Using special NTSC receiver to detect when co-channel interfering NTSC signal accompanies a digital tv signal

A method comprising the steps of:

• receiving a digital television signal apt at times to be accompanied by a co-channel interfering analog television signal having a video portion;
• synchrodyning the video portion of any said co-channel interfering analog television signal to baseband, for generating an in-phase demodulation result including first artifacts of said digital television signal, and for generating a quadrature-phase demodulation result including second artifacts of said digital television signal;
• phase shifting said quadrature-phase demodulation results by 90.degree. at frequencies in a prescribed frequency range extending below 750 kilohertz;
• linearly combining the resulting phase-shifted quadrature-phase demodulation results with said in-phase quadrature-phase demodulation results, to generate a linear combining result substantially free of said first and second artifacts of said digital television signal in said prescribed frequency range;
• and
• detecting the amplitude of said linear combining result in said prescribed frequency range exceeding a prescribed value, for generating an indication that said digital television signal is accompanied by co-channel interfering analog television signal of substantial amplitude.

A digital television receiver including circuitry for detecting times when analog television signal of substantial amplitude occupies a television broadcast channel, which circuitry comprises:

• input circuitry for selecting from a television broadcast channel a vestigial sideband amplitude-modulation signal descriptive of a video signal portion of any analog television signal that occupies said television broadcast channel, converting the selected vestigial sideband amplitude-modulation signal to an intermediate frequency signal, and amplifying said intermediate frequency signal to provide an amplified intermediate frequency signal, said vestigial sideband amplitude-modulation signal as originally received by said input circuitry including a video carrier and full sideband in addition to a vestigial sideband;
• video synchrodyning circuitry for synchronously detecting said amplified intermediate frequency signal with respect to said video carrier signal and with respect to a carrier in quadrature phase with said video carrier signal, for generating an in-phase synchronous detection response and for generating a quadrature-phase synchronous detection response;
• first phase shift circuitry for phase shifting by substantially 90.degree. all frequency components of said quadrature-phase synchronous detection response above a prescribed frequency to generate a first phase shift circuitry response;
• first linear combining circuitry linearly combining said in-phase synchronous detection response and said first phase shift circuitry response for recovering first linear combining circuitry response to a portion of said video signal described both in said full sideband and said vestigial sideband, which first linear combining circuitry response is substantially free of response to any digital television signal that occupies said television broadcast channel;
• and
• a threshold detector for determining when said first linear combining circuitry response exceeds a prescribed threshold value, for generating indications that co-channel analog television signal is of substantial amplitude.