| 摘要 |
<p>A circuit for use with a vectorscope to indicte SC/H phase of a composite video signal comprises an oscillator (28) for generating a continuous wave signal at subcarrier frequency and having its phase locked to the horizontal sync point of the video signal, and a switch (30) for connection between the output terminal of the filter (14) of the vectorscope and the second inputs of the demodulators (18,20) of the vectorscope and for connection also to the oscillator, for alternately connecting the subcarrier burst from the filter of the vectorscope and the sync locked continuous wave from the oscillator to the demodulators. When the subcarrier burst is connected to the demodulators, the vectorscope provides an indication at a predetermined angle of the polar display of the vectorscope, whereas when the sync locked continuous wave signal is connected to the demodulators the vectorscope provides an indication at a position of the polar display that is angularly spaced from a predetermined axis of the polar display by an angle that depends upon the difference in phase, within the subcarrier cycle, between the sync locked continuous wave signal and the subcarrier burst.</p> |
