Note: Descriptions are shown in the official language in which they were submitted.
3~
SPECIFICATION
Background of the Invention
The present application relates generally to sound
transmission systems, and more particularly to a receiver
for the stereophonic sound transmission system described in
copending Canadian parent Application No. 271,170 filed
February 7, 1977.
The transmission of stereophonic sound together
with a conventional television picture transmission greatly
enhances the realism and entertainment value of the program
being transmitted. Various systems and apparatus have been
proposed for such transmissions including various compatible
subcarrier-type systems wherein left-plus-right (L~R)
information is conveyed on the regular frequency-modulated
sound channel of a composite television broadcast signal,
and left-minus-right (L-R) information is conveyed on a sub-
carrier.
One such system, which was described in "Simul-taneous
Transmission of Two Television Sound Channels", NHK Laboratories
Notes, Serial No. 132, February 1970, by Yasutaka Numaguchi,
Yashitaka Ikeda, and Osamu Akiyama, conveyed L-R information
on a single-sideband amplitude-modulated subcarrier frequency-
modulated on the standard NTSC aural carrier. To simpliEy
the synchronous detection required for demodulating the sub-
carrier in this system, the subcarrier was generated at a
frequency of 23.625 KHz, or one and one-half times the 15.75
KHz horizontal scanniny frequency of UOS~ monochrome television
broadcasts, enabling the missing subcarrier to be generated
- 1- `~
~5~3~ (
,
in the receiver L-R demodulator by sampling the horizontal
deflection signal. This system was found to he unsatisfactory,
primarily because of insufficient subchannel bandwidth, poor
channel separation and ambiguity in development of the
left (L) -and right (R) audio signals at the receiver.
Another system proposed for stereophonic television
sound transmission utilized a frequency-modulated subcarrier
centered at ~1 5 KHz, or twice the horizontal scanning fre-
quency. This subcarrier, when frequency-modulated on the
NTSC-standard aural carrier, pro~ided an L-R bandwidth of
12 KHz However, when it was attempted to add stereophonic
demodulation capability to-the 4.5 MHz soun~ channel of
standard intercarrier-type television receivers to recover
the L-R component, video signal component contamination
resulted to an extent that satisfactory L-R audio signals
could not be obtained without extensive modification of the
receivers. Applying such subcarrier signals to conventional
splLt-sound receivers, wherein separate intermediate frequency
~IF) channels are provided for video and sound components,
is not practical since the 410 25 MHz sound IF output of
conventiona~ mo~ern TV tuners is abo~e the range at which
presently employed sound channel IF filters can achieve the
required effectiveness.
Another system, which was proposed in U.S. Patent
3,099,707 to R. B. Dome, utilized an amplitude-modulated
suppressed-carrier subcarrier component, centered at 23.525 KHz
. _ .
~5(33~
- to avoi~ inter~erence with harmonics of the horiæontal
... . .
scanning signal, frequency-modulated on the sound carrier.
To facilitate regenerating the subcarrier for demodulation
. :. .
` purposes at the receiver a 39.375 KHz pilot signal was trans-
-- 5 mitted which, when combined with the 15.75 KHz horizontal
.
scanning signal present in the receiver, resulted-in genera-
~- tion of the suppressed 23.625 KHz carrier. This system did
. . .
not provide-satisfactory performance in that the bandwidth of
the L-R channel was limited to ~ KHz with symmetrical side-
.: .
bands. Attempting to r,crease available bandwidth by the use
. . ~ . .
- o~ assymmetrical sidebands was not practical because this
~ introduced a principa~ harmonic of the horizontal scanning
- signal into the upper sideband of the L-R comp~nent.
.
- - Two additional systems, which d~fered from those
: .~ , . . .
proposed in the afore-described systems in that they employed
a subcarrier centered at 31.5 KHz, or twice the horizontal
.
.-:.
- scanning frequency, were shown in U.S. Patents 3,046,329 to
; . ,
. Reesor and 3,219,759 to R. B. Dome. The first system
was a single-sideband system which necessitated the provision
of complex filtering and demodulation circuitry in th~
` receiver i~ unacceptably narrow L-R channel bandwidt~ was
-- to be avoided_ The second system, like other intercarrier
systems, was susceptible to`video signal component contamin-
- ation in the sound channel. Furthermore, both of these
2 systems required connection to or at least non-destructive
sampling of the horizontal deflection signal within the
.
. . .
-- 3 --
33~
receiver, necessitating in the case of an add-on adapter a
modification of the receiver and the provision of an additional
cable to a converter, thereby increasing installation cost
and reducing the versatility of the converter.
In contrast, the system of the aforesaid parent
Application No. 271,170 utilizes an amplitude-modulated double-
sideband suppressed-carrier 38 KHz subcarrier L-R component
frequency-modulated on the main aural carrier together with a
15 KHz bandwidth L+R component and a 19 KHz pilot carrierO
This forms a composite signal which is similar to that employed
in stereophonic FM broadcasts in the United States. The use
of this system simplifies the demodulation process at the
receiver, and provides a signal which is compatible with
conventional non-stereophonic sound television receivers. Also,
the proposed system lends itself to use with self-contained
converters of a design and construction which may be readily
utilized in conjunction with existing monochrome or color
television receivers.
It is a general object of the present invention
to provide a new and improved receiver for stereophonic
sound information transmitted in conjunction with a standard
television transmission, and which has improved performance
and is not unduly subject to interference from an accompanying
video transmission.
-- 4 --
It is another object of the present invention
to provide receiving apparatus for receiving a subcarrier-
type compatible stereophonic transmission which apparatus
can be conveniently installed on an existing television
receiver with minimal modifications to the receiver.
It is another object of the present invention to
provide apparatus for receiving a subcarrier-type stereophonic
television sound transmission which can be economically
constructed using standard commercially available components.
Summary~of the Invention
The invention is directed to a receiver for
receiving stereophonic sound transmissions included on a
television broadcast channel of defined frequency limits,
wherein the sound transmissions comprise a sound carrier
frequency-modulated by a composi-te signal including a first
component representative of the sum of the left and right
source signals, a second amplitude-modulated subcarrier
component representative of the difference between the left
and right signals, the subcarrier component having upper and
lower sidebands centered about a suppressed carrier, and a
pilot component representative of the phase and frequency of
the suppressed carrier. The receiver includes tuner means for
converting the television broadcast channel to an intermediate
frequency channel including an intermediate frequency sound
signal sound bandpass filter means for separating the sound
signal from the intermediate frequency channel, sour-d detector
means for deriving from the intermediate frequency sound signal
a composite signal including the first, second and -third
components, and stereo demodulator means for deriving -the left
and right source signals from the composite signal.
Brief Description of the Drawings
The features of the present invention which are
believed to be novel are set forth with particularlity in the
appended claims. The invention, together with the further
objects and advantages thereof, may best be understood by
reference to the following description taken in conjunction
with the accompanying drawings, in the several figures of
which like reference numerals identify like elements, and
in which:
Figure 1 is a functional block di.agram of the
transmitting portion of a stereophonic television sound
transmission system constructed in accordance with the
invention of aforesaid Application No. 271,170.
Figure 2 is a graphic presentation of the frequency
spectrum of a standard U.S. television channel.
Figure 3 is a graphic presentation of the composite
signal generated by thestereophonic television sound trans-
mission system of Application No. 271,170.
Figure 4 is a functional block diagram of a stereo-
phonic multiplex generator for use in the stereophonictelevision sound transmission system of Application No. 271,170.
Figure 5 is a graphic presentation of the frequency
spectrum of a television sound channel showing the effect
thereon of stereophonic sound transmission in accordance
with Application No. 271,170.
Figure 6 is a functional block diagram of a single-
conversion converter for allowing reception of stereophonic
television sound transmissions in accordance with the present
invention.
~ 6 --
~æ
Figure 7 is a functional block diagram of a stereo
demodulator for use in thestereophonic television sound
transmission system of the Application NoO 271,170.
Figure 8 is a functional block diagram of a double-
conversion converter for use in receiving stereophonic tele-
vision sound transmissions in accordance with Application
No. 271,170.
Figure 9 is a rear elevational view of a television
receiver and adapter in accordance with the present invention.
Figure 10 is a functional block diagram of the
stereophonic television sound adapter shown in Figure 9.
Figure 11 is a rear elevational view of a television
receiver and adapter for adapting the receiver to receive
stereophonic television sound transmissions in accordance
with the invention.
Figure 12 is a functional block diagram of the
stereophonic television sound adapter shown in Figure 11.
Figure 13 is a functional block diagram of a
television receiver incorporating means for receiving stereo-
phonic television sound transmissions in accordance withthe present invention.
Figure 14 is a functional block diagram of a
converter for allowing reception of stereophonic television
sound transmissions in accordance with the present invention
on a standard stereophonic FM broadcast receiver.
Figure 15A is a schematic diagram partially in
functional block form of the transmitter portion of a system
for bilingual television sound transmission in accordance
with Application No. 271,170.
-- 7 --
æ
Figure 15s is a schematic diagram partially in
functional block form of the receiver portion of a system
for bilingual television sound -transmission in accordance
with the invention.
Figure 16 is a graphic presentation of various
transmission standards as applicable to a stereophonic
television sound transmission system cons-tructed in accordance
with Application No. 271,170.
Figure 17A is a schematic diagram partially in
functional block form of the transmitter portion of a system
for L-R component enhancement in accordance with Application
No. 271,170.
Figure 17B is a schematic diagram partially in
functional block form of the receiver portion of a system
for L-R component enhancement in accordance with the invention.
Figure 18A is a functional block diagram of the
transmitter portion of the stereophonic television sound
transmission system of Application No. 271,170 incorporating
means for L-R component enhancement for improved performance.
Figure 18B is a functional block diagram of the
receiver portion of the stereophonic television sound trans-
mission system of the invention incorporating means for
compensating for L-R component enhancement.
Figure l9A is a functional block diagram of the
transmitter portion of the stereophonic television sound
transmission system of Application No. 271,170 showing means
for Dolby type B encoding incorporated therein.
Figure l9B is a functional block diagram of the
receiver portion of the stereophonic television sound trans-
mission system of the invention showing means for Dolby type
B decoding incorporated therein.
-- 8 --
Figure l9C is a functional block diagram of a
Dolby type B signal processing stage suitable for use in
the stereophonic television sound transmission system of
the invention.
Description of the Preferred Embodiment
Referring to the Figures, and particularly to
Figures 1-6, a stereophonic sound transmission system con-
strueted in aeeordanee with Applieation No. 271,170 may be
employed in eonjunetion with an aural transmitter 20 and a
visual transmitter 21, whieh may be eonventional in design
and eonstruetion. The radio frequeney (RF) output signals
from the two transmitters are combined in a eonventional RF
signal diplexer 22 and radiated by means of a eommon trans-
mitting antenna 23.
g _
3æ
Video source signals such as may be obtained from
cameras, film chains, video tape recorders or the like, are
applied to the visual transmitter 21 to produce an amplitude-
modulated (AM) RF output signal 24 having the bandpass
characteristic shown in Figure 2. At the same time, left (L)
and right (R) stereophonic audio signals, such as may be
obtained from microphones, tape decks, turntables, or the
like, and which typically represent the sound level at two
different locations in the scene being televised, are applied
to the aural transmitter 20. Within this transmitter these
signals are applied to an stereo multiplex generator 25,
which may be similar in construction and operation to those
utilized in conventional stereo FM broadcast transmitters.
The output of generator, in accordance with the invention,
consists of a composite signal which includes a direct L+R
audio component 27, a double-sideband L-R component 28
consisting of upper and lower sidebands 28a and 28b centered
at 38 KHz, and a 19 KHz pilot component 29, as shown in
Figure 3. This signal is applied to conventional RF modu-
later and amplifier circuits 30 within aural transmitter 20
to develop a frequency modulated RF output signal 26 centered
4.5 MHz from the video signal, as shown in Figure 2.
Referring to Figure 4, the multiplex generator 25
may include a pair of pre-emphasis networks 31a and 31b and
a pair of 17 KHz low pass filters 33a and 33b for the L and R
channels, respectively. As is well known to the art, the
- 10 -
~4~3æ
pre-emphasis circuits impose a frequency response character-
istic on the L and R signals which emphasizes the hlgher
frequencies to improve the signal-to-noise ratio of the
transmitted program. The low pass filters 33a and 33b serve
to prevent input signals exceeding 17 KHz from affecting the
19 KHz pilot and L-R components. The left and right audio
signals from filters 33a and 33b are applied to a synchronous
switching stage 35 wherein they are alternately sampled to
develop the L+R and L-R components in a manner well known
to the art. The operation of switching stage 35 is controlled
by a 38 KHz square wave signal, which is generated by means
of a 76 KHz crystal-controlled oscillator stage 36 and a
2:1 frequency divider stage 37.
After further filtering in a 60 KHz low pass filter
38 to remove harmonics which may exist in the composite signal
above 53 KHz, the output of the synchronous switching stage 35
is combined with a 19 KHz pilot signal in a summing stage 39.
The pilot signal is derived by means of an additional 2:1
frequency divider stage 40 and 19 KHz low pass filter and
phase adjustment stage 41 to assure precise time coincidence
with the 38 KHz sampling action of switching stage 35.
Referring to Figure 3, in the basic system contem-
plated by Application No. 271,170 the L+R signal component
27 generated by stereo multiplex generator 25 preferably has
a frequency range extending from 50 Hz to 15 KHz and an
amplitude sufficient to produce a maximum sound carrier
frequency deviation of 22.5 KHz. The L-R signal component
28 consists of lower and upper side band components 28a and
28b preferably centered about a 38 KHz suppressed carrier and
30 extending from 23 to 37.95 KHz and 38.05 to 53 KHz, respectively,
each having an amplitude sufficient to produce a maximum
frequency deviation of 11.25 KHz in the sound carrier. The
-- 11 --
19 KHz pilot component 29, which is preferably centered between
the lower sideband 28b and the L+R component 27, is trans-
mitted at a frequency deviation of 2.5 KHz in the sound
carrier. As shown in Figure 3, for the illustrated embodi~
ment the total bandwidth required by the composite signal
is 53 KHz and, by reason of the amplitude limitations imposed
on the L+R, L-R, and pilot components, the maximum deviation
of the sound carrier is 25 KHz.
In accordance with the invention of Application
No. 271,170, the frequency of the pilot carrier may be increased
to 5/4 the horizontal scanning rate of the video transmission
(Fh), or 19.6875 KHz in the case of U.S. monochrome transmission
and 19.66783 KHz in the case of U.S. color transmissions.
This centers the suppressed carrier between the second and
third harmonics of the horizontal scanning frequency, which
harmonics have been found to be a principal cause of inter-
ference in prior-art stereophonic sound systems which lacked
adequate video component rejection. This also reduces
interference to the L R component to a single component at
approximately 7.8 KHz instead of three components at 6.5 KHz,
- 12 -
~æ
9~Z5 KHz an~ 2_75 KHz (~eat between the 6.5 KHz and 9.25 KHz
components perceived by a listener) as with a 19 K~z pilot
. .
carrier.
A stereophonic sound converter 50 for receiving
. .. .
stereophonic sound transmissions in accordance with the inven-
- tion is shown in Figure 6. This converter operates indepen-
dently of the television receiver, having an input for direct
connection to a conventional television antenna 5L and L and R
... .
audio outputs for connection to an external stereo amplifier
--~ 10 and speaker system. The RF signals intercepted by antenna 51.... .
~-- are app-ied to a user-adjustable tuner 52 within the converter
wherein the desired television channel is selected, amplified
.
~- and converted to a suitable intermediate frequency, in this
case 10.7 MHz. The intermediate frequency (IF) signal is
applied to an IF amplifier stage 53, wherein additional
. ~ . . . .
` amplification and limiting are provided. The amplified IF
signal is applied to a conventional FM detector stage 54
; wherein a composite audio signal having L~, L-R and pilot
signal components as depicted in Figure 3 is developed in a
2~ manner w~ll known to the art. In addition, detector 54 may
:
`-- also develop an automatic frequency control ~AFC) voltage for
~ .
- application to tuner 52 to maintain the tuner properly tuned
to the desired station, and the IF amplifier stage 53 may
- develop an AGC signal which is applied to tuner 52 to main-
` 25 tain a constant signal level.
.
- 13 -
In order to obtain the L and R audio signals neces-
sary for driving a stereo amplifier and speaker system the
; . composite signal from detector 54 is applied to a stereo
d.emodulator 55, which serves as the counterpart of the stereo
multiplex generator 25 in the transmitter. In its most basic
form the demodulator 55 may include a commercial integrated-
circuit (IC) type stereo demodulator circuit similar to those
con~only employed in stereo FM broadcast receivers together
with necessary de-emphasis circuits for the L and R audio
outputs of the circuit. Referring to Figure 7, within such
a demodulator circuit the composite signal lS typically
ampiified by a buffer amplifier 60 and applied to a phase-
detector 61, which comprises part of a phase-lock loop. The
phase-lock loop includes a low-pass filter 62, a DC amplifier
63, a voltage-controlled 76 KHz oscillator 64, two 2:1 fre-
quency divider stages 65 and 66, and a phase correction
circuit 67 ~hose output is presented as a second input to
the phase detector 61 for compàrison with the composite input
signal. The phase-lock loop is designed to lock onto the
lg KH~ pilot carrier and produce its 38 KHz second harmonic
in correct phase to control synchronous switch stage 6g.
The synchronous switch 68 alternately samples the
composite stereo input signaI at a 38 KHz rate, synchronized
precisely in time and in the same sequence as the correspondinq
samples are assembled by the synchronous switch 35 in stereo
multiplex generator 25 (Yigure 1) in forming the composite
- 14 ~
3~
signal at the transmitter. As is well known to the art,
this results in the L and R audio signals being developed at
the output of the swi~ch, and these derived audio signals are
applied to:respective ones of two'de-emphasis networks 69a
and 69b wherein a predetermined frequency response charac-
teristic'is introduced-to compensate for the pre-emphasis
characteristic introduced at the transmitter. ~he L and R
audio outputs of the synchronous switch may be applied to an
external stereo amplifier and speaker system, or alternatively
applied 'o suitable audio amplifier stages and~or spea~ers
'provlded within the converter.
-A dual-convers~on version of the stereophonic
television sound converter is shown in Figure 8. In this
embodiment tuner 52 converts the selected television broadcast
signal to an IF signal which includes a video component
centered'at 45.75 MHz an~ a sound component centered at 41.25
MHz. This signal lS applied to a 41.25 MHz bandpass fil-ter 70
wherein the sound component 1S separated and applied to a mixer
stage 71. ' Within mixer stage 71 the IF sound component is
combined with a 51.95-MHz, or alternatively, a 30.55 ~lHz
continuous wave signal from an oscillator stage 72 to develop
a second IF signal at 10.7 MHz. This signal is ampliried and
amplit-ude-limited in a conventional 10.7 ~1Hz IF amplifier
stage 53 prior to application to an FM detector 54, wherein
'25 it is converted to a composite stereo signal having L1R,
L-R and pi,lot signal components as depicted in Figure 3. In
-- 15 --
.
addition, as in the previously described single-conversion
converter of Figure 6, IF amplifier stage 53 may develop an
AGC voltage for application to tuner 52 and detector 54 may
develop an AEC voltage for centering the IF frequency, the
AFC voltage being applied to either oscillator 72 or tuner 52.
Image rejection for the dual-conversion converter
is superior to that for the 10.7 MHz IF single-conversion
converter, in that the 41.25 MHz first intermediate frequency
provides greater separation between the frequency of the
received signal and the lmage frequencies to which the
rèceiver is subject. With the 41.25 MHz IF channel the
receiver is subject to a primary image band 82.5 MHz removed
from the- received broadcast and a potential secondary inter-
ference band 20.625 MHz removed from the broadcast which
can be doubled in the mixer and thereby pass through the IF
amplifier. Fortunately, frequencies that far removed are
efficiently rejected by normal tuner selectivity in the
dual-conversion converter. However, in the single-conversion
converter these primary and secondary frequency bands are
removed from the desired signal by only ~1.4 MHz and 5 ~5
MH~ respectively, the proximity of the latter being such
that a portion of the video spectrum of the selected channel
and an adjacent channel may pass through the IF amplifier
to contaminate the sound channel. Therefore, the tuner
for the single-conversion converter must exhibit far greater
selectivity than that utilized in the dual-ccnversion converter.
S~3;~
The problem o~ discriminating against secondary interference
in the single-conversion recei~er may be ameliorated by
adopting an IF slightly greater than 10.7 MHz.
- For convenience, it may be desirable to perform
channel selection at the converter for both the sound and
video portions of a broadcast. To this end, the converter
may take the form of an adapter 90 such as than shown in
Figures 9 and lO This adapter includes suitable RF circuitry
for receiving both the audio and video portions of the signal,
1~ and for concurrently supplying the converted video IF signal
to a convèntional television receiver 94 to permit reproduction
of the ~ideo scene. The adapter 90, which, except for an
additional RF output circuit extending to the television
receiver, may be similar in design and construction to the
lS converter shown in Fi~ure 8, includes a tuner 91 for converting
signals intercepted by the television receiving antenna 51 to
an intermediate frequency. The intermediate frequency signals,
corresponding to those commonly employed in a television
receiver, i.e. 41.25 MHz ~or the sound carrier and 45.75 L~rz
for the video carrier, are amplified in an RF amplifie~ 92 and
coupled through a coa~ial cable ~3 to the television receiver
94. Within the television receiver the coaxial cable 93 .~ay
terminate in an isolation network 95 which serves to couple
- the signals to UHF input of the VHF tuner 96a of the .ele-
~25 vlsion receiver~ The VHF tuner is coupled in a conventional
manner to the main chassis 99 of the receiver, which r,ay be
- 17 -
:
. conventio~a~ i~ a~l respects. The main chassis dev~lops a
.. video output signal for driving a picture tube 97.
. . .
The balance of the stereophonic sound converter 90
is similar in construction and operation to the converter
S shown in Figure 8. As in the converter of Figure 8, the
- 41.25-MHz output of RF amplifier 92 is applied through a
-. 41.25 MHz bandpass filter 70 to the input of mixer stage 71.
.- There, the IF s.ignal is heterodyned with a continuous wave
-
51.95 M~æ signal developed by an oscillator 72. As a result
a 10.7 ~Hz-IP sisnal lS developed whicn is applied to the
10.7 ~Hz I~ amplifier 53. The amplified IF output signal
` . rom this stage is applied to an FM detector 54 wherein a
- composite audlo signal having L~R, L-R and pilot components
is derived. The composite audio signal is applied to a stereo
` lS. demodulator stage 55 wherein L and R audio signals are
~ . . .
developed for application to external stereophonic amplifying
.equipment (not shown).
In operation, tuner 91, which may consist of a con-
ventional turret or bandswitch type discrete channel tuner
of the type cormrnonly incorporated in consumer television
receivers~ i5 set to a desired channel and the intermediate
frequency output from the tuner is routed through RF amplifier
92,-cable 9~ and isolation network 95 t~ the input of the
. television receiver VHF tuner 96a. This interconnection can
usually be readily accornplished, since the V~IF tuner 96a is
ordinarily connected to the UHP tuner 96b by means of a
- 18 -
~ s~
coaxiaL cabl~ 93a having plugs on at least one end, so that
it is only necessary to unplug this cable and plug in the
isolation network to complete the connection. The isolation
network serve~ to isolate or decouple chassis grounds as
well as to match impedances. VHF tuner 96a, when switched
to its UHF position, serves only to pass the signal from the
adapter 90 to the television receiver main chassis 99.
Adapter 90 develops L and R audio signals while
the television receiver 94 operates in a normal manner to
produce a picture on picture tube 97. Since the oper~tion
of the television receiver has no effect upon the reception
of the stereophonic sound signal, instability or poor signal
quality within the receiver cannot depreciate the quality of
; the reproduced sound. The automatic gain control (AGC) cir-
cuits of the television receiver remain in effective control
-- of video level with this arrangement, and while tuner 91 is
adjusted`to optimize picture quality in television receiver 94,
the quality of the reproduced sound is automatically and inde-
pende~tly optimized by AFC and AGC circuits of the adapter.
~0 It will be appreciated that instead of the multi-
channel tuner, it is also passible to utilize a single-channel
tuner for receiving a special interest channel. Obviously,
this arrangement results in simplification and reduced manu-
facturing costs for the adapter, making the pac~a~ attractive
for promotional and special interest uses.
.
A version of the stereophonic sound adapter for
. use in conjunction with a conventional television receiver,
wherein channel-selection is accomplished within the receiver,
is shown in Figures 11 and 12. This arrangement allows the
option of concealing the adapter 100 wi~hin the television
receiver cabinet, as shown in Figure 11. The television
receiver, as shown in Figure 12, may be conventional ln design
. and construction, comprising a receiving antenna 51, a tuner
101, television receiver circuits 103, a picture tube 104,
and a loudspeaker .105. To facilitate operation of the adapter
the intermediate frequency.output-signal from tuner 101 i5
non-destructively sampled by 2 pick-up 102 and conveyed through
. a coaxial cable 106 to the input of a variable-gain RF amplifier
107. The pick-up 102 may consist of a high-impedance voltage
. 15 pick-up coupled to the signal path, or alternatively a low-
impedance current pick-up which may be inserted in series with
. the signal path-by unplugging the existing cable between the
tuner and the main chassis and plugging in the pick-up.
The output of RF amplifier 107 is applied through
. 20 a 41.25 MHz sound~ bandpass ~ilter 108 to a first mixing or
heterodynin~ stage 109, wherein this signal is heterodyned
with a 51.95 MHz continuous wave signal supplied by an oscil-
lator 112 to develop a 10.7 ~z IF signal. As in the previously
- described converters, this signal is applied to a 10.7 MHz IF
.25 amplifier 113 wherein it is amplified and amplitude-limited,
and from there to a conventional FM detector stage 114. The
- 20.- .
(
3~
-- composite output signal from detector 114 is applied to the-
... .
-, non-inverting input of a differential amplifier 118, whose
- output is in turn applied to a stereo demodulator stage 119
, to develop L and R audio output signals for connection to an
, 5 external stereophonic audio amplifier system.
;. .
To provide'improved performance, an optional 45.75
,, ` MHz video bandpass filter 110 may be connected to the output
' of amplif'ier lU7. The video signal passed by thls filter is
' ' mixed in a second mixing or heterodyning stage 111 with the
,' 10 51.95 MHz continuous wave signal developed by oscillator 112
. . .
. .
to form a 6.2 MHz IF signal. The 6.2 MHz siynal is amplified
and amplitude-limited in a 6.2 MHz IF amplifier 115 and
' applied to a conventional FM detector stage 116 wherein an
'~ output signal indicative of frequency shift in the video
', 15 channel i5 developed The output slgnal is applied through
a 500 Hz Low-pass filter 117 to the inverting input of differ-
ential amplifier 118, causing the output of this amplifier
to correspond to the difference between the composite signal
`~ from detèctor 114 and the low frequency signal from'detector
``~ 20 116.
The effect of subtracting the low frequency audio
component optionally derived from the 6.2 l~lHz IF signal is
to cancel out some or all of the effects of any extraneous FM
' modulation which exists in the sound channel at the output of
television receiver tuner 101 as a result of microphonics or
AC power supply'harmonics~ The 6.2 MHz circuits are effective
.
- 21 -
3~
for this purpase because the video carrier is relatively free
of fre~uency modulation components below 500 Hz, therefore
any such frequency modulation finding its way into FM detector
116 is necessarily due to an undesirable effect common to
both signal paths, and therefore should be cancelled out of
the principal sound channel by differential amplifier 118.
The necessary circuitry for receiving stereophonic
television sound broadcasts transmitted in accordance with
the invention may also be provided as an integral part of a
television receiver, as shown in Figure 13. In the illus-
trated receiver, which except ~or its sound channel may be
conventional in structure and operation, televisio~ trans-
missions are intercepted by an antenna 51, and amplified and
converted by a conventional tuner 120 to an intermediate
frequency. The 45.75 MHz video portion o~ the IF signal is
amplified by a 45.75 ~z video IP amplifier 121, and then
applied to a video detector 122 wherein viaeo information in
the intercepted signal is derived. The video signal from
detector 122 is ampllfied in a conventional video amplifier
stage lZ3- an~ applied to a picture tube 124 to control the
brightness of the electron beam thereon. The horizontal and
vertical scanning of the electron beam is controlled by con-
ventional deflection circuits 125 which receive sync;lronizin~
pulses from video detector 122.
The sound signal appears at the output of tuner 120
as a 41,25 MHz IF siqnal. This signal is separated from the
_ 22 -
45 75 MHz video siynal by a 41 25 M}Iz sound bandpass filter
126 and applied to a mixing or heterodyning stage 127. In
mixing stage 127 the 41.25 M~Iz sound IF signal i5 com~ined
with a 30 55 MHz, or alternatively a 51.95 MHz, continuous
wave signal from an oscillator 128 to develop a 10.7 ~z IF
signal. This signal is ampllfied and amplitude limited in a
10.7 MHz IF amplifier stage 129 and applied to an FM detector
130 wherein a composite signal containing L*R~ L-R and pilot
components as depicted in Figure 3 is developed. The com-
.
posite signal is applied to a stereo demodulator 131 wherein
L and R audio signals are developed FM detector 13Q also
develops an AFC voltage which is applied to appropriate fre-
quency control circuitry in oscillator stage 128 to maintain
the I0.7 MHz IF signal centered in the IF channel regardless
- .
` 15 of the fine tuning of tuner 120.
The stereo demodulator 131, which contains both
- stereo demodulation and de-emphasis circuitry, such as those
described in connection with the previously described stereo-
phonic sound converters and adapters, reproduces from the
composite s~gna-l the R and L au~io signals developed at the
program source. These audio signals are applied to respective
inputs of audio amplifiers 132 and 133 wherein they are
amplified to a level suitable for driving respective-loud
speakers 134 and 135. Preferably, these speakers are located
to the right and left of picture tube 124 as shown to provide
a realistic stereo effect during viewing of the television
receiver.
.
- 23 -
3~
The sound channel of the television receiver is dual-
conversion in designt the first conversion stage being con-
tained in the tuner 120. For this application, with present
technology, a single-conversion sound channel would be sub-
5- stantially .inferior ~y reason of the high IF output frequency
(41.25 MHz) of available television tuners, and the difficulty
of building filters, amplitude-limiters, and FM detectors
capable of operating at that frequency while meeting the
stringent requirements of the IF amplifiers for high-fidelity
stereophonlc sound. Co.. ~ined-bandwidtll, pass-Dand phase-
. linearity, and skirt attenuation design requirements are
outside.of practical technical and/or consumer market economic
ranges, using present-day RLC, ceramic, and crystal filters,
. although it is contemplated that new filter technology may
15. ultimately meet these fiiter requirements. Lowering the
41.25 MHz output frequency of modern television tuners is
not an attractive alternative, since superior image rejection
and adequate video channel bandwidth are important advantages
of the higher IF fre~uency.
A dual-conversion sound channel retains the superior
image-rejection advantage o the standard 4L.25 MHz tuner
outpu~ frequency, while simultaneously exploiting the advan-
- tages of a low second-conversi.on IF output frequency to
achieve improved limiting and FM detection. Further~ore, a
dual-conversion sound channel more effectively isolates the
video and sound channels while independently optimizing both
. _ ~4 -
3~
by means of AFC and AGC signals derived in the respective
channels. It will be appreciated that frequencies other than
10.7 MHz may be utilized for the second IF channel for optimum
performance, the principal advantage of the 10.7 MHZ fre-
quency being for the present the ready availability of 10.7
MHz IF amplifier components.
Stereophonic television sound signals transmitted
in accordance with the invention of Application No. 271,170
can be received by a conventional FM stereo broadcast receiver
by means of the adapter 80 shown in Figure 14. The converter
includes an RF amplifier 81 to which the RF signal intercepted
by the receiving antenna 51 is applied, and a mixer stage 82,
wherein the amplified signal is heterodyned with a continuous
wave output signal from an oscillator 83. The RF amplifier 81,
mixer 82 and oscillator 83 together function as a tuner 84,
the operating frequency of RF amplifer 81 being adjusted to
the desired television broadcast channel and the frequency
of oscillator 83 being adjusted to operate at a frequency
removed from the television channel sound carrier such that
the sound difference frequency, when tripled, fall within
the 88-108 MHz FM broadcast band. In the illustrated embodi-
ment this intermediate frequency is 30 MHZ.
The intermediate frequency output signal from mixer
82 is applied to a 30 MHZ IF amplifier stage 85 wherein it is
amplified and amplitude-limited prior to being applied to a
- 25 -
1~45;~32
- tripler and~gQ M~z filter stage 36. To maintain the con- verter 80 centered on the desired channel the 30 ~1Hz output
- signal from IF amplifier stage 85 may be applied to an FM
- detector ~7 to develop an AFC signal for application to
oscillator 33~ The output of tripler 86, ~hich constitutes
a stereophonic signal having modulation characteristics
~-. similar to those of a standard stereophonic FM broadcast
signal, is applied to the antenna input terminal of a.con-
ventional FM stereo tuner (not shown). The output of the FM
stereo tuner, which consists of L and R audio output signals,
may be applied to a conventional stereo amplifier~ and then
to left and right loudspeakers which preferably are placed
on either side of the television screen on ~-Ihich the video
portion of the received broadcast is being viewed. A selector
switch (not shown) may be included in the output circuitry
........... ...... of adapter 80 to facilitate connecting the FM tuner to an
FM receiving antenna (not shown) when the adapter is not in use.
Since the frequency deviation of the third harmonic
of the 30 MHz IF signal is three times the 25 KHz maximum
d~iation of th~ TV sound carrier, the 75 ~Hz ma~im~m deviation
`prescribed for standard F~I broadcasts is obtainec in.the
resulting 90 MHz signal. For example, assuminS reception of
TV channel 11, the sound carrier of the received signal is
located at 203.75 MHz and the video carrier is at 199.25 MHz.
. 25 This dictates an oscillator frequency of 233.75 MHz, resulting
in an intermediate frequency video carrier at 3~.5 M~z and a
,
~ 26 -
3~:
sound carrier at 30 M~lz The 34.5 MHz video c'arrier is
eliminateh in the IF amplifier stage, leaving only .the 30
MHz sound carrier for tripling to 90 MHz in tripler 86, and
reception on F~ ~roadcast channel Zll at 90 MHz. Since the
, 5 pilot is, in accordance with the inventiont established at
; 19 XHz the same demodulator circuits utilized in the tuner
for demodulating standard F~ broadcasts serve to demodulate
the stereophonic television sound- signal,
Filters for use in the 30 MHz IF amplifier 85 are
within the practical design capabilities of recent surface-
wave technology, and provide a particularly good application
for a filter~of sin X configuration. F~ detection at the
30 MHz frequency is not a problem in this application, since
; that functlon is performed externally within the stereo tuner.
-The technique of increasing frequency'deviation
- by utilizing,a harmonic of the desired signal provides the
: basis for improving the performance of the limitor'and dis-
criminator stages of an FM receiver. This i5 because
increasing the frequency deviation of the modulated inter-
. mediate carrier'effectively increases the level of the developed
output signal. To illustrate application of this technique,
the second converslon from 41.25 MHz to 10.7 ~z in the pre-
viously described television.sound converter of Figure 8 can,
be accomplished by selecting 46.60 MHz as the frequency of
oscillator 72, thereby obtaini.ng a difference frequency Oc
5.35 MHz at the output of mixer 71. The 10.7 MHz IF amplifier
,
_ 27 -
iQ3;~:
53, beins now tuned to the second harmonic of 5.35 MHz,
provides twice the fre~uency deviation of the transmitted
signal to the FM detector 54 The amplitude of the 10.7 ~z
second harmoni.c thus extracted need not equal the amplitude
of the 5.35 MHz fundamental to rece~.ve the full ~enefit of
the increase'd'deviation for maximum signal-to-noise improvement.
All that is required is that it exceeds the minimum threshold
-level of IE amp~ifier 53 so that good limiting action is
obtained. It should be obvious to those skilled in the art
that by designing for other suitably lower difference frequency
outputs from mixer. 71, still higher order harmonics can be
extracted by the 10.7 MHz IF amplifier, yielding proportion-
ately increased frequency deviations.
In accordance with another aspect of the invention,
15 the stereopho'nic television sound system of the invention can
- be utilized for bi-lingual progran~ing.. As shown in Figure l5A,
assum'ing that the sound portion of a television broadcast is
to be broadcast simultaneously in two different lanyuages
A and B, the A sound source is connected through resistances
2a 150 and 151 to th-e inverting inputs of first and second
*ifferential ampli~iers 152 and 153, respectively The. B
; sound source is connected throu~h a resistance 154 to the
inverting input of amplifier 152 and through a resistance 155
to the non-inverting input of amplifier 153. The non-inverting
inputs of amplifiers 152 and 153 are connected to ~round by
resistances 156 and 157, respectively, and the inverting
.
~ 28 -
~ S~3~ l
inputs are connected to the outputs of their respective
amplifiers by-resistors 158 and 159, respectively. The
outputs of amplifiers 152 and 153 are connected to the L
and R audio inputs o~ the system stereo generator 147, which
may be identical in construction and operation to the stereo
generator 25 shown in Figure 4~
As a result of this matrixing arrangement language
B modulates what was formerly the 38 KHz L-R sub-carrier
channel, and Ianguage A modulates what was formerly the L+R
main channel. The l9 KHz pilot component is transmittea as
it was during the transmission o~ stereophonic program material.
At the receiver, as shown in Figure 15B, the L and R
audio outputs o~ the system stereo demodulator 148 are applied
through respectiv~ resistances 160 and 161 to the inverting
and non-inverting inputs of a differential amplifier 162.
- The output of amplifier 162 is coupled back to the inverting
input terminal by a resistance 164 and the non-inverting
input is connected to ground by a resistance 163. Resis-
tances 160, 16L, 163 and 164 form a matrix in combination with
amplifier 162 to generate- a signal corresponding to language B
at the output o~ the amplifier. Language A can be obtained
` at either of the output terminals of the stereo demodulator
- 148 by conditioning the demodulator for monophonic operation.
A three-pole three-position mode selection switch 165 may be
provided to select the signal to be amplified by an external
.two channel audio amplifier 166 and applied to loudspeakers 167
.
- 29 -
3~
and 168, and to condition the demodulator for monophonic
operation during reception of language A.
With this arrangement, it is contemplated that
language A would normally be the majority or domestic
language, since the L+R channel on which it is conveyed is
compatibly received by existing monaural television receivers.
At the receiving end the matrixing circuitry can be constructed
as an adapter 149 which can be readily added to or incorporated
in existing receivers, such as those depicted in Figures 6
and 8, to enable selective reproduction of either language A
or language B. It should also be noted that the bilingual
system can also be used in conjunction with standard FM stereo
broadcasts. In this case the adapter 149 is connected between
the L and R audio outputs of the stereo FM receiver and the
stereo amplifying system.
From the preceding discussion it will be realized
that the basic stereophonic television sound transmission
system described requires only the addition of a stereo FM
multiplex generator to existing television sound transmission
equipment, and the addition of a converter or adapter to existing
television receiving equipment. However, by modifying certain
parameters of the heretofore described system in accordance with
further features to be subsequently described, improved sound
transmission ispossible in conjunction with such existing
equipment. Such modifications are feasible at this time since
commercial
- 30 -
P3~
.
: .
. .
- stereophonic television ~roadcasts are presently non-existent,
: .
and engineering standards concerning such broadcasts have
not been established. Therefore, in anticipation of, and
. . .
as a basis for establishing such standards, it is appropriate
to examine the characteristics of the modulated sound carrier
. .
... .
generated by the tra~smission system in detail to determine
what standards provi~e for optimum transr~lission of stereo-
, ...... . . .
.
- phonic sound without detri~ent to picture quality.
Referring to Figure 3, the maximum ~re~uency devi-
. .: . . .
~` 10 ation of either L-R component is 50% that of the L+R main
.
.
channel component, this reduction being the result of the L-R
energy being spread over two sidebands which span twice the
--- bandwidth of the main channel. This has the effect of reducing
the modulation lndices of the L-R channel relative to the main
. . - .
h 15 channel. Moreover, the L-R channel modulation indices are
. . .
~ further reduced by the well known l/f decrease of the modula-
: . .
tion index with increasing modulation frequency. This is
graphically illustrated in logarithmic format by r igure 16,
wherein Curve A is a plot o~ modulation index vs. modulation
frequency (measured fro~ .he sound carrier) for the conditio~
of constant maximum frequency deviation (22 5 K~z). Curve R
- is a similar plot, except ~at, in accordance ~ith the above-
~, .
mentioned maximum frecuency deviation limi.s o, FigLre 3,
the modulation indices of the L-R channel are de~ressed 50
- 25 ~6 db.), while the L~R resion remains identical to Curve A.
- 31 -
~i 93~:
Modulation index curve B represents modulation at
the 100% level for the proposed transmission system based
upon a uniorm audio spectral energy distribution. In
practice the distribution of energy peaXs in audio program
material falls off with increasing frequency. I'his is shown
by Curve C, wherein only lower frequency peaks attain the
100~ modulation level of curve B. The form of curve C is
actualIy that of a de-emphasis network having a time constant
of 25 microseconds, that curve having been found to best
10~ approximate the energy distribution in modern audio programs
as shown by Ray M. Dolby, Optimum Use of Noise Reduction in
` ~M Broadcasting, Journal of the Audio ~ngineering Society,
Vol. 21, No. S, June 1973, and D. P. Robinson, Dolby B-Type
Noise Reduction for FM Broadcasts, Journal of the Audic
Engineering Society, Vol. 21, No. 5, June 1973. These
references demonstrate that the conventional 75 microsecond
time constant presently prescribed by U.S. F~l radio standards
is outmoded, being based on the frequency distribution of
program material as it existed at an earlier time using
~0 equipment and methods which are now obsolete.
Referring a~ain to Figure 16, curves X and Y depict
the effect on frequency response o~ pre-emphasis networks
having respective time constants of 75 and 2S microseconds-
- in the L~R region, the L-R region having ~een omitted for
reasons of clarity. Curve D illustrates .he effect of a 75
microsecond pre-emphasis network on modern program material
- 32 -
~ L5~3;~ \ -
- (~s represented by curve C). Curve D is obtained by sub-
. tracting curve C from cur~e X, with curve ~ as the baseline.
Over-modulation is that portion of cur~e D which exceeds the !~
100% modulation line (curve B), being prominent at high
audio modulation frequencies of both the L~R and L-R bands.
It can be concluded from curve D that the result
of applying the 75 microsecond pre-emphasis required by U.S.
standards in present-day FM broadcasting has been overcom-
pensation of the high frequencies, requiring either amplitude F
~ limiting of hLgh ~requencies r or substantial under-modulating
of mid and low frequencies to a~oid over-modulation of the
transmitter. The penalty in the irst instance is diminished
high frequency response when the program material is de-
emphasized prior to reproduction at the receiver. The penalty
in the latter instance is reduced broadcast coverage.
In interpreting curve D it shollld be considered that
- the ordinate in Figure 16 is the phase modulation angle of
the sound carrier, which may be taken as an indication of
tolerance by the transmitted signal to noise interference
0 along the transmitter-receiver radiation path. Curve D
reveals that, when 75 microsecond pre-emphasis i5 applied,
the overall noise tolerance of the L-R sidebands ls substan-
tially inferior to that of the main channel, the modulation
frequencies in the region of 38 XHz being particularly defi-
cient. This latter deficiency, together with over-modulation
of high audio frequencies, are present-day proble~is of FM
stereo broadcasting.
- . ~ 33 - -
3~
~ 75 microsecond pre-emphasis were to be adopted
as a standard for stereophonic television sound, the problem
of L-R noise susceptibility would be more serious than is
presently the case for F,~l stereo broadcasts, since FM broad-
- 5 cast standards specify 75 KHz as the maximum frequency
deviation, whereas the corresponding ma~imum specified for
television sound transmission is only 25 KHz. For comparison,
curve E, which results from subtracting curve C from curve Y,
with curve B as the baseline, shows the effect of applying
25 microsecond pre em,phasis to the representative program
content of curve C. It will be noted that curve E coincides
at all frequencies with curve B,which can be interpreted as
lndicating that, with 25 microsecond pre-emphasis, the program
can be transmitted with essentially 100% modulation at all
frequencies. It is also apparent that adopting 25 micro-
second pre-emphasis eliminates the 38 KHz-centered noise
tolerance deficiency and high frequency over-modulation
characteristics of 75 microsecond pre-emphasis. However,
the overall L-R noise suscèptibility (relative to that of the
L+R main channel) remains low, as evidenced by the 9.6 dh
drop between the-low point (15 XHz) of the L+R channel and
the high point (23 KHz) of the L-R channel, as shown in
Figure 16.
. . . One way to increase L-R noise tolerance is shown
by curve F, wherein L-R signal amplitude has been increase`d
by a factor of 3` (9.6 db) relatlve to curve E, ~hile the L+R
_ 3~ -
3~
channel amplitude remains unchanged. Another way to improve
L-R noise susceptibility is illustrated by curve G, which
results from first enchancing the L-R portion of curve E by
a factor of 2 (6 db), then applying 7.5 microsecond pr~-
emphasis to the overall stereo composite signal. This raises
the relative modulation level of the stereo channel and
adjusts the slope to increase the relative signal strength
at the high frequency end of the L-R spectrum. Since pre-
emphasis having a 7.5 microsecond time constant has its
greatest effect above approximately 20 KHz, its effect on
the L~R main channel is minimal.
Thus, the basic transmission system of Application
No. 271,170 may be significantly improved with respect to
sound fidelity, broadcast coverage, and signal-to-noise ratio
of the L-R channel, by 1) employing an audio pre-emphasis
time constant of 25 microseconds rather than the 75 microsecond
time constant required by the present FM and TV broadcast
standards, 2) enhancing only the L-R component of the composite
stereo signal, while maintaining the L+R main channel unchanged,
and 3) applying additional pre-emphasis to the composite
signal in combination with selective enhancement of the L-R
region to adjust or eliminate, as desired, the negative slope
of the L-R spectrum.
Figure 17A shows cixcuitry for enchancing the ampli-
tude of the L-R channel without affecting the L+R main channel,
~5~;~Z
thereby achieving the improved signal-to-noise performance
- characteristic of curve F in Figure 16. In this further
-- aspect of the invention, additional circuitry 140 is inter-
posed between the L and R program sound sources and the L
` 5 and R audio inputs of the stereo multiplex generator 25 at
- the studio. Alternatively, this additional circuitry could
be incorporated within the multiplex generator itself, the
internal pre-emphasis circuits of which may be converted to
a 25 microsecond tlme constant. At the receiver additional
- 10 circuitry lkl, which compensates for the effect of the trans-
mitter circuitry 140, is added as shown in Figure 17B. The
receiver compensating circuitry 141 may be a simple adapter
- connected between the L and R audio output terminals of the
- receiver and the corresponding inputs of an external stereo
amplifier/speaker system, or may be incorporated within the
stereo demodulator 55 of the receiver.
The enhancement circuit shown in Figure 17A utilizes
t~Q differential amplifiers 142 and 142, Amplifier 142 has
its nan-inverting input connected to the L sound source and
amp~Ifie~ l~ has its non-in~ertlng input connected-to the
R sound source_ The output of amplifier 142, henceforth
designated L', is~connected to the left audio input of the
multiplex generator 25, and by an impedance Za to its invert-
ing input. The output of amplifier 143, henceforth designated
R', is similarly connected to the right audio input of the
multiplex generator 25 and by an impedance Zb-to its inverting
.. . . . . _ . . . . .............. . .. . . .
_ 36 _
\
3Z
input. The inverting inputs of amplifiers 142 and L43 are
interco~nected by an impedance Zc.
If the three impedances are arranged to be resistive
and of equal value (Za = Zb = Zc), the L-R audio amplitude
increases by a factor of 3, while-the L+R audio amplitude
remains unchanged. This follows since
` - L' = 2L-R R' = 2R-L
- and L ' + ~ = 1 and L - R' = 3
L+R L-R
1-0 The relationship is shown in the following tabulation for
~arious input combinations wherein each unit corresponds to
an 11.25 KHz deviation:
TABLE 1
L R L' R' L+R L'+R' L-R L'-R'
1 1 - 1 1 2 2 0 0
0 0 2 6
1 0 2 -1 1 1 1 3
~ 1 -1 2 1 1 -1 -3
As shown in Figure 17B, a compensating circuit 141 consisting
2~ o three Lmpedances Za', Zb', and Zc' and a pair of audia
amplifTers ~44 and 145 may be provided at the receiver to
restore the h and R audio signals to the amplitude relation-
ship they ha* prior to the L-R enhancement introduced at the
transmitter. The L' audio output signal from demodulator 55
is coupled to the input of audio amplifier 144 by impedance Za'
and the R' audio output signal from the demodulator is coupled
37
3~
by impedance Zb' to the input of audio amplifier 145. The
inputs of amplifiers 144 and 145 are connected together by
impedance Zc'. As in the previously described en~odiments,
the composite signal developed within the converter or
S adapter is applied to demodulator 55.
If, as illustrated above, Za, Zb, and Zc are made
equal and resistive in the enhancement circuit, Za', Zb', and
Zc' will also necessarily be equal and resistive, although
they need not have the same absolute resistance as Za, Zb
and Zc. ~ith this arrangement the L and R audio output signals
at the recei~er will be restored to the same amplitude reIa-
. tionship they had prior to L-R enhancement at the transmitter.
. - Although the 9.6 db tfactor of 3~ enhancement of
the L-R channel has been shown and discussed, it will be
lS appreciated that by selecting other .values for Za,.Zb, and Zc
at the studio, a greater or lesser enhancement of the L-R
component can be achieved. For example, for the factor of 2
(6 db) enhancement illustrated by curve G of Figure 16, it
is necessary that R~-ZRb=2Ra with the result that
2~ L/ = 3L-R R' - 3R-L
` 2~ . 2.
and L+R -- =1 and L R = 2
As described earlier, curve G results from the
-25 combination of a 6 db enhancement, with a 25 microsecond audio
pre-emphasis then applied prior to multiplexi~g, followed by
- 38 -
3~
... . .
.. an additional pre-emphasis applied to the composite stereo
. signal after multiplexing. As shown in Figure 18A the enhance-
. .
- ment and audio pre-emphasis may be accomplished by means of a
.. circuit 170 situated ahead of the system stereo multiplex
.... .
S generator 25 (assuming no pre-emphasis within the generator),
.. . . . .
. while a pre-emphasis network 171 for the composite signal may
. be located between the multiplex generator and the trans-
- mitter modulator/ampli~ier circuits 30. For curve G the
... : . .
.~ composite signal pre-emphasis time constant is 7.5 micro-
seconds.
. . .
. Referring to Figure 18B, to compensate at the
. . .
.. receiver for the pre-emphasis of the comp~site signal intro-
~- duced a~ the. transmitter, a 7.5 micros.econd de-emphasis
- network 17.2 may be incorporated ahead of the demodulator 55.lS. Audio de-emphasis an~ the earlier described audio de-enhance-
. ment circuits of the receiver may be incorporated in a
; . circuit 173 at the output of demodulator 55.
. It will be appreciated that t~e composite signal
conditions represented by curves F and G of Fisure 16 have
; 20 been presente~ as a.means of illustrating various techniques
which may ~ combined-in v~rious degrees to shape the com-
posite.signal spectrum so as to.optimize stereophonic per-
formance, It.is anticipated that such techniques would
ultimately be defined as parameters in a yet to be adopted
stereophonlc te1evision sound standard.
.
. . .
. . - 39 -
It should be understood that the curves of Figure 16
are idealizations, in that they represent the case for
sinusoidal signals oE prescribed amplitude plotted one fre-
quency at a time. By contrast, audio programs originate as
S complex signals of constantly changing amplitude and frequency.
Modulation envelopes of complex signals tend to blend as a
continuum and to thereby obscure details of the underlying
signals such as the 8 K~z gap which separates the L~R and
L-R components ln Figure 16. For this reason, observed complex
10 . signal modulated spectra Lor the stereophonic tel~vision sound
transmission system appear somewhat as shown in the expanded
television sound channel portrayed in Figure 5. Referring
to that figure, modulation of the sound carrier with a com-
posite signal, such as developed in the basic (non-enhanced)
`transmission system of this invention, results in the genera-
tion .of an RF signal 180 at the upper end of the television
channel (Figure 2) having a maximum overall bandwidth of
S0 K~z during monophonic (L+R only~ transmissions, and an RF
signa.l.181 having a maximum overall bandwi~th of 106 KHz
during stereophonic transmissions. Mod~latio~ of the sound
carrier Z6 with the composite signal developed when the basic
transmission system.has bee~ modifi.ed to incor~orate .the~
circuitry o~ Figure 17A, so as to provide 9.6 db enhancement
of the L-R component, results in generation of an RF signal
182 having shoulders above those of the envelope of RF
.signal 181.
. _ _ . . ... . .
0 -
3æ
--- The conditions depicted in Figures 3 and 16
- represent extreme modulation limits for the L~R and L-R
. :.:. .
components, those limits being mutually exclusive in the
- .
sense of being attainable in only one channeI at a time,
`: 5 and only under the uncommon circumstance that the other
:.......... .
channel equates to 0 at that instant. For this reason, most
.
--- of the audio content of the program is developed at levels
.*
-- below these limits, being susceptible therefore to environ-
, .......................... .
mental ~lectrical`noise to a still greater degree. One way
. , .
to furt~er improve noise rejection for`the low level signals
.;.` . .
` is to process the program content before transmission in a
- m~nner that raises the average modulation of the composite
~.................. . .
- signa~ closer- to 100%, and to then compensate at the receiver
with reciprocally matched circuitry to restore the original
conditions. One such system, the Dolby Type B noise reduction
system, is findlng increasing commerclal application in FM
stereophonic radio broadcasting for reducing high frequency
over-modulation and raising the average modulation level of
~` the transmitted program.
Basically, the tran~er chaIacteristic of the Dolby
B system i5 such as to enhance low-level high frequency
` signals, the degree of low level enhancement increasing as
: .
a functio~ Q~ frequency. Since the Dolby B system has been
-- detailed promlnently in the literature, only the relevant,
`- 25 qualitative features are noted herein. Tnese are summarized
in Table 2, wherein various configurations, consisting of
,
- 41 -
.
~L~4~3;~ ( -
various transmisslon pre-emphasis and reception de-emphasis
time constants are compared with and without Dolby Type B
transmission and reception units with respect to maximum
modulation level for high fidelity transmission (0 db = 100
modulation),.high fidelity capability, and relative signal-
to-noise performance.
TABLE 2
i~aximum
Configuration Relative Net
System Transmitter Receiver l~lodulation High Relative
~ ~o. Pre-Emphasis* De-Emphasis* Level Fidelitv S~N
1 75 75 -8.3 db Yes -0 9 dh
2 25 75 0 db No ~2.7 db
3 25 25 0 db Yes -~2.7 db
4 Dolby & 75 75-8.~ db No ---
Dolby ~ Z5 75 0 db No
15 : 6 Dolby & 25 25 0 db No ---
7 Dolby & 25 25 & Dolby 0 db Yes ~12.3 db
* = microseconds
As.can he seen in Table 2, the canfigurations are
listed ln order o~ increasing signal-to-noise (S/N) ratio,
20 . althou~hj for configurations 4, 5 and 6 the improvement has
dubious merit~ since the result ls distortion of the audio
signal due to over-emphacis o~ high frequencies. Only. configu-
rations 1, 3, and 7, for which the de-emphasis is truly
complementary, are capable of high fidelity reproduction; and
of these, only configurations 3 and 7 can transmit.all of ~he
program material at essentially full modulation. Configuration 3
. . . .. .
_ _ .
- 42 -
3~ ~ -
was described ~arlier as the preferred 25 microsecond pre-
emphasis/de-emphasis system for the basic transmission/reception
system of the invention. Configuration 7, which incorporates
complementary Dolby noise-reduction, may be implemented in the
system of configuration 3 without modification, resulting in
a 13.2 db S/N improvement as compared with the complementary
75 microsecond pre-emphasis standard for FM broadcasts.
Furthermore, without system modification, Dolby Type B noise
reduction may be combined with the previously described L-R
io enhancement and composite signal pre-emphasis techniques for
further noise reduction to the extent permitted by whatever
frequency deviatlon limits are ultimately adopted as a
standard for stereophonic television sound transmission~
Figure l9A shows the application of Dolby Type B
noise reduction to the ~asic transmission system of the inven-
tion, wherein two Dolby B processors 185 and 1~6 at the
transmitter encode the pre-emphasized L and R audio signals
prior to application to the system stereo generator 25. At
the receiver, as shown in Figure 19B, the L and R audio outputs
of the rece~r stereo demodulator stage 55 are applied through
respective de-emphasis net~orks 187 and 188, which i~ accord-
ance with the previous discussion have a time constant of
25 microseconds, to respective Dolby B processors 189 and 1~0,
which decode the L and R audio signals. The de-emphasis
networks and processors together provide compensation to
restore the de-emphasized L and R audio signals from demodulator
55 to their original relationship.
.
- 43 -
33~
., .
The encoding and decoding processors may be basically
identical in construction, differing only in the manner in
whlch the input signal is routed, as shown in Figure l9C.
The input signal traverses two paths; a main path through a
combining network 191 and an lnverter 192, and a secondary
path through a voltage-responsive variable-frequency filter
193, a signal amplifier 194, and an overshoot suppressor 195.
The main path passes the input signal essentially unchanged_
` The secondary path is essentially a filter which passes only
low-level, high-frequency components of the input~signal
During encoding, the output of the secondary path is added
to the main path, boosting the low-level, high-frequency por-
tions of the input signal. During decoding, the output of
- - the secondary path is subtracted from the main signal path
output, a result of the secondary path input belng sensed as
-the inverted output of the processor. Decoding thus removes
the same inormation to the same degree as was inserted
during encoding.
The characteristics of the secondary path variable
filter 1~ are determined by a feedback loop consisting of a
contro~ amp~fier 196 and recti~ier/integrator 197. For
signal amplitudes which do not exceed a fixed threshold, no
feedback signaL is generated, and the transfer function of the
filter is simply that of a fixed, 500 Hz high pass filter.
The threshold level is fixed at approximately 40 db below
Dolby level, an internationally standardized reference
3æ ~ -
corresponding to a ~requency deviation of + 37.5 K~lz for FM
broadcasting. A similar reference would be established for
television sound at 50% of the maximum frequency deviation
allowed for the television sound carrier. The gain of control
amplifler 196 is a non-linear function of frequency, so that
as signal amplitudes increase above the threshold level nega-
tive feedback raises the.variable filter cut~off frequency
in a non-linear manner, reaching a constant, maximally.
restricted ~andwidth for input signals near Dolby level. The
10. overall effect is negligible at low frequencles an~ at levels
approachLng full modulation, but increases with increasing
frequency and decreasing amplitude.
. . In another en~odlment of the invention, enhancement
of low level signals is accomplished in a manner similar to
15 that of .the ~olby Type B system, in that the degree of enhance-
ment also lncreases as a function of f~equency. However,
unlike Dolby Type B, enhancement of low level signals occurs
only above 2Q K~zr an~ i5 applied to the composite signal,
being provided immediately following the system stereo multi-
plex generator.25, as with the pre-emphasis netwark 1~l
provided for the composite signal in Figure 18A~ Complementaxy
circuitry i5` installed at the receiver just prior to the s.tereo
demodulator SS, as with the composite signal de-emphasis
circuits 172 in Figure 18B. Applied in this way, only the
.25 L-R channel is affected, the effect being to raise the modu-
lation level (and hence the noise tolerance) of low level L-~
. . .
- 45 -
S~3;2
signals without increasing the modulation level of high-level
signals.
Since neither commercial stereophonic television
. broadcasts nor suitable con~ercial receivers for receiving
5 such broadcasts are in existence, no problem of obsolescence
of existing equipment exists in adopting the proposed compatible
transmission system. Receivers ~r adapters to reproduce
television stereophonic sound in accordance with the trans-
mission/reception system of the invention may be manufactured
with the preferred 25 microsecond time constant, and may in
fact immediately incorporate Dolby Type B receptIon circuitry,
since such circuitry is already commercially availabIe in
economical ~C packaging. As for compatibIlity with existing
monophonic tele~ision receivers which have 75 microsecond
sound de-emphasis, it is doubtful that any unfavorable change
in tAe repraduced sound signal could be perceived, sInce
very few television receivers are capable of high fidelity
receptlon At any rate, F~ broadcasts using the previously
discussed.configuration.5 system indicate that many monophonic
TV listenersl would actually perceive the sound quality as
. `improved bècause ofi its -increased high frequency con~ent
25
- 46 -
~ S~32
To better enable Dolby-B processor-equipped receivers
tq receive non-Doiby transmissions a remote switching
signal may be added to the composite signal to control the
decoding processors within the receivers. This switching
signal may take the form of a sub-sonic fixed-frequency
signal in the 10 to 25 Hz range which may be selectively
added to either the composite signal output of the stereo
generator 25 ~Figure 1) or generated within the stereo
generator whenever the transmission includes Dolby-B process-
ing. The subsonic tone would be detected by a c~ntrol tone
sensing circuit 198 (Figure l9c~ ln the receiver and utilized
to control the Dolby-B processing stage(s) therein.
-For example, a subsonic 20 Hz signal could be
generated at a fixed amplitude corresponding to 25 Hz fre-
quency deviation o~ the sound carrier The corresponding
modulation index (1 25) of this inaudible component would be
60 db below the 100~ modulation level (25 XHz), being there-
fore innocuous to other intelligence and functions At the
receiver, a narrow-band frequency detector would respond
to this 20 Hz component by generating a DC control signal
suitable for switching or otherwlse conditioning the receiver
for Dolby-~ decoding.
The above-described subsonic switching signal
requires no additional bandwidth and, combined with the con-
trol possibilities of the 19 Khz pilot component constitutes
a flexible means for conveying conditions of transmission.
- 47 -
3~
, . .
:::
-'- This is shown in Table 3 which illustrates the four trans-
- mission format conditions possible with the pilot and
-.
- subsonic signals.
. . .
-. TABLE 3
.:.
Control Signals
.
--' Pilot Subsonic Signal Transmission Format
~- Yes - No Stereophonic, 25 Microsecond
--- preemphasis
:........ . .
-- No No Monophonic, 75 Microsecond
-' - preemphasis
-:. 10
-.` No Yes Monophonic, Dolby, 25 Micro-
-. second preemphasis
~.~ Yes Yes ' Stereophonic, Dolby with 25
-`~' Microsecond preemphasis
. It will be appreciated that more than one subsonic switching
i .
-- signal may'be transmLtted to accomplish additional control
. 15
- functions, and that the aboue-described subsonic switching
-~ technique is also applicable to conventlonal FM stereo broad-
~ cast transmissions.
. .
' The system of the present invention enables
.. . .
. stereophonic'sound to be broadcast over commercial television
: 20
`~ channels and' faithfully and compatably reproduced in cQn-
.`~ ' junctio~ wit~ conventional existing television receivers.
~ The system requires a minimal amount of additicnal transmittin~
~ equipment an~ minimal modification of èxisting receiving
`` equip~ent. With modification the system may also pro~ide
`- for compatable broadcast and reception of bilingual television
.
` programming.
.
'l'elevision sound transmissions in accordance
with the lnvention may be received on converters, either'of
- 48 -
3~:
the type wherein channel selection is accomplished independ-
ently of the associated television receiver on which video
information is being reproduced, or by means of the tuner
contained in the receiver. Such converter may provide a low
.level audio signal for amplific.ation on an external stereo
amplifier/speaker system, or may provide high level audio
and/or speakers for direct sound reproduction. A variation
of the converter allows reception by means of a conventional
stereo FM broadcast receiver. Alternatively, adapters may
be integrally installed in existing-or newly constructed
television rece;vers to achieve stereophonic sound reception.
. Improvements in the signal-to-noise ratio of the
basic t~a~smission system are possible by
. 1) Adopting a 25 microsecond preemphasis/deemphasis
.15 time constant,
.2) Enhancing the L-R component of the composite
.signal,
3) Applying preemphasis and deemphasi~ to the
composite signal,
`~ . 4~ -Applying Dolby ~ype- signal processing to the
L and ~ au~io si~nals, and~o~
5) Applyin~ Dolby Type-~ s~gnal pxocessing to the
L-R component a~ the co~posite signal.
Further improvement is contemplated by selecting a pilot
component having a frequency equal to 5/4 F~l to reduce the
. _ : . . . . . . .
- 49 -
number o~ audible interference bands which result from hori-
zontal scanning frequency harmonics within the L-R component
sidebands.
'' '
'
.
.. . . . . _ _ . . . . . . . . . - 50 -
