Importance

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Abstract

Digital data having a symbol rate that is a multiple of horizontal scan rate are buried in broadcast television signals. In a digital signal receiver the data are separated from composite video signal by quadrature video detection followed by comb filtering. The comb filtering is most economically realized by digital sampling at symbol rates. The regeneration of clocking signals at symbol rate, and at multiples of symbol rate where oversampling analog-to-digital conversion (ADC) techniques are employed, is done using a controlled oscillator with automatic frequency and phase control (AFPC) responding to the horizontal synchronizing pulses transmitted in the broadcast television signals. The horizontal synchronizing pulses are usually much larger than noise, so the controlled oscillator frequency and phase is rapidly adjusted following energization or channel change of the digital signal receiver. A fine adjustment of the phase of the oscillations from the controlled oscillator is made to minimize intersymbol error. This adjustment is made indirectly, by adjustably delaying the horizontal synchronizing pulses applied to the AFPC, so there is no discontinuity in the application of clocking signals to ADC circuitry and to comb filtering circuitry.

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First Claim