| 摘要 |
<p>Each television in a cable system has a multi-standard receiver (120) for receiving signals in predefined formats, each employing a hidden coupling convertor (104). Each television includes a raster scan CRT and control circuitry (112) that controls the sweep pattern generated by the CRT, and which control circuitry includes an h sync circuit that synchronizes each sweep of the CRT electron beams with an h sync signal in the received signal, and generates a distinctive voltage at the beginning of each sweep of the CRT. A filter internal to the convertor extracts the h sync component of the TV signal on a user-selected channel. A comparator compares the extracted h sync with any signals received from an external antenna, the resultant signal indicating whether the TV is both turned on and tuned to the selected channel.</p> |
