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BACKGROUND OF THE INVENTION
The present invention relates to a new and improved television audio
receiver system and is more particularly directed to apparatus and a
method for receiving and reproducing stereo television sound.
Under present television broadcasting standards, a band of frequencies
approximately 80 KHz wide is designated within each 6 MHz television
channel for the transmission of the audio component of a television
signal. Within this band of frequencies, an RF main audio carrier signal
is frequency modulated by an audio baseband signal for producing a main
aural audio transmission signal. The transmitted main aural audio signal
is received by a television receiver that converts the RF audio carrier
signal to a signal having a frequency centered at 4.5 MHz. The converted
4.5 MHz sound carrier is then processed by an FM detector to reproduce the
main aural audio signal that was used to frequency modulate the RF audio
carrier at the transmitter.
The Federal Communications Commission has recently approved a standard for
broadcasting stereophonic television sound. The standard approved uses the
Zenith Broadcast Delivery System and the dbx noise-reduction system. This
system was, in part, an outgrowth of the well-known techniques for
transmitting stereophonic audio signals which has been popular in radio
broadcasting for some time. The basic FCC-approved system for stereophonic
radio broadcasting is disclosed in U.S. Pat. No. 3,257,511 to R. Adler, et
al. In this system, the arithmetic sum of left (L) and right (R) audio
source signals (L+R), commonly referred to as the main channel modulation,
is used to directly frequency modulate the RF carrier signal. The
difference between the left and right stereophonically related signals
(L-R) is used to amplitude modulate a 38 KHz subcarrier signal in a
suppressed carrier fashion with the resultant double-sideband signal being
impressed as frequency modulation on the radiated RF carrier. In addition,
a pilot subcarrier signal of 19 KHz is transmitted for synchronization of
the FM receiver. The FM receiver extracts the 19 KHz pilot subcarrier,
doubles its frequency, and applies the resulting 38 KHz signal to a
synchronous detector where the (L-R) difference signal is recovered from
the amplitude modulated 38 KHz stereophonic subcarrier. The recovered
(L-R) modulation is then suitably matrixed with the (L+R) main channel
modulation in order to recover the original left and right stereophonic
signals.
The foregoing stereophonic radio broadcasting system often also includes an
SCA component which allows broadcasters to provide a subscription
background music service. The SCA component comprises a 67 KHz subcarrier
frequency modulated by the background channel program, the frequency
modulated subcarrier being used to frequency modulate the main RF carrier
signal together with the stereophonic modulation.
Various systems and apparatus have been proposed for the transmission of
stereophonic sound together with a conventional television picture
transmission. These systems normally utilize the radio broadcasting
stereophonic transmission techniques discussed above but with, in most
cases, different subcarrier frequencies selected for their compatibility
with the transmitted video signal. One such prior art system is disclosed
in U.S. Pat. No. 4,048,654 to Wegner. This patent discloses a transmission
system in which a composite baseband signal identical to that employed in
FM stereophonic radio broadcasting is employed to frequency modulate the
main sound carrier of a television transmission signal. Thus, the proposed
composite baseband signal includes an (L+R) main channel component, an
amplitude modulated double-sideband suppressed-carrier 38 KHz subcarrier
(L-R) component and a 19 KHz pilot component. In another embodiment, the
use of a subcarrier signal having a frequency (f.sub.H) characterizing the
transmitted video signal is proposed in lieu of the 38 KHz (L-R) channel
subcarrier to reduce interference from the video component of the
television signal.
Another system, which was proposed in U.S. Pat. No. 3,099,707 to R. B.
Dome, also employed the conventional stereophonic radio broadcasting
system but with an (L-R) channel subcarrier equal to 1.5f.sub.H and a
pilot signal equal to 2.5f.sub.H. These frequencies were selected to
minimize the effect of the video components of the television signal
appearing in the recovered sidebands of the (L-R) channel signal.
U.S. Pat. No. 3,046,329 to Reesor discloses yet another similar system in
which the composite baseband signal used to frequency modulate the main
sound carrier includes only the main channel (L+R) component and the upper
sidebands of the (L-R) channel signal amplitude modulated on a subcarrier
having a frequency of 2f.sub.H. Other prior art system for stereophonic
television sound transmission have proposed the use of frequency modulated
subcarriers for the (L-R) stereo channel typically centered at 2f.sub.H,
although a center frequency of 1.5f.sub.H has also been proposed.
As previously mentioned, in addition to transmitting stereophonic sound
components on the main aural carrier of a transmitted television signal,
it is also desirable to transmit additional information thereby more
completely exercising the available audio bandwidth within a television
channel. For example, the transmission of a second audio program ("SAP")
signal would enable a viewer to selectively operate a television receiver
for reproducing the audio signals associated with the transmitted
stereophonic information, or alternatively, the audio signals associated
with the transmitted second audio program which may comprise, e.g., a
foreign language version of the television program.
One prior art proposal for providing a second language capability in
connection with a transmitted television signal is disclosed in previously
mentioned U.S. Pat. No. 4,048,654 to Wegner in which the two channels of a
stereophonic-like signal are employed. In particular, the (L+R) main
channel signal is used to transmit a first language audio signal and the
(L-R) stereo channel signal is used to transmit a second language audio
signal. U.S. Pat. No. 3,221,098 to Feldman discloses a transmission system
allowing for the simultaneous broadcast of a single television program
having up to four or more different language soundtracks by forming a
composite baseband signal consisting of four or more different subcarrier
signals each amplitude modulated with a different language audio signal,
the composite baseband signal being used to frequency modulate the main RF
audio carrier. Yet another proposed second language system uses a
frequency modulated subcarrier baseband signal centered at 2f.sub.H for
both stereophonic sound transmission and for second language transmission.
A pilot signal, modulated with one of two different frequencies, is used
to indicate which service is being broadcast.
The foregoing systems and techniques for transmitting different audio
signals in conjunction with a standard television transmission were not
adopted in the U.S. for a number of reasons including, in certain cases,
poor performance and, in others, incompatibility with U.S. television
transmission standards.
The concept behind the Zenith stereo broadcast system adopted in the U.S.
is disclosed in U.S. Pat. No. 4,405,944 to Eilers et al. This system
comprises an audio transmission system that is fully compatible with U.S.
television broadcasting standards and is capable of providing stereophonic
sound transmission together with a second audio program service.
In the Zenith stereo broadcast delivery system, audio information is
located in the region from about 4.4 to 4.6 MHz above the video carrier of
a television channel allocation. The audio portion takes up only about
0.20 MHz, which is small compared to the large portion of bandwidth
occupied by the video (luminance and chroma) signal. In the past, a
monophonic audio channel was transmitted as an (L+R) FM signal with a
frequency range of 50-15,000 Hz. In the Zenith system, a pilot signal has
been added at the horizontal scanning line frequency f.sub.H (15.734 kHz)
to allow new stereo receivers to locate a second channel for stereo, which
resides from 16.47-46.47 kHz (centered at 2f.sub.H) from the bottom of the
audio allocation. This second channel is the key to receiving stereo
sound, as it is an (L-R) AM signal with the same frequency range as the
mono channel. Stereo is achieved when the L-R and L+R signals are
combined.
A third channel, the second audio program or "SAP", is provided in the
Zenith system for bilingual programming and other commentary. The SAP
channel is FM and extends from about 65 to 95 kHz (centered at 5f.sub.H)
with a frequency range of 50 Hz to 12 kHz. Professional channels which may
be used for voice or data can be inserted into the remaining audio space
of about 98.2 kHz to 106.5 kHz (centered at 6.5f.sub.H). Several types of
sound channel processing for these audio signals at the home television
receiver are known.
One such processing technique is provided by a "separate aural carrier
receiver", in which the aural carrier is processed separate from the
visual carrier. Since the aural carrier is transmitted without incidental
phase modulation ("ICPM"), none can reach the FM detector so that this
receiver can be free of all video related buzz.
A second known receiver for television sound channel processing is referred
to as the "split sound receiver". This method of sound processing was used
in the early days of television before intercarrier detection was
introduced. The video and sound portions of a received television signal
are down converted to a lower frequency and the sound component of the
composite signal is pulled off and processed to provide an audio output.
In the split sound receiver technique, tuner-introduced ICPM can cause low
frequency noise in the sound output.
Neither separate aural carrier receiver techniques nor split sound receiver
techniques can be used in a cable television environment due to the high
FM noise in the oscillators used to down convert the television signal.
Expensive oscillators with separate tuning systems would be required to
overcome this problem, and thus the techniques are not economically viable
in cable television systems.
A third known type of sound channel processing is referred to as the
"quasi-split sound receiver". In this technique, separate processing of
the sound and video signals is used, but with synchronous detection
combined with intercarrier sound detection. Such a receiver is disclosed
in U.S Pat. No. 4,405,944 referred to above. Nyquist ICPM is eliminated in
the quasi-split sound receiver by a specially designed IF filter with
symmetrical response centered at the video carrier. Although this type of
receiver is relatively immune to tuner-introduced ICPM, microphonics,
local oscillator phase noise, reverse mixer feedthrough to the tuner of
local oscillator, and to video related frequency modulation caused by the
AFC/AFT circuits, it suffers from distortion caused by interfering
harmonics of the television horizontal line frequency. Such harmonics fall
within the pilot signal, the (L-R) subchannel and the SAP signals.
It would be advantageous to provide an apparatus and method for receiving
stereo broadcast television sound which avoids such interference. Such an
apparatus and method should be able to be used in the cable television
environment and remain uneffected by harmonics of the television
horizontal line frequency, as well as phase noise due to jitter in the
cable television converter and local oscillator tuning loop.
The present invention provides an apparatus and method with these
advantages, through the use of two separate receivers for the (L+R) signal
and the pilot, (L-R), and SAP signals. The result is a substantially
improved quality of television stereo sound reception.
SUMMARY OF THE INVENTION
In accordance with the present invention, an apparatus and method are
provided for receiving and reproducing stereo television sound. A
transmitted television signal includes an audio component comprising a
main carrier signal frequency modulated in accordance with a composite
modulation function. This function has a first component comprising the
sum of first and second stereophonically related audio signals, and a
second component comprising a double sideband suppressed carrier signal,
formed by amplitude modulating a first subcarrier having a frequency
2f.sub.H in accordance with the difference between said stereophonically
related audio signals. The frequency f.sub.H is the horizontal scanning
line frequency associated with the horizontal synchronization signal of
the transmitted television signal.
The apparatus comprises input means responsive to the transmitted
television signal for developing a first signal corresponding to the
composite modulation function. Intercarrier detector means are coupled to
receive the first signal for detecting the portion thereof corresponding
to the first component of the composite modulation function and producing
a first audio output signal comprising the sum of the stereophonically
related audio signals. Independent FM detector means are coupled to
receive the first signal for detecting the portion thereof corresponding
to the second component of the composite modulation function and producing
a second audio output signal comprising the difference of the
stereophonically related audio signals. Means are coupled to receive the
first and second audio output signals for producing therefrom a first
channel audio output and a second channel audio output.
The apparatus can further comprise means coupled to the input means for
converting the first signal to a first intermediate frequency for input to
the intercarrier detector, and for converting the first signal to a second
intermediate frequency for input to the independent FM detector means.
The composite modulation function can include a third component comprising
a second subcarrier having a frequency 5f.sub.H modulated in accordance
with a third audio signal. The independent FM detector means can then be
used to selectively detect the portion of the first signal corresponding
to the third component and produce a third audio output signal therefrom.
At least one of the first and second audio output signals can be delayed to
provide equalization therebetween. Such delay enables the first channel
audio output and second channel audio output to be in proper phase.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphic representation of the frequency spectrum of the
composite baseband audio signal which is part of a transmitted television
signal; and
FIG. 2 is a functional block diagram of the receiver apparatus of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, FIG. 1 is a graphical representation of the
multichannel sound baseband signal 10 approved for use in television
broadcast signals. The baseband signal includes a main channel component
12 occupying a band which extends from 50 Hz-15 KHz. The main channel is
modulated by the left channel plus right channel (L+R) audio. A subcarrier
stereo subchannel 16 centered at twice the television horizontal scanning
frequency f.sub.H of 15.734 KHz is suppressed carrier amplitude modulated
by the left minus right (L-R) audio channels. A second audio program
channel 18 is provided at 5f.sub.H. A pilot subcarrier is inserted at the
horizontal scanning frequency f.sub.H. The pilot signal is used in prior
art receivers to facilitate recovery of the (L-R) subcarrier by
synchronization of the FM receiver. In such prior art receivers, the pilot
signal at f.sub.H is extracted, doubled, and applied to a synchronous
detector where the (L-R) difference signal is recovered from the amplitude
modulated subcarrier at 2f.sub.H. This technique, which uses the
intercarrier detector also used to detect the (L+R) signal, is prone to
intercarrier phase modulation whereby video information is detected by the
sound detector. Such video information interferes with the pilot and
subcarrier areas at f.sub.H, 2f.sub.h, etc.
The present invention overcomes this problem by using intercarrier
detection to arrive at the (L+R) signal together with an independent FM
detector, not phase locked to the video signal, for receiving the pilot,
the (L-R), and the SAP signals. In such manner, harmonics of the
television horizontal line frequency are avoided. The desired output of
the independent FM detector is of a higher frequency than that of the
intercarrier detector, where phase noise due to jitter in a cable
television converter and local oscillator tuning loops is minimal.
FIG. 2 illustrates a cable television stereo sound adapter for reproducing
stereophonic sound transmissions in accordance with the present invention.
Those skilled in the art will appreciate that the teachings of the present
invention can be used to receive stereophonic sound from direct broadcast
television signals as well as over cable television systems. The
television signal containing the composite modulation function illustrated
in FIG. 1 is received at terminal 20 which is coupled to a bandpass filter
22 and a microprocessor 66. The signal input at terminal 20 can include
data, e.g., from a cable television remote control, that is used by
microprocessor 66 to execute various functions selected by a user. For
example, a user can select to receive the main stereophonic signal or the
alternate SAP channel, and microprocessor 66 will output a signal on line
67 to execute the user's choice. Remote control of the sound volume is
accomplished via a digital to analog converter 70 that is accessed by
microprocessor 66 to control a volume control circuit 60. A display 68
associated with microprocessor 66 is provided to give a user visual
feedback as to the selections made via the remote control.
Bandpass filter 22 is used to separate the video and multichannel sound RF
signals from any other signals (such as the data signals referred to
above) present at input terminal 20. The video and multichannel sound RF
signal is passed from bandpass filter 22 to an RF amplifier 24. The
amplified RF signal is then coupled to a mixer 26 which, in combination
with local oscillator 28, converts the frequency of the amplified RF
signal to a new "intermediate" frequency. If, for example, the signal
output from RF amplifier 24 contains components at 41.25 and 45.75 MHz,
and local oscillator 28 runs at 30.55 MHz, the signal output from mixer 26
will contain frequency components at 15.2 MHz and 10.7 MHz.
The output from mixer 26 is input to a bandpass filter 32 with a center
frequency of 15.2 MHz (video carrier) and a bandpass filter 34 with a
center frequency at 10.7 MHz (audio carrier). Bandpass filter 32 can, for
example, comprise a simple L-C filter while bandpass filter 34 is
preferably a ceramic filter.
The output of bandpass filter 32 is input to a video detector 30, which
also includes an automatic frequency control ("AFC") and automatic gain
control ("AGC") detector. The AFC and AGC signals are applied to local
oscillator 28 and RF amplifier 24, respectively. Video detector 30 also
receives the output of bandpass filter 34. The resulting output from
detector 30 contains the main channel component 12 of the audio carrier
signal having a center frequency of at 4.5 MHz (15.2 MHz-10.7 MHz). The
converted 4.5 MHz sound carrier is then processed by an FM detector 48,
after being filtered by a 4.5 MHz bandpass filter 46, to reproduce the
(L+R) audio signal that was used to frequency modulate the audio carrier
at the television signal transmitter.
The output from FM detector 48 is input to a delay equalizer 56 that
provides equalization between the (L+R) signal and the (L-R) signal that
is retrieved by a separate independent FM detector as described below.
After the (L+R) signal is delayed by an appropriate time period, it is
passed through a 15 KHz low pass filter to limit the signal to the audio
frequencies to be ultimately reproduced. This signal is then input to a
conventional matrix 54 that combines it with the (L-R) signal to reproduce
a right channel audio signal at terminal 62 and a left channel audio
signal at terminal 64.
In order to recover the pilot, (L-R), and SAP signals, the output of
bandpass filter 34 is passed through a second (preferably ceramic)
bandpass filter 36 that provides additional filtering for high fidelity
sound reproduction. The output of filter 36 is input to a 10.7 MHz FM
detector 38, that recovers the (L-R) and SAP components 16 and 18,
respectively, illustrated in FIG. 1. The detected signal is passed to a
demodulator 40 that selectively demodulates the AM (L-R) signal at 31.5
KHz (2f.sub.H) or the FM SAP signal at 78 KHz (5f.sub.H). Demodulator 40
utilizes a 50 KHz low pass filter 42 for demodulating the (L-R) component
or, alternately, a 78 KHz bandpass filter 44 to demodulate the SAP
component. Selection of the component to be demodulated is made via a mode
control signal outputted by microprocessor 66 on line 67.
The demodulated output from demodulator 40 is input to a 15 KHz low pass
filter 50 that limits the signal to the band of audio frequencies to be
reproduced. The signal is then input to a standard dbx noise reduction
circuit 52 and passed to matrix 54 where the (L-R) signal is combined with
the (L+R) signal to produce the desired right and left audio channels.
It will now be appreciated that the present invention provides reproduction
of stereo broadcast television sound without distortion due to video
information interfering with the pilot signal, (L-R) subcarrier, and SAP
subcarrier components of the multichannel sound baseband signal. In
accordance with the present invention, the best (L+R) channel performance
is achieved using a quasi-parallel intercarrier detector. Improved (L-R)
channel and SAP channel performance is achieved by using a separate sound
detector instead of the same intercarrier detector used for recovering the
(L+R) component. In this manner, interfering harmonics of the television
horizontal line frequency with vertical scan frequency sidebands that fall
within the pilot, the (L-R) subchannel, and the SAP signals are prevented
from distorting the stereo audio or SAP output. By using the higher
frequency components only of the separate FM detector, where phase noise
due to jitter is minimal, and recovering the (L+R) channel using a
standard intercarrier detector at lower frequencies (where phase noise
would be severe), the quality of television stereo sound reception is
greatly improved.
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