four separate channels of information.
An important objective of the invention has been achieved in the
embodiments shown in FIGS. 7 and 9, in which each of the four
loudspeakers control circuits is controlled depending on the
dominance of the respective signal, and entirely independently of
the other three control circuits. Thus, it is possible to turn "on"
one circuit at a time. This is not the case with the embodiment in
FIG. 11, which, while possessing the virtue of greater simplicity
does not exhibit as precise a degree of control as the other two
circuits.
It is again emphasized that although the stereo-phonic disc record
has been selected as the medium with which to explain the recording
and playback techniques, it is to be understood that the playback
method and apparatus responds in the same manner to similar
composite signals regardless of the medium from which they are
transduced.
* * * * *