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.
