Method and apparatus for NTSC signal interference cancellation using recursive digital notch filters

A system for attenuating NTSC interference present in an in-phase television signal and a quadrature-phase television signal, each of said in-phase and quadrature-phase television signals including a HDTV signal and an NTSC interference signal, the system comprising:

• a complex recursive digital notch filter having a first input for receiving the in-phase television signal, a second input for receiving the quadrature-phase television signal, an in-phase signal output, a quadrature-phase signal output, and a notch centered at the carrier frequency of a preselected NTSC signal component of the in-phase and quadrature phase television signals for attenuating the preselected NTSC signal component of the in-phase and quadrature-phase television signals;
• wherein the complex digital notch filter includes:
• a complex recursive passband filter circuit having an in-phase signal input coupled to the first input of said complex recursive digital notch filter, a quadrature phase signal input coupled to the second input of said complex recursive digital notch filter, an in-phase NTSC interference signal component output, a quadrature-phase NTSC interference signal component output and a passband centered at the carrier frequency of the preselected NTSC signal component for isolating the preselected NTSC signal component by passing the preselected NTSC signal component;
• and
• a first summer having a summing input coupled to the first input of said complex recursive digital notch filter, a subtracting input coupled to the in-phase NTSC interference signal component output of the complex recursive passband filter circuit, and an output coupled to the in-phase signal output of said complex recursive digital notch filter, said first summer for subtracting an in-phase NTSC interference component signal output by the complex recursive passband filter circuit from the in-phase television signal to attenuate the preselected NTSC signal component of the in-phase television signal.

A method of attenuating an NTSC interference signal from a complex television signal, the complex television signal including an in-phase television signal and a quadrature-phase television signal, each of said in-phase and quadrature phase television signals including a HDTV signal, the method comprising the steps of:

• providing a complex recursive digital notch filter having a complex television signal input, a complex filtered television signal output, and a notch centered at the frequency of a preselected NTSC interference signal component;
• supplying the complex television signal to the complex input of the complex recursive digital notch filter;
• monitoring the complex television signal to detect the presence of the preselected NTSC interference signal component;
• and
• operating the complex recursive digital notch filter to filter the complex television signal to attenuate the preselected NTSC interference signal component only when the presence of the preselected NTSC signal interference signal component is detected.

A method of attenuating an NTSC interference signal present in a complex television signal, the complex television signal including an in-phase television signal and a quadrature-phase television signal, each of said in-phase and quadrature phase television signals including a HDTV signal and an NTSC interference signal, the method comprising the steps of:

• providing a complex recursive digital notch filter having a complex television signal input, a complex filtered television signal output, and a notch centered at a frequency of a preselected NTSC interference signal component;
• supplying the complex television signal to the complex input of the complex recursive digital notch filter;
• and
• operating the complex recursive digital notch filter to filter the complex television signal to attenuate the preselected NTSC signal component and to generate a complex filtered television signal;
• the step of operating the complex recursive digital notch filter including the steps of:
• estimating the power level of the preselected NTSC signal component;
• adaptively adjusting the gain of the complex recursive digital notch filter as a function of the estimated power level of the preselected NTSC interference signal component to remove approximately all of the preselected NTSC interference signal component.

A filter for removing a preselected NTSC interference signal component from an intercarrier frequency (I.F.) television signal, the filter comprising:

• a fixed gain recursive digital notch filter having a notch centered at the nominal carrier frequency of a preselected NTSC signal component of the television signal and adapted for coupling to a tuner for receiving the intercarrier frequency television signal, for isolating the preselected NTSC interference signal component from the received television signal by passing the preselected NTSC signal component and for outputting the isolated NTSC signal component;
• a summing circuit adapted for coupling to the tuner and selectively coupled to the recursive digital notch filter for receiving the isolated NTSC signal component and for subtracting the isolated NTSC signal component from the intercarrier frequency television signal;
• and
• means for detecting the carrier frequency of the preselected NTSC signal component and for generating a frequency error Signal when the detected carrier frequency of the preselected NTSC signal component differs from said nominal carrier frequency.

A filter for removing a preselected NTSC interference signal component from an intercarrier frequency (I.F.) television signal, the filter comprising:

• an adjustable gain recursive digital notch filter having a notch centered at the nominal carrier frequency of a preselected NTSC interference signal component of the television signal and adapted for coupling to a tuner for receiving the intercarrier frequency television signal, for isolating the preselected NTSC interference signal component from the received television signal by passing the preselected NTSC interference signal component and for outputting the isolated NTSC interference signal component;
• a summing circuit adapted for coupling to the tuner and coupled to the recursive digital notch filter for receiving the isolated NTSC interference signal component and for subtracting the isolated NTSC interference signal component from the intercarrier frequency television signal;
• means for detecting the carrier frequency of the preselected NTSC interference signal component and for generating a frequency error signal when the detected carrier frequency of the preselected NTSC interference signal component differs from said nominal carrier frequency;
• and
• means for adjusting the intercarrier frequency of the television signal as a function of the frequency error signal, to thereby adjust the carrier frequency of the preselected NTSC interference signal component to correspond to the nominal carrier frequency of the preselected NTSC interference signal component.