Video decoder having lock algorithm that distinguishes between a noisy television signal input and a video recorder signal

Abstract

A phase-locked loop is provided which is operable to lock the sampling clock (pixel clock) to the incoming horizontal sync pulse contained within composite video information. Given the input signal is a VCR signal or a normal noise-free signal, there exists two modes of operation, coarse lock mode and fine lock mode, which are used in controlling the phase-locked loop. In the coarse lock mode, coarse corrections are made to a horizontal discrete time oscillator so that a fast lock may be achieve using the fine lock mode. Coarse corrections are based on a normalized sum of weighted pixels collected within a narrow gate window. Lock is achieved when the falling edge is centered within the window. Given the input signal is a television signal having noise, there exists one mode of operation where the flat window phase detector is used instead of coarse lock mode to bring the sync edge to fall within the window, where the flat window normalization constant is tuned.