U.S. patent number 3,818,136 [Application Number 05/248,285] was granted by the patent office on 1974-06-18 for four-channel front-to-back balance control.
This patent grant is currently assigned to Zenith Radio Corporation. Invention is credited to Wayne M. Schott.
United States Patent |
3,818,136 |
Schott |
June 18, 1974 |
FOUR-CHANNEL FRONT-TO-BACK BALANCE CONTROL
Abstract
A front-to-back balance control circuit in a quadraphonic
playback system for adjusting the sound output levels of a front
pair of speakers relative to a back pair while simultaneously
varying the phase differential between the back speakers. The
balance control utilizes a ganged potentiometer arrangement wherein
a potentiometer is serially-connected with each front speaker to
concurrently adjust the sound output of each speaker to
corresponding levels. The ganged potentiometer arrangement also
provides a variable impedance signal path common to the back
speakers for varying their sound output levels inversely in
response to adjustments in the sound output level of the front
speakers. The common signal path is further effective to serve as a
variable coefficient matrixing means for applying portions of a
first and a second input signal to each of the back speakers.
Inventors: |
Schott; Wayne M. (Des Plaines,
IL) |
Assignee: |
Zenith Radio Corporation
(Chicago, IL)
|
Family
ID: |
22938458 |
Appl.
No.: |
05/248,285 |
Filed: |
April 27, 1972 |
Current U.S.
Class: |
381/21; 369/89;
381/109 |
Current CPC
Class: |
H04H
20/89 (20130101); H04S 3/02 (20130101) |
Current International
Class: |
H04H
5/00 (20060101); H04S 3/00 (20060101); H04S
3/02 (20060101); H04n 005/00 () |
Field of
Search: |
;179/1GQ,1G,1UL,1.4ST,1.1TD,1GA,1GP,1D |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cooper; William C.
Assistant Examiner: D'Amico; Thomas
Attorney, Agent or Firm: Camasto; Nicholas A. O'Connor;
Cornelius J.
Claims
I claim:
1. A control circuit for use in an audio playback system which
includes an amplifier stage having right and left outputs issuing
respective right and left output signals, a right front sound
reproducer and a right back sound reproducer each having a first
terminal coupled to said right output, a left front sound
reproducer and a left back sound reproducer each having a first
terminal coupled to said second output, said control circuit
comprising:
a pair of potentiometers individually associated with an assigned
one of said front sound reproducers, each of said potentiometers
having a first terminal coupled to a second terminal of its
assigned front sound reproducers,
each potentiometer also having a second terminal coupled to a
junction and each potentiometer further having a wiper arm coupled
to a plane of reference potential to constitute a first segment of
each potentiometer between its first terminal and its wiper arm as
a series impedance for coupling its assigned front sound reproducer
to reference potential;
impedance means intercoupling a second terminal of said right back
sound reproducer and a second terminal of said left back sound
reproducer;
a matrix network comprising second segments of said potentiometers
disposed between their wiper arms and said junction and a matrixing
impedance coupled between said junction and said intercoupling
impedance means,
said potentiometer second segments, said matrixing impedance and
said intercoupling impedance means establishing a common signal
path coupling said second terminals of said back sound reproducers
to reference potential to permit application of portions of said
first and second output signals to each of said back sound
reproducers;
and control means coupled to said potentiometer wiper arms for
varying the impedance of said potentiometer second segments
included in said matrix network to adjust the output levels of said
back sound reproducers as well as the magnitude of said first and
second output signals applied to said left back and right back
sound reproducers, respectively, while simultaneously varying the
impedance of said potentiometer first segments forming said series
impedances to adjust the output level of said front sound
reproducers relative to the output level of said back sound
reproducers.
2. A control circuit as set forth in claim 1 in which said wiper
arms are interconnected through said control means so that when the
impedance of said matrix segments is increased, the impedance of
said series segments is decreased, and vice versa.
3. A control circuit as set forth in claim 1 in which said matrix
segments of said potentiometers are effectively connected in
parallel.
4. A control circuit as set forth in claim 1 in which said
impedance means intercoupling said right back and left back sound
reproducers comprises an adjustable resistor having one terminal
coupled to said second terminal of one of said back sound
reproducers and a wiper arm coupled to said second terminal of said
other back sound reproducer and, through said matrixing impedance,
to said junction, for changing the phase difference between said
first and second output signals applied to said rear sound
reproducers to substantially zero as the output level of said rear
sound reproducers approaches a maximum,
and in which said wiper arm of said intercoupling impedance is
mechanically coupled to said wiper arms of said potentiometers for
conjoint displacement therewith.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to quadriphonic sound
reproduction systems and more pariticularly to a front-to-back
balance control circuit for adjusting the sound output level of the
"front" sound reproducers relative to that of the "rear" sound
reproducers while simultaneously varying the phase differential
between the "left-rear" and "right-rear" sound reproducers in a
predetermined manner.
It is well known that the human auditory system can distinguish
between sounds emanating from points (i.e., front, back, left
and/or right) on an imaginary 360.degree. circle of variable radius
surrounding the listener. Furthermore, the listener can also detect
ambient sounds having a reduced amplitude, differential phase
shift, and time delay resulting from sound reflections which are
dependent on the acoustical characteristics of the room in which
the sound source is located. Recorded ambient sound, when part of,
or added to stereo records or tape can result in greatly enhanced
sound reproduction to the listener.
Heretofore, stereophonic sound reproduction systems have provided
means for coupling two channels of signal information representing
"right-front" and "left-front" sound sources to a pair of sound
reproducers, or speakers, resulting in a more realistic sound image
to the listener. With such a system, however, the listener is able
to distinguish only between sounds coming from the "right-front" or
the "left-front." Any ambient or so-called reverberation sound
effect will depend on sound reflections from the rear and side
walls of the room. Accordingly, the manner in which the room is
furnished can greatly affect what the listener hears; for example,
in a carpeted room having drapes and a great deal of furniture, the
sound is dispersed and absorbed, thereby minimizing the
reverberation effect. Thus, even though the original recording may
have been that of an orchestra performing in a large concert hall,
it will not be apparent when reproduced on the listener's
stereophonic system. The opposite may be true, however, in a room
having bare walls, no carpeting and little furniture where the
sound it almost totally reflected by the walls to effect
reverberation.
A logical extension of the stereophonic concept is to provide a
quadraphonic sound reproduction system having one speaker
positioned in each quadrant of the room. Thus, a true "concert
hall" effect may be obtained by reproducing the ambient sound or
reverberation from the "rear" speakers. While conceptually a
discrete four-channel system would appear to be the most desirable
means of accomplishing this, the integration of such a system with
existing phonographs, tape players and FM stereo tuners would be
difficult if not impossible. For example, most present-day tape
recorders provide only two recording and two playback heads thereby
rendering them useless for quadraphonic purposes, while most
phonographs are likewise capable of reproducing only two channels
of sound. Furthermore, the FM stereo multiplex broadcasting
techniques currently in widespread use are subject to rules
promulgated by the Federal Communications Commission which provide
for only two discrete channels. Although the design of new
four-channel equipment may not be difficult, the fact remains that
such revisions would make most of today's stereophonic systems
obsolete.
Accordingly, some emphasis has shifted to psuedo four-channel
systems which recover quadraphonic signals from matrixed
two-channel information and are therefore compatible with presently
available phonographs, tape players and FM stereo tuners. These
systems are generally based on the encoding or matrixing of four
discrete sound channels into two composite signals which may then
be stored on records and tape or broadcast to an FM multiplex
tuner. Upon application to a quadraphonic playback system, the
signals are amplified and then coupled to a matrixing network
wherein the original four discrete signals are reconstituted for
application to the sound reproducers. Prior art quadriphonic sound
reproduction systems, unfortunately, have generally required an
expensive, specially-designed transformer to "matrix" the composite
signals.
As in conventional stereophonic systems, it is desirable to include
a balance control for adjusting the sound output levels of the
"right-front" and "left-front" speakers relative to one another. In
addition, the quadraphonic system adds a further dimension; that
is, besides maintaining balance between the two "front" speakers,
proper balance should be maintained between the sound output levels
of the "front" speakers and the "rear" pair. If, however, the
"front" speakers are reduced to zero sound output, the 180.degree.
phase differential between the "right-rear" and "left-rear"
speakers required to reproduce the reverberation effect of the
concert hall is harsh and displeasing to the listener.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a novel
front-to-back balance control circuit for a quadraphonic sound
reproduction system.
A more particular object of the invention is to provide a
front-to-back balance control circuit which may be used to adjust
the sound output levels of the "front" and "rear" pair of sound
reproducers relative to one another.
A further object of the invention is to provide a front-to-back
balance control circuit which varies the phase differential between
the rear sound reproducers in accordance with the relative sound
output levels of the "front" and "rear" sound reproducers.
Another object of the invention is to provide a front-to-back
balance control circuit incorporating an inexpensive simplified
matrixing network for reconstructing four channels of audio
information from two input signals.
In accordance with the present invention, a control circuit in a
quadraphonic sound reproduction system is provided for adjusting
the sound output levels of a primary pair of sound reproducers
relative to an auxiliary pair. The control circuit of the present
invention contemplates the application of first and second input
signals to the amplifier stage of an audio playback system. The
amplifier stage has a primary sound reproducer with an associated
series resistance coupled between each output and a plane of
reference potential and an auxiliary sound reproducer coupled to
each output. A resistive network couples the auxiliary sound
reproducers to the plane of reference potential providing each of
them with a signal path common to the other auxiliary sound
reproducers. The common signal path is effective to serve as a
matrixing means for applying portions of the first and second input
signals to each of the auxiliary sound reproducers. Further,
front-to-back balance means are also provided for simultaneously
varying the impedances of the common signal path and the series
resistances of the primary sound reproducers to control the sound
output level of the auxiliary sound reproducers.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of this invention which are believed to be novel are
set forth with particularity in the appended claims. The invention
together with its further objects and advantages thereof, may be
best understood, however, by reference to the following description
taken in conjunction with the accompanying drawings, which like
reference numerals refer to like elements in the several figures
and in which:
FIG. 1a is an illustrative block diagram of a quadriphonic sound
reproduction system;
FIG. 1b is an illustrative block diagram of a quadriphonic playback
system; and
FIG. 2 is a combined schematic and block diagram of an FM multiplex
receiver which includes a front-to-back balance control in
accordance with the present invention.
PREFERRED EMBODIMENT OF THE INVENTION
The quadraphonic sound reproduction system shown in FIG. 1a employs
a matrixing network to encode four discrete channels of sound
information into two composite signals for purposes of information
storage and/or FM multiplex broadcast transmissions. A room 10
(e.g., a concert hall) having a sound source located therein is
depicted to have four microphones 11, i.e., right-front (R.sub.f),
left-front (L.sub.f), right-back (R.sub.b), and left-back
(L.sub.b), positioned to "pick-up" sounds emanating from the
corresponding quadrant of the room. Accordingly, if the sound
source (e.g., an orchestra) is positioned in the front-center of
the room, as illustrated in FIG. 1a, the front pair of microphones,
(R.sub.f) and (L.sub. f), may be directed toward the sound source
to receive sound waves directly therefrom. The rear microphones,
R.sub.b and L.sub.b, face away from the sound source to "pick-up"
reflected sound waves from the rear and side walls.
The sound waves detected by the four microphones 11 are, in turn,
converted into corresponding electrical signals, designated here
for convenience as R.sub.f, L.sub.f, R.sub.b and L.sub.b and
applied to an encoder network 12. In a typical quadraphonic system,
the encoder 12 matrixes the four input signals into two composite
signals, R' and L', with the following amplitude and phase
relationships;
1. R' = +1.22 R.sub.f + 0.38 L.sub.f + 1.22 R.sub.b - 0.62
L.sub.b
2. L' = +0.38 R.sub.f + 1.22 L.sub.f - 0.62 R.sub.b + 1.22
L.sub.b
The composite signals, R' and L', may then be coupled to a
tape/phono recording unit 13 where they are stored on conventional
stereophonic records or recorded on stereophonic tapes.
Alternatively, since the four discrete channels are encoded into
two composite signals, the R' and L' composite signals may be
broadcast by an FM multiplex transmitter 14 for reception by a
conventional FM stereo multiplex receiver such as that shown in
FIG. 1b.
The quadraphonic playback system, shown in FIG. 1b, is effective to
dematrix the R' and L' composite signals developed by the
quadraphonic transmission system of FIG. 1a into four channels of
audio information.
Operationally, broadcast signals from the FM multiplex transmitter
14 are intercepted by the antenna 15 of an FM multiplex receiver 16
wherein the received radio-frequency signals are processed and
converted into the respective R' and L' composite signals. The R'
and L' signals are then applied to an audio amplifier 17 where they
are further amplified. In the alternative, when the composite
signals have been stored on stereophonic tapes or records, as
previously mentioned, a conventional stereophonic phonograph or
tape player may be used to reproduce the R' and L' composite
signals which are then, in turn, applied to the audio amplifier 17.
There is provision in the audio amplifier 17 for manual switching,
as desired by the listener, between input composite signals derived
from the FM multiplex reciever 16 or the tape and/or phono playback
system 18.
The R' and L' composite signals are then coupled to a decoder 19
where they are matrixed. While it is feasible to matrix the
composite signals into four decoded output signals, it has been
empirically determined that decoding of the composite signals for
application to right-front (RF) and left-front (LF) speakers 20,
respectively, provides no significant advantages under most
conditions. Thus, the signals applied to the right-front and
left-front speakers 20 are:
1. RF = R' = +1.22 R.sub.f + 0.38 L.sub.f + 1.22 R.sub.b - 0.62
L.sub.b
2. LF = L' = +0.38 R.sub.f + 1.22 L.sub.f - 0.62 R.sub.b + 1.22
L.sub.b
For this reason, the composite signals are matrixed to provide only
the RB and LB signals applied to the right-back (RB) and left-back
(LB) speakers 20, respectively. This is accomplished when the R'
and L' signals are matrixed according to the equations:
3. RB = +0.68 R' - 0.53 L'
4. lb = -0.53 r' + 0.68 l'
combining the matrixing equations (1) and (2) with composite signal
equations (3) and (4) it is found that the signals applied to the
right-back and left-back speakers 20 are:
5. RB = 0.63 R.sub.f - 0.39 L.sub.f + 1.16 R.sub.b - 1.07
L.sub.b
6. LB = 0.39 R.sub.f + 0.63 L.sub.f - 1.07 R.sub.b + 1.16
L.sub.b
Subsequently, the four derived output signals (i.e., RF, LF, RB and
LB) available at the output terminals of decoder 19 are applied to
the corresponding speakers 20, designatd RF, LF, RB and LB,
positioned in the quadrants of listening room 21. The speakers 20
reproduce the audio information from the signals applied thereto,
simulating the "concert hall" effect.
One advantage of this type of quadriphonic playback system is that
conventional stereophonic records, tapes and FM multiplex
transmissions may be reprodced with some "enhancement" thereby
producing an acceptable illusion of four channel stereo when, in
fact, encoded signals are not being used. The degree of
enhancement, however, will vary between the various materials being
reproduced, but this does mean that not all present stereophonic
audio information storage techniques and hardware are obsolete.
Referring now to FIG. 2, shown therein is an FM multiplex receiver
which is adaptable as a quadriphonic playback system incorporating
a front-to-back balance control in accordance with the present
invention. The receiver includes an antenna 22 which intercepts
frequency-modulated (FM) signals and couples them in a conventional
manner to a radio-frequency (RF) amplifier and converter stage 23
wherein the signals are amplified and translated to an
intermediate-frequency. The resultant IF signal is then coupled to
an intermediate-frequency (IF) amplifier stage 24, which may
comprise a plurality of individual tuned amplifier and limiting
circuits for subsequent amplification.
The IF output signal from IF amplifier 12 is coupled to an FM
detector and filter 25 wherein the received carrier transmission is
demodulated to obtain a modulation signal at the detector output.
The signal from FM detector 25 consists of three components: an
audio frequency component representing the sum (L' + R') of the
left and right composite audio signals, a 38 kHz suppressed-carrier
subcarrier component representing the difference (L' - R') of the
two composite audio signals, and a 19 kHz pilot-carrier component.
The output signal from FM detector 25 is amplified by an amplifier
26 and applied to the biplex demodulator 27. In order to detect the
suppressed-carrier amplitude-modulated subcarrier, it is necessary
to apply to the biplex demodulator 27 a 38 kHz demodulation or
switching signal which corresponds to the carrier component of the
suppressed-carrier subcarrier signal. The 19 kHz pilot-carrier is
amplified by a 19 kHz amplifier and doubler 28 wherein the
amplified 38 kHz switching signal is generated. The regenerated 38
kHz demodulating signal is then applied to the biplex demodulator
27. The biplex demodulator 27 reinserts the 38 kHz subcarrier
switching signal into the (L' - R') 38 kHz suppressed-carrier
subcarrier component from amplifier 26 and demodulates this signal
into the (L' - R') audio signal. Demodulator 27 further matrixes
the (L' + R') audio signal from amplifier 26 and the (L' - R')
audio signal to produce audio signals of the form L' and R', which
correspond to the desired left and right composite signals. The R'
and L' composite signals are then applied through a switching
network 30 to a right channel audio amplifier 31 and a left-channel
audio amplifier 32, respectively. The right and left channel audio
amplifiers 31 and 32, in turn, drive the right-front (RF) and
left-front (LF) speakers, 33 and 34, respectively.
Similarly, the R' and L' composite signals may be recovered from
the tape-phono playback unit 29 and coupled to a switching network
30 having composite signals from biplex demodulator 27 also applied
thereto. Accordingly, the listener may manually select his source
of listening material, i.e., records, tapes, or FM multiplex
broadcast transmissions.
As thus far described, the receiver is conventional in general
construction and operation such that further and more particular
consideration may now be given to that portion of the receiver
which relates to the preferred embodiment of the present invention,
and in general constitutes a front-to-back balance control.
In accordance with the present invention, the amplified R'
composite signal is coupled directly from the right channel audio
amplifier 31 to a right-front (RF) speaker 33 where it is converted
into sound waves emanating therefrom. The other terminal of the RF
speaker 33 is coupled to ground through a variable impedance,
identified as .DELTA.Z.sub.RF in FIG. 2, comprising a portion of
the potentiometer 35 determined by the positioning of its wiper arm
35a. Similarly, the amplified L' composite signal is coupled from
the left channel audio amplifier 32 to a leftfront (LF) speaker 34
having a resistive network identical to that just described
interposed between its other terminal and ground. That is, the LF
speaker 34 is coupled to ground through a variable impedance,
.DELTA.Z.sub.LF, determined by the positioning of wiper arm 36a of
potentiometer 36. In the embodiment shown in FIG. 2, the
potentiometers 35, 36 are ganged such that corresponding
adjustments of wiper arms 35a and 36a are accomplished
simultaneously. Thus, the variable impedance (.DELTA.Z.sub.RF)
interposed between the RF speaker 33 and ground is identical to
that (.DELTA.Z.sub.LF) interposed between the LF speaker 34 and
ground. Accordingly, the relative sound output levels of the two
front speakers 33, 34 are correspondingly increased or decreased. A
resistor 37 is coupled between the RF speaker 33 and a tap 35t on
the potentiometer 35 to effect differing tapers over two
predetermined impedance ranges. Likewise, a resistor 38 spans the
portion of potentiometer 36 between a tap 36t and the LF speaker
34.
The R' composite signal is further coupled from the right channel
audio amplifier 31 to the right-back (RB) speaker 39, while the L'
composite signal is also coupled from the left channel audio
amplifier 32 to the left-back (LB) speaker 40. Potentiometer 41,
which is ganged with potentiometers 35, 36, is connected to the
other terminal of the LB speaker 40. Its wiper arm 41a is coupled
to the second terminal of the RB speaker 39 interconnecting the two
back speakers to complete the L' - R' signal path. The variable
impedance interposed between the two back speakers 39, 40 by
potentiometer 41 controls the level of L' - R' signals coupled to
each of the back speakers. That is, when potentiometer 41 is
adjusted to it's "maximum impedance" the L' - R' signal path is
essentially an open circuit and L' - R' signals will not be coupled
to the back speakers 39, 40.
A resistor 42 connected to the wiper arm 41a also couples the back
speakers 39, 40 to ground through a variable impedance,
.DELTA.Z.sub.BACK, comprising the parallel impedances,
.DELTA.Z.sub.RB and .DELTA.Z.sub.LB, which are determined by the
positioning of wiper arms 35a and 36a, respectively. The common
signal path to ground provided by the variable impedance
.DELTA.Z.sub.BACK and resistor 42 provides the back speakers 39, 40
with an L' + R' sum-type signal having the desired matrixing
coefficients required for decoding the R' and L' composite signals
into the RB and LB signals applied to the RB speaker 39 and the LB
speaker 40, respectively. When the variable impedance
.DELTA.Z.sub.BACK is varied by the repositioning of wiper arms 35a,
36a the resistance of the common signal path interposed between the
back speakers 39, 40 and ground is adjusted to correspondingly
change the relative sound output levels of the two back speakers.
Furthermore, by varying the impedance of the common signal path the
matrixing coefficients are similarly changed such that there is
nearly zero phase difference between the signals applied to the RB
and LB speakers 39, 40, respectively, when the variable impedance
.DELTA.Z.sub.BACK is minimized.
Operationally, the three ganged potentiometers 35, 36, and 41 may
be simultaneously adjusted by a single manual control to adjust the
sound output levels of the front speakers 33, 34 relative to the
back speakers 39, 40 while varying the phase differential between
the back speakers. For example, when the wiper arms 35a, 36a and
41a are moved to their extreme counterclockwise positions, and
variable impedances .DELTA.Z.sub.RF and .DELTA.Z.sub.LF in series
with the RF and the LF speakers 33, 34, respectively, will be
small. Accordingly, the R' and L' composite signals applied across
the front speakers will be minimally attenuated. The variable
impedance .DELTA.Z.sub.BACK, however, will be maximized due to the
variable impedances .DELTA.Z.sub.RB and .DELTA.Z.sub.LB such that
there will be minimal sound output from the back speakers 39, 40
resulting from the matrixing of the R' and L' composite signals
through the common signal path to ground. Also, the series
impedance provided by potentiometer 41 and serially connected
between the back speakers 39, 40 will effectively present an open
circuit to any L' - R' difference signals.
As the wiper arms 35a, 36a, and 41a are moved to an optimum
position defined by the taps 35t, 36t on potentiometers 35, 36 the
impedances .DELTA.Z.sub.RF and .DELTA.Z.sub.LF are correspondingly
increased thus decreasing the sound output level of the front
speakers 33, 34. Simultaneously, the resistance of potentiometer 41
is decreased as wiper arm 41a is moved to tap 41t to allow
reproduction by the back speakers 39, 40 of the decoded signals, RB
and LB, applied thereto. The impedance .DELTA.Z.sub.BACK provides a
common signal path to ground to matrix the R' and L' composite
signals.
Finally, when the potentiometers 35, 36 and 41 are adjusted to
their extreme clockwise position, the impedances .DELTA.Z.sub.RF
and .DELTA.Z.sub.LF are maximized to attenuate the R' and L'
composite signals applied to the RF and LF speakers 33, 34,
respectively, attenuating greatly the sound output from the front
speakers. The impedance .DELTA.Z.sub.BACK is minimized such that
the back speakers 39, 40 are, in essence, coupled closely to ground
through resistor 42. Accordingly, the matrixing coefficients are
changed such that decoding of the R' and L' composite signals will
not be effected. Accordingly, for the most part, R' composite
signal will be applied to the RB speaker 39 while L' composite
signal will be applied to the LB speaker 40. This reduces the phase
differential between the back speakers to nearly zero.
Accordingly, there has been shown a means for adjusting the sound
output levels of the two front speakers relative to a pair of back
speakers so that a proper balance may be maintained. Further
provision has been made for automatically reducing the phase
differential between the back speakers to nearly zero when the
front speakers are effectively "shut-off" thereby eliminating any
harsh or displeasing sounds to the listener. Also, the present
invention provides an inexpensive matrixing network for
reconstructing the four channels of audio information from the two
composite input signals.
While a particular embodiment of the present invention has been
shown and described, it will be obvious to those skilled in the art
that various changes and modifications may be made without
departing from the invention in its broader aspects. The aim in the
appended claims is to cover all such changes and modifications as
may fall within the true spirit and scope of the invention.
* * * * *