Television sound signal processor

An IF signal filter in the IF sound channel of a television receiver exhibits peak amplitude responses which are unsymmetrical and slightly offset from the nominal picture and sound carrier frequencies by an amount dictated by the amount of tilt introduced by the tuner and the IF signal processing section. As a result, a filtered IF signal applied to a sound demodulating system exhibits a substantially symmetrical characteristic with respect to both the sound and picture carrier frequencies, yielding reduced audio buzz and harmonic distortion.

This invention concerns apparatus in a television receiver for improving 
the quality of sound reproduced by the receiver. 
The need for improved quality of sound reproduced by a television receiver 
has increased with the advent of wider bandwidth signals processed by the 
sound channel of television receivers designed with multichannel sound 
capability. Multichannel sound capability, such as is associated with 
stereophonic and bilingual broadcasting, involves the use of one or more 
audio subcarriers for forming an aural baseband signal. The aural baseband 
signal requires that the television audio signal bandwidth be increased to 
approximately 110 KHz or more as compared to the 15 KHz bandwidth of a 
monophonic audio program. As a result of the wider bandwidth of the sound 
processing channel of the television receiver, objectionable audio buzz 
produced in the sound channel tends to be more noticeable. Harmonic 
distortion of the sound signal is also more noticeable in wideband 
systems, and should be reduced along with audio buzz. 
Audio buzz, a well-known phenomenon, may be defined as the result of 
picture related modulation which is transferred to the sound signal. More 
specifically, buzz results when picture related phase modulated components 
of the picture carrier are transferred to the sound channel. Considerable 
buzz occurs at the horizontal line scanning frequency and multiples 
thereof. Although buzz is more noticeable in receivers with wider 
bandwidth multichannel (e.g., stereophonic) sound capability, buzz may 
also occur in monophonic systems in amounts sufficient to be 
objectionable. The phenomenon of audio buzz in a television receiver 
system is discussed in detail in an article by P. Fockens et al. titled 
"Intercarrier Buzz Phenomena Analysis and Cures", published in IEEE 
Transactions on Consumer Electronics, Vol. 27, August 1981, pp. 381-396. 
In a television receiver sound channel, harmonic distortion can be produced 
as a result of demodulating an FM sound carrier, such as a 4.5 MHz 
intercarrier sound signal, when such carrier exhibits nonsymmetrical 
sidebands. 
In a disclosed embodiment of the invention, apparatus for improving the 
quality of reproduced sound is associated with an intermediate frequency 
(IF) sound detection system, sometimes called "quasi-parallel" IF, in 
which sound and video signals are separately demodulated in different 
channels. In the sound channel the picture IF carrier is mixed with the 
sound IF carrier signal to form a 4.5 MHz intercarrier sound signal. The 
intercarrier sound signal is subsequently demodulated to produce an audio 
signal which, after processing, is conveyed to a sound reproducing 
loudspeaker. 
The sound channel includes a filter which receives the sound and picture 
carriers prior to demodulation. In accordance with the principles of the 
present invention, the sound channel filter exhibits an 
amplitude-versus-frequency transfer function with a peak amplitude 
response at a frequency which is slightly offset from the sound carrier 
frequency. In a disclosed embodiment, the sound channel filter also 
exhibits a peak amplitude response at a frequency which is slightly offset 
from the picture carrier frequency. The offset peak frequencies assist to 
substantially compensate for an amplitude-versus-frequency "tilt" 
characteristic such as is often associated with preceding IF signal 
processing circuits including the tuner of the receiver. The sound channel 
filter provides an input to a sound demodulating system which is 
substantially symmmetrical with respect to both the sound and picture 
carrier frequencies as required to produce a properly demodulated sound 
signal. The described arrangement according to the principles of the 
present invention not only significantly reduces sound buzz, but also 
reduces distortion in the demodulated sound signal. 
In an illustrated preferred embodiment of the invention, the sound channel 
filter is a Surface Acoustic Wave (SAW) IF filter having a transfer 
function which is nonsymmetrical with respect to each of the offset 
frequency peaks.

The single FIGURE of the drawing illustrates a portion of a television 
receiver including IF sound signal processing apparatus in accordance with 
the present invention. 
A broadcast television signal received by an antenna 10 is applied to a 
tuner 12. Tuner 12 selectively translates the RF signal of a selected TV 
channel to an intermediate frequency (IF) signal including picture and 
sound carriers at, e.g., 45.75 MHz and 41.25 MHz, respectively, in the 
NTSC system. The IF picture carrier is an amplitude modulated (AM) 
vestigial sideband signal containing the composite video information. The 
IF sound carrier is a frequency modulated (FM) signal. Tuner 12 exhibits 
an amplitude (A) versus frequency (f) transfer function with respect to 
the picture and sound carriers as shown by the insert above tuner 12. The 
tuner response exhibits a "tilt" characteristic, i.e., a deviation from a 
flat bandpass characteristic around frequencies of interest, as indicated 
by a difference in amplitude around the sound carrier at 41.25 MHz and 
around the picture carrier at 45.75 MHz. The amplitude response of the 
tuner exhibits a "haystack" characteristic in its passband, placing the 
picture carrier and the sound carrier on opposite slopes of the 
"haystack". As a result, the sidelobes of the sound and picture carriers 
are not symmetrical about their respective carrier. 
The IF output signal from tuner 12 is applied to a network 14 including a 
preamplifier and a 47.25 MHz adjacent channel sound trap which prevents 
automatic fine tuning (AFT) circuits of the receiver from locking onto the 
adjacent channel sound carrier signal rather than the carrier signal to 
which the receiver is tuned. The adjacent channel sound trap also exhibits 
a tilt characteristic around the 41.25 MHz sound carrier and around the 
45.75 MHz picture carrier, as shown by the insert above network 14. The 
output signal from network 14 exhibits a combined tilt characteristic 
including that of network 14 and tuner 12. The combined tilt 
characteristic affects the symmetry of the aural sidebands around the 
sound carrier, and the symmetry of the video sidebands around the picture 
carrier. 
The IF signal from the output of network 14 is split into two separate 
channels for sound and picture information demodulation in accordance with 
the quasi-parallel principle by applying it to two bandpass filters 15 and 
16. The output signal from network 14 is applied to filter 15 in a picture 
(video) signal demodulation channel, and to filter 16 in a sound signal 
demodulation channel. Filters 15 and 16 in this example are of the surface 
acoustic wave (SAW) type, but may be of the discrete 
inductance-capacitance type. However, filters 15 and 16 preferably are SAW 
filters rather than discrete inductance-capacitance type filters because 
the structure and parameters of a SAW filter permit easier, more 
predictable tailoring of the filter response to achieve a desired 
combination of peak frequency, shape and slope characteristics. 
Filter 15 has a response, as shown in the insert above filter 15, which 
matches the vestigial sideband video IF signal and which attenuates the 
41.25 MHz sound carrier signal. A differential output signal from filter 
15 is applied to differential inputs of a video demodulator network 20 
which in this example is included in an integrated circuit 18 such as type 
T2899 manufactured by Toshiba Corporation. A bandpass filter tank circuit 
21 operatively associated with video demodulator 20 is tuned to the 
picture carrier frequency of 45.75 MHz. Demodulator 20 includes circuits 
which, in conjunction with bandpass filter 21, form a quasi-synchronous 
detector that produces an output baseband composite video signal. The 
baseband video signal is applied to video signal processing circuits in a 
video processor 29 for producing R, G and B color image representative 
signals as known. 
As will be discussed subsequently, in accordance with the principles of the 
present invention sound channel filter 16 has a response, or transfer 
function, as shown in the insert below filter 16. Filter 16 exhibits a 
double tuned response with a first peak amplitude response which is 
nonsymmetrical around a frequency of 41.18 MHz slightly offset from the 
sound carrier frequency of 41.25 MHz, and a second peak amplitude response 
which is nonsymmetrical around a frequency of 45.95 MHz slightly offset 
from the picture carrier frequency of 45.75 MHz. 
A differential output signal from filter 16 is applied to differential 
inputs of a sound demodulator network 22 also included in integrated 
circuit 18. A bandpass filter tank circuit 25 operatively associated with 
sound demodulator 22 is tuned to the 45.75 MHz picture carrier frequency. 
Demodulator 22 includes circuits which, in conjunction with bandpass 
filter 25, form a mixer for providing a 4.5 MHz FM intercarrier sound 
signal from the sound and picture carriers. A bandpass filter 23, which 
exhibits a substantially symmetrical amplitude characteristic with respect 
to the 4.5 MHz center frequency of the intercarrier signal, couples the 
4.5 MHz FM intercarrier sound signal from the output of demodulator 22 to 
an FM demodulator 24. A demodulated baseband audio signal from the output 
of FM demodulator 24 is applied to a sound signal processor network 28 
which includes, for example, a stereo decoder which provides L (left) and 
R (right) baseband audio signals to audio amplifiers (not shown). 
The amplitude-versus-frequency response (transfer characteristic) of filter 
16 in the sound channel compensates for the overall system IF "tilt" 
exhibited by preceding IF signal processing circuits, so that a resulting 
signal applied to the input of sound demodulator (mixer) 22 is 
substantially symmetrical in amplitude with respect to the 45.75 MHz 
picture carrier, and a resulting signal applied to the input of FM 
demodulator 24 is substantially symmetrical in amplitude with respect to 
the 4.5 MHz aural intercarrier signal The amplitude symmetry of the 
demodulator input signal with respect to the 45.75 MHz picture carrier 
results in significantly reduced audio buzz In this regard it is noted 
that buzz results from incidental phase modulation produced when the video 
IF signal component having unequal (unsymmetrical) sidebands is mixed with 
the sound IF component. The peak amplitude response of filter 16 at 45.95 
MHz, in the vicinity of the 45.75 MHz picture carrier, compensates for the 
effect of the IF tilt of preceding circuits upon the picture carrier 
frequency, such that the picture carrier presented to the input of sound 
demodulator 22 desirably exhibits symmetrical sidebands rather than 
unsymmetrical sidebands likely to produce audio buzz. 
The amplitude symmetry of the input signal to FM demodulator 24 with 
respect to the 4.5 MHz intercarrier signal advantageously reduces the 
harmonic distortion which would otherwise be produced by the sidebands of 
the 4.5 MHz FM aural intercarrier signal having a nonsymmetrical amplitude 
response around the 4.5 MHz carrier. The amplitude symmetry of the input 
signal to FM demodulator 24 with respect to the 4.5 MHz intercarrier 
signal advantageously reduces the harmonic distortion inherent to the 
decoding process of an FM modulated signal. 
Thus there is disclosed an IF sound channel with an IF filter exhibiting 
unsymmetrical peak amplitude responses which are slightly offset from the 
picture and sound carrier frequencies, in accordance with the amount of 
tilt in the IF signal passband encompassing the sound and picture carrier 
frequencies, so that a resulting filtered IF signal as applied to an input 
of a demodulator system exhibits a substantially symmetrical amplitude 
characteristic with respect to both the sound and picture carrier 
frequencies. As a consequence of the disclosed filter arrangement, the 
television receiver exhibits improved sound quality with reduced audio 
buzz and harmonic distortion. Apparatus employing the present invention 
may be used in a television receiver as discussed, as well as in a video 
cassette recorder.