Signal to noise ratio enhancement using baseband signals in an FM television system

Abstract

In an FM television signal angle modulated with luminance and chrominance information, there exists a high degree of correlation between picture elements occurring one line period apart. In an effort to steer the pass band of a tunable bandpass filter to the instantaneous frequency of the incoming television signal, a steering signal is developed which utilizes this high degree of correlation. For NTSC systems, luminance information is delayed by one-line period (63.5 us in the U.S.A.) and combined with suitable delayed chrominance information to form the filter's steering signal. For MAC systems, one-line delayed luminance and two-line delayed chrominance information are alternately applied to the filter as its steering signal. By steering the pass band of the filter to the incoming signal, maximum signal will be captured with minimum noise, enhancing the signal-to-noise ratio of a color television system.

What is claimed is: 
     
       1. An apparatus for improving the signal-to-noise ratio of a television signal angle modulated with line and field scanned luminance information, comprising: limiter means receiving the television signal, for limiting the amplitude of the television signal to a predetermined value;   a voltage tuned bandpass filter, having a center frequency tunable by a tuning signal, to filter the limited signal from said limiter means;   discriminator means for demodulating the filtered signal;   feedback means for delivering the demodulated luminance information to said voltage tuned bandpass filter as the tuning signal, said feedback means comprising delay means for delaying the luminance information by one line.   
     
     
       2. An apparatus as claimed in claim 1, wherein the tuning signal includes substantially all of the demodulated luminance information. 
     
     
       3. An apparatus as claimed in claim 1, wherein the television signal is further angle modulated with line and field scanned chrominance information, and wherein said feedback means further comprises means for combining the one-line delayed luminance information with the chrominance information. 
     
     
       4. An apparatus for improving the signal-to-noise ratio of a television signal angle modulated with line and field scanned luminance and chrominance information, comprising: limiter means receiving the television signal, for limiting the amplitude of the television signal to a predetermined value;   a voltage tuned bandpass filter, having a center frequency tunable by a tuning signal, to filter the limited signal from said limiter means;   discriminator means for demodulating the filtered signal;   feedback means, comprising: a first delay means for delaying the luminance information by one-line,   a second delay means for delaying the chrominance information by two-lines, and   controllable switching means for selectively applying either the one-line delayed luminance information signal or the two-line delayed chrominance information signal to said voltage tuned bandpass filter as the tuning signal.     
     
     
       5. An apparatus as claimed in claim 4, further comprising: means for generating a control signal to be applied to said controllable switching means. .[.6. A method of tuning a tunable filter, to the instantaneous frequency band occupied by a previous incoming amplitude limited television signal angle modulated with line and field scanned luminance information, by a tuning signal, said method comprising:   filtering the limited signal by said tunable filter about a center frequency controllable by the tuning signal;   demodulating the filtered signal;   delaying the demodulated signal by a period substantially corresponding to one-line period, producing a tuning signal; and   applying the tuning signal to said tunable filter so as to adjust said filter's center frequency based on incoming signal's instantaneous center   
     
     
        frequency which occurred one-line period prior in time..].7. A method of tuning a tunable filter, to the instantaneous frequency band occupied by a previous incoming amplitude limited television signal angle modulated with line and field scanned luminance and chrominance information, by a tuning signal, said method comprising: filtering the limited signal by said tunable filter about a center frequency controllable by the tuning signal;   demodulating the filtered signal;   delaying the demodulated signal by a period substantially corresponding to one-line period, producing a one-line delayed signal;   delaying the one-line delayed signal by a period substantially corresponding to one-line period, producing a two-line delayed signal;   alternately applying the one-line delayed signal and the two-line delayed signal to said tunable filter so as to adjust said filter's center frequency based on the incoming signal's instantaneous center frequency which occurred alternately one-line period and two-line periods prior in   
     
     
        time. 8. An apparatus for improving the signal-to-noise ratio of a television signal angle modulated with line and field scanned luminance information, comprising: a voltage tuned bandpass filter, having a center frequency tunable by a tuning signal, to filter an amplitude limited signal;   discriminator means for demodulating the filtered signal;   feedback means for delivering the demodulated luminance information to said voltage tuned bandpass filter as the tuning signal, said feedback means comprising delay means for delaying the luminance information by one line.   
     
     
        . An apparatus as claimed in claim 8, wherein the tuning signal includes 
     
     
        substantially all of the demodulated luminance information. 10. An apparatus as claimed in claim 8, wherein the television signal is further angle modulated with line and field scanned chrominance information, and wherein said feedback means further comprises means for combining the one-line delayed luminance information with the chrominance information. 
     
     
          . An apparatus for improving the signal-to-noise ratio of a television signal angle modulated with line and field scanned luminance and chrominane information, comprising: a voltage tuned bandpass filter, having a center frequency tunable by a tuning signal, to filter an amplitude limited signal;   discriminator means for demodulating the filtered signal;   feedback means, comprising: a first delay means for delaying the luminance information by one-line,   a second delay means for delaying the chrominance information by two-lines, and   controllable switching means for selectively applying either the one-line delayed luminance information signal or the two-line delayed chrominance information signal to said voltage tuned bandpass filter as the tuning     
     
     
        signal. 12. An apparatus as claimed in claim 11, further comprising: means for generating a control signal to be applied to said controllable   
     
     
        switching means. .Iadd.13.  A method of tuning a tunable filter, to the instantaneous frequency band occupied by a previous incoming amplitude limited television signal angle modulated with line and field scanned luminance information, by a tuning signal, said method comprising: filtering the limited signal by said tunable filter about a center frequency controllable by the tuning signal;   demodulating the filtered signal;   delaying the luminance information of the demodulated signal by a period substantially corresponding to one-line period, producing a tuning signal; and   applying the tuning signal to said tunable filter so as to adjust said filter's center frequency based on incoming signal's instantaneous center frequency which occurred one-line period prior in time. .Iaddend.