| 摘要 |
A television receiver is provided with the circuitry for displaying a second programme in a portion, e.g., one corner, of the viewing screen whilst a first programme is being displayed on the remainder of the screen. Application No. 53437/77 is referred to. An interference suppression circuit is provided to prevent superimposition of the tuner switching signal on the audio channel. This circuit includes a sample-and-hold stage (14) to which the incoming audio signal NF of the first programme is fed and which is controlled from the horizontal deflection circuit via a frequency multiplier (17) and a pulse shaper (16). This arrangement blocks the sound signal, the output signal from the sample-and-hold stage being continuously applied to the receiver's audio output amplifier through a low pass filter (15) whose upper cut-off frequency is less than half the cut-off frequency of the sampling signal. The sample-and-hold stage stores the last audio-signal value occurring prior to the beginning of the switching of the tuner to the record programme for a time which is approximately equal to the run of the duration of the square-wave pulses switching the tuner and the transient response time of the audio i-f filter. The sample-and-hold stage samples the audio signal constantly at a frequency which is such an integral multiple of the frequency of the square-wave pulses switching the tuner that the holding time of the sample-and-hold stage remains just longer than the total interference time. <IMAGE> |
