| 摘要 |
<p>A vertical synchronisation signal detector is disclosed for detecting a vertical synchronisation signal in a composite video signal. The detector comprises an input (1) for receiving the composite video signal, a first detector (4, 16, 22) for detecting equalisation pulses occurring in the composite video signal and for generating a first detection signal upon detection of an equalisation pulse, a second detector (4, 16, 20) for detecting horizontal sync pulses occurring in the composite video signal and for generating a second detection signal upon detection of a horizontal sync pulse, a third detector (4, 12) for detecting negative going pulses occurring in the composite video signal that have a longer duration than the horizontal sync pulses and for generating a third detection pulse upon detection of such a pulse, a first signal combination unit (FF1) for generating a fourth detection signal (VBI) in response to at least one first and second detection signal generated by the first and second detector respectively, a first counter (58) for counting, when being enabled, the number of occurrences of second detection signals generated by the second detector and for generating a first control signal upon the detection of n occurrences of such second detection signals, where n is a constant integer larger than 1. Further, a second signal combination unit (AND) is provided for combining the third and fourth detection signals so as to obtain a second control signal. A third signal combination unit (FF2) receives the first control signal generated by the first counter and the second control signal from the second signal combination unit and generates in response thereto an output signal which is indicative of the occurrence of the vertical synchronisation signal. The first counter is in an enabled state upon the presence of the output signal of the third signal combination unit (FF2).</p> |
