| 摘要 |
Radio receivers for receiving DTV signals, in accordance with the Advanced Television Systems Committee (ATSC) standard, or analog TV, in accordance with the National Television Sub-Committee (NTSC) standard, each use a single first detector for both types of signal. This single first detector supplies its output signals to an intermediate-frequency amplifier chain for TV signals and to another intermediate-frequency amplifier chain for analog TV signals. The response of the IF amplifier chain for DTV signals is synchrodyned to baseband, supplied to symbol decoding circuitry, and used by first automatic gain control circuitry to develop AGC for amplifier stages in that IF amplifier chain when DTV signals are received. The response of the IF amplifier chain for analog TV signals is supplied to a video detector, and second automatic gain control circuitry develops AGC for amplifier stages in that IF amplifier chain from the composite video signal reproduced by the video detector when analog TV signals are received. In some of the radio receivers the sound carrier of the NTSC signal has a separate, further intermediate-frequency amplifier chain. The absence of 4.5 MHz intercarrier sound is detected when DTV signals are received to automatically fine tune the first detector in response to pilot carrier and to obtain delayed AGC for the radio-frequency amplifier from the first AGC circuitry. The presence of intercarrier sound is detected when analog TV signals are received, to automatically fine tune the first detector in response to video carrier and to obtain delayed AGC for the radio-frequency amplifier from the second AGC circuitry.
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