null  null
United States Patent [191
Jul. 5, 1983
[75] Inventor:
phase for all angles of elevation and azimuth of the
incident sound and which can be decoded to give good
Jonathan Halliday, Winchester,
[73] Assignee: Independent Broadcasting Authority,
London, England
quality stereo reception. The third channel contains the
signal indicative of the “front-back” information and
this signal is chosen to yield the best quality of surround
sound reception consistent with minimum interference
[21] Appl. No.: 218,081
[22] Filed:
Dec. 19, 1980
to the quality of mono and stereo reception for a given
type or range of program material. Preferably, the third
Int. cu .............................................. .. 1104s 3/00
of the other two channels. The three audio channels can
US. c1. ....................... .. 179/1 GQ; 179/1 GD
Field of Search ......... .. 179/1 GD, 1 GH, 1 GM,
be derived directly from three directional microphones
channel is phase shifted by 90° with respect to the phase
179/1 GQ; 369/89
References Cited
or can be derived from, the Ambisonics ‘B’ format in
which case an encoding matrix of the following form is
Primary Examiner-A. D. Pellinen
Attorney, Agent, or Firm—Laubscher, Philpitt &
—0.3954j 0.3954;
where 2, A and T are the signals in the three audio
channels and W, X and Y are the input signals to the
7/1972 Halpem ......................... .. 179/1 GD
7/1974 Christensen et al. ......... .. [79/1 GQ
6/1978 Gerzon .......................... .. 179/1 GQ
encoding matrix.
A system for surround sound utilizes three audio chan
nels two of which are either in phase of 180° out of
x=w ficose.
ms 0
10 Claims, 6 Drawing Figures
y=v 1? sin 9.
cos 0
US. Patent
Jul. 5, 1983
Sheet 2 of 3
E.3. WV.
_<HE‘551,2$.3 8 .
M25.8 :.
5$263.S: m.523 27¢-;
mxas. .
a28.l53 .
\ 0.8.
T being de?ned in terms of the actual or intended direc
tion of a sound to be reproduced, wherein the signals 2
and A are generated by an encoding means and are
The present invention relates to sound reproduction
either in phase or 180° out of phase for all angles of
elevation and azimuth of the incident sound whereby, in
use, the best compatible signal can be chosen for a given
type or range of programme material when received by
and more particularly to a system for producing sur
While the present invention is applicable to recording
monophonic and stereophonic receivers; and wherein
and playback as well as to transmission and distribution
the signal T is generated by the encoding means and
may, in use, be chosen, having regard to the imperfec
tions of real receivers, to yield the best quality of sur
round-sound reception consistent with minimum inter
the remainder of the speci?cation will be directed to
transmission and reception of a sound broadcast and
decoders which such reception may share in common
with such playback.
ference to the quality of mono and stereo reception for
a given type or range of programme material. It is un
derstood that all three audio signals are pre-emphasised
according to the standard locally in force for stereo
For several years surround-sound systems have been
investigated. All these systems, for broadcasting or
- transmissions.
recording on a recording medium such as a tape or disc,
Phase shifts may be introduced between the T signal
provide additional encoded information that is intended
to enable the listener, if he wishes, to locate the relative
positions of the instruments, voices and sounds at the
and the Z and A signals.
This new proposal differs from the earlier proposal
described above in that surround-sound decoding is not
time of the transmission or recording and so obtain,
possible using the 2 and A signal alone; however, since
more completely than with conventional monophonic
or stereophonic systems, an illusion of hearing the
sounds in their original directions and/or in apparent
phase differences are not present between these two
signals, no compromise is made with the quality of
stereo reception. The new proposal has the advantage
that it is capable of being used with existing mono and
stereo receivers to provide an acceptable quality of
directions chosen by the producer.
The invention takes as a basis the Ambisonic ‘B’ for
mat which provides four basic signals, namely:
W—an omnidirectional signal;
X—a signal which together with W is indicative of the
The transformation from ‘B’ format to our 3-channel
transmission format may be achieved by an encoding
front-back position;
matrix such as the one set out below:
Y—a signal which together with W is indicative of the
side-to-side position;
Z-a signal which together with W is indicative of
From these four signals it has been previously pro
posed to produce by linear mixing three signals, namely:
2-a sum signal equivalent to mono;
A-a difference signal which when taken with the 2
signal gives stereo and may also be decoded to give
horizontal surround-sound of limited quality;
T——a third signal which with the other two signals gives
horizontal surround-sound of good quality.
Experimental broadcast transmissions have been
—O.3954j 0.3954] ' 0
l [X]
' After reception of the signal the reverse transforma
tion is performed using a decoding matrix, which is the
inverse of the encoding matrix, to yield signals W’, X’,
Y’ equivalent to the original B-format signals. These are
then in turn decoded to yield loudspeaker drive signals
using known Ambisonic techniques.
that kind it was necessary to make a compromise,
This system is not a universal system and exact results
cannot be achieved by transmission or distribution sys
namely a diminution in the quality of the sound received
by a listener having only stereo equipment. This diminu
tems and/or decoders using less than 3 full bandwidth
audio channels or their multiplex or digital equivalent.
made during 1978 using such a system. In systems of 45
tion is due to the presence of phase differences necessar
ily introduced between the 2 and A signals (and conse
quently between the received stereo ‘left’ and ‘right’
signals) in order to make surround-sound decoding
possible from these two signals alone.
It is an object of the present invention toprovide an
improved surround-sound system based on the work of
Details of how the ‘B’ format signals are produced
have not been included since it is considered that this is
well known to those skilled in the art.
In order that the present invention be more readily
understood, embodiments thereof will now be de
scribed by way of example with reference to the accom
panying drawings, in which:
FIG. 1 is a block diagram representation of a trans
The present invention provides a system for transmis
mitter of one embodiment of the present invention;
sion and reception of horizontal surround-sound by 60 FIG. 2 is a diagrammatic representation of an alterna
frequency modulation of a carrier, wherein the modu
tive for part of the embodiment shown in FIG. 1;
lating signal contains a monophonic audio signal (here
FIG. 3 is a block diagram representation of a receiver
termed Z), a subcarrier modulated by an audio signal
for receiving the signal transmitted by the transmitter of
equivalent to the stereo difference signal (here termed
FIG. 1;
A) of a stereophonic broadcast, a pilot tone at half the 65
FIG. 4 shows a block diagram of a part of the re
subcarrier frequency, and in addition a second subcar
ceiver shown in FIG. 3;
rier in quadrature with the ?rst and modulated by a
FIG. 5 shows a representation of a part of a transmit
third audio signal (here termed T), the signals 2, A and
,ter for monitoring and controlling F.M. deviation, and
FIG. 6 shows a circuit diagram of a part of the circuit
shown in FIG. 1.
where the only direct (as opposed to reflected or rever
berant) sound sources are placed in front of the listener
and those where direct sound sources are placed all
The following embodiment provides a system for
transmitting and receiving 3 channels 2, A, T which are
derivatives of the channels W, X and Y of Ambisonic
all types of material its stereo compatibility must be a
technology. Primarily the system is designed for opti
mum results via FM radio, but in certain respects it is
also applicable to other means of dissemination, such as
3-channel analogue tape, or digital recordings with at
least 3 audio channels. The problem is to encode (or
mix) the 3 channels in such a way that those listening in
stereo or mono to the same broadcast will hear good
stereo or mono sound respectively. This is done by
transmitting 3 channels (known as E, A and T) which
are related to the original 3 channels by a matrix known
as the encoding matrix; 2, A and T are multiplexed
together and transmitted according to a speci?cation
which is essentially the Zenith-GE pilot-tone system
with an additional term for the T channel. The 2 signal
is that which is heard by a mono listener. The 2 and A
round. Hence if one encoding matrix is chosen to cover
compromise. The arrangement to be described incorpo
rates such a compromise. It is this choosing of the best
2 and A signals which is equally applicable to other
media such as 3-channel analogue tape or digital record
The choice of third channel is also a compromise
which depends on the type of programme material and
on the choice of stereo channels. The present embodi
ment incorporates such a choice, which has been made
bearing in mind the properties of multiplex FM trans
mission and reception. This speci?c choice is, therefore,
not equally applicable to other media. This choice for
multiplex FM transmission is made with the object of
giving best signal/noise ratio in surround-sound recep
tion consistent with minimum loss of coverage area in
stereo and mono reception and minimum loss of quality
signals together yield the two channels heard in stereo
due to the imperfections of existing receivers.
reception according to the formulae:
It will be noted that the signals, W, X, Y from which
25 2, A and T are derived are by de?nition signals which
indicate the intended location of the reproduced sound
in the horizontal plane. Nothing in this, however, pre
cludes the use of height information in generating these
(where k is arbitrary). The 2, A anclT signals together
are used only by those listening in surround-sound.
It has previously been proposed (by NRDC/Ambi
sonics) to transmit signals 2, A and T related to W, X
and Y by a speci?cation (known as System UHJ) having
the properties that 2 yields a good compatible mono
signal, 2 and A together yield stereo of moderate com
patibility and are also capable of being decoded (into
modi?ed signals W’, X’, Y’) to yield horizontal sur
round-sound of limited quality, and 2, A and T together
can be decoded to yield horizontal surround-sound of
signals W X Y by previous processing according to
Ambisonic principles, for example by tilting the re
sponse pattern of a microphone array. Neither does the
present embodiment preclude the additional transmis
sion of height information by the use of a fourth or
further channels added to the multiplexed signal.
Referring now to the drawings FIG. 1 shows one
way in which a transmitter for surround-sound accord
ing to the present proposal may be constructed. Block 1
represents an array of a plurality of microphones or
some other signal source which can be arranged to
good quality. The advantage of such a system is that 40 produce three outputs W, X and Y de?ned according to
Ambisonic technology where W is an omnidirectional
good surround-sound may be obtained even if T is re
ceived within a reduced audio bandwidth, which im
proves the signal-to-noise ratio in surround-sound re
signal, X = Moos 0 cos d), and Y=
to many listeners.
2-sin 0 cos 11>.
The angle 0 is the azimuth of the sound direction, mea
ception; but the major disadvantage is that the quality of
sured anticlockwise from centre-front, and d) is its angle
stereo reception is impaired by the necessary presence
of elevation. These three signals are fed to an encoding
of phase shifts between the two audio channels, which
circuit where they are transformed into signals 2, A and
for some types of programme material are unacceptable
According to the present embodiment signals 2, A
When the requirements are taken into account that
matrix such that 2 yields good compatible mono, Z and
A together yield good compatible stereo, and 2, A and
T together can be decoded to yield good horizontal
any sound direction, that the transmitted signals 2, A
and T contain no component representative of height
(i.e. the Z signal is not used), and that sounds symmetri
cally placed to left and right in surround-sound are
reproduced with a similar left-right symmetry in stereo
reception, the possible choices of encoding matrix are
restricted. When the requirements mentioned above for
choice of the third channel for FM multiplex transmis
and T are transmitted, related to W, X and Y by a new 50 there are no phase shifts between the stereo channels for
surround-sound. T, like 2 and A, needs to be received
with the full (normally 15 kHz) audio bandwidth, so
that the signal-to-noise ratio in surround-sound recep
tion is somewhat worse than in the former proposal.
However, the present proposal has the decisive advan
tage of better stereo compatibility. This is so because
there are no audio phase shifts between 2 and A. Sys
tems of this kind are of a fundamentally different class
from systems of the former kind because there is no
attempt to convey front/back information in the stereo
channels. Without this technical constraint it is possible
to choose the encoding matrix so as to produce the most 65
compatible stereo (and mono) signals. Unfortunately,
the optimum mix varies widely between different types
of source material, in particular between presentations
sion are also taken into account, it is found that the best
general form of the encoding matrix is:
EH2 2m
where a, b, f, g and h are real and j represents a broad
band phase advance of B 90°.
In FIG. 1 the encoding circuit is shown ascontaining
10 to another terminal of the circuit 11 whose outputs
L1+R1 and L|—R1 represent the 2 and A signals re
a linear mixer and ampli?er circuit 2 to which the W
and X signals are applied and from which signals
spectively. The microphone To has a directional sensi
tivity proportional to 1-B cos 0 cos d) and its output is
aW+bX and gW+hX are generated. The signal
aW+bX is fed througha'n all-pass ?lter network‘3 to
produce the 2 signal while the gW+hX signal is passed
through another all-pass ?lter network 4 which relative
fed through all pass ?lter network 14 which relative to
networks 10 and 12 adds a 90° phase advance and
thence to a multiplication network of coef?cient )1 to
to network 3 introduces a 90° phase advance to produce
produce the T signal. Thereafter, the E, A and T signals
the T signal. The Y signal is fed to a multiplying circuit
are multiplexed and transmitted as before. The pre
5 of co-ef?cient f and then through an all-pass ?ter 0
ferred values of a, ,B, 'y and 6 are as follows: a: 1.097;
network 6 identical to network 3 to produce the A
B=l.4l4; 'y=—0.879, 6=8l° where 6 is the 5 angle
signal. It should be understood that the equations im
between L, and R0 microphones.
plied by the encoding matrix permit the introduction of
FIG. 3 shows a block diagram of a receiver which is
a frequency-dependent phase shift x(w) common to all
in conjunction with an Ambisonic decoder and
the networks 3, 4, 6, and indeed any ?lters or other 5
hence the signal path is the opposite to that shown in
circuitry in the transmission/reception chain may also
FIG. 1.
introduce arbitrary phase shifts common to all 3 chan
The r.f. signal is ‘received in an F.M. receiver 20
nels, provided that the amplitude and phase relation
which feeds a demultiplexer 21 part of which will be
ships between the 3 channels remain as de?ned by the
encoding matrix. The 2, A and T signals at the output of 20 described in more detail later with reference to FIG. 4.
The output of the demultiplexer 21 is the three de
the encoding circuit are fed to a multiplexing circuit 7
emphasised signals 2', A’ and T’ which are fed to a
and thence to an FM transmitter section 8. The trans
decoding matrix, consisting of two identical all-pass
mitted multiplex signal is 2+A sin 2m0t+T cos
?lter networks 22, 23 for the 2’ and A’ signals respec
2coot+0.l sin wot (m==2 rr>< 19 kHz).
It has been found that the preferred coef?cients are as 25 tively, and an all-pass ?lter network 24 for the T’ signal
which generates a phase shift lagging by 90° relative to
the phase shift of networks 22, 23. The outputs from the
networks 22 and 24 are fed to a linear mixing and ampli
a = 09
b = 0.1092
f = 06897
g = —0.3954
h =
fying network 25 where signals A2"+CT’=W and
30 D2"+FT’=,X’ are produced. The output from the
FIG. 6 shows the circuit diagram for the linear mixer
and ampli?er circuit 2 of FIG. 1. The circuit is based on
two operational ampli?ers 60 and 61. The W signal is
fed via an input terminal 62 and a resistor 63 to the
non-inverting input of the ampli?er 60 and via a resistor
64 to the inverting input of the ampli?er 61. The X
network 23 is fed through a multiplication network 27
of co-ef?cient H to form the Y’ signal. The W’, X' and
Y’ signals are then fed to a conventional Ambisonic
decoder 28 which produces the loudspeaker feed signals
in known manner.
The coef?cients A C D F H are related to the terms
a b f g h in the encoding matrix by the equations
signal is fed via an input terminal 65 and a resistor 66 to
the non-inverting input of the ampli?er 61 which is
earthed via a resistor 69 and via a resistor 67 to the
non-inverting input of the ampli?er 60.
The output of the ampli?er 61 is fed back via a feed
back resistor 68 to the inverting input of the ampli?er 61
and is also fed to the network 4. The output of the
ampli?er 60 is fed to the network 3 and-is also con
nected to two resistors 70 and 71 connected in series
between the output and earth. The junction between the
resistors 70 and 71 is connected to the inverting input of
the ampli?er 60.
With the resistors having the standard preferred val 50
ues as given in FIG. 6 the outputs from the networks 3
Turning now to FIG. 4, this shows a dual-mode phase
lock loop which forms part of the demultiplexing cir
cuit 21. It is to be remembered that the input to the
demultiplexer is a pilot-tone system multiplexed trans
mission signal. This signal is fed directly to audio de
modulators 40, to an input of a multiplying circuit 41
and to an input of another multiplying circuit 42. The
output from the circuit 41 is fed to a dc. ampli?er 43.
The output of the circuit 42 is fed to a phase lock loop
ampli?er 44 whose output drives a voltage controlled
oscillator 45 which is used to control a divider circuit 46
and 4 will be a good approximation to the encoding
which has one output representative of sin wt which is
matrix speci?ed above.
fed to another input of the circuit 41 and another output
An alternative way to produce an approximation to
the encoding of the 2, A and T signals is shown in FIG. 55 representative of cos wt which is fed to another input of
the circuit 42. In this case, to is approximately equal to
2. In this Figure, it is not necessary to ?rstly produce
21rx pilot tone frequency. The circuit 41 and ampli?er
the W, X and Y signals. Three directional microphones
43 thus form an in-phase detector of the input signal
L,,, R,,, and T0 are represented by their polar diagrams
while the .circuit 42 and ampli?er 44 form a quadrature
and once again, as in FIG. 1, azimuth is represented by
0 and elevation by 4). The three mcirophones are placed 60 detector.
as closely together as possible. ,
Further outputs from the divider 46 representative of
The microphone L0 has a directional sensitivity pro
sin 2wT and cos 200T are used in the demodulators 40.
portional to l+a cos (0——8) cos 4) where a is a constant
Three channel reception ideally requires a higher
and its output is fed through an all pass ?lter network 10
standard of separation between the A and T signals
~to oneI input’vof an add/subtract circuit 11. The micro 65 (in-phase and quadrature signals) than is normally at
phone Ro has aidirectiona'l sensitivity proportional to
tained in conventional stereo receivers. In the circuit
l+a cos (0+8) vcos d) and its output is fed through
shown in FIG. 4, the pilot tone frequency phase is mea
another all pass ?lter network 12 identical to network
sured with a high degree of precision. This is achieved
by connecting the output of the ampli?er 43 to the
phase lock loop ampli?er 44 so as to switch the phase
It is to be noted that the circuit of FIG. 5 may be used
with other than the presently proposed three-channel
lock loop to “fast” if no pilot tone is detected by the
in-phase detector. Thus, the phase lock loop initially has
a wide bandwidth with low d.c. loop gain until the pilot
tone is detected whereupon the phase lock loop is
Programme material may sometimes be available not
in the form of B-format but in a 4-channel format using
signals known as LF, RF, LB and RB (representing
switched to a narrow bandwidth with a high d.c. loop
left-front, right-front, left-back and right-back respec
gain for normal running.
tively). Commonly, in such a format sounds intended to
be reproduced in the directions LF, RF etc. are repre
The output of the ampli?er 43 is also used to provide
the FM deviation at the'transmitter or in the studio or
sented by signals in the channels LF, RF etc. respec
tively alone, while sounds in intermediate directions are
represented by signals only in the two channels between
which the sound direction lies. Signals encoded in this
format (known as “pairwise panning”) cannot be ex
elsewhere. In stereo transmissions the deviation is sim
actly converted to B-format by a linear “ conversion
ply related to the levels of the separate left and right
channels; these levels are therefore used both for driv
ing level meters (PPM’s) used for manual control and
within transmission limiters which automatically pro
matrix, but an approximate compromise is‘ possible. It
has previously been proposed that this approximate
a mono/stereo indication as well as to provide a switch
ing signal to the audio demodulators 40 to switch them
to “mono” if no pilot-tone is detected.
FIG. 5 shows a circuit for monitoring and control of
conversion to B-format be done by using a matrix of the
tect the transmitters from over-modulation. In three
channel transmission there is no such simple relation
ship; the deviation is given by a complicated function of
the three signals which includes, in particular, the ex
pression (A2+T2)5. FIG. 5 is thus essentially an ana
logue computer for calculating the deviation from the
three signals 2, A and T, containing in particular an
arrangement for generating a voltage proportional to
with K lying between 0.707 and 1.
In the present embodiment it is proposed that pair
wise-panned material be converted approximately into
B-format according to this previous proposal with K
equal to or close to 0.888. After the resulting signals W
X Y have been encoded into transmission signals 2, A,
T by use of the encoding matrix with values a b f g h
given above, the overall result is equivalent to use of the
the expression (A2+T2)§.
In FIG. 5 the 21 signal is fed through a full wave
recti?er 50 which provides a signal indicative of the
modulus of 2|, which modulus signal is fed to one input
of an adder circuit 51. The A1 and T1 signals are fed to
adding and amplifying networks indicated by reference
numeral 52 which has a number of outputs for a range of
discrete values of a parameter 6 spaced uniformly from
0° to 360°. At each output there is present a voltage
representative of the value of A1 cos €+T1 sin e for a
particular value of 6. These outputs are combined via
diodes 53 and fed to one end of an earthed resistor 40
0.0222,,‘ -O.3732j —0.3732j
whereby the most positive value of A1 cos e+T1 sin 21
is selected. This signal represents (A12+T1Z)5 and is fed
The received signals e, A and T are de-emphasised
to the adder circuit 51 to produce a signal D: |
and then decoded in the same way as is described with
2| +(A12+ T12)%. The practical circuit includes means,
not shown here, for making allowance for the voltage
offsets introduced by the diodes 53.
Alternatively, the circuit 52 may be provided will full
wave recti?cation means in the A1 and T1 signal paths
whereby each output provides a signal of the form [A1 |
reference to FIG. 3 to yield signals W’, X’, Y’; these
signals are then fed into the B-format input of an Ambi
sonic decoder. The overall result of this is that at low
frequencies the loudspeakers in a square layout are
driven according to the matrix
cos 6+ lT| sin e in which case the range of values of e 50
is only from 0° to 90". In either case, any desired degree
of accuracy is obtained by spacing the values of e suf?
ciently close together.
The signal D is shown as being used to drive a peak
detecting circuit 55 which in turn drives a meter. Addi
tionally, or as an alternative, the signal D is used for
automatic limiting via a dc. coupled threshold detector
and loop ampli?er circuit 56 whose output is a control
voltage which is fed to a variable gain ampli?er 57 in
each of the input signal paths.
_ 00.9909 -0.7250
but that at frequencies above about 400 Hz the shelf
?lters in the Ambisonic decoder change the matrix to:
Other automatic limiting arrangements may be used
in conjunction with the means described for obtaining
the signal D e.g. a delay-line type, or a variable pre
_- 0.1777
1.0359 -0.6279
emphasis type. The variable gain ampli?ers 57 may
alternatively operate on three signals derived by some 65
A simple reduced-performance decoder could use an
linear combination of the signals 21, A1, T1, these latter
intermediate, frequency-independent matrix and the
signals being then regenerated by linear mixing to yield
same decoder would be used for transmissions derived
according to either this four channel source format or
the same overall effect.
4,392,019 ,
the three channel source format as proposed with refer
ence to FIG. 1.
‘ ‘
__ starting from the three audio signals and using the
'' “formula,
1. In a system for transmission and reception offhori~
zontal surround-‘sound by ‘modulation of "a‘ carrier,
wherein the modulating signal contains a monophonic
D= |2| +(A2+T2)l, and
_ (e)_means‘for selecting the most positive of a set of
audio signal 2, a subcarrier modulated by an audio
signal equivalent to the stereo difference signal A of a
signals of the form
stereophonic broadcast, a pilot tone at half the subcar- ' ‘
rier frequency, and a second subcarrier in quadrature
with the ?rst and modulated by a third audio signal T,
the signals 2, A and T being de?ned in terms of the
direction of a sound to be reproduced, the improvement
which comprises
' where c has a range of values from 0° to 360°.
7. Apparatus as de?ned in claim 1, and further com
(d) means for analog computation of the envelope
amplitude of the multiplexed signal, exclusive of
the pilot-tone, starting from the three audio signals
using the formula
(a) means for generating the signal 2;
(b) means for generating the signal A at a phase angle
with respect to the signal 2 which is selected from
an in phase relationship and a 180° out of phase
relationship for all angular values of elevation and
azimuth; and
(c) means for generating the signal T with a phase 20
(e) means for selecting the most positive of a set of
shift of 90° with respect to the signals 2 and A.
2. Apparatus as de?ned in claim 1, wherein said
means for generating the signals 2, A, and T includes a
matrix of the form:
' signals of the form
where 6 has a range of values from 0° to 90°.
a b
8. Apparatus as de?ned in claim 6, and further com
prising a peak reading meter arrangement connected to
the output of said selection means for indicating F.M.
30 deviation.
9. Apparatus as de?ned in claim 6, wherein said trans~
where a, b, f, g, h, are real and multiplication by j signi
mitter comprises means for automatically limiting F.M.
?es a broadband phase advance of 90°, and where W, X
and Y comprise source signals having amplitude ratios deviation, said automatic limiting means comprising a
‘peak detecting circuit connected to the output of said
l:\/2 cos 0 cos ¢:\/2_sin 0 cos (I), 0 being the azimuth
35 selecting means and variable gain elements for the three
and qb the angle of elevation.
audio signals controlled in response to the output from
3. Apparatus as de?ned in claim 2, wherein said ma
gi hj 0
trix has the speci?c value:
said peak detecting circuit.
10. A system for producing a horizonal surround
—0.3954j 0.3954j
sound signal, comprising
(a) means (1) for sensing sound in an area and for
producing an omnidirectional sound signal (W), a
depth sound signal (X), and a horizontal sound
signal (Y), said depth and horizontal sound signals
4. Apparatus as de?ned in claim 1, wherein said gen
erating means comprises three substantially coincident 45
microphones connected with signal processing net
works to provide said 2, A and T signals.
5. Apparatus as de?ned in claim 1, and further com
(d) means for receiving the frequency-modulated
signal and for demultiplexing the three audio chan
(e) phase-locked loop means locked to the pilot-tone
for recovering the sub-carrier;
(0 means for monitoring detection of a signal by said 55
phase-locked loop means; and
(g) means for setting the bandwidth of said phase
locked loop means to be wide when no signal is
detected by said monitoring means, whereby said
phase-locked loop means acquires lock of the pilot
tone, and for setting the bandwidth to be narrow
when a signal is detected by said monitoring means
with said phase-locked loop means having high d.c.
loop gain.
6. Apparatus as de?ned in claim 1, and further com 65
(d) means for analog computation of the envelope of
the multiplexed signal, exclusive of the pilot tone,
being a function of said omnidirectional sound
(b) means for encoding and modulating said omnidi
rectional, depth, and horizontal sound signals to
produce a monophonic audio signal (2), a ?rst
subcarrier modulated by an audio signal equivalent
to a stereo difference audio signal (A) of a ‘stereo
phonic broad broadcast, a pilot tone at half the
subcarrier frequency, and a second subcarrier in
quadrature with said ?rst subcarrier and modulated
by a third audio signal (T), the signals 2, A, and T
being de?ned in terms of the direction of the sound
being reproduced, said encoding means comprising
(1) mixer and ampli?er means (2) for processing
said omnidirectional and depth sound signals;
(2) multiplier means (5) for processing said hori
zontal sound signal;
(3) ?rst ?lter means (3) connected with one output
of said mixer and ampli?er means to produce
said monophonic audio signal 2,
(4) second ?lter means (6) similar to said‘ ?rst ?lter
means and connected with the output of said
multiplier means to produce said difference sig
'nal A at a phase angle with respect to said signal
2 which is selected from an in phase ‘relationship
" 12
duce 053i‘! third audio sigflal
for all angular values of elevation and azimuth;
of 9.0 with respect to Said Signals. 2 an? A}
an _
with a Phase Shift
(c) multiplexer means (7) for multiplexing said signals
i e, A, and T, whereby a high quality stereo signal is
(5) third ?lter means (4) connected with another 5
output of said mixer and ampli?er means to pro-
produced for transmission
“ I
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