| 摘要 |
Timing in a time division multiplex video cassette recorder is improved and skew jump error is virtually eliminated by recording video signals with a superimposed pilot signal synchronized to-but having a frequency greater than-the incoming horizontal sync pulse frequency. During playback, a local pilot signal-similar to the previously recorded superimposed pilot signal-and a local horizontal sync signal control the de-multiplexing process and timing. The local pilot and horizontal sync signals are controlled by a three-tiered feedback timing control loop: a phase lock loop varies a local clock signal in order to correct very fine time differences between the recorded and local pilot signals; a digital loop changes the frequency ratio between the clock signal and the local pilot signal to correct larger time differences between the recorded and local horizontal sync pulse signals; and a look-ahead up/down counter counts and stores the net cumulative change in the pulse repetition frequency ratio. The ratio is changed by the entire cumulative amount during (or just prior to) the first horizontal line (only) of each video field as a very large automatic correction which anticipates skew jump error. The phase lock loop is virtually unaffected by interruption of the digital control loop (due to temporary drop-outs of the imcoming horizontal sync pulse signal), and the systm timing is therefore impervious to dropouts or noise.
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