| 摘要 |
1445159 Detectors; noise suppression; television RCA CORPORATION 26 Oct 1973 [6 Nov 1972] 49893/73 Headings H3T H4F and H4L In a radio receiver, which in the embodiment comprises a television receiver, using a synchronous detector 109, Fig. 1, noise pulses such as 50, Fig. 2a, result in the detected signal containing both a positive 56 and a negative pulse 57, Fig. 2b; the latter pulse constitutes whiterthan-white and is objectionable. According to the invention, the receiver also includes an envelope detector 123; the output from this only includes a positive noise pulse 60, Fig. 2c and this is extracted by a threshold detector 125 and applied to a blanking circuit 127 which, for a period of the noise pulse, clamps the output from the synchronous detector 109 to a predetermined value, e.g. black or grey level (Figs. 2e, 2f, 2g, not shown). The sound recovery circuits 121 may, as shown, be fed from the envelope detector 123, or alternatively from the output of the blanking circuit 127, which may also feed the sync separator 111; as shown, the latter is fed directly from the synchronous detector 109. Detector circuits.-The video signal is fed from the I.F. amplifier to a bandpass filter 201, Fig. 3, having a balanced output feeding into the emitter-followers 203, 205. Emitter-followers 203, 205 feed via amplifiers 209, 211 a sharplytuned circuit 213 which extracts the carrier frequency and feeds it via cascaded emitterfollowers 215, 219 and 217, 221 respectively to one pair of inputs of the synchronous detector 225: the modulated signal is fed to the other pair of inputs from the emitter-followers 203, 205. The blanced output at terminals 227, 229 from the synchronous detector 225 are fed to the two inputs of an amplifier 231 which produces an unbalanced output via emitterfollower 233 which is fed via resistor 261 and emitter-follower 263 to the video output terminal. I.F. signal from emitter-follower 205 is also fed via an amplifier 235 to transistor 239 which constitutes the envelope detector: the output from this is applied via emitter-follower 245 to threshold circuit 250. Threshold circuit 250 is so adjusted that its output comprises noise pulses and these are fed to transistor 260 which, during the noise pulses, clamps the base of emitter-follower 263 to a predetermined potential.
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