Note: Descriptions are shown in the official language in which they were submitted.
- ~ -
TRIPHONIC SOUND SYSTEM
BACKGROUND OF THE INVENTION
~ This invention relates to a triphonic sound
5 transmission system that ;s particularly compatible with
existing monophonic and biphon;c reeeivers.
To accommodate the increasing public awareness
and interest in home reproduction o multi-channel sound,
the process of selecting a standard transmission system
for stereophonic sound for television is currently under-
way. This activity, intially undertaken by the broadcast
and consumer electronics industries~ will eventually in-
clude the Federal Communicaticns Commission (FCC) and the
consumer marketplace, and, in turn, creates 8 need for
improving mul~i-channel service, in particular, the pro-
vision of a triphonic sound system for television broad-
casting.
Multi-channel sound transmisslon had its prac-
tical beginning with the experiments at Bell Laboratories
in the early 1~3Q's described by JO C. Steinberg and W. B.
Snow in an article entitled "Symposium on Wire Tr~ns-
mission of Symphonic Music and its Reproduction in Audi-
~5 tory Perspective: Physical Factors" publi~hed in the Bell
System Technical Journal, Vol. XIIIa No. 2, April 1934~
Following the work of the National StereophGnie Radio Com-
mittee established by ~he Electronics Indus~ries Asso-
ciation in 1959, the present-day system for FM stereo-
phonic radio bro~dcasting was authorized by the FCC in
1961. Further research in the past decade has led to the
development of a number of proposed systems both for AM
stereophonic broadcasting and FM surround-sound broad-
casting.
. . .
C-1476
57~
Interest in multi channel sound with visual
smageS was given strong impetus by Walt Disney's pioneer-
ing movie 'IFantasia", first released in 1940. Today the
specification for 35 millimeter cinematic film provides
for four tra~ks of audio recording, and the 70 millimeter
standard provides for six. With such well-established
precedents in the film industry, and the routine trans-
mission of filmed programs by television broadcasters,
consideration is now being given to methods for ~rans-
mitting more than a single audio channel with a televisionpicture. While it may be argued that the audio needs of
the cinema and the televis~on media are different, and, in
partic~l~r, that the viewing s~reen size9 aspect ratio,
. ~udience seating, transmission band width limitations,
timeliness, and production cos~s seem to suggest the use
of the simplest possiblè audio system for today's tele-
vision, larger-screen home receivers are already gaining
in popularity, and serious studies are underway toward the
~stablishment of wide-screen high definition service,
thereby creating the requirement to consider the audio
needs in the near- and longer-term future and to provide
the technical means to meet such fu~ure needs.
Since any transmission system for s~ereophonic
~S television sound must be compatible with existing ser-
vice, all systems being considered begin with a mono~hon;c
æum signal (M) on the existing baseband channel, and a
s~ereophonic difference signal (S) to enable ~eparation
of the monophonic signal into its lef~ and right com-
ponents at ~he home receivPr. In the existing two-channel
stereophonic system approved by the FCC, and also in those
systems being considered for transmission of television
stereophonic so~nd, a symmetrical matrix7 expressed by
C-1476
7~
--3--
tbe following equations, is employed:
M=(L~0.7C)~(0.7C~)
S=(L-R)
In the home receiver, the signals applied to the left and
right loudspeakers are derived by the addition and sub-
straction (and normalization of gain3 of these combined
signals:
Left Channel = M -~ S = L + 0.7C
Right Channel = M - S = R ~ 0.7C
1~ .
While it is generally not customary to show the
center front term C in the matrix equation, i~ is included
here to demonstrate some interesting properties important
to multi-channel sound broadcasting wi~h televisicn. Re-
gardless of how many special audio effects may be employedor how wide a viewing screen may be used, the important
dialogue and other prominent audio signals conventionally
have been, and undoubt~dly will continue to be, placed at
the center of ~be picture. For traditional two-channel
loudspeaker playback, the center signal is presented as a
virtual, or phantom, image created by the acous~ic power
summation of sound of equal amplitude and phase from each
of the two loudspeakers. If the left, center~ and right
signals appeared at equal intensity in the original pro-
2S gram, such balance will be maintained in the stereophon$chome listening home environment. In the monophonic lis
tening mode, however, the equal voltage components of the
center signal in the left and right channels will add
~rithmetically~ causing the sum signal to be presented as
L ~ 1.4C + R. This equation illustrates the well-known 3dB
center imbalance common to monophonic playback of all
stereophonic systems which use traditional amplitude pan-
ning controls. Although an inevitable consequence of the
matrix process, the result is a desirable increase in
the prominance of the center channel, especially in pres-
ence of side stage effects. With two channel repro-
~ C 1~76
~D257
--4--
duction, where the center image is at normal level, a
listener can perform such discrimination easily7 even in
- the pre~ence of competing soundsO
That the important center signal is d;splayed as
a phantom image is unfortunate in that while the image ls
reasonably well defined for a lis~ener rigidly positioned
on the line of symmetry between the two loudspeakers, its
location for other listener pos~t~ons is vague and un-
stable. Any motion of the listener~s head causes apparen~
~hangesr At best, image location will be vague; at worst,
i~ will appear to move with the slightest motlon of the
head. While-this appears not to significan~ly detract
from the enjoyment of music alone, it presents a more
15 serious problem when the sound is accompani~d by visual
images. Even for a lis~cener positioned along the line of
loudspeaker symmetry, the image w;ll appear to rise as it
is panned from left through center to righ~. Although this
elevatlon of the center image has been recognized since
first reported in 1959, the effect has not yet been ade-
quately explained. However, the degree of elevation
appears to be related to the angles subtended from the
listener ~o the speakers, being least prominent when the
angle is small, but settlin& overhead when the listener is
~5 directly between the loudspeakers.
Even with the significant body o locali-
zation theory ~hat has been advanced in the las~ 20 years
or so; and despite the fact that the traditional model
requires careful seating of the listener, the traditional
model still remains the only one in practical use. One
reason may be that it permits the use of simple production
techniques and relativ21y inexpensive equipment, but more
importantly, is that different and far more complex pan-
ning functions would be required for each listener posi-
tion and orientation in the lis~ening room. Briefly
summarizing psychoacoustics localiæation principles~ the
C-1476
2 5
--5--
basic model of localization assumes tbat ~he geometry of
the humsn head is sy~metrical from left to right, and that
- the he~ring acui~y in the two ears is equa]. Thus, a
center image will be perceived when the outputs of the two
loudspeakeIs as ~eceived at the ~ars are equal in ampli-
tude and phase. When the head is turned, various other
factors come into account. FIG. 1 of the accompanying
drawings, taken from an articlP entitled "Measurement of
Diffraction and Interaural Delay of a Progressive Sound
Wave Caused by the Human Head" published by applicant and
Messrs. Abbagnaro and Bauer in J. Acoustical Society of
America, Vol. 58, No. 3, September 1975, illustrates the
effect of the head on a single-sound wave front arriving
at an angl~ of 90 from the front of the listener. At low
frequencies, the sound to the far ear is delayed by
approximately 0.8 milliseconds, and, furthermore, the
head acting as a baffle, causes a rise in sound pressure
at the near ear and a decrease in sound pressure at the far
ear. As the left-right difference curve in FIG. 1 illus-
trates, the overall amplitude difference between thesound at the two ears in this case differs from OdB at low
frequencies to approximately -15dB at lOkHz. Delays and
pressure responses for other head orientations are of
lesser magnitude, b~t st;ll significan~.
That listeners rely on interaural time differ-
ence cues in localization has long been recognized. In
1959, in an article en~itled "A Compatible Stereophonic
Sound System", Bell La~oratories Record, November 1959,
F~ K. Becker proposed a stereophonic matrix which u~ed
tim~-of-arrival information to vary the apparent location
of images between two loudspeakers. Time differences at
the ears were studied in detail by D. Mo Leakey, resulting
în a general localization theory based on phase differ~
ences at low frequencies and time differences between
sound envelopes a~ higher frequencies; the results of this
study are described in an article entitled "Some Measure
' C-1476
::~2~Z~
--6--
ments on the ~ffects of Interchannel Intensity and Time
Differences in Two~Shannel Sound Systems", Journal of the
Acoustical Society of AmeTica, Vol. 31, No. 7,
July 1959.While a panning function based on the ~ ~ve-
mentioned criteria could be employed in a stereophonicmixing system, it is elear that such a function could be
idealized only for listeners on the line of symmetry
between the loudspeakers.
lOOther researcbers ~ave studied the effect of
varying the amplitude between stereophonic loudspeakers
to position phantom images. An article entitled "Phasor
Analysis of Some Stereophonic Phenomena" published by
B. B. Bauer in the Journal of the Acoustical Society of
i5~merica, Vol. 333 No. 11, November 1961, describes the now
famous "Stereophonic Law of Sin~s" which provided one of
the first means to quantify such panning. Bauer derived
the following relationship:
Si~ sin ~A = (SL ~ SR~/(SL + SR), approximately~ where
~I is the azmith angle of the virtual image, and A is
the azmith angle of the real sources, and SL and SR are the
strengths of the signals applied ~o the left and right
loudspeakers, respec~ively. FIG. 2 illustrates the use o~
the "Law of Sines" for the case o~ two loudspeakers at an
angle of 90 ~o the listener. Bauer's law is not com-
pletely accurate, since it applies only to low frequencies
below 500 Hz and is constrained to the use of in-phase
signals. While the slope of the curve shown in FIG. 2 has
been questioned by some researchers, most confirm the end-
point of 20 dB separation required for a fully discreteimage.
Given the apparent impossibility of satisfying
the perceptual requirement for listeners at arbitrary
locations in a room, it is not surprising that early
practioners of stereophony characterized the center image
problem as the "hole in ~he middle." Most early attempts
at solving the problem involved t~e derivation of a sum
C~ 76
~7--
signal ~L~R) and spplying it ~o a third loudspeaker
loca~ed at ~he center. S~ch a "tri~rontal" approach does
lndeed stabilize the center image, especially if ~he gain
of the center channel is increased by 3dB with respect to
the left or right channels as recommended by Klipsch in his
article "Three-Channel Stereo Playback of Two Tracks
Derived From Three Microphones", I.R E. TransO Audio,
Vol. 7, March-April 1959. However, this approach causes
a drama~ic shrinkage of ~he apparent width of the stereo-
phonic stage; following Bauer's "Law of Sines" as illus-
trated in FIG. 3, the original 90 width would be reduced
to 45 when the signal level in all loudspeakers is equal.
If the center channel gain is reduced by 3dB, the maximum
stage width will be increased to 73, but of course at the
expensP of reduced stabili~y of the center image. Later
experimen~ation with quadraphonic matrices some~imes en-
coded the center-front ;mage by separating the left and
rîght components of this signal by 9V; less shifting of
the center-front image occurs in such a display, probably
because the image itself appears so wide as to be un-
acceptable for important music or dialogue.
One solution which appears to be quite sat;s-
factory from ~he listener's poin~ of view (hearing) is
employed routinely in the cinema, in which important
dia~ogue is usually assigned to a discrete center channel
feeding a center~screen loudspeaker. While the method
requires slightly more complex mixing and recording a-
cilities, it is direct in its approach and provides
satisfactory reproduction of impor~ant signals. Although
~he addition of a new loudspeaker in~erposed between the
left and right loudspeakers provides the opportunity to
pan additional virtual images at the near left and near
right locations, for non-dialogue ef fects it appears
~uite satisfactory ~o simply pan from lef~ to right~
especially for rapidly moving or non-discrete effects.
C-1476
2 57
--8--
The three-channel technique provides a sensible solution
which allows every member in a theatre audience to ex-
perience sound images at the proper location, and suggests
the desirability of incorporating a discrete cen~er-sound
channel in telev;sion reproduction.
Among YarioUS pruposals ~hat have heretofore
been advanced for three-c~annel FM stereo transmission
systems, the one described in Halpern U.S. Pat. No.
3,679,832 is illustrative. In this system, three in-
dependent sources o stereophonically related audio fre-
quency waves are added together to obtain a sum signalO
Each audio ~requency wave is also used to amplitude-
modulate a respective subcarrier signal, the subcarrier
signals being of the same frequency and spaced 120 apart
in phase. A suppresse~-carrier, double~sideband modu-
lation of each subcarrier is employed, with the frequency
of the subcarrier signals being sufficiently high as to
assure a frequency gap between the lower sidebands of the
modulated subcarrier signals and the sum signal. To
achieve the desired compatibility with monophonic and
two-channel stereophonic FM receivers, the amplitude of
each double-sideband suppressed-carrier signal is mul-
tiplied by a factor of 2/ ~ . A conventional low-level
phase reference pilot signal, lying within the frequency
gap, ;s employed for receiver detection purposes. A
second pilot signal, of one-third the ampl;tude oE the
third ~armonic of the phase reference pilo~, is utilized
to achieve three-channel receiver compatibility with a
monophonic or two-channel stereophonic broadcast. The
sum signal, the three double-sideband suppressed-carrier
signals, and the two pilot signals are frequency modu-
lated onto a high frequency FM carrier for transmiss;on
purposes.
~ C-1476
5~7~
The composite, f~equency modulated, carrier
signal i~ t~ansmit~ed to one or more receivers, which may
be either of ~he conventional monophonic or two-channel
stereophoni~ type or preferably a ~hree-channel stereo
receiver, each adapted to receive and reproduce the
three-channel broadcast in accordance with its respec-
tive mode of operation. Compatibility of the three-
channel stereophonic receiver with one-channel or two-
channel broadcast is achieved by the use of the second
pilot signal. In the absence of this pilot, the three-
channel receiver operates in a conventional manner to
reproduce a monophonic or ~wo-channel stereophonic
broadcast. The second pilot signal is used as an indi-
cator for a three-channel broadeast and when the same îs
received by a three-channel receiver it serves to switch
the latter into a three channel stereophonic reception
mode. Thus, a three-channel broadcast is compatible with
a one, two, or three-channel receiver, while the three-
channel receiver is compatib~e with a one, two, or three-
channel broadcast.
This system ;s relatively complex in that ;trequires two pilot signals and a phase-shift network for
establishing the 120 phase relationship between the
subcarrier signals, and has the disadvantage that all
three of the independent source signals are modulated to
enable recovery of third-channel information, some of
which information gets into the output of a two-channel
receiver.
C-1476
:~'25~S7~
- 1 o -
SUMMARY OF TffE INVENTION
_
- It is a primary object of the present inven~ion
~o provide a triphonic transmission sys~em that is fully
compatible with existing monophonic receivers and with
new television receivers that may employ only two loud-
speakers.
A related object of the invention is to provide
a triphonic transmission system that will provide center-
channel quality equivalent in all respects to that of the
left 2nd right channels without significant degradation
or compromise.of existing monophonic or future biphonic
~ervice and coverage~
In accordance with the present invention, three
independent sources of stereophonically related audio
frequency ~aves characterized as L(left) 3 R(right) and
C(center), are matrixed to obtain three signals exhibit-
~O ing the matrix equations:(l)M = L ~ 1.4C ~ R; (2)S=L-R; and
(3)T - -1.4C. Each of audio frequency waves S and T is used
to amplitude~modulate a respective subcarrier signal~
the subcarrier signals being of the same frequency and
spaced 90 apart in phase. Suppressed-carrier, double-
sideband modulation of each subcarrier is employed, withthe frequency of the subcarrier signals being suffi-
ciently high as to assure a frequency gap below the lower
sidebands of the modulated subcarrier signals and the M
signal. A conventional low-level phase reerence pilot
signal, lying within the aforementioned frequency gap, is
employed for receiver detection purposesO The afore-
mentioned M signal, the two double-sideband suppressed-
carrier signals, and the pilot signal are frequency modu-
lated onto a high frequency FM carrier for transmission
purposes.
C-1476
~ 5~
The composite, requency modulated, carrier
s;gnal is transmitted to one or more remote receivers,
which may be of the conven~ional monophonic or two-channel
stereophonic type, or preferably a triphonic receiver
constructed in accordance with 4he :invention. Typically,
a plurality of receivers of each type will receive and
reproduce the three signals, each in accordance with its
respective mode of operation. A conven~ional monophonic
receiver decodes only ~he sum signal (M~, and a two-
channel receiver reproduces the transmitted M signal inboth loudspeakers for mon ophonic reception, and the tra-
ditional stereophonic signals for the biphonic and tri-
phonic modes. For a third category of receiver having a
large screen and three widely-spaced lo~dspeakers, a
choice of reproduction is available. For reproduction of
monophonic transmissions, the sum signal ~M) can be util
ized in all three lolidspeakers, although at reduced level
in the flanking loudspeakers so as to avoid "pulling" the
sound image away from its desirable cen~er location. For
biphonic broadcasts, the M signal may be used for the
center loudspeaker, and the conveDtional left and right
signals for the ~lanking loudspeakers. Finallg, the
reproduction of triphonic broadcasts resul~s in the dis~
play of left, center, and right signals by respective
loudspeakers; in this case, the signal T is fed directly
to the center loudspeaker, and is also used to electri-
cally subtract the center signal components from the leEt
and right channels, resulting in fully discrete per-
formance.
.
C-1~76
~lZ~ZS~7~L
- 1 2 -
BRIE- o~S _I~TION OF THE DRAWINGS
The invention will be more fully appreciated
. from the following de~ailed description when consid~red
in connection with the accompanying drawings in
which: ,
FIG. 1, to which reference has already been
made, is a plot illustrating the effect of the head on a
single-sound wave front arriving at an angle of 90 from
the front of the listener;
FIG. 2, to which previous reference has been
made, is a plot showing the use of the "Law of Sines" for
the case of two lo~dspeakers at an angle of 90 to the
listener;
FIG. 3, previously referrred to, illustrates an
application of the "Law of Sines";
FIG. 4 is a requency diagram of the composite
baseband signal developed in accordance with the prin-
ciples of the present invention;
FIG. 5 is a simplified block diagram of a trans-
mitting terminal for generating the composite signal of
FI~. 4;
FIG. 6 is a simplified block diagram of a re-
ceivîng terminal in accordance with the inventionS and
FIG. 7 is a pictorial diagram illustrating the
reception mode hierarchy in accordance wi~h ~he prin-
ciples of the invention.
- C-1476
5 7
13-
DETAILED DESCRIPTION
_ .
Before describing the present invan~ion, it maybe useful ~a briefly r~view the basic principles of the
existing two-channel s~ereo system approved by the FCC, as
well as multi-channel television sound systems presently
under consideratîon for future broadcast service in the
United States. In the current radio system, the stereo-
phonically rela~ed signals ~hat are added together con-
stitute a "monophonic channel" which consists of a (~+R)signal of 50 to 15,000 Hz, where L and R represent the left
and right independent audio signals or channels9 as noted
earlier, each of the L and R signals may also include a
0.7C component. It is this combined signal that is
reproduced by a standard monaural FM receiver, hence the
descriptive term "monophonic channel" and the use herein
of the letter M to identify this channel. To this is added
a double-sideband suppressed 38 kHz subcarrier signal
Ssin~st, where S=(L-R), ~long with a pilot of l9kHz. The0 composite modulation signal can be written as:
em = M ~ Ssin~st + psin(~t/2)
where ~S=2~fS and s=38kHz, and p is the amplitude of the
l9k~lz pilot. Looking at the baseband sp2ctrum, one would
find a monophonic channel M from about 50 H~ to 15kHz, a
19kHz pilot, and a stereophonic channel Ssin~st signal
from 23 to 53kHz. If SCA tSubsidiary Communication
Authorization) is also being transmitted, there is an SCA
frequency modulated subcarrier band rom 59.5 to 74.5kHz.
Three multi-channel television sound sytems are
presently under consideration for future broadcast ser-
vice in the United States. These three sy~tems are
described in some detail in a July 1982 Electronics
Industries Association report entitled "Multi-channel
Television Sound: The Basis for Selection of a Single
C-1~76
~LZ~7:~
-14-
Stand~rd", but ~uffice lt eO s~y for ~p~e~ent p~rpose~ ~ba~
hree propose the tTan~mi6sion of a ~ter~oph~nic
~u~c~rrier for ~Do-channel sud~o progr~ g, ~ ~econd
aLIdio prog~ SAP) f9r ~ddltional l~ngu~ge or other
5 ~ervice, ~nd a third ~ubcarrier for non-publie t~l@metry
or el~ctronic new~ g~hering ~ENG3 u8~ ubc~rrler~
~re loc~ed ~t requ~ncies wh~ch are ~nteger OT frdCeiOnal
multiple~ of the NTSC televi~ion lhorizont~l ~ync~ro~i-
æa~iorl fr~quency (f~-15,743Hz). A syscem propo~ed by che
10 Electron~c Indu~er~e~ As~ociation of Japan u1:ilize~ fre~
quency modula~ion of the ~tereophonic 6ubcArrier, ~hale
the other tlJO, ~ropose~ by Telesoni~s, Inc. and
Zenith Radio Corporation, respectively, utilize double-sideband
suppres~ed carrier amplitude modulation, similar to that
employed in standard FM stereophonic radio broadcasts.
A6 has been noted earlier, io the pre~ent sy~tem
~n independent third or triphonic ~ignal ~ ~ provided for
reproduction by ~ center loudspe~ker ~nd al~o to be used
to electrically sub~ract the center ~i~nal component5
from the left &nd righe channel8 to give ~ fully di~crete
perormance. There are ewo choiees9 ln ~he three propo~ed
~ulti-channel televi~ion sound ~y~ems, for the potential
loc~tion of this new ~rip~onic ~ign~lg T~ Any of ~h~ ebree
~ystems could ~ccommoda~e the ~ignal T ~n the SAP channel,
25 ~lthough wiel: varying degrees of audio fidelityu The two
sy~tems which u~e an amplitude modulated ~ter~ophonic
~ubcarrier provide an al~ernate means for transmi~sion of
the T ~ignal, in that in ei~her one the new ~ignal T c~n
be incorporated as quadrature D~odul~tion of the ~m~ ~up-
30 pr~ssed carrier that c~rri~ ~he stereophonic diffe~encesig~aal S~tL-R3J The ~iphor~ic sy~tem of the pre~ent
invention will be desc~ib~d irl the context of the Tele~
sonics and ~enith m-~lti-ch~nnel televi6ion ~ound sy~tems
which diffes, for pre~ent purposes~ only in the Tequeney
35 o it~ ~tereophonic pilot tone9 wh~ch ~ fg~ or the Zenith
~ystem ~nd 5/4f~ for the Tele~nic~ i9yl3te!m~
b~ ~
'~''
:ILZ6~25~7~
-15-
In the triphonic ~ound system of the present
invention, to the monophonic channel are added two double- -
sideband kfH kHz signals (where k is 2.0 or 2.5~, one
corresponding to a difference signal consisting of ~L-R~
and the other consisting of a signal (T=-1.4C) and spaced
90 apart in phase, along with a pilot signal having a
frequency of either f~ or 5/4fH (for the Zenith and
Telesonics systems, respectively) all as shown in FIG. 4.
In accordance with the ~enith and Telesonics design
specifications, the amplitude o each o the doubl~-
sideband signals is twice the amplitude of the monophonic
channel signal, and tbe pilot, in turn, has a somewhat
smaller amplitude. Thus, the composite baseband signal of
this triphonic sound system can be written as follows:
em=(L~1.4C+R) ~ psin~t ~ (L-R~sin 2~t
+ (-1.4C)cos 2~t (Equation 1)
where L, R and C are independent audio channels,w= ~ kfH
(~H - 15,734kHz and k = 2.0 or 2.5) and p is the amplitude
of the pilot signal.
The transmitter for ~enera~ing this composite
signal is illustrated in the block diagram of FIG. 5~ For
purposes of simplicity, some of the more conventional
transmitter circuits (e.g., pre emphasis networks, car-
rier frequency source, and carrier frequency modulator~have not been shown and will be mentioned only briefly,
~here necessary, in the following description. The three
audio frequency s;gnals L, C, and R, derived from three
independent sources ~not shown~, are applied by pre-
30 emphasis networks (not shown) to the inputs of a conven-
tional matrix netwo~k 10 consisting, for examplet of a
network of summing amplifiers arranged to produee at the
output of tbe mat~ix three audio signals as follows: (1)
(L+1.4C+R), (L-R), and (-1~4C). The monophonic signal (M)
35 is appl ied as one input to an adder 12 t and the stereo-
phonic difference ~ignal (L-R~ and the triphon;c signal
C-1476
-16- .
(-1.4~ are applied to the inputs of respective modulators
14 and 169 the outputs of which sre also delivered to adder
12 where they are linearly combined with ~he monophonic
signal.
The subcarrier and pilot signal are derived from
a carrier generator 18, which is synchronized with and
clocked by a signal fH (the television horizontal syn-
chronization frequency) derived from the video signal to
be transmitted along with the audio signals, and which is
designed to provide an output sinewave signal S having a
frequency of kfH kHz, where, again, k i5 either 2.0 or 2.5,
depending upon whether the Zenith or Telesonics system is
used. The carrier ~enerator includes any one of the known
arrangements for providing a 90 phase displacement be-
tween the subcarrier output signals applied to the re-
spective modulators, as indicated in FlGo 5. The modu-
lators 14 and 16 comprise suppressed-carrier ~mplitude
modul~tors of known construction which serve to ampli-
tude-modulate the two subcarriers with respective audio
requency signals so as to produce the two double-side-
band, suppressed-carrier, amplitude-modulated subcarrier
signals (L-R)sin 2~t and (-1.4Cjcos 2~t. These latter
signals are then combined in adder 12 with the monophonic
signal M and a sinewave pilot signal of frequency k/2 fH
derived from carrier generator 18. The composite signal
produced at the output o~ adder 12, set orth in Equation
1 above, is then applied to the FM exciter of the trans-
mitter (not shown) and frequency modula~ed onto a high
3n frequency FM carrier for transmission purposes.
A triphonic receiver, in accordance with the
invention, is shown in the ~lock diagram of FIG. 6 and,
a~ain, for purposes of simplicity, some of tbe ~ore
C-1476
~ 2 5 7
-17-
conventional FM receiver circuits ~e.g., RF and IF stages,
discrimina~or~ and de-emphasîs netwo~ks~ have not been
shown and will be only briefly mentioned as necessary. In
~ddition to reproducing a triphonic broadcast, in the
manner to be descri~ed, the receiver is fully compatible
with conventional monophonic and two-channel ~biphonic)
~tereophonic broadcasts. A received FM signal is ampli-
f;ed in the RF and IF stages ~not shown) of a receiver/-
demultiplexer 20, and demodulated in any of the known FM
detection circuits (not shown) and demultiplexed to de-
rive the audio signals contained in the received FM
signal.
When a monaural broadcast is being received 7 the
15 output of the demultiplexer comprises the monaural signal
M consisting of (L+1.4C~R). This signal i6 applied as a
first input to both an adder 22 and a subtrac~or 249 the
outputs of which are applied to a first input of an adder
26 and an adder 28, respectively. In the absence of
signals applied to the second inputs o~ subtractor 24 and
adders 22? 267 and 28, the monophonic M signal (i.e.,
[L+1.4C~R]) appear~ at the output of each of adders 26 ~nd
28~ one of which may be selected by suitable switching (not
shown) for reproduction.
For a received two-channel stereo signal, the M
and S signals will be derived at the output of the
demultiplexer; as before, the M signal is applied to one
input of each of adder ~2 and subtractor 24, and the S
signal (L-R) is applied as a second input to adder 22 and
is subtracted from the ~ignal M in sub~ractor 24. As a
result, the output of adder 22 comprises the signal
(2L+1.4C~, and absent a signal at the second input of adder
26, the output of adder 26 will be (2L~1.4C), the amplitude
of which is then reduced by one-half to obtain a signal
tL~0.7C~ for application to the left loudspeaker. Simi-
larly, subtraction of the difference signal (L-R) from the
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monophonic signal yields a signal (2R~1.4C), and since
this signal likewise is not modified by adder 28, it
appears at the output of 2dder 28; again, reducing the
. amplitude of this signal by one-half ~ields ~he signal
(R+0.7C~ fo~ reproduction by the right loudspeaker of the
two-channel system. All of the above is typical of the
mode of operation of a conventional ~wo-channel FM ~e-
ceiver.
For a received triphonic signal, that is, a
composite signal including the new T signal (-1.4C), the
M, S, and T signals all appear at the output of demulti-
plexer 20; the M and S signals are applied to adder 22 and
subtractor 24 as before, and the T signal is applied to a
splitter circuit 30, a known matrix ne~work designed ~o
pass the (-1.4C) signal through to two separate outputs
for application to the s~cond input of each of adders 26
and 28, and to alter the amplitude of the T signal and
deliver to a third output terminal the signal 2C which,
after suitable reduction in amplitude, is fed directly to
the center loudspeaker of a triphonic reproduction sys-
tem. The linear addition in adder 26 of the signals
(2L+1.4C) and (-1.4C) yields a signal 2L and, similarly,
the addition in adder 28 of the signals (2R+1.4C) and
~5 (-1.4C) yields the discrete signal 2R; thus, after suit-
a~le reduction in amplitude, discrete L and R signals are
available or application to the left and right loud-
speakers9 respectively, of the triphonic sound repro-
duction system.
3~
The reception mode hierarchy described above is
seen in FIG. 7 which shows the three types of television
receivers in which the three different transmit modes
would be encountered 7 namely, a current conventional
television set 30 having a single loudspeaker, a biphonic
receiver 32 equipped with two loudspeakers for stereo-
phonic reproduction of television sound, and a system
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~ S'7~
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likely to ~ave future p~ominance having a large screen
display 34, a pair of outboard left and right loudspeakers
36 and 38, and a center loudspeaker 40 pdsitioned cen-
trally of and below the viewing screen In the first case,
as explained above, regardless of whether the trans-
mission is monophonic7 biphonic, or triphonic in accord-
ance with the present invention, the monaural M signal is
reproduced by the single loudspeaker. The two-channel
reproduction capability of receiver 32 displays the mon-
aural signal M on each of its loudspeakers when thetransmiss;on is monophonic, and for both biphonic and
triphonic transmissions displays the signal (L+0.7C~ on
its left loudspeaker and the signal (R+0.7C) on its right
loudspeaker. Finally, for the receiver having a large
screen and three loudspeakers, the audio designer has a
number of choices. For r~production of monophonic trans-
missions, it is possible ~o utilize the M signal in all
three loudspeakers, although at reduced level in the
flanking loud speakers 36 and 28 so as to avoid "pulling"
the sound image away from its desirable center location.
Employing these flanking loudspeakers in the illusrated
out;of-phase condition, may add somewhat to a feeling of
increased ambience. For biphonic broadcasts, the M signal
may be used for the center loudspeaker, and the con-
~5 ~entional left and right signals for the flanking loudspeakers; here, too, an out~of-phase presentation may
minimize slightly the impression of a shrunken stage
wid~h. Fina~ly, for triphonic broadcasts, the discrete L
and R signals are applied to a respective flanking loud-
~0 speaker snd the discrete C signal is fed directly to thecenter loudspeaker to provide accurate display of the
three signals, comparable to that obtained in cinema sound
systems.
Desirably the system according to the invention
includes an identification signal to permit automatic
switching of receivers to the triphonic reception mode.
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~ 2 ~ ~ ~'7
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Such a signal can be incorporated in the video signal or
within the audio baseband spectrum. One oiE ac least ~wo
- possibilities is to use a second pilot signal utilizing
one-tbird am~litude of ~che third harmonic of the main
5 pilot as suggested by Halpern in the aforementioned U.S.
Pat. No. 3 9 679,832, which does not increase the instan-
taneous frequency deviation of the FM carrier. Alter-
natively, depending on the baseband configuration se-
lected~ it may be preferable to employ amplitude modula-
tion of the first pilot; a subharmonic frequency of the
pilot should be selected to provide sidebands far enough
beyond the eapture range of receiver pilot detec~ors, yet
low enough in.frequency that the resultant sidebands about
the pilot do not fall within the main or stereophonic
channels.
It wîll have become apparent from the foregoing
that the distinctive requirements for satisfactory multi
channel sound reproduction in television make it de-
sirable to extend the scope of the sound systems currently
being considered for broadcast service. Although the un-
stable center sound image does not present a severe
handicap în the reproduction of sound without pictures,
this is not the case in telev;sion, particularly those
with wide-screen displays and widely spaced loudspeakers;
such systems demand a stable center sound, clearly sug-
gesting that new television audio service must follow the
example of the cinema rather than that of audio recording
or FM r~dio broadcasting. The described triphonic system
according to the present invention addresses and sat-
isfies this need in that it is easily transmitted, with
little or no penalty in station modulation capability or
area of broadcast coverage. The system offers the
potential for minimizing program production and editing
costs, since the major portion of sound-track program will
undoubtedly continue to be eenter-channel dialogue. Fin-
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ally, since the triphonic system is hierarchical, it
offers broadcasters and receiver manufacturers alike an
- unusual opportunity for flexibility in selection ~f op-
erational modes.
S
The foregoing disclosure is intended to be
merely illustra~ive of the principles of ~he present
inven~ion and numerous modifications or alterati~ns might
be made therein without departing from the spirit and
scope of the invention. For example, although the T signal
is described as having a value of -1.4C, it is obvious that
the value may be ~1.4C, which would require that adders 26
and 28 instead be subtra~ting circuits to obtain the same
results.
C-1476
