| 摘要 |
In a television receiver a downconverter responding to a selected television signal respectively supplies its first and second output signals as the input signals of first and second IF amplifiers, respectively. The downconverter (12) includes a local oscillator (13) for generating oscillations at a frequency controlled in response to the AFT signal. The first IF amplifier (17) includes a first adjacent-channel sound carrier trap, in consequence whereof the first-IF-amplifier response is substantially reduced for the downconverted picture carrier, and an in-channel sound carrier trap. Responsive to the first-IF-amplifier response, a video detector (18, 19) generates a composite video signal in which sound beats are suppressed to relatively low level by the first adjacent-channel sound carrier trap. A second IF amplifier (21) includes a second adjacent-channel sound carrier trap. The second-IF-amplifier response, from which in-channel sound carrier is not trapped, is not as much reduced for the downconverted picture carrier by the second adjacent-channel sound carrier trap as the first-IF-amplifier response is by the first adjacent-channel sound carrier trap. From the second-IF-amplifier response, intercarrier detection circuitry detects an intercarrier-sound-IF response, which is amplified by an intercarrier sound-IF amplifier (22) and then detected by an FM sound detector (26). Responsive to the second-IF-amplifier response, an automatic fine tuning (AFT) detector generates the AFT signal used to control the frequency of the oscillations generated by the local oscillator (13) that the downconverter includes. |
