U.S. patent number 3,784,744 [Application Number 05/118,271] was granted by the patent office on 1974-01-08 for decoders for quadruphonic sound utilizing wave matching logic.
This patent grant is currently assigned to Columbia Broadcasting System, Inc.. Invention is credited to Benjamin B. Bauer.
United States Patent |
3,784,744 |
Bauer |
January 8, 1974 |
DECODERS FOR QUADRUPHONIC SOUND UTILIZING WAVE MATCHING LOGIC
Abstract
Apparatus for decoding four separate channels of information
transduced from a medium having only two separate tracks and
presenting it on four loudspeakers to give the listener the
illusion of sound coming from a corresponding number of separate
sources. The realism is enhanced by a decoding system which accepts
the two outputs from the medium, which may be a disc record, one
for each track, separates them into four independent channels each
carrying predominantly the information contained in the originally
recorded sound signals, and, utilizing a novel wave-matching
technique, derives control signals for controlling the gains of
amplifiers associated with the four looudspeakers. The use of the
wave-matching technique improves the separation of the four
independent channels, particularly the generally "front" from the
generally "back" signals, thereby to enhance the realism of
four-channel simulation.
Inventors: |
Bauer; Benjamin B. (Stamford,
CT) |
Assignee: |
Columbia Broadcasting System,
Inc. (New York, NY)
|
Family
ID: |
22377557 |
Appl.
No.: |
05/118,271 |
Filed: |
February 24, 1971 |
Current U.S.
Class: |
381/22;
G9B/20.003 |
Current CPC
Class: |
G11B
20/00992 (20130101); H04S 3/02 (20130101) |
Current International
Class: |
G11B
20/00 (20060101); H04S 3/00 (20060101); H04S
3/02 (20060101); H04r 005/00 () |
Field of
Search: |
;179/1G,1GP,15BT,1GA,1.1TD,1.4ST
;330/124,127,129,131,134,135,138,140,141 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
4 Channels and Compatibility by Schiebes Audio Engineering Society
Preprint Oct. 12-15, 1970..
|
Primary Examiner: Cooper; William C.
Assistant Examiner: D'Amico; Thomas
Attorney, Agent or Firm: Olson; Spencer E.
Parent Case Text
CROSS-REFERENCE TO OTHER APPLICATIONS
This invention is related to the subject matter of the following
applications, all of which are assigned to the assignee of the
present invention: Ser. No. 40,510 filed May 26, 1970, now
abandoned in favor of Ser. No. 164,675 filed July 21, 1971; Ser.
No. 44,196, filed June 8, 1970, now U.S. Pat. No. 3708631; Ser. No.
44,224 filed June 8, 1970, now abandoned in favor of Ser. No.
251,544 filed Apr. 21, 1972, now abandoned in favor of Ser. No.
328,814 filed Mar. 10, 1973; Ser. No. 44,224 also now abandoned in
favor of Ser. No. 185,204 filed Sept. 30, 1971; Ser. No. 44,224
also now abandoned in favor of Ser. No. 185,050 filed Sept. 30,
1971; Ser. No. 81,858, filed Oct. 19, 1970, now abandoned in favor
of Ser. No. 251,636 filed May 8, 1972; and Ser. No. 112,168 filed
Feb. 3, 1971 now U.S. Pat. No. 3,745,252.
Claims
I claim:
1. In combination:
decoder apparatus adapted to receive first and second composite
signals L.sub.T and R.sub.T respectively containing dominant
signals L.sub.f and R.sub.f and each including two sub-dominant
signal components L.sub.b and R.sub.b, said signal component
L.sub.b being of substantially equal magnitude and in substantially
quadrature relationship in said first and second composite signals,
and in one of said first and second composite signals in leading
relationship with that in the other, and said signal component
R.sub.b being of substantially equal magnitude and in substantially
quadrature relationship in said first and second composite signals,
and in said one of said first and second signals in lagging
relationship with that in the other, and including at least two
phase-shifting networks operative to shift the phase of one of said
first and second composite signals relative to the other by
substantially 90.degree. to cause the signal components L.sub.b and
R.sub.b in one of said relatively phase-shifted composite signals
to be in phase coincidence or in phase opposition with
corresponding signal components in the other relatively
phase-shifted composite signal, combining networks operative to
derive from said relatively phase-shifted composite signals third
and fourth composite signals respectively containing dominant
signal components L.sub.b and R.sub.b and each including two
subdominant signal components L.sub.f and R.sub.f, said signal
component L.sub.f being of substantially equal magnitude and in
phase coincidence or phase opposition in said third and fourth
composite signals, and said signal component R.sub.f being of
substantially equal magnitude and in phase coincidence or phase
opposition in said third and fourth composite signals, and means
for applying composite signals respectively containing dominant
signal components L.sub.f, R.sub.f, L.sub.b and R.sub.b to first,
second, third and fourth gain control amplifiers, respectively, for
reproduction over four corresponding loudspeaker circuits, and
a control circuit for enhancing the realism of the four channel
sound produced by the loudspeakers, siad control circuit
comprising:
first circuit means for comparing said relatively phase-shifted
first and second composite signals and operative to produce a
signal indicative of whether they contain substantially equal
amplitude signals in phase coincidence or phase opposition,
second circuit means for comparing said third and fourth composite
signals and operative to produce a signal indicative of whether
they contain substantially equal amplitude signals in phase
coincidence or phase opposition, and
third circuit means operative in response to signals from said
first and second circuit means to enhance the gain of said first
and second gain control amplifiers relative to said third and
fourth gain control amplifiers when said relatively phase-shifted
first and second composite signals do not contain said signal
components L.sub.b and R.sub.b, and to enhance the gain of said
third and fourth gain control amplifiers relative to said first and
second gain control amplifiers when said third and fourth signals
do not contain said signal components L.sub.f and R.sub.f.
2. The combination defined by claim 1 wherein said first circuit
means includes first and second rectifiers for separately
rectifying said relatively phase-shifted first and second composite
signals and means for subtracting one from the other the rectified
outputs therefrom to produce a first difference signal, and said
second circuit means includes third and fourth rectifiers for
separately rectifying said third and fourth composite signals and
means for subtracting one from the other the rectified outputs
therefrom to produce a second difference signal.
3. The combination defined in claim 2 wherein said rectifiers are
full-wave rectifiers and each includes means for integrating the
output thereof.
4. The combination defined by claim 2 further including fifth and
sixth rectifiers for separately rectifying said first and second
difference signals, respectively.
5. The combination defined by claim 4 wherein said rectifiers are
full-wave rectifiers and each includes means for integrating the
output thereof.
6. The combination defined by claim 2 wherein said third circuit
means includes first and second subtracting means to both of which
said first and second difference signals are applied in
complementary subtracting relationship and operative to produce
first and second control signals, respectively.
7. The combination defined by claim 6 wherein said control circuit
further includes first and second wave-shaping networks having
substantially identical transfer functions for separately shaping
said first and second difference signals.
8. The combination defined by claim 7 further including fifth and
sixth rectifiers for separately rectifying said shaped first and
second difference signals, respectively.
9. A system for processing sound reproducing signals comprising, in
combination,
decoder means adapted to receive first and second composite signals
L.sub.T and R.sub.T respectively containing dominant signals
L.sub.f and R.sub.f and each including two sub-dominant signal
components L.sub.b and R.sub.b, said signal components L.sub.b and
R.sub.b in said first composite signal being in substantially
quadrature relationship with the corresponding signal components in
said second composite signal, and including
decoding means including at least first and second all-pass
phase-shifting networks to which said first and second composite
signals are respectively applied for shifting the phase of one of
said first and second composite signals relative to the other by
substantially 90.degree. and signal-combining networks for
combining said relatively phase-shifted first and second composite
signals to derive third and fourth composite signals respectively
containing dominant signal components L.sub.b and R.sub.b,
means for applying composite signals respectively containing said
signal components L.sub.f, L.sub.b, R.sub.b and R.sub.f as
predominant components to first, second, third and fourth gain
control means, respectively, for reproduction over four
corresponding loudspeaker circuits,
control circuit means operative in response to at least said
relatively phase-shifted first and second composite signals to
detect whether said relatively phase-shifted first and second
composite signals contain substantially equal amplitude signal
components in phase coincidence or in phase opposition and to
produce a control signal operative to enhance the gain of the first
and second gain control means when said relatively phase-shifted
first and second composite signals do not contain said signal
components L.sub.b and R.sub.b, and
means for applying said control signal to said first and second
gain control means for controlling the gain thereof.
10. The combination in accordance with claim 9 wherein said
third and fourth composite signals respectively contain dominant
signal components L.sub.b and R.sub.b and two subdominant signal
components L.sub.f and R.sub.f, said signal component R.sub.f being
of substantially equal magnitude and in phase coincidence or phase
opposition in said third and fourth composite signals, and said
signal component R.sub.f being of substantially equal magnitude and
in phase coincidence or phase opposition in said third and fourth
composite signals, and said control circuit means further
comprises
first circuit means for comparing said first and second composite
signals and operative to produce a signal indicative of whether
they contain substantially equal amplitude signals in phase
coincidence or phase opposition, and
second circuit means for comparing said third and fourth composite
signals and operative to produce a signal indicative of whether
they contain substantially equal amplitude signals in phase
coincidence or phase opposition.
11. A system for receiving and processing sound reproducing signals
comprising
first, second, third and fourth gain control amplifiers connected
to respectively receive first, second, third and fourth composite
signals for reproduction over four corresponding loudspeaker
circuits, the first, second, third and fourth composite signals
containing respectively a dominant signal component a and
subdominant signal components c and d, a dominant signal component
b and sub-dominant signal components c and d, a dominant signal
component c and sub-dominant signal components a and b, and a
dominant signal component d and sub-dominant signal components a
and b, and
a control circuit for the gain control amplifiers connected to
receive said composite signals in a form such that in the first and
second composite signals and also in the third and fourth composite
signals like sub-dominant signal components are of substantially
equal magnitude and are substantially in phase coincidence or phase
oposition, the control circuit comprising:
a first circuit for comparing the first and second composite
signals and operative to produce a signal indicative of whether
they contain substantially equal amplitude signals which are either
in phase coincidence or phase opposition,
a second circuit for comparing the third and fourth composite
signals and operative to produce a signal indicative of whether
they contain substantially equal amplitude signals which are either
in phase coincidence or in phase opposition, and
a third circuit operative in response to signals from the first and
second circuits to enhance the gain of the first and second gain
control amplifiers relative to the third and fourth gain control
amplifiers when the first and second composite signals do not
contain the signal components c and d and to enhance the gain of
the third and fourth gain control amplifiers relative to the first
and second gain control amplifiers when the third and fourth
composite signals do not contain the signal components a and b.
12. A system in accordance with claim 11, further including decoder
apparatus for deriving the said first, second, third and fourth
composite signals from first and second input signals respectively
containing dominant signals a and b and each including two
subdominant signal components c and d, the two signal components c
being of substantially equal magnitude and in substantially
quadrature phase relationship and the two signal components d being
of substantially equal magnitude and in substantially quadrature
phase relationship, the signal components c and d in one of the
input signals being in leading and lagging relationship,
respectively with the signal components c and d in the other of the
input signals,
said decoder apparatus including all-pass phase-shifting means
operative to shift the phase of at least one of the input signals
to obtain phase-shifted signals wherein the signal components c and
d in one are in phase coincidence or in phase opposition with
corresponding signal components in the other, and
signal combining networks operative to derive from the said
phase-shifted signals the first, second, third and fourth composite
signals for application to said gain control amplifiers.
13. In apparatus for reproducing on three or more sound-reproducing
devices a like number of directional audio information signals
contained in first and second composite signals each of which
includes at least three of said audio information signals in
preselected amplitude and phase relationships, two of which are
common to both composite signals with said common signals in one of
said composite signals in substantially quadrature relationship
with corresponding ones of said common signals in the other of said
composite signals, the combination comprising:
decoding circuit means including at least first and second all-pass
phase-shifting means connected to receive said first and second
composite signals and operative in response thereto to shift the
phase of one of said first and second composite signals relative to
the other by substantially 90.degree. and to produce third and
fourth composite signals each of which includes a combination of at
least three of said audio information signals and respectively
containing a different one of said common signals as its
predominant component,
signal amplitude-modifying means connected to receive and operative
to couple said first and second relatively phase-shifted first and
second composite signals and said third and fourth composite
signals to respective ones of said sound-reproducing devices,
control signal generating means connected to receive and operative
in response to at least said relatively phase-shifted first and
second composite signals to detect whether said relatively
phase-shifted first and second composite signals contain
substantially equal amplitude signal components in phase
coincidence or in phase opposition and to produce a first control
signal operative to enhance the gain of the signal
amplitude-modifying means receiving said first and second
relatively phase-shifted composite signals when said relatively
phase-shifted first and second composite signals do not contain
said common signals either in phase coincidence or in phase
opposition, and
means for applying said first control signal to the signal
amplitude-modifying means receiving said relatively phase-shifted
first and second composite signals.
14. Apparatus according to claim 13 wherein said control signal
generating means further comprises:
means connected to receive and operative to compare said third and
fourth composite signals and to produce a second control signal
when said third and fourth composite signals do not contain
substantially equal signals either in phase coincidence or in phase
opposition, and
means for applying said second control signal to the signal
amplitude-modifying means receiving said third and fourth composite
signals.
15. Apparatus according to claim 14 wherein said means for applying
said first and second control signals to said signal
amplitude-modifying means, respectively, includes means for
subtracting said second control signal from said first control
signal, and means for subtracting said first control signal from
said second control signal.
16. Apparatus according to claim 13, wherein said control signal
generating means includes
means connected to receive said relatively phase-shifted first and
second composite signals and operative to derive therefrom first,
second, third and fourth auxiliary composite signals respectively
containing in the same relative phase relationship the signal
components contained in said relatively phase-shifted first and
second composite signals and in said third and fourth composite
signals,
first circuit means connected to receive and operative to compare
said first and second auxiliary composite signals and to produce a
first signal indicative of whether they contain substantially equal
amplitude signals in phase coincidence or in phase opposition,
second circuit means connected to receive and operative to compare
said third and fourth auxiliary composite signals and to produce a
second signal indicative of whether they contain substantially
equal amplitude signals in phase coincidence or in phase
opposition,
means for subtracting said second signal from said first signal to
produce said first control signal, and
means for subtracting said first signal from said second signal to
produce said second control signal.
17. Apparatus according to claim 16, wherein said first circuit
means includes means for separately rectifying said first and
second auxiliary composite signals and means for subtracting one
from the other the rectified output signals therefrom to produce a
first difference signal, and said second circuit means includes
means for separately rectifying said third and fourth auxiliary
composite signals and means for subtracting one from the other the
rectified output signals therefrom to produce a second difference
signal.
18. In a system for reproducing on four sound-reproducing devices a
like number of directional audio information signals contained in
first and second composite signals each of which includes at least
three of said audio information signals in preselected amplitude
and phase relationships, two of which are common to both composite
signals with said common signals in one of said composite signals
in substantially quadrature relationship with corresponding ones of
said common signals in the other of said composite signals,
including decoding means connected to receive said first and second
composite signals and operative to shift the phase of one of said
composite signals relative to the other by substantially 90.degree.
and to produce first, second, third and fourth output composite
signals respectively containing a different one of said audio
information signals as its predominant component and signal
amplitude-modifying means connected to receive and operative to
couple said first, second, third and fourth output composite
signals to respective ones of said sound-reproducing devices, a
logic circuit for identifying the signal or signals instantaneously
present in said output composite signals and for producing signals
for controlling said signal amplitude-modifying means to enhance
said instantaneously present signal or signals at said
sound-reproducing devices relative to other signals, and logic
circuit comprising:
means connected to receive and operative in response to at least
said relatively phase-shifted first and second composite signals to
detect whether said relatively phase-shifted first and second
composite signals contain substantially equal amplitude signal
components in either phase coincidence or in phase opposition, and
to produce a first control signal operative when applied to the
signal amplitude-modifying means receiving said first and second
output composite signals to enhance the gain thereof when said
relatively phase-shifted first and second composite signals do not
contain said common signals either in phase coincidence or in phase
opposition.
19. A logic circuit according to claim 18 further including
circuit means connected to receive and operative to compare said
third and fourth output composite signals and to produce a second
control signal operative when applied to the signal
ampltiude-modifying means receiving said third and fourth output
composite signals to enhance the gain thereof when said third and
fourth output composite signals do not contain substantially equal
signals either in phase coincidence or in phase opposition.
20. A logic circuit according to claim 18, wherein said
last-mentioned means includes
cicuit means operative in response to said relatively phase-shifted
first and second composite signals to produce first, second, third
and fourth auxiliary composite signals respectively containing in
the same relative phase relationship the signal components
contained in said first, second, third and fourth output composite
signals, and
means connected to receive and operative to compare said first and
second auxiliary composite signals.
Description
BACKGROUND OF THE INVENTION
This invention relates to apparatus for recording and reproducing
four separate channels of information on a medium having only two
independent tracks, and more particularly to apparatus for
reproducing such information and presenting it on four loudspeakers
to give the listener the illusion of sound coming from a
corresponding number of separate sources. More particularly, the
present invention is concerned with an improved decoder for
four-channel sound, also known as "quadruphonic sound", recorded on
a two-track medium in accordance with the method described in the
aforementioned co-pending application Ser. No. 44,224, and similar
systems.
The recording method disclosed in application Ser. No. 44,224 is
based on an encoding function which results from passing four
signals associated with the four channels of sound (which, for
convenience, are identified as L.sub.f, L.sub.b, R.sub.f, and
R.sub.b, for "left-front", "left-back", "right-front" and
"right-back", respectively) through six all-pass phase-shifting
networks and thereafter combining them in appropriate proportions
to produce two composite signals, L.sub.T and R.sub.T. These
composite signals may be transmitted over two lines, or recorded
and reproduced over a two-channel recording medium, such as a
stereophonic disc record, and subsequently transduced and presented
for decoding into four simulated channels of sound by suitable
decoding apparatus, one form of which is described in the aforesaid
application Ser. No. 44,224. Without here going into a detailed
description of the encoder, suffice it to say that the phasors
corresponding to the composite signals L.sub.T and R.sub.T bear the
relationship portrayed in FIG. 1 as phasor groups 10 and 12. It
will be noted that the signals L.sub.T and R.sub.T contain
predominant signals L.sub.f and R.sub.f, respectively, with each
containing a lesser proportion of L.sub.b and R.sub.b signals at
90.degree. out of phase relative to each other, with L.sub.b
leading the L.sub.f signal in L.sub.T and R.sub.b leading the
R.sub.f signal in R.sub.T. It will be recognized that the signals
L.sub.f, L.sub.b, R.sub.f and R.sub.b are usually incoherent since
they proceed from different instrumental or vocal groups and,
accordingly, the phasor diagrams 10 and 12 (and others to which
reference will be made as the description proceeds) represent the
phase relationships of the common in-phase frequency components of
the original signals.
It should also be noted that usually the channels L.sub.f and
R.sub.f are associated with left and right loudspeakers
respectively arranged at the two front corners of a room or
listening area, while the signals L.sub.b and R.sub.b are
associated with respective loudspeakers positioned at the left and
right back corners of the listening area.
It is shown in application Ser. No. 44,224 that to accomplish
decoding of the signals L.sub.T and R.sub.T depicted in FIG. 1,
which typically will have been recovered from a tape or disc record
by conventional playback methods and applied to the decoder
directly, or after broadcasting and reception through a two-channel
broadcasting system, they are first passed through respective
all-pass phase-shifting networks (herein referred to as
.psi.-networks) which introduce phase angle shifts, as a function
of frequency, without substantial alteration of the amplitude of
signals, identified by the expressions (.psi. + 90.degree.) and
(.psi. + 0.degree.), respectively, the expression in parenthesis
identifying the angle vs. frequency function of each network. The
reference angle .psi. is arbitrarily chosen, the only requirement
being that this reference angle is substantially the same in all
phase-shift networks embodied in the decoder. Although a convention
that the phase-shift angles are lagging has been observed, leading
phase-shifts could also be employed, as long as the same convention
is observed throughout the decoder. This phase-shift operation
(which is also utilized in the present decoder) causes the phasors
appearing at the output terminals A and B of the .psi.-networks to
appear in the positions depicted by the diagrams 14 and 16. It will
be observed that the magnitude and relative internal positions of
the various frequency components have been retained (although in
reality they are displaced by the reference angle .psi. which is a
function of frequency, from the phasor groups 10 and 12). To
signify this relative phase displacement, the phasors appearing in
groups 14 and 16 are distinguished by primes and, in the
description to follow, each time a signal undergoes a phase-shift
in a .psi.-netowrk, another prime is added to its symbol. Thus, the
phasor group 14 contains L'.sub.f as a predominant signal, with the
signals .707L'.sub.b and .707R'.sub.b appearing in the same
relative relationship with respect to L'.sub.f, and phasor group 16
contains R'.sub.f as a predominant signal along with the signals
.707R'.sub.b and .707L'.sub.b, also in the same relative phase
relationship as found in the phasor groups 10 and 12. By virtue of
the 90.degree. differential phase-shift, the phasor groups 14 and
16 are positioned in phase with respect to each other in a manner
to be linearly added and subtracted so as to derive two additional
signals at terminals C and D, represented by phasor groups 18 and
20, respectively. The signal 18 contains predominantly the L'.sub.b
signal, and signal 20 contains predominantly the signal R'.sub.b,
with both containing signals .707L'.sub.f and .707R'.sub.f in the
relaive phase relationships illustrated by their respective
phasors. It is evident, then, that by the relatively simple process
of decoding disclosed in the aforementioned co-pending application
(which is also used in one embodiment of the decoder to be
described herein), there are produced four signals, 14, 16, 18 and
20, which respectively predominantly contain the original signals
L.sub.f, R.sub.f, L.sub.b and R.sub.b. These decoded signals are
not "pure" or discrete original signals, however, each being
"diluted" or "contaiminated" by two other signals. Nevertheless,
when all four channels of the original program contain musical
signals in concert, and the four decoded signals are reproduced
over respective loudspeakers placed in the corners of the room or
listening area, then as far as the listener is concerned there is
sufficient "mixing" of the sounds in the room that the resulting
overall sound effect is quite similar to the sound of the original
four discrete channels, and a credible simulation of the original
four-channel program results.
There are situations, however, in which it is desirable to provide
an illusion of greater independence or purity of the decoded
signals; for example, when the original sound is present in one or
two channels only, it is desirable to automatically attenuate the
gain in those channels which originally were inactive, thereby to
enhance the separation of the channels which are present. Two
different logic and control systems for achieving this type of
audible spatial enhancement are described in co-pending
applications Ser. Nos. 44,224 and 81,858. It is a primary object of
the present invention to obtain greater quadruphonic realism than
that attainable with previously described methods while, at the
same time, simplifying the circuitry for accomplishing it.
SUMMARY OF THE INVENTION
Briefly, in accordance with one aspect of the invention, a
simplification and reduction in cost of the decoder are achieved by
providing at the input to the decoder two pairs of phase-shifting
networks (instead of the two used in the decoders described in the
aforementioned applications), the networks of each pair being
operative to introduce a relative shift of 90.degree. between
signals applied thereto, and applying the two composite input
signals in parallel to the respective pairs, so as to produce four
output signals bearing such a phase relationship to each other as
to be combined by appropriate addition and subtraction to produce
four signals predominantly containing left-front, left-back,
right-front and right-back information and having a phase
relationship to each other favorable for application, without
further phase-shifting, to their respective loudspeaker systems.
Thus, effective decoding is achieved using only four phase-shifting
networks, whereas in the decoders using two phase-shifting networks
at the input required four additional phase-shifting networks, or a
total of six, to achieve the same favorable phase relationship
between the signals applied to the four loudspeakers.
Another aspect of the invention is concerned with improved logic
circuitry for controlling the gains of the amplifiers associated
with the four separate loudspeakers to enhance the realism of
four-channel simulation. In accordance with this feature of the
invention, four signals produced in the decoder and resembling the
signals ultimately applied to the loudspeakers, are separately
rectified and the resulting voltage waveforms compared and shaped
to produce a pair of gain control signals which are utilized to
control the gains of gain control amplifiers in the respective
loudspeaker circuits. One of the signals controls in unison the
gains of the amplifiers associated with the two front loudspeakers,
and the other controls the two back loudspeakers in unison, the
relative phases of the signals representing the independent
channels, together with the nature of the control signals derived
by wave-matching, improves the separation between the front and
back sounds to enhance the realism of four-channel simulation.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention, and a
better understanding of its construction and operation, will be had
from the following detailed description, taken in conjunction with
the accompanying drawings, in which:
FIGS. 1A and 1B taken together is a schematic diagram of one
embodiment of decoding apparatus embodying the invention, to which
brief reference has already been made in discussing the background
of the invention;
FIG. 1C is a series of phasor diagrams useful in explaining the
operation of the invention;
FIG. 2 illustrates the control characteristic of a portion of the
circuit of FIG. 1;
FIG. 3A and 3B taken together are curves useful in explaining the
operation of the circuit of FIG. 1; and
FIGS. 4A and 4B taken together is a schematic diagram of another
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The decoder of the present invention is similar in some respects to
the decoder described in the aforementioned application Ser. No.
44,244, certain details of which, in turn, are described in
application Ser. No. 44,196, as well as to the decoder described in
application Ser. No. 81,858. The disclosures of these applications
are incorporated herein by reference to the extent necessary for
understanding the operation of certain aspects of the herein
described decoder; however, the description to follow is believed
to be sufficiently complete to enable one skilled in the art to
understand its operation without recourse to the co-pending
applications.
Reverting to FIG. 1, the present decoder is similar to those
described in the aforementioned applications in that the composite
signals L.sub.T and R.sub.T are applied to corresponding input
terminals 22 and 24, subjected to a relative phase-shift of
90.degree. by a pair of .psi.-networks 26 and 28 to produce at
terminals A and B the signals represented by phasor diagrams 14 and
16, respectively, and adding and subtracting the signals 14 and 16
in a pair of summing networks 30 and 32, each of which may be a
simple resistor matrix, to derive signals at terminals C and D
depicted by the phasor diagrams 18 and 20. The present circuit
differs, however, from those described in the referenced
applications in that it includes an improved logic and control
circuit which results in greater enhancement of quadruphonic
realism.
In accordance with the invention, the instantaneous amplitudes of
the signals delivered to the four loudspeakers (in a manner to be
described) are controlled by logic circuitry contained within the
dotted line enclosure 34 in which a manner that a listener is given
the illusion of four separate sources of sound (provided, of
course, that four sources are present), and providing relatively
good channel separation when less than four original signals are
present in the composite L.sub.T and R.sub.T input signals. This is
accomplished by applying the signals appearing at terminals A, B, C
and D, depicted by phasor groups 14, 16, 18 and 20, respectively,
to respective rectifiers 36, 38, 40 and 42 (which are preferably
full-wave rectifiers) and smoothing the rectified signals with
leaky integrators comprising capacitors 44, 46, 48 and 50 and
resistors 52, 54, 56 and 58 respectively in parallel therewith. The
decay-time characteristic of the four rectifier integrator circuits
are substantially the same, the rise time being relatively rapid,
of the order of 0.001 second, while the decay-time should be
relatively longer, of the order of 20 milliseconds, although these
times may be varied considerably.
The voltage produced by rectifier 38 is subtracted from the voltage
produced by rectifier 36 in a subtracting circuit 60, which may
simply be a summing resistor or junction, and the voltage produced
by rectifier 42 is similarly subtracted from the voltage delivered
by rectifier 40 in a similar subtracting circuit 62. The resulting
signals from subtracting circuits 60 and 62 are again rectified by
respective rectifiers 64 and 66, which are also preferably
full-wave rectifiers, and integrated by respective R-C circuits 68
and 70. The rise time of these circuits, which are substantially
equal, is of the order of 10 milliseconds, and a decay time of the
order of 400 milliseconds has been found satisfactory, although it
has been determined by experiment that these values may be varied
over an appreciable range without significant loss of
performance.
The voltages appearing at junctions 72 and 74 may then be shaped in
a pair of wave-shaping networks 76 and 78, respectively, each of
which has a logarithmic transfer characteristic, and then applied
in mutually reversed arrangement to a pair of subtracting circuits
80 and 82. That is, the output of network 78 is applied to the
subtractive terminal of subtracting circuit 80 and to the adding
terminal of circuit 82, and the output of network 76 is applied to
the negative terminal of circuit 82 and to the positive terminal of
subtractor 80. It may be noted at this juncture that in some cases
the wave-shaping networks 76 and 78 may not be necessary, in which
case the voltages appearing at junctions 72 and 74 would be applied
directly to the subtracting circuits 80 and 82 in the manner
illustrated. Or, alternatively, the shaping networks 76 and 78 may
be placed in circuit between subtracting circuit 60 and 62 and
their associated rectifiers 64 and 66, the objective being to
maintanin the relative amplitudes of the control signals as a
function of position and relative magnitudes of the signals
L.sub.f, L.sub.b, R.sub.f and R.sub.b regardless of their total
amplitudes. In either arrangement, the subtractor circuits 80 and
82 respectively deliver control voltages E.sub.b and E.sub.f
(having characteristics to be described hereinafter), the E.sub.f
signal being used to control the gain of the loudspeaker circuits
which handle the front loudspeaker signals, and the voltage E.sub.b
being used to control the gain of the back loudspeaker
circuits.
While the signals appearing at terminals A, B, C, and D, which, it
will be remembered, correspond to the original quadruphonic signals
L.sub.f, L.sub.b, R.sub.b and R.sub.f, respectively, may be applied
to their respective loudspeaker circuits and still achieve
satisfactory operation of the decoding and control circuit 34, it
is preferable that they be applied through four additional
.psi.-networks 84, 86, 88 and 90 which provide additional
differential phase shifts so that the corresponding signal phasors
92, 94, 96 and 98 are in a relative phase relationship with each
other more favorable to maintenance of the integrity of image
formation at the adjacent loudspeaker pairs. It is to be
understood, however, that reasonably satisfactory, but not ideal,
performance is obtainable without .psi.-networks 84 - 90. As an
alternative, the input signals to the left-front and right-front
loudspeakers may be taken from terminals A' and B' (at the inputs
of .psi.-networks 26 and 28) to help maintain the integrity of the
phasors at the front loudspeakers when the .psi.-networks 84 - 90
are not used.
The signals delivered by .psi.-networks 84, 86, 88 and 90 are
respectively applied to the input terminal of gain control
amplifiers 100, 102, 104 and 106, the outputs of which are applied
to loudspeakers 108, 110, 112 and 114, respectively. These
loudspeakers, which carry phase-shifted replicas of the signals
L.sub.f, L.sub.b, R.sub.b and R.sub.f, respectively, (which are
designated L".sub.f, L".sub.b, R".sub.b and R".sub.f, respectively)
are placed in the corresponding corners of a room or listening area
116 as shown. Any other arrangement serving the taste of a
particular listener may, of course, be used.
Each of the gain control amplifiers 100 - 106 has a control
electrode 118, 120, 122 and 124, respectively, application of a
control voltage to which varies the gain of the amplifier. The gain
control amplifiers may embody a variable gain vacuum tube, a
suitable variable gain transistor, or any suitable vario-losser
circuit, or other well known technique for providing variable gain
in response to an applied voltage. For reasons to be explained, the
control voltage E.sub.f from subtractor circuit 82 is applied in
parallel to control terminals 118 and 124 of amplifiers 100 and
106, respectively, to control the gains of the front loudspeakers,
and the control signal E.sub.b is applied in parallel to the
control electrodes 120 and 122 to control the amplification of
signals applied to respective back loudspeakers. A suitable
relationship of the gains of amplifiers 100 - 106 as a function of
the voltage applied to their respective gain control terminals is
depicted by the curve 128 in FIG. 2. It is seen in this exemplary
case that when the gain control voltage is zero, the amplification
factor is approximately 80 percent of maximum. When the applied
control voltage E.sub.b or E.sub.f, as the case may be, is strongly
positive, the amplification factor approaches the 100 percent
value, and if the control voltage is negative, the amplification
factor decreases until at some value, -E.sub.c, the amplifier is
cut off and removes the signal from its respective loudspeaker. The
circuit is so arranged that when there is no input signal, or when
all signals are present at equal strength simultaneously, the
control voltages E.sub.b and E.sub.f are at or near zero, causing
all of the loudspeaker circuits to be at about 80 percent gain.
This characteristic may be varied within reasonable limits,
however, and it should also be understood that the shape of curve
128 is also subject to some variation without departing from the
spirit of the invention.
The gain control amplifiers 100 - 106 have time constant circuits
incorporated therein having a characteristic to allow the rapid
application of an increasing control voltage in order that the
amplification factor can be rapidly increased, but which allows the
control voltage to decrease relatively slowly (as by slow discharge
of a capacitor) so as to prevent the amplification factor from
decreasing or being "released" too rapidly. In a system which has
been satisfactorily operated, a rise time of the order of 0.02
second and a release time of the order of 0.80 second have been
found suitable. It is to be understood, however, that these values
may vary over wide limits without significantly impairing the
operation of the system.
The operating principle of the decoder and logic circuitry
illustrated in FIG. 1 is based upon the phasor relationship of the
signals depicted by the phasor diagrams 14, 16, 18 and 20, which
are reproduced to a larger scale in FIG. 1C better to visualize the
action of the logic circuit. If, for example, a left front signal
L'.sub.f only is present at terminal A, no signal appears at
terminal B, as seen from inspection of right-front phasor group 16.
It is seen, however, that the signal L'.sub.f does appear in the
two phasor groups 18 and 20, in the form of two identical phasors
.707L'.sub.f. If, on the other hand, a right-front signal R'.sub.f
is present in the phasor group 16, then this signal is not present
in phasor group 14, but appears in equal magnitudes in phasor
groups 18 and 20, but in an out-of-phase relationship. If both
L'.sub.f and R'.sub.f are present simultaneously, then, since they
are different sounds, they will be uncorrelated at terminals A and
B, but will appear in either a positive or a negative correlation
at terminals C and D. On the other hand, by similar reasoning, it
may be shown that if only either one or both of signals L'.sub.b
and R'.sub.b are present, they will be uncorrelated at the
terminals C and D, but will be in either direct or inverse
correlation at terminals A and B. An important aspect of the
present invention in the utilization of a novel principle for
recognizing these correlated or uncorrelated conditions without the
use of conventional multiplying and integrating circuits. Rather,
recognition of the correlated or uncorrelated condition is
accomplished by instantaneously matching the waveforms of signals
as they appear in the decoder.
Referring to FIG. 1C, let it be assumed that only the left-front
signal L.sub.f exists, which after passage through the
.psi.-networks 26, appears as phasor L'.sub.f, shown in solid,
heavy line. After rectification by the rectifier 36, this signal
will produce a voltage for application to the positive terminal of
subtracting circuit 60. Since no signal is present at terminal B,
there will be no voltage developed by rectifier 38 and,
accordingly, after subtraction in the circuit 60, a current due
solely to the signal L'.sub.f will appear at its output which, in
turn, will result in a voltage being generated at junction 72 of
integrating circuit 68.
Turning attention now to the phasor groups 18 and 20, it is seen
that the two signals .707L'.sub.f, shown in heavy solid lines, are
identical in both phase and intensity. Therefore, the voltages at
the output terminals of full-wave rectifiers 40 and 42 contributed
by these two signals will be identical in magnitude and waveform
with the consequence that when subtracted, one from the other, in
circuit 62, zero current, and therefore zero voltage at terminal
74, results. It will be evident that after the signals appearing at
terminal 72 and 74 are shaped by their respective shaping networks
76 and 78 and thereafter subtracted in the subtracting circuits 80
and 82, a negative voltage E.sub.b appears at the output of
subtractor 80. This voltage is applied in parallel to the gain
control terminals 120 and 122 of amplifiers 102 and 104,
respectively, causing these amplifiers, which actuate the back
loudspeakers 110 and 112 to be partially or completely turned off,
while at the same time amplifiers 100 and 106 feeding the front
loudspeakers 108 and 114 will attain full gain condition.
Therefore, the signal L.sub.f will appear to emanate principally
from loudspeaker 108.
By similar reasoning, it is seen that if the signal R'.sub.f only
is present, identified by a heavy broken line in phasor group 16,
only two other phasors will appear in the system, namely the phasor
.707R'.sub.f at terminal C, and a corresponding .707R'.sub.f signal
at terminal D, in the opposite direction. In this case, then, the
output of rectifier 36 is zero, while the output of rectifier 38 is
maximum, corresponding to the phasor R'.sub.f. Subtraction of these
two signals in subtractor 60 causes a current to flow toward the
subtractor, but since rectifier 64 is a full-wave rectifier, the
voltage developed at junction 72 will be a positive voltage, as
before.
Turning again to phasor groups 18 and 20, it is seen that the
phasors .707R'.sub.f at terminals C and D are oppositely directed;
however, because rectifiers 40 and 42 are full-wave rectifiers, the
voltages generated at their respective outputs will have the same
polarity and will be very nearly identical. This is demonstrated in
FIG. 3 which portrays, by way of example, an impulse contained in
the signal R'.sub.f applied to the system and appearing as two
signals of opposite phase at terminals C and D. The positively
directed signal at terminal C is shown in FIG. 3A as a decaying
sine wave 130. Upon rectification, the portions of the wave below
the time axis are reversed and appear above the time axis as shown
by the corresponding dotted-line curves. After being smoothed by
the integrating circuit, consisting of capacitor 48 and resistor
56, the resulting voltage is of the form shown in heavy dotted
lines and identified as e.sub.40. Turning now to FIG. 3B, the
initial impulse at terminal D is of the same magnitude as the one
shown in FIG. 3A, but is reversed in phase. Again, after full-wave
rectification, and smoothing by capacitor 50 and resistor 58, the
voltage appearing at the output of rectifier 42 has the same
magnitude and sense as the voltage appearing at the output of
rectifier 40. Subtraction of one from the other in subtractor 62
produces a zero output, and consequently zero voltage at junction
74, as in the previous case. Thus, with either a L'.sub.f or
R'.sub.f signal acting alone, there is a positive voltage at
junction 72 and zero voltage at junction 74, resulting in turning
on of the front loudspeakers 108 and 114 and turning off of the
back loudspeakers 110 and 112.
When both L'.sub.f and R'.sub.f signals are present, remembering
that these two signals are incoherent (even if they may be in
concert), their instantaneous peak values as a function of time do
not occur simultaneously; thus, after rectification by rectifiers
36 and 38 and subtraction at subtractor 60 there will be a net
current through rectifier 64, resulting in a net voltage at
junction 72. On the other hand, voltages developed by rectifiers 40
and 42 will continue to exhibit the same identity as described
above with the result that the net output voltage at junction 74 is
zero. In other words, even if there are two separate and distinct
signals L'.sub.f and R'.sub.f supplied to the circuit in concert,
only the front loudspeakers will be activated with increased gain
and the back loudspeakers will be attenuated.
A converse action takes place when either one or both of signals
L'.sub.b or R'.sub.b are applied to the decoder. In this case,
there is a net control voltage at junction 74, and a zero voltage
at the junction 72, resulting in a negative control signal E.sub.f
and a positive control signal E.sub.b, whereby the back
loudspeakers 110 and 112 are turned on, and the front loudspeakers
108 and 114 are turned off. It follows that as the various signals
appear and disappear the appropriate gain control amplifiers are
turned on and off in in response thereto.
Referring now to FIG. 4, there is shown in schematic form an
alternative decoder matrix in which a simplification of the
circuitry is achieved without sacrifice of performance and a
modified form of the just-described logic circuit. It will be
remembered that in order for the circuit of FIG. 1 to perform the
decoding function it is necessary to first introduce a relative
phase shift of 90.degree. with .psi.-networks 26 and 28 to place
the signals L'.sub.f and R'.sub.f in quadrature. This relationship,
per se, is not undesirable, except when these two phasors contain a
common center signal. In order to form a virtual image of this
center signal, in-between the two front loudspeakers, it is
desirable that the two front signals remain in phase. This
relationship may be obtained by various stratagems, the one used in
the system of FIG. 1 being the incorporation of the four additional
.psi.-networks 84, 86, 88 and 90. As has been suggested
hereinabove, in a decoder which is not intended for audition of the
highest quality it is permissible to dispense with .psi.-networks
84 - 90, instead connecting the two front loudspeakers (i.e., 108
and 114) to terminals A' and B' instead of to terminals A and B, so
that the two front loudspeakers receive the full, unmodified
signals L.sub.T and R.sub.T, respectively, in which the signals
L.sub.f and R.sub.f predominate in proper phase relationship.
However, even if this is done, there is a .psi.-function phase
angle between the two front loudspeakers and the two rear
loudspeakers, resulting in a certain degree of side and back image
blurring and dissymetry. This latter effect is not sufficient to
appreciably diminish the quality of the quadruphonic image and is
the type of compromise which may be accepted in lower cost
equipment. In the best professional equipment, however, it is
important that the phasor groups are presented in a most favorable
phase relationship.
This goal is accomplished, in the embodiment of the decoder shown
in FIG. 4A. This decoder achieves the same relative phase
relationship of the phasors applied to the loudspeakers obtained in
the embodiment of FIG. 1A, but with a saving of two .psi.-networks;
that is, the system of FIG. 4A, requires only four, instead of six,
.psi.-networks.
In this decoder, two composite quadruphonic signals L.sub.T and
R.sub.T, recovered from a two-channel medium, and respectively
protrayed by phasor groups 10 and 12, are applied to the input
terminals 150 and 152. These signals are in all respects identical
to the corresponding signals applied to the input terminals of the
decoder of FIG. 1A. Unlike the system of FIG. 1A, however, in which
two .psi.-networks are used to properly position the phases of the
two signals for subsequent manipulation, the decoder of FIG. 4A
utilizes four such networks, 154, 156 158 and 160, two of which
provide a phase shift of (.psi.+0.degree.), and the other two
introducing a phase shift of (.psi.+90.degree.), the L.sub.T signal
being applied in parallel to .psi.-networks 154 and 156, and the
R.sub.T signal being applied in parallel to .psi.-networks 158 and
160. By reason of the relative 90.degree. phase shift, the phasor
groups 162 and 164 appearing at the outputs of .psi.-networks 154
and 156, corresponding to the L.sub.T signal, are in quadrature
relationship, and similarly, the phasor groups 166 and 168
appearing at the outputs of .psi.-networks 158 and 160,
respectively, corresponding to the R.sub.T signal, are also in
phase quadrature. Because the phase angle .psi. generally varies
with frequency, the phasor groups 162, 164, 166 and 168 do not bear
a fixed angular relationship with respect to phasor groups 10 and
12, but inasmuch as the reference angle .psi. is the same for all
of the networks, it is permissible to treat them as bearing a fixed
relationship with respect to each other. It will be remembered that
the signals L'.sub.f, L'.sub.b, R'.sub.b and R'.sub.f are usually
complex program signals and, therefore, the phasor relationships
within each phasor group represents the relationships of the same
frequency components of the signals.
As in the circuit of FIG. 1A, the object of the decoder is to
derive from the incoming signals L.sub.T and R.sub.T four separate
signals predominantly containing the signals L'.sub.f , L'.sub.b ,
R'.sub.b and R'.sub.f , respectively, and to reproduce them over
respective loudspeakers 170, 172, 174 and 176. To this end, the
signals represented by phasor groups 162 and 168 are applied
without change to respective gain control amplifiers 178 and 180,
the outputs of which are applied to loudspeakers 170 and 176,
respectively. A signal predominantly containing L'.sub.b is derived
by adding .707 of the output of .psi.-network 156 and .707 of the
output of .psi.-network 160 in a summing circuit 182, the output of
which may be represented by the phasor group 184, in which the
signal L'.sub.b predominates, with the signals L'.sub.f and
R'.sub.f also being present but at a 3db lower level. This signal
is amplified by gain control amplifier 186 and applied to
loudspeaker 172. A signal predominantly containing the R'.sub.b
signal is obtained by adding in summing circuit 188 .707 of each of
the outputs of .psi.-networks 154 and 158, the resultant signal,
represented by phasor group 190, being applied to a respective
control amplifier 192 for application to loudspeaker 174. Thus, the
loudspeakers 170, 172, 174 and 176 carry signals which have
predominant information from the left-front, left-back, right-back
and right-front channels, respectively. As with the decoder of FIG.
1A, each of these signals is contaminated with information from two
other signals, but the contaminating signals being part of the same
original program, their contribution is not unpleasant and, indeed,
often provides an improvement in "ambience" or spaciousness of the
musical selection.
It will be noted that the phasor groups 162, 184, 190 and 168
exhibit relative phase-shift relationships identical to the
corresponding phasors of FIG. 1A, this desired relationship having
been obtained with only four .psi.-networks, whereas the system of
FIG. 1A requires six .psi.-networks to achieve the same result.
Thus, the decoder of FIG. 4A offers the advantage of greater
simplicity, with accompanying reduced cost. The arrangement
comprising the four .psi.-networks and two summing circuits
constitutes a satisfactory decoder for connection through suitable
amplifiers and loudspeakers to produce a highly realistic and
satisfactory rendition of the original quadruphonic program.
As with the decoder of FIG. 1A, in the interest of achieving the
illusion of greater independence or "purity" of the decoded
signals, it may be desirable to provide a logic and control circuit
to provide "enhancement" of the individual predominant signals. The
logic and control circuit 34 of FIG. 1B may be used for this
purpose, or the alternative control circuit within the dotted-line
enclosure 200 in FIG. 4B may be used. The two signals at the
outputs of .psi.-networks 160 and 156, represented by phasor
diagrams 168 and 164, respectively, are applied to a pair of gain
control amplifiers 202 and 204, the gains of which are made to
closely track each other by application of a common control signal
to their respective gain control terminals 206 and 208. Although
different in magnitude, thp outputs of these amplifiers,
represented by phasors 168' and 164', have the same phase position
as the similarly numbered phasors at the outputs of .psi.-networks
160 and 156. These phasor groups exhibit predominant left and right
front channel signals L'.sub.f and R'.sub.f, respectively, each
also containing .707 of the signals L'.sub.b and R'.sub.b.
By adding .707 of the output of amplifier 202 and .707 of the
output of amplifier 204 in a summing circuit 210 a new signal is
obtained which predominantly contains the left-back signal
L'.sub.b. Similarly, by algebraically adding .707 of the output of
amplifier 204 and -.707 of the output of amplifier 202 in summing
network 212, another signal is obtained which contains
predominantly the signal R'.sub.b . It will be observed that the
latter two signals have the same phase relationship as phasors 184
and 190 in the decoder and, accordingly, have been similarly
identified as 184' and 190'. It will also be noted that phasors
168', 190' and 164' have the same relative magnitudes and phase
positions as phasors 16, 18, 20 and 14 in FIG. 1A, from which it
follows that the "wave-matching" recognition function described in
connection with FIG. 1B is equally applicable here. This function
is accomplished by rectifying (preferably full-wave) the signals
168' and 164' with rectifiers 214 and 216, respectively, and
smoothing the rectified signals with respective R-C integrating
circuits 218 and 220. The output voltages at the terminals 222 and
224 of the integrators are then subtracted from each other in a
subtracting circuit 226, the difference shaped and limited in
magnitude by a shaping network 228 (having the illustrated transfer
function), again rectified with full-wave rectifier 230, and
integrated with an integrator comprising a capacitor 232, a rise
time control resistor 234, and a release or decay time control
resistor 236. The signals 184' and 190' are similarly full-wave
rectified in rectifiers 240 and 242, respectively, and integrated
with respective R-C circuits 244 and 246, and the output voltages
appearing at their respective output terminals 248 and 250 are
subtracted from each other in subtracting circuit 252. The
difference signal is shaped and limited by shaping network 254,
having the same transfer function as shaping network 228, full-wave
rectified by rectifier 256, and integrated in a circuit including
resistors 258 and 260 and capacitor 262. The release (or discharge)
time of integrators 218, 220, 244 and 246, as established by the
product of their respective capacitances and resistances,
preferably is of the order of 20 milliseconds, but this number is
not critical.
The charge time of capacitors 232 and 262, established by resistors
236 and 260, respectively, and their decay times, established by
resistors 234 and 258, respectively, are preferably of the order of
20 milliseconds and 250 milliseconds, respectively but these
values, likewise, may vary over a considerable range without
deteriorating system performance.
The outputs of the integrators, appearing at output terminals 264
and 266, respectively, are subtracted in an inversely complementary
manner by a pair of subtracting circuits 268 and 270 which produce
at their outputs control signals E.sub.f and E.sub.b, respectively.
The E.sub.f control signal is applied in parallel to the control
terminals of amplifiers 178 and 180, which respectively control the
signal applied to the front loudspeakers 170 and 176, and the
E.sub.b control signal is applied in parallel to the control
terminals of amplifiers 186 and 192, which control the amplitude of
signals applied to the back loudspeakers 172 and 174, respectively.
The gain control amplifiers, of a type similar to those described
in connection with FIG. 1A, have a gain control characteristic
similar to that illustrated in FIG. 2, although it should be
understood that this is by way of example only. As in the system of
FIG. 1, the gain control function is so established that the gain
can rise relatively rapidly (e.g., in approximately 20
milliseconds) in response to a positively changing gain control
voltage, but to diminish slowly (e.g., in approximately 800
milliseconds) when the gain control signal decreases. As with the
other time control functions, these values may vary considerably
without affecting the principle of operation, and, in essence, are
adjusted to suit the artistic preference of the average
listener.
It is desirable to maintain the range of the control voltages
E.sub.f and E.sub.b within the limits indicated in FIG. 2, namely
between -E.sub.c for cut-off of amplification, and E.sub.m for
maximum gain, as there is no significant value in applying control
voltages to the gain control amplifiers in excess of these limits.
In the present embodiment, the amplitude of the control signals is
achieved by summing the outputs appearing at the output terminals
222, 224, 248 and 250 in a summing circuit 272, and applying this
sum signal in parallel to the gain control terminals 206 and 208 of
automatic gain control amplifiers 202 and 204. In this manner, the
gains of amplifiers 202 and 204 are automatically adjusted to
accommodate for changes in amplitude of the input signals. For
example, when the amplitude of signals L.sub.T and R.sub.T applied
to input terminals 10 and 12 is low, the sum signal is in a
direction to increase the gain of the amplifiers, whereas if the
input level is high, the gain of the amplifiers is rapidly reduced
so that the sum of the signals at terminals 222, 224, 248 and 250
is maintained at a relatively constant level.
The limits of the control signals E.sub.f and E.sub.b are further
established by the wave-shaping networks 228 and 254, the transfer
characteristics of which may be selected experimentally to obtain
the most efficacious and pleasing action. A good guideline to this
adjustment is that when a single signal L.sub.f and R.sub.f is
applied to input terminals 150 or 152, the gain of the front
loudspeaker amplifiers 178 and 180 is increased to a maximum, and
the gain of the back loudspeaker amplifiers 186 and 192 is just
reduced to zero. For convenience in making the adjustment, it is
preferable to provide a gain control adjusting knob for adjusting
the transfer function of the wave-shaping networks 228 and 254. It
is to be understood that the wave-shaping networks need not be
placed as illustrated but, instead, may be placed in the output
lines of subtractors 268 and 170.
* * * * *