U.S. patent number 3,798,373 [Application Number 05/155,976] was granted by the patent office on 1974-03-19 for apparatus for reproducing quadraphonic sound.
This patent grant is currently assigned to Columbia Broadcasting System, Inc.. Invention is credited to Benjamin B. Bauer.
United States Patent |
3,798,373 |
Bauer |
March 19, 1974 |
APPARATUS FOR REPRODUCING QUADRAPHONIC SOUND
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 stereophonic disc
record, separates them into four independent channels each carrying
predominantly the information contained in the four original
recorded sound signals, and, utilizing a wave-matching technique
derives control signals for controlling the gains of amplifiers
associated with the four loudspeakers. The control circuitry
improves the separation of the four independent channels,
particularly the generally "front" from the generally "back"
signals. In another aspect of the encoder, the front-to-back
separation is improved by intermixing some of the "left" output
signal into the "right" output signal, and vice versa, at both the
front and the back sets of decoder output terminals.
Inventors: |
Bauer; Benjamin B. (Stamford,
CT) |
Assignee: |
Columbia Broadcasting System,
Inc. (New York, NY)
|
Family
ID: |
22557545 |
Appl.
No.: |
05/155,976 |
Filed: |
June 23, 1971 |
Current U.S.
Class: |
381/22 |
Current CPC
Class: |
H04S
3/02 (20130101) |
Current International
Class: |
H04S
3/00 (20060101); H04S 3/02 (20060101); H04r
005/00 () |
Field of
Search: |
;179/15BT,16,16P,1.4ST,1.1TD,1GQ |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Four Channels and Compatibility, by Scheiber. Audio Engineering
Society Preprint, Oct. 12-15 1970..
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: D'Amico; Thomas
Attorney, Agent or Firm: Olson; Spencer E.
Claims
I claim:
1. In apparatus for reproducing on separate loudspeakers four
individual audio information signals contained in first and second
composite signals each of which contains up to three of said audio
information signals, two of which are common in said first and
second composite signals and said common signals in said first
composite signal being in substantially quadrature phase
relationsiip with corresponding common signals in said second
composite signal, the combination comprising:
matrix decoding means including
first and second input circuits to which said first and second
composite signals are respectively applied, said input circuits
including means for shifting the phase of one of said composite
signals relative to the other by substantially 90.degree. for
positioning said common signals in one of said relatively
phase-shifted composite signals either in phase coincidence of in
phase opposition with corresponding ones of said common signals in
the other of said relatively phase-shifted composite signals,
first, second third and fourth output channels,
means for coupling said first and second composite signals to said
first and second output channels, respectively,
means for selectively combining predetermined proportions of said
relatively phase-shifted first and second composite signals to
produce third and fourth composite signals each containing a
different one of said common signals as its predominant component,
and
means for coupling said third and fourth composite signals to said
third and fourth output channels, respectively;
means for combining predetermined fractions of the composite
signals in said first and second output channels with the composite
signals in said second and first output channels, respectively,
operative to produce first and second composite output signals
respectively containing different ones of two of said audio
information signals as predominant components, and
means for combining predetermined fractions of the composite
signals in said third and fourth output channels with the composite
signals in said fourth and third output channels, respectively,
operative to produce third and fourth composite output signals
repsectively containing different ones of the other two of said
audio information signals as predominant components.
2. Apparatus according to claim 1, wherein the said fractions are
equal.
3. Apparatus according to claim 1, wherein the said fractions are
unequal.
4. Apparatus according to claim 1, wherein at least some of the
said fractions have a value in the range of 0.25 and 0.35.
5. In apparatus for reproducing on separate loudspeakers four
individual audio information signals L.sub.f, R.sub.f, L.sub.b and
R.sub.b, respectively, to the extent they are contained in first
and second composite signals L.sub.T and R.sub.T respectively
containing dominant L.sub.f and R.sub.f component signals and each
containing to the extent they are present sub-dominant L.sub.b and
R.sub.b signal components with a phase-shift angle of substantially
90.degree. between said L.sub.b components and between said R.sub.b
components and with the L.sub.b component in one of said composite
signals leading the L.sub.b component in the other and with the
R.sub.b component in said one composite signal lagging the R.sub.b
component in the other, the combination comprising:
matrix decoding means including
first and second input circuits to which said L.sub.t and R.sub.t
composite signals are respectively applied, said first and second
input circuits respectively including first and second pairs of
all-pass phase-shifting networks to both of which the corresponding
input signals is applied, a first phase-shifting network of each
pair being operative to shift the phase of the applied signal by a
predetermined reference angle and the second phase-shifting network
of each pair being operative to shift the phase of the applied
signal by an angle differing from said reference angle by
substantially 90.degree.,
first, second, third and fourth output channels adapted to be
coupled to the loudspeakers positioned at the left front, right
front, left back and right back corners, respectively, of said
listening area,
means for coupling the composite signals from the first network of
said first and second pairs, respectively containing said L.sub.f
and R.sub.f components as predominant components, to said first and
second output channels, respectively,
first means for combining substantially equal proportions of the
composite signals from the second network of said first pair and
from the first network of said second pair and for coupling to said
third channel a composite signal containing said L.sub.b component
as its predominant component, and
second means for combining substantially equal proportions of the
composite signals from the first network of said first pair and
from the second network of said second pair and for coupling to
said fourth channel a composite signal containing said R.sub.b
component as its predominant component;
means for combining predetermined fractions of the composite
signals in said first and second channels with the composite
signals in said second and first channels, respectively, to produce
first and second composite output signals respectively containing
said L.sub.f and R.sub.f components as predominant components;
and
means for combining predetermined fractions of the composite
signals in said third and fourth channels with the composite
signals contained in said fourth and third channels, respectively,
for producing third and fourth composite output signals
respectively containing said L.sub.b and R.sub.b components as
predominant components.
6. Apparatus according to claim 5, wherein the said fractions are
equal.
7. Apparatus according to claim 5, wherein the said fractions are
unequal.
8. Apparatus according to claim 5, wherein at least some of the
said fractions have a value in the range between 0.25 and 0.35.
9. Apparatus according to claim 5, wherein said first, second,
third and fourth output channels respectively include first,
second, third and fourth gain control amplifiers connected for
respectively coupling said first, second, third and fourth
composite output signals to a respective loudspeaker, and further
including a control circuit for controlling the gains of said gain
control amplifiers to enhance the realism of the four channel sound
reproduced by the loudspeakers, said control circuit
comprising:
control signal generating means connected to receive composite
signals from the second network of said first pair and from the
first network of said second pair and operative to derive therefrom
first, second, third and fourth auxiliary composite signals of
substantially constant amplitude regardless of the amplitudes of
said composite signals and respectively containing the information
contained in the composite signals in said first, second, third and
fourth channels,
first circuit means for comparing said first and second auxiliary
composite signals and operative to produce a first signal
indicative of whether they contain substantially equal amplitude
signals in phase coincidence or in phase opposition,
second circuit means for comparing said third and fourth auxiliary
signals and operative to produce a second signal indicative of
whether they contain substantially equal amplitude signals in phase
coincidence or in phase opposition, and
means for combining said first and second signals and operative to
produce and apply to said gain control amplifiers a control signal
for enhancing the gain of said first and second gain control
amplifiers relative to said third and fourth gain control
amplifiers when said first and second auxiliary 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 auxiliary signals do not contain said signal components
L.sub.f and R.sub.f.
10. Apparatus in accordance with claim 9, further including
means for comparing the sum and the difference of said first
auxiliary signal and the composite signal from the first network of
said first pair and operative to produce a second control signal of
a given polarity or opposite polarity depending on whether the sum
exceeds the difference, or vice versa, and
means for combining said second control signal with the control
signal from said control signal generating means for application to
said gain control amplifiers to enhance the separation between
signals contained in said first or said second channels, or in both
of said first and second channels, relative to signals contained in
said third and said fourth channels, or in both of said third and
fourth channels.
11. Apparatus according to claim 10, wherein said last-mentioned
combining means includes means for limiting the amplitude of said
second control signal to a predetermined amplitude.
12. Apparatus according to claim 11, wherein said control signal
generating means includes
first and second auxiliary gain control amplifiers each having
input and output terminals and a control electrode,
means coupling the composite signals from the second network of
said first pair and from the first network of said second pair to
the input terminals of said first and second auxiliary gain control
amplifiers, respectively, said first and second auxiliary gain
control amplifiers being operative to produce first and second
auxiliary composite signals, respectively, at their respective
output terminals corresponding to the composite signals applied
thereto,
means connected to the output terminals of said first and second
auxiliary gain control amplifiers for selectively combining the
auxiliary composite signals appearing thereat and operative to
produce third and fourth auxiliary composite signals containing the
information contained in the composite signals in said third and
fourth channels, respectively, and
circuit means connected to the control electrodes of said auxiliary
gain control amplifiers and operative in response to at least one
of said first, second, third and fourth auxiliary composite signals
to maintain substantially constant the amplitude of said firt,
second, third and fourth signals regardless of changes in amplitude
of said L.sub.T and R.sub.T signals.
13. Apparatus according to claim 12, further including a third
auxiliary gain control amplifier for coupling the composite signal
from the first network of said first pair to said means for
comparing the sum and difference of it and said first auxiliary
signal, said third auxiliary gain control amplifier having input
and output terminals and a control electrode, and means coupling
the composite signal from the first network of said first pair to
the input terminal of said third auxiliary gain control amplifier,
and wherein said last-mentioned circuit means is connected to the
control electrode of said third auxiliary gain control amplifier
and operative to maintain substantially constant the amplitude of
the amplified composite signal appearing at the output terminal of
said third auxiliary gain control amplifier regardless of changes
in amplitude of said L.sub.T signal.
14. Apparatus according to claim 12, wherein said comparing circuit
means comprises,
means for separately rectifying said first, second, third and
fourth auxiliary composite signals, and
first and second signal subtracting junctions to which said
rectified first and second and said rectified third and fourth
signals are respectively applied and which are respectively
operative to produce said first and second signals respectively
proportional to the difference between said first and second
rectified signals and between said third and fourth rectified
signals.
15. Apparatus according to claim 13 wherein said means for
comparing the sum and difference of said first auxiliary signal and
the composite signal from the first network of said first pair
comprises
a summing junction and a subtracting junction, each having first
and second input terminals and an output terminal,
means connecting the auxiliary composite signals from said first
and said third auxiliary gain control amplifiers to the first and
second input terminals, respectively, of both of said summing and
subtracting junctions,
means for separately rectifying the signals appearing at the output
terminals of said summing and subtracting junctions,
means for comparing said rectified signals and operative to produce
said second control signal with said given polarity when said
L.sub.T and R.sub.T composite signals contain generally "front"
signals and to produce said second control signal with opposite
polarity when said L.sub.T and R.sub.T composite signals contain
generally "back" signals, and
means for limiting the amplitude of said second control signal
before combination with the control signal from said control signal
generating means to a predetermined amplitude.
16. Apparatus according to claim 5, wherein said first, second,
third and fourth output channels respectively include first,
second, third and fourth gain control amplifiers for respectively
coupling said first, second, third and fourth composite output
signals for a respective loudspeaker, and further including a
control circuit for producing a control signal for selectively
controlling the gains of said gain control amplifiers to enhance
the front-to-back channel separation, said control circuit
comprising:
means for comparing the sum and the difference of the composite
signals from the first network of each of said pairs and operative
to produce a control signal of a given polarity or of opposite
polarity depending on whether the sum exceeds the difference, or
vice versa, and
means for applying said control signal with said given polarity to
said first and second gain control amplifiers and with said
opposite polarity to said third and fourth gain control amplifiers
and operative to increase the gain of said first and second
amplifiers relative to the gain of said third and fourth amplifiers
when said L.sub.T and R.sub.T composite signals contian generally
front signals and to increase the gain of said third and fourth
amplifiers relative to said first and second amplifiers when said
L.sub.T and R.sub.T composite signals contain generally back
signals.
17. Apparatus according to claim 16, wherein said sum and
difference comparing means comprises
a summing junction and a subtracting junction, each having first
and second terminals and an output terminal,
means connecting the composite signals from the first network of
each of said pairs to the first and second input terminals,
respectively, of both of said summing and subtracting
junctions,
means for separately rectifying the signals appearing at the output
terminals of said summing and subtracting junctions, and
means for comparing said rectified signals.
18. In apparatus for reproducing on four sound-reproducing devices
four directional audio information signals respectively designated
L.sub.f, R.sub.f, L.sub.b and R.sub.b contained in said first and
second composite signals respectively containing to the extent they
are present dominant L.sub.f and R.sub.f component signals and each
including to the extent they are present sub-dominant L.sub.b and
R.sub.b component signals, with the L.sub.b and R.sub.b component
signals in one of said composite signals in quadrature relationship
with the corresponding component signals in the other composite
signal, the combination comprising:
decoding circuit means including first and second pairs of all-pass
phase-shifting networks connected to receive said first and second
composite signals, respectively, a first phase shifting network of
each pair being operative to shift the phase of the applied signal
by a predetermined reference angle and a second phase-shifting
network of each pair being operative to shift the phase of the
applied signal by an angle differing from said reference angle by
substantially 90.degree., and means for selectively combining
predetermined portions of said relatively phase-shifted first and
second composite signals to produce third and fourth composite
signals respectively containing said L.sub.b and said R.sub.b
component signals as its predominant component,
signal-coupling means connected to receive and operative to couple
composite signals respectively containing said L.sub.f, R.sub.f,
L.sub.b and R.sub.b component signals as its predominant signal to
respective ones of said sound-reproducing devices said
signal-coupling means including signal amplitude-modifying means
for separately adjusting the amplitude of the composite signal
applied thereto, and
a control circuit for producing and applying to said signal
amplitude-modifying means a control signal to enhance the amplitude
of the signal or signals applied to said sound-reproduction devices
which instantaneously contain audio information signals which
predominate relative to the other signals applied to said
sound-reproduction devices, said control circuit including:
means for deriving a first set of control-signal-producing signals
from like phase-shifting networks of said first and second
pairs,
means for deriving a second set of control-signal-producing signals
including the output signal from the other phase-shifting network
of one of said pairs and one of the composite signals of said first
set of control-signal-producing signals,
means connected to receive said second set of
control-signal-producing signals and operative to derive therefrom
first, second, third and fourth auxiliary composite signals
respectively containing the signal information contained in said
first, second, third and fourth composite signals,
first circuit means for comparing said first and second auxiliary
composite signals and operative to produce a first signal
indicative of whether they contain substantially equal amplitude
signals in phase coincidence or in phase opposition,
second circuit means for comparing said third and fourth auxiliary
signals and controlling to produce a second signal indicative of
whether they contain substantially equal amplitude signals front
phase coincidence or in phase opposition, and
means for combining said first and second signals and operative to
produce and apply to said signal-amplitude-modifying means a
control signal for enhancing the gain of said first and second
signal amplitude-modifying means relative to said third and fourth
signal amplitude-modifying means when said first and second
auxiliary signal do not contain L.sub.b and R.sub.b signal
components and to enhance the gain of said third and fourth signal
amplitude-modifying means relative to said first and second signal
amplitude-modifying means when said third and fourth auxiliary
signals do not contain L.sub.f and R.sub.f signal components.
19. Apparatus in accordance with claim 18, further comprising
means for summing and differencing the signals of said first set of
control-signal-producing signals to produce sum and difference
signals, and
means for comparing the absolute magnitudes of said sum and
difference signals to produce and apply to said signal
amplitude-modifying means a second control signal of a given
polarity or opposite polarity depending upon whether the sum
exceeds the difference, or vice versa.
20. Apparatus according to claim 19, wherein said last-mentioned
means includes means for limiting the amplitude of said second
control signal to a predetermined amplitude.
21. Apparatus according to claim 18 wherein said control signal
generating means includes
first and second auxiliary gain control amplifiers each having
input and output terminals and a control electrode and connected to
receive at their respective input terminals the signals of said
second set of control-signal-producing signals, said first and
second auxiliary gain control amplifiers being operative to produce
first and second auxiliary composite signals at their respective
output terminals corresponding to the composite signals applied
thereto,
means connected to the output terminals of said first and second
auxiliary gain control amplifiers for selectively combining the
auxiliary composite signals appearing thereat and operative to
produce third and fourth auxiliary composite signals containing the
information contained in the composite signals in said third and
fourth channels, respectively, and
circuit means connected to the control electrodes of said auxiliary
gain control amplifiers and operative in response to at least one
of said first, second, third and fourth auxiliary composite signals
to maintain substantially constant the amplitude of said first,
second, third and fourth auxiliary signals regardless of changes in
the amplitude of said first and second composite signals.
22. Apparatus according to claim 21, including a third auxiliary
gain control amplifier to the input terminal of which one of the
control signal-producing signals to said first set is applied, and
wherein said last-mentioned circuit means is operative to maintain
substantially constant the amplitude of the output composite signal
from said third auxiliary gain control amplifier regardless of
changes in amplitude of said first and second composite
signals.
23. Apparatus according to claim 22, wherein said means for
comparing the absolute magnitudes of said sum and difference
signals comprises
a summing junction and a subtracting junction, each having first
and second input terminals and an output terminal,
means connecting the composite output signals from said first and
third auxiliary gain-control amplifiers to the first and second
input terminals, respectively, of both said summing junction and
said subtracting junction,
means for separately rectifying the signals appearing at the output
terminals of said summing and subtracting junctions,
means for comparing said rectified signals and operative to produce
said second control signal with said given polarity when said first
and second composite signals contain generally "front" signals and
to produce said second control signal with opposite polarity when
said first and second composite signals contain generally back
signals, and
means for limiting the amplitude of said second control signal to
substantially the amplitude of said first control signal.
24. In apparatus for reproducing four individual audio information
signals respectively designated L.sub.f, R.sub.f, L.sub.b and
R.sub.b contained in first and second composite signals
respectively containing to the extent they are present dominant
L.sub.f and R.sub.f component signals and each including to the
extent they are present sub-dominant L.sub.b and R.sub.b component
signals with a phase-shift angle of substantially 90.degree.
between said L.sub.b component signals and between said R.sub.b
component signals, the combination comprising:
decoding circuit means connected to receive said first and second
composite signals and operative in response thereto to produce
third and fourth composite signals respectively containing
predominant L.sub.b and R.sub.b component signals and each
including sub-dominant L.sub.f and R.sub.f component signals, said
decoding circuit means including first and second pairs of all-pass
phase-shifting networks to which said first and second composite
signals are respectively applied, a first phase-shifting network of
each pair being operative to shift the phase of the applied signal
by a predetermined reference angle and the second phase-shifting
angle of each pair being operative to shift the phase of the
applied signal by an angle differing from said reference angle by
substantially 90.degree.,
signal amplitude-modifying means connected to receive and operative
to couple said first, second, third and fourth composite signals to
respective ones of said sound-reproducing means,
control signal generating means for producing a control signal for
selectively controling the transmission characteristic of said
signal amplitude-modifying means to enhance the separation between
frnt and back channel signals, said control circuit including:
means for comparing the sum and the difference of the composite
output signals from the first network of each of said pairs and
operative to produce a control signal of a given polarity or of
opposite polarity depending on whether the sum exceeds the
difference, or vice versa, and
means for applying said control signal with said given polarity to
said first and second signal amplitude-modifying means and with
said opposite polarity to said third and fourth gain signal
amplitude-modifying means and operative to increase the gain of
said first and second signal amplitude-modifying means relative to
the gain of said third and fourth signal amplitude modifying-means
when said first and second composite signals contain generally
front signals and to increase the gain of said third and fourth
signal amplitude-modifying means relative to said first and second
signal amplitude-modifying means when said first and second
composite signals contain generally back signals.
25. Apparatus according to claim 24, wherein said sum and
difference comparing means comprises
a summing junction and a subtracting junction each having first and
second input terminals and an output terminal,
means for coupling the composite signals from the first network of
each of said pairs of phase-shifting networks to the first and
second input terminals, respectively, of both said summing junction
and said subtracting junction,
means for separately rectifying the signals appearing at the output
terminals of said summing and subtracting junctions, and
means for comparing said rectified signals.
Description
CROSS-REFERENCE TO OTHER APPLICATIONS
This invention is related to the subject matter of the following
co-pending applications all of which are assigned to the assignee
of the present application: Ser. No. 44,196, filed June 8, 1970,
now abandoned in favor of continuation Ser. No. 251,544, filed Apr.
12, 1972, which in turn has now been abandoned in favor of
continuation-in-part application Ser. No. 328,814 filed Mar. 10,
1973, which has also been abandoned in favor of
continuation-in-part application Ser. No. 384,334 filed July 31,
1973; Ser. No. 81,858, filed Oct. 19, 1970, now abandoned in favor
of continuation application Ser. No. 251,636, filed May 8, 1972;
Ser. No. 112,168, filed Feb. 3, 1971, now Pat. No. 3,745,252; Ser.
No. 118,271, filed Feb. 24, 1971; and Ser. No. 124,135, filed Mar.
15, 1971.
BACKGROUND OF THE INVENTION
This invention relates to systems 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 a decoder for improving the
realism of sound decoded from a matrixed quadraphonic record,
recorded on a two-track medium in accordance with the method
described in aforementioned co-pending application Ser. No. 124,135
and similar systems.
Briefly, in a matrixed quadraphonic record, four usually
independent channels, L.sub.f, L.sub.b, R.sub.f and R.sub.b, which
are intended to be reproduced on respective loudspeakers positioned
at the left front, left back, right front, and right back corners,
respectively, of a room or listening area, are combined into two
channels by a device known as a quadraphonic encoder. A suitable
encoder for this purpose is illustrated in FIG. 8 of the
aforementioned application Ser. No. 124,135, but it is to be
understood that the decoder to be descried herein is operative to
reproduce signals encoded with encoders of other configurations,
for example, the encoder described in co-pending application Ser.
No. 384,334. The encoder produces two composite signals that can be
recorded on a two-track medium, such as magnetic tape or a disc
record, utilizing conventional recording techniques. The two output
channels, which for convenience will hereinafter be designated
R.sub.T and L.sub.T (for total or transmitted left and right
signal, respectively) may be recovered from a phonograph record
with a conventional phonograph pickup, or alternatively,
transmitted directly from the encoder, and applied to a decoder
which transforms them into four new signals, predominant components
of which correspond to the original signals L.sub.f, L.sub.b,
R.sub.f and R.sub.b applied to the encoder, except that they may
have a different phase orientation than the original signals.
An essential feature of the decoder is a combination of all-pass
phase-shifting networks, usually employed in groups of two or more,
for positioning the components of the two composite signals to
permit combination thereof by addition and subtraction. Each
network of a group has a basic phase-shift angle, .PSI., which is a
function of frequency, and an incremental angle, .DELTA., which is
essentially constant over the frequency range of interest. The
angle .DELTA. is normally zero to 90.degree., although it will be
evident from the description to follow that other values may be
used with equivalent results.
The nature of the encoded signals, and the significance of the
phase-shifting networks, will be better understood from a brief
description of the encoder illustrated in FIG. 8 of the
aforementioned application Ser. No. 124,135, which is repeated
herein as FIG. 1. The encoder has four input terminals 10, 12, 14
and 16 to which four input signals L.sub.f, L.sub.b, R.sub.f and
R.sub.b, depicted as in-phase signals of equal amplitude, are
respectively applied. The total L.sub.f signal is added in a
summing junction 18 to -0.707 of the R.sub.b signal, the output of
this summing junction being applied to a phase-shifting network 20
which introduces a reference phase-shift .PSI. which, as was noted
earilier, is a function of frequency. The full R.sub.f signal at
terminal 16 is added in summing network 22 to .707 of the L.sub.b
signal appearing at input terminal 12, and the output passed
through the .psi.-network 24, which also provides the reference
phase-shift .psi.. The L.sub.b and R.sub.b signals are also applied
to respective .psi.-networks 26 and 28, each of which provides a
phase shift of .psi. + 90.degree.. It should here be noted that the
angular notation used refers to lagging angles, but as long as
there is consistency in notation, it makes no difference to the
operation of the system whether the angles are lagging or leading.
The full signal appearing at the output of network 20 is added in a
summing circuit 30 to -0.707 of the signal appearing at the output
of network 26 to produce at its output terminal 32 a composite
signal designated L.sub.T. Similarly, the full signal from network
24 is added in summing junction 34 to 0.707 of the signal from
network 28, the latter in this case being in the positive sense.
The signal appearing at the output terminal 36 is the composite
signal R.sub.T. The signals L.sub.T and R.sub.T may be recorded on
any two-channel medium such as a two-track tape or stereophonic
record for later reproduction, or may be transmitted by FM
multiplex radio.
The composite signals appearing at output terminals 32 and 36 are
portrayed as phasor groups 38 and 40, respectively, which may be
characterized in complex notation, as follows:
L.sub.T = L.sub.f -0.707R.sub.b + j 0.707L.sub.b ( 1)
and
R.sub.T = R.sub.f + 0.707L.sub.b - j0.707R.sub.b ( 2)
In the interest of providing better realism of image placement when
the record is played on a conventional stereophonic phonograph over
two loudspeakers, it is preferable that the phasor 0.707L.sub.b in
phasor group 40 lags behind the similarly numbered phasor in phasor
group 38, and conversely, to arrange phasor 0.707R.sub.b in phasor
group 38 to lag behind the corresponding phasor in group 40.
Co-pending application Ser. No. 118,271 describes a system for
decoding of the signals L.sub.T and R.sub.T depicted in FIG. 1, in
which they are respectively applied to a pair of phase-shifting
networks, one network of each pair introducing a phase-shift of
(.psi. + 0.degree.), and the other network of each pair introducing
a phase-shift of (.omega. + 90.degree.). By reason of the relative
90.degree. phase-shift, the two phasor groups appearing at the
outputs of the .psi.-networks to which the L.sub.T signal is
applied are in quadrature relationship, as are the two phasor
groups appearing at the outputs of the .psi.-networks to which the
R.sub.T signal is applied. Thus, the phasors at the outputs of the
four .psi.-networks are properly positioned for selective addition
and subtraction to derive four separate output signals
predominantly containing the original signals L.sub.f, L.sub.b,
R.sub.b and R.sub.f, respectively, for reproduction over four
corresponding loudspeakers. These decoded signals are not "pure" or
discrete original signals, however, each being "diluted" 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
the 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 are inactive, thereby to
enhance the separation of the channels which are present. It is
also desirable that the decoder distinguish between front and back,
especially as it relates to the center front and center back
signals. Three different logic and control systems for achieving
signal enhancement are described in co-pending applications Ser.
Nos. 384,334 and 251,636. It is a primary object of the present
invention to obtain greater quadraphonic realism than that
attainable with previously described methods while, at the same
time, simplifying the circuitry for accomplishing it.
SUMMARY OF THE INVENTION
The foregoing and other objects of the invention are achieved
according to one aspect thereof, by an improved logic for
controlling the gains of the four output amplifiers of the decoder
in response to signals appearing in the decoder to enhance the
realism of the four-channel reproduction. It being a characteristic
of the decoder that regardless of the bearing of a signal
originally applied to the encoder, upon reproduction the
predominant signal appears at the output of the correct loudspeaker
and unwanted side effect signals from other channels associated
therewith are equal and either in-phase or in quadrature phase
relationship, the logic is designed to compare the voltages in
adjacent channels and to derive control signals for reducing the
gains of the output amplifiers for such channels whenever the
voltages are in-phase or in quadrature phase condition, thereby to
eliminate the side-effect signals. That is, the logic senses the
presence of side-effect signals and causes the gains of the
appropriate output amplifiers to be correspondingly increased so
that the total accustical power contributed by the active
loudspeakers resmins unchanged.
Substantially constant amplitude signals for comparison of the
voltage in adjacent channels, regardless of changes in level of the
program, are obtained by applying a pair of signals derived from
the decoder respectively corresponding to the composite input
signal R.sub.T and the composite signal L.sub.T shifted by
90.degree. from its input phase condition, to respective gain
control amplifiers which include means for maintaining the
amplitude of their outputs substantially constant. The output
signals from these auxiliary gain control amplifiers are
selectively added and subtracted to produce two additional signals,
also of substantially constant amplitude, resembling two signals
produced in the decoder and ultimately applied to two of the four
loudspeakers. The four signals thus produced are separately
rectified and the resulting wave forms compared in pairs in a pair
of subtracting junctions, for example, the outputs of the junctions
again rectified, and one of the rectified outputs subtracted from
the other two produce a signal of one polarity or the other
indicative of the presence of side-effect signals. Signals of one
polarity control in unison the gains of the gain control amplifiers
associated with the two "front" loudspeakers, and signals of
opposite polarity control the two "back" loudspeakers in
unison.
The ability of the logic to discriminate between generally front
and generally back signals is enhanced by comparing the
substantially constant amplitude signal from one of the
above-described gain control amplifiers, preferably the one to
which the R.sub.T composite signal is applied, with the output of a
third auxiliary gain control amplifier (which also has a
substantially constant amplitude output) to which the L.sub.T
composite signal is applied. The comparison is made by obtaining
the sum and the difference of the two outputs, separately
rectifying the sum and difference signals, and obtaining the
difference of the two rectified signals. That portion of the latter
difference signal which exceeds a predetermined level is added to
the control signal generated by the wave-matching logic described
in the preceding paragraph for application to the output gain
control amplifiers.
In accordance with another aspect of the invention, the
front-to-back discrimination of the decoder is enhanced, without
the use of a logic and control circuit, by blending or mixing the
signals in some of the output channels of the matrix decoder with
signal in some of the other channels. More particularly, a fraction
of the signal in the "left front" channel is added to the "right
front" signal, and a like or different fraction of the signal in
the "right front" channel is muxed with the "left front" signal.
Similarly, fractions of the signals in each of the "left back" and
"right back" channels are intermixed with the full signal in the
other.
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:
FIG. 1 is a schematic diagram of an encoder for encoding four
original sound signals into two composite signals, to which
reference has already been made in discussing the background of the
invention;
FIGS. 2A and 2B taken together is a schematic diagram of decoding
apparatus embodying the invention;
FIG. 3 is a plot of the output of the front-back logic circuitry of
the system of FIG. 2 as a function of "panning" a constant signal
around a circle;
FIG. 4 is a plot of the output of the wave-matching logic of the
system of FIG. 2 as a function of "panning" a constant signal
throughout a circle;
FIG. 5 is a plot of the values of the phasors for the four channels
as a function of the bearing angle for eight positions of a panned
signal; and
FIG. 6 is a series of phasor diagrams useful in explaining the
operation of the channel intermixing feature of the decoder of FIG.
2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The decoder of the present invention, illustrated in FIG. 2, is
similar in many respects to the decoder described in aforementioned
application Ser. No. 384,334 certain details of which, in turn, are
described in application Ser. No. 118,271. The input signals to the
decoder, designated L.sub.T and R.sub.T and depicted by the phasor
groups 38 and 40, respectively, are applied to respective input
terminals 42 and 44. The L.sub.T signal is applied in parallel to a
pair of all-pass phase shifting networks 46 and 48 which introduce
phase shifts of (.psi. + 0.degree.) and (.psi. + 90.degree.),
respectively, and the R.sub.T signal is similarly applied to a pair
of all-pass networks 50 and 52 which provide phase shifts of (.psi.
+ 90.degree.) and (.psi. + 0.degree.). respectively. The four
phase-shift networks are operative to produce four signals at the
output terminals or leads 54, 56, 58 and 60, depicted by the phasor
groups 62, 64, 66 and 68, respectively. To distinguish these
signals from the encoded input signals, and to signify that they
have passed through a set of phase-shift networks with a common
phase-shift .psi., the designations of the components of the
phase-shifted signals are primed.
The signals appearing on conductors 56 and 60 are each multiplied
by the coefficient 0.707 and added together at a summing junction
70 to produce a new signal at the output terminal 71 thereof, while
the signals on conductors 54 and 58 are multiplied by the
coefficient -0.707 and added together in a summing junction 72 to
produce a second new signal at its output terminal 73. In the
decoder described in application Ser. No. 118,271, the signals on
conductors 54, 71, 73 and 60 are amplified by amplifiers 74, 76, 78
and 80, respectively, and applied to corresponding loudspeakers 82,
84, 86 and 88 where they are reproduced as sounds which correspond
to the configuration of phasor groups 90, 92, 94 and 96,
respectively.
In accordance with one aspect of the present invention, applicant
has recognized that quadraphonic realism can be improved by
blending or mixing the outputs of some of the channels with the
outputs of other channels before amplification. This blending
operation is accomplished be means of four summing junctions 98,
100, 102 and 104 each of which is operative to add the signal from
one channel to a fraction of the signal of another channel. More
specifically, junction 98 adds a fraction, designated n, of the
signal appearing on conductor 60 to the signal appearing on
conductor 54, the output of the summing junction being applied to
amplifier 74. Similarly, the junction 100 adds a fraction, n', of
the signal on conductor 54 to the signal on conductor 60, the sum
being applied to amplifier 80. Similarly, summing junction 102 adds
a fraction, designated m, of the signal appearing on conductor 73
to the signal appearing on conductor 71 and applies the sum to
amplifier 76, and summing junction 104 adds a fraction, m', of the
signal appearing on conductor 71 to the signal appearing on
conductor 73 prior to application to amplifier 78. It is
convenient, and satisfactory in many circumstances, to make the
fractions n, n', m and m' equal, but they may be of differing
values in others. In general, it has been found that beneficial
results are obtained by choosing values for the fractions m and n
between 25 and 50 percent, it being understood, however, that other
fractional amounts may be used. It should be pointed out at this
juncture that the phasor groups 90, 92, 94 and 96 are, for clarity,
shown for the condition where the fractions m and n are zero, that
is, for a matrix without "blend", and that the logic and control
circuitry to be described hereinbelow, which is an extension of the
control circuitry described in co-pending applications Ser. Nos.
118,271 and 124,135, can be used with a decoding matrix with or
without "blend"; the effect on the corresponding phasor groups of
intermixing some of the left output into the right channel and vice
versa, in both the front and back sets of loudspeakers, will be
described later with reference to FIG. 6, following a description
of the balance of the system of FIG. 2.
It will be noted that the phasor groups 90, 92, 94 and 96 which
characterize the sounds emanating from the four loudspeakers 82,
84, 86 and 88, respectively, and which are usually placed in a
listening room or area so that the signals L.sub.f ', L.sub.b ',
R.sub.b " and R.sub.f " are localized at the left front, left back,
right back and right front corners, contain dominant signals
L.sub.f ', L.sub.b ', R.sub.b ' and R.sub.f '; however, they each
also contain diluting or side-effect signals from two other
channels. Although these side-effect signals are relatively
unobjectionable in the thus far described matrix configuration, the
perfection of quadraphonic sound reproduction is enhanced if the
gains of those channels which contain only side-effect signals are
controllably diminished. This can be accomplished by using gain
controlled amplifiers instead of fixed gain amplifiers for
amplifiers 74, 76, 78 and 80 and varying the gains thereof in
response to a control voltage derived by an electronic logic
system. A number of different forms of electronic logic for this
purpose are described in certain of the above-mentioned co-pending
applications. The present invention, in another aspect thereof, is
concerned with an improved and simplified logic which will now be
described in the environment of the improved matrix illustrated in
FIG. 2A.
Referring now to FIG. 2B, the electronic logic according to the
present invention is operative to develop control signals for the
gain control amplifiers by operating on three signals developed in
the matrix of FIG. 2A, preferably the signals appearing on
conductors 60, 56 and 54. The reasons for selecting these three in
preference to others will become evident as the description
proceeds. To insure that the signals to be operated upon by the
logic are relatively uniform regardless of the signal strength of
the program being reproduced, the signals from conductors 60, 56
and 54 are first coupled through respective, substantially
identical high pass filters 110, 112 and 114 designed to reject
frequencies below about 50Hz-frequencies which normally should not
be involved in the logic action. The transmission characteristic of
the filters above the cutoff point is preferably adjusted so as to
optimize the logic control action in accordance with the
sensitivity of the ear to the loudness of various sounds. The
signals delivered by the filters are applied to the input terminals
of respective gain control amplifiers 116, 118 and 120 which have
identical or closely similar gain versus control voltage
characteristics. It will be observed that the signals from
conductors 60 and 56 applied to amplifiers 116 and 118,
respectively, are obtained from the outputs of all pass phase-shift
networks 52 and 48, respectively, whereby corresponding components
of the R.sub.T and L.sub.T composite signals are shifted in phase
relative to each other by 90.degree.. This phase relationship
permits the signals delivered by gain control amplifiers 116 and
118 to be added and subtracted to derive two new signals having
properties advantageous to the desired performance of the logic.
Specifically, 0.707 of each of the signals from amplifiers 116 and
118, appearing at terminals 130 and 132, respectively, are added in
a summing junction 122 to produce at its output a new signal
represented by the phasor group 124, in which the component L.sub.b
' is predominant. Similarly, -0.707 of the signal from amplifier
118 is added in another summing junction 126 to 0.707 of the signal
from amplifier 116 to produce at its output terminal another new
signal represented by the phasor group 128, in which the component
R.sub.b ' is predominant. At the same time, the predominant
component of the signals appearing at terminals 130 and 132 are
R.sub.f ' and L.sub.f ', respectively.
The four signals just described are rectified by respective
rectifiers 134, 136, 138 and 140, which are preferably full-wave
rectifiers, each of which includes respective time constant
circuits 142, 144, 146 and 148, each designed to provide a rapid
attack time, of the order of about 1 millisecond, and a relatively
slower decay time, of the order of about 20 milliseconds. The four
rectified signals are added together in a summing junction 150 and
the sum signal is applied to the control electrodes 116a, 118a and
120a of gain control amplifiers 116, 118 and 120. Application of
the sum of the rectified signals in the illustrated feedback
relationship automatically and simultaneously adjust the gains of
the amplifiers in response to changes in the strength of the
signals being processed, thereby to maintain the amplitude of the
rectified signals essentially constant.
It will be noted that although only the signals from amplifiers 116
and 118 are used to develop the gain control voltage, the same gain
control voltage applied to the control electrode 120a of amplifier
120 also maintains its output level essentially constant.
Although a particular circuit arrangement has been described for
feeding information back to the gain control amplifiers 116, 118
and 120 so as to obtain a constancy of output signals, it will be
understood that other techniques can be used for this purpose
without departing from the spirit of the invention. For example, it
is possible to use a single time constant element for all four of
the rectified signals, and/or an OR gate may be used to select the
strongest of the four signals for application to the control
electrodes of the three gain control amplifiers.
Summarizing the function of the control circuit thus far described,
it selects for the logic operation the signals represented by
phasor groups 68, 64 and 62 and maintains them all at a relatively
constant level. It will be observed that in two of these phasor
groups, namely, in groups 68 and 64, the signal components 0.707
L.sub.b ' and 0.707R.sub.b ' are either in phase coincidence or in
phase opposition; consequently, as taught in applicant's co-pending
application Ser. No. 118,271, the disclosure of which is hereby
incorporated by reference, these signals can be utilized in a
wave-matching arrangement to ascertain if either an L.sub.b ' or an
R.sub.b ' signal component is present. Also, as has already been
noted when these two signals are added and subtracted in junctions
122 and 126, two new signals, represented by phasor groups 124 and
128, are obtained in which the components 0.707L.sub.f ' and
0.707R.sub.f ' are also either in phase coincidence or in phase
opposition, and thus can be used to ascertain if an L.sub.f ' or an
R.sub.f ' signal component is present in the circuit.
Following the principles of the wave-matching technique described
in application Ser. No. 118,271 for ascertaining the presence in
the circuit of L.sub.b ', R.sub.b ', L.sub.f ' or R.sub.f ' signal
components, four signals that are applied to rectifiers 134, 136,
138 and 140 to develop the gain control signal are also applied to
rectifiers 152, 154, 156 and 158, respectively, which are
preferably full-wave rectifiers and the rectified outputs therefrom
then subtracted in pairs in subtracting junctions 160 and 162.
Specifically, the rectified signal appearing at terminal 132 is
subtracted in junction 160 from the rectified signal appearing at
terminal 130 and the rectified signal represented by phasor group
124 is subtracted in junction 162 from the rectified signal
corresponding to phasor group 128.
It will be noted that rectifiers 152-158 are shown as not having
resistor and capacitor time constant circuits; this is deliberate
to indicate that these rectifiers desirably have a short time
constant. In fact, if ideal phase-shift networks were available, no
time constant elements would be required because the waves to be
matched would be in perfect alignment with each other; however,
because of circuit imperfections, these rectifiers may be designed
to have a relatively short time constant, of the order of a
fraction of a millisecond to a few milliseconds, which may, in
fact, be provided by the capacitance of the circuit leads.
The output signals from junctions 160 and 162 are again rectified
by rectifiers 164 and 166, respectively, (which preferably are also
full-wave rectifiers) having associated time constant circuits 168
and 170 the resistance and capacitance values of which are selected
to provide a rise time of the order of 1 millisecond and a decay
time of the order to 20 milliseconds. It should be understood,
however, that wide variations in these values may be used without
substantially affecting the operation of the invention. The output
signal from rectifier 166 is subtracted in a subtracting junction
172 from the output signal from rectifier 164 and the difference
signal appearing at its output terminal 174, which represents the
contribution of the wave-matching logic to the control signals for
the gain control amplifiers 74-80 in FIG. 2A, is applied as one
input to a summing junction 176.
To briefly review the action of the wave-matching logic (which is
decribed in detail in co-pending application Ser. No 118,271), if,
for example, either or both of the L.sub.f or R.sub.f signals are
present, since they would be present in precisely equal amounts at
the outputs of junctions 122 and 126, the wave-matching of the
rectified signals in junction 162 would result in a zero signal
output. At the same time, since the L.sub.f and R.sub.f signals at
the terminals 130 and 132 are completely different and incoherent,
wave-matching of the rectified signals applied to junction 160 will
not cause cancellation, and an output would be produced. For this
signal condition, then, rectification of the outputs of junctions
160 and 162, and subtraction thereof in junction 172, will produce
a positive signal at terminal 174. Other signal conditions may
result in correlation and cancellation in junction 160 so as to
produce zero output therefrom, while at the same time the signals
applied to junction 162 may be incoherent and thus produce an
output signal, with the consequence that the output signal from
junction 172 would be of negative polarity. The signal delivered by
junction 172 (which may be positive or negative) applied to one
input of summing junction 176 (the second input to which will be
subsequently described) and the output thereof is applied in
parallel to transmission elements 178 and 180.
Transmission element 178 is a network that is operative to transmit
a control signal therethrough without change in sign and has a
transfer function designed to avoid overloading the controls of the
gain control amplifiers 74-80. Transmission element 180 is similar
to transmission element 178 except that it includes means for
inverting the sense of signals applied to it. Therefore, the
signals delivered by transmission element 178 and 180 in response
to a given input signal are of the same magnitude but of opposite
polarity. The output signal from transmission element 178 is
applied via conductor 179 to the control electrodes of gain control
amplifiers 74 and 80, and the output of transmission element 180 is
applied over conductor 181 to the gain control electrodes of
amplifiers 76 and 78. Thus, a positive signal appearing at the
output of junction 176 passes through transmission element 178
without change of sign, and upon application to the control
electrodes of amplifiers 74 and 80 increases their gains and
enhances the signals L.sub.f ' and R.sub.f " emanating from
loudspeakers 82 and 88, respectively. At the same time, the
positive signal at the output of junction 176 is inverted by
transmission element 180 with the consequence that when it is
applied to the control electrodes of amplifiers 76 and 78, the
gains thereof are decreased, thereby to attenuate the side-effect
signals in their associated loudspeakers 84 and 86. Conversely,
when either one or both of signals L.sub.b ' or R.sub.b ' are
present, there is a net control voltage at the output of rectifier
166, and a zero voltage at the output of rectifier 164, resulting
in a negative control signal at the output terminal 174 of junction
172 (and 176). The positive control signal resulting from inversion
in transmission element 180 when applied to the control electrodes
of amplifiers 76 and 78 increases their gains and enhances the
signals L.sub.b " and R.sub.b " reproduced by loudspeakers 84 and
86. At the same time, the negative control signal from junction
176, applied without change of sign through transmission element
178 to the control electrodes of amplifiers 74 and 80, reduces the
gain of the front loudspeakers.
The gain control amplifiers 74-80 preferably have time constants
such as to permit relatively rapid increase in gain in response to
application of positively going control signals and a relatively
slow decrease in gain when the gain control signal decreases, in
accordance with the teaching of applicant's aforementioned
co-pending application Ser. No. 118,271.
The action of the present wave-matching logic is as described in
application Ser. No. 118,271 except for the utilization of
automatic gain control amplifiers 116 and 118 to provide signals of
substantially constant amplitude for wave-matching, and the manner
in which the output signal from the wave-matching logic interacts
with the output of a second logic circuit for providing improved
separation between front and back signals, now to be described.
The front-back logic utilized in the present system is similar in
some respects to the front-back logic described in applicant's
aforementioned co-pending application Ser. No. 124,135, and the
disclosure thereof is hereby incorporated by reference. The signals
for the front-back logic are derived from the outputs of automatic
gain control amplifiers 116 and 120, one of which it will be noted
is common with one of those used to derive the signals for the
wave-matching logic, and which provide relatively constant level
output signals corresponding to the phasor groups 68 and 62,
respectively. It will be noted that L.sub.f ' and R.sub.f ' signals
in these two phasor groups are in phase with the consequence that
if a front center signal is applied to the L.sub.f and R.sub.f
terminals of the encoder of FIG. 1, equal amounts of such a front
center signal would appear coincident with the L.sub.f ' and
R.sub.f ' component signals in these phasor groups. As described in
co-pending application Ser. No. 124,135, addition and subtraction
of these phasor groups, under the circumstance in which there is a
center front signal present, causes a greater total signal upon
addition and a smaller total signal upon subtraction. In contrast,
if a center back signal were to be applied to the terminals L.sub.b
and R.sub.b of the encoder of FIG. 1, such signals would appear out
of phase in phasor groups 62 and 68 with the result that a smaller
total signal results upon addition and a larger total signal is
obtained upon subtraction. These differences in the magnitude of
the output signals are utilized, in keeping with the teachings of
application Ser. No. 124,135 to ascertain whether there is a center
front or a center back signal in the composite signals R.sub.T and
L.sub.T to be decoded.
To this end, the output signals from gain control amplifiers 116
and 120 are applied to the two inputs of an adding junction 190,
and also to the two inputs of a subtracting junction 192 in which
the signal from the amplifier 116 is subtracted from the signal
appearing at the output of amplifier 120. The sum signal appearing
at the output terminal 194 of the summing junction, and the
difference signal appearing at the output 196 of the subtracting
junction, are rectified by respective rectifiers 198 and 200 (which
are preferably full-wave rectifiers) which may be provided with
time constant circuits 202 and 204, respectively. The outputs of
rectifiers 198 and 200 are applied to the positive and negative
terminals, respectively, of subtracting junction 206 which produces
a difference signal at its output terminal 208 which is available
to perform a logic function.
The front-back logic circuit and its function thus far described is
similar to that in co-pending application Ser. No. 124,135 with one
important exception. In the earlier application the voltage input
control of the front-back logic was provided with logarithmic
amplifiers which, because of the limitations of practical
amplifiers of this type limited the sensitivity and range of
control that could be exercised with signals of widely varying
levels. The use of automatic gain control amplifiers 116 and 120 to
keep the signal level at a high, substantially constant value
greatly enhances the action of the front-back logic.
The present embodiment of the front-back logic also differs from
the previously described embodiment in the significant respect that
the output of subtracting junction 206 is applied to a parallel
back-to-back junction of rectifiers 210 and 212, and the output of
this junction after suitable amplification by an amplifier 214,
applied to the other terminal of summing junction 176. The
rectifiers 210 and 212 are appropriately biased (not shown) to
function as a "slicer" to permit only front-back control signals
exceeding a predetermined amplitude to be applied to the junction
176. The purpose and operation of this "slicing" action will now be
explained.
To better understand the need for the slicer and its proper
adjustment, it should be kept in mind that the front-back control
logic should act properly not only with center front and center
back signals, but also must not introduce undesirable actions for
other types of signals that might be present in the composite
signals L.sub.T and R.sub.T applied to the decoder. Consider, for
example, a signal panned around the four corner terminals of the
encoder of FIG. 1. The effect of such a signal can be calculated by
taking into account the fact that a center front (or 0.degree.
bearing) signal C.sub.f appears as 0.707C.sub.f at both the left
front (L.sub.f) and right front (R.sub.f) input terminals of the
encoder. As the signal is panned to the right, the left front
signal drops to zero and the right front signal increases to unity,
following the cosine and sine laws, respectively. As panning
continues from front right to back right, the signal input into the
front channel of the encoder decreases according to the cosine law,
while the input to the right back channel increases following the
sine law, and so forth, as the signal is panned from back right to
back left to front left. The total signal power into the encoder
remains constant, since the sum of sine squared and cosine squared
equals one. The amplitudes of the signals developed at the
terminals L.sub.T and R.sub.T of the encoder for an input signal
panned in this way have been calculated; this information permits
the output voltage produced by the front-back logic to be
calculated. The results of these calculations are presented in the
following TABLE I in which:
a. The first column is the bearing angle of panning in degrees,
with 0.degree. corresponding to center front; 45.degree. to right
front; 135.degree. to right back; and 180.degree. to center back,
etc.
b. The second column gives the relative signal voltage at the
L.sub.T terminal and its phase position.
c. The third column gives the relative signal voltage at the
R.sub.T terminal and its phase position.
d. The fourth column gives the absolute value of the phasor sum of
the left and right signals L.sub.T and R.sub.T.
e. The fifth column gives the absolute value of the phasor
difference of L.sub.T and R.sub.T (absolute values are used in the
computation since the phasors are rectified prior to subtraction,
this being tantamount to obtaining an absolute value.
f. The sixth column gives the difference between the sum and
difference columns (this corresponds to the signal at the output of
subtracting junction 206 in FIG. 2B). ##SPC1##
The data presented in TABLE I are plotted in FIG. 3, in which the
solid-line circle represents signals of unity amplitude. It is seen
that the plot contains two large lobes 220 and 222 centered about
0.degree. and 180.degree., respectively, and two smaller lobes 224
and 226 centered about 90.degree. and 270.degree., respectively.
The two larger lobes, which have maxima of 1.414, are properly
positioned to act in a logical manner to emphasize respective front
or back center signals; however, the smaller lobes at 90.degree.
and 270.degree. are undesirable and, in fact, deleterious to
front-back logic action since they would tend to displace a side
signal to the front or rear, depending upon the direction of
arrival of the signal, obviously an undesirable result.
Accordingly, the influence of the side lobes 224 and 226 is
desirably eliminated, this being accomplished by the slicer
arrangement of diodes 210 and 212 in which the diodes are properly
biased to form a voltage barrier operative to suppress all signals
which are equal to or smaller in amplitude than the maximum
amplitude of the side lobes. The action of the slicer is depicted
by the dash-line circle 228, which signifies that all signals lower
in amplitude than that represented by the circle are suppressed.
Since the slicing action also reduces the amplitude of the desired
main lobe signals, it may be necessary to amplify the output from
the slicer in amplifier 214 before application to the summing
junction 176.
By a similar calculation, the voltage generated at the output
terminal 174 of the wave-matching logic has been calculated and the
results plotted in FIG. 4. As in the FIG. 3 plot, the solid line
circle 230 represents unity voltage, and it is seen that there are
four symmetrical lobes 232, 234, 236 and 238 each having a maxima
of unity and centered at 45.degree., 135.degree., 225.degree. and
315.degree., respectively. Superimposed on the plot, in dash-lines,
are those portions of the lobes 220 and 222 (FIG. 3) not suppressed
by the slicer, identified by reference numerals 220' and 222',
respectively. It will be noted that the latter lobes effectively
"fill" the null spaces between the lobes of the signals from the
wave-matching logic for front and back signal orientations. It may
here be noted that the gain of amplifier 214 is adjusted so that
the coaction of the wave matching logic with the front back logic
is properly matched. Thus, the sum of the signal produced by the
wave-matching logic (the action of which on the gain control
amplifiers 74-80 has been described previously) has added to it in
junction 176 the signals from the front-back logic represented by
lobes 220' and 222' . The resulting sum signal is applied through
transmission elements 178 and/or 180 to the control electrodes of
the gain control amplifier to achieve enchanced separation between
front and back signals.
It will be noted from the phasor diagrams 90-96 in Fig. 2A that the
matrix decoder system is completely symmetrical. This symmetry
helps to preserve the balance, say, between the primary sound
energy on a concert stage and the perceived energy of the hall.
This important aspect of symmetry can also be noted when the
signals from TABLE I are applied to the decoder. The following
TABLE II gives the relative voltages and phase angles of the
signals appearing at the outputs of the four loudspeakers 82-88.
Not shown in the table is the fact that the total power level
remains completely constant at 3dB level regardless of the bearing
angle of the panned signal. ##SPC2##
It will be noticed that for 45.degree., 135.degree., 225.degree.
and 315.degree. bearings, which correspond to the R.sub.f, R.sub.b,
L.sub.b and L.sub.f inputs, respectively, the relative voltages at
the corresponding loudspeakers are unity, while those in the
opposite loudspeakers are at 0.707, which corresponds with the
values shown in phasor diagrams 90-96 in FIG. 2A.
Amplitudes the phasors of the signals from TABLE II are depicted in
FIG. 5 for eight positions of the panning control, spaced
45.degree. apart. The direction of the arrows in the central square
refer to the panning position (that is, the arrow labeled 0.degree.
represents a center front signal, which would be applied equally to
the left front and right front channels, and the 180.degree.
position would represent a center back signal which would be
applied equally to the left back and right back channels), and the
arrows appearing in each of the eight surrounding squares, reading
clockwise from the upper left-hand corner of each square, represent
the relative amplitude and phase of the left front, right front,
right back and left back signal components. Thus, in the square
labeled R.sub.f, with the panning position at 45.degree. the left
front signal at the output terminal of the right front channel is
zero, the right front signal is of unity amplitude, the right back
signal is at right angles to the right front signal and has a
relative amplitude of 0.707, and the left back signal also has a
relative amplitude of 0.707 and is in phase with the right front
phasor and at right angles to the right back phasor. It will be
noted that this phase relationship between the component signals is
the same as shown in phasor group 96 in FIG. 2A, and that the
phasors appearing in the squares labeled R.sub.b, and L.sub.b and
L.sub.f correspond to phasor groups 94, 92 and 90, respectively.
Starting, for example, with the 0.degree. position, (corresponding
to the center front signal) it is seen that the desired signals at
the front loudspeakers are equal and in phase, while the undesired
side-effect signals are equal and out-of-phase. If the latter two
signals are attenuated, then only the desired front signals remain.
Moving next to the 45.degree. position, which corresponds to input
to the right front channel, it is noticed that the side-effect
signals are equal and at 90.degree. to each other. At the
90.degree. positions, the desired signals are of equal amplitude
(0.866) and are within 70.degree. of each other, while the
side-effect signals in both cases are equal and out-of-phase. Upon
examination of the remaining positions it will be seen that in
every case the unwanted signals are equal and either in quadrature
or out-of-phase relationship. This diagram illustrates, then, that
a logic designed to compare the voltages in adjacent channels and
to control the gain of the output amplifiers for such channels to
be attenuated whenever these voltages are in quadrature or in an
out-of-phase condition, side-effect signals will be completely
eliminated, while at the same time the wanted signals are properly
emphasized. This is precisely the action of the combined
wave-matching logic and the front-back logic described above.
With the background provided by the diagram of FIG. 5, reference is
again made to FIG. 2A for explanation of the function of the
blending or adding junctions 98, 100, 102 and 104. It has been seen
from FIG. 5 that the magnitudes of signals in all four loudspeakers
are the same for the 0.degree. and 180.degree. panning positions,
which correspond to the center front and center back signals into
the encoder, respectively. This is not of consequence when logic
circuitry is used with the matrix decoder to suppress the unwanted
side-effect out-of-phase signals, but in the interest of providing
a lower cost decoder (one not having logic and control circuitry),
it is desirable to improve the front-to-back discrimination of the
matrix itself. The manner in which this is accomplished may be seen
from FIG. 6 where the phasors 90-96 are reproduced, in solid lines,
and enlarged to show more detail, and the dash-line phasors are the
ones resulting from cross-blending portions of the phasors from
L.sub.f " into R.sub.f ", and vice versa, and from cross-blending
portions of the phasors from L.sub.b " into R.sub.b ", and vice
versa. The amount of cross-blend illustrated in FIG. 6 is 50
percent, primarily for clarity of illustration, the usual amount of
blending being in the vicinity of 25 and 30 percent for m and n.
Thus, considering the upper left-hand phasor diagram, it will be
seen that the phasors .35R.sub.b and 0.35L.sub.b and 0.5 R.sub.f,
each representing 50 precent of its corresponding phasor in phasor
group 96, have been superimposed in the same relative phase
relationship. The phasors resulting from adding the solid and
dash-line phasors are shown in dotted lines. For example, when
solid line phasor .7L.sub.b ' is added to dash-line phasor
.35L.sub.b, the dotted line phasor .79L.sub.b ' results. It will be
seen, also, that the cross-blending causes a center front signal to
be increased to a value of 1.06C.sub.f ' in the two front channels,
and a center back signal in the two front channels to be greatly
diminished, namely, to 0.353C.sub.b '. Conversely, a center back
signal is increased to 1.06C.sub.b ' in the back channels, and a
center front signal C.sub.f ' is reduced in the back channels to
0.353C.sub.f. Thus it is seen that the intermixing of signals
between channels by the junctions 98-104 establishes a
front-to-back information differential even without the use of
logic control circuitry. Moreover, the initial favorable
positioning of the front and back phasors resulting from
intermixing simplifies the task to be performed by logic circuitry
and thus serves an important purpose whether used simply as a
feature of the matrix decoder, or as a feature of a matrix decoder
combined with more sophisticated, and consequently more expensive,
logic and control circuitry.
It will be observed that since the matrix of FIG. 2A is a linear
additive device, and the cross-blending operation but another
additive operation, the circuitry of the matrix may be simplified
somewhat without degrading the above-described operation. For
example, since the input to gain control amplifier 76 is composed
of
0.707E.sub.56 + 0.707E.sub.60 - 0.707mE.sub.54 - 0.707mE.sub.58 (
3)
and the input to amplifier 78 is
- 0.707E.sub.54 - 0.707E.sub.58 + 0.707mE.sub.56 + 0.707mE.sub.60 (
4)
junctions 70 and 72 can be designed to perform the required
operations thereby to eliminate the need for junctions 102 and
104.
Although a preferred embodiment of the invention has been
illustrated and described, various modifications will now be
suggested to ones skilled in the art. For example, although the
front-back logic of FIG. 2B is best carried out with signal
voltages derived from conductors 60 and 54, and the wave-matching
logic is best performed with signals derived from conductors 60 and
56, somewhat less adequate wave-matching action can be obtained
from voltages also taken from conductors 60 and 54. It follows,
therefore, that it is possible to operate both the wave-matching
logic and front-back logic with but a single pair of gain control
amplifiers, for example, amplifiers 116 and 120, thereby to reduce
the cost of the system. Although there would be some degradation in
performance, this implementation might be commercially attractive
for use in the less expensive sound reproducing systems.
* * * * *