GB1410771A - Audio frequency signal changing circuits - Google Patents

Abstract

1410771 Reproducing music; stereophonic systems NIPPON COLUMBIA KK 8 Aug 1972 [11 Aug 1971 30 Nov 1971 14 April 1972 25 May 1972] 36996/72 Headings H4J and H4R An audio frequency band such as a voice frequency band contained in an audio frequency signal from a pre-recorded tape, record radio or T.V. is selectively attenuated or removed and a second audio frequency signal, e.g. from a microphone, is added to the reproduced signal in place of the attenuated or removed band. This second audio frequency signal has a range occupying at least part of the attenuated or removed band. A switch is responsive to the presence of the second audio frequency signal to provide a control signal for bringing the attenuating or removing means into operation. The switch includes a delay circuit to delay provision of the control signal for a predetermined time after detection of the second audio frequency signal to ensure that transient signals generated during switching are not present in the mixed signal. Stereophonically related signals IL, IR (Fig. 5) are applied to speakers 4L, 4R through line amplifiers 3Lb, 3Rb, respectively. Signal IL is also connected to speaker 4L through a band stop filter (122) (800 to 5000 Hz) and line amplifier 3La. Similarly signal IR is connected to speaker 4R through a similar band elimination filter 12R and line amplifier 3Ra. The output from a microphone 6 is supplied through an amplifier 7 to a band stop filter 17 (800 to 5000 Hz) and then through a high pass filter 19 (3000 + Hz) to a buffer amplifier 20 and rectifier circuit 21. When circuit 21 provides an output, a Schmidt trigger 22 actuates a switching circuit 23. The arrangement is such that, when no sound is applied to the microphone 6, switching circuits 23 grounds the power source terminals of amplifiers 3La and 3Ra so that signals IL, IR are supplied to speakers 4L, 4R by amplifiers 3Lb, 3Rb. When voice signals are applied to microphone 6, switching circuit 23 is actuated to ground the power source terminals of amplifiers 3Lb and 3Rb while the power source terminals of, amplifiers 3La, 3Ra are energized. Thus signals IL, IR after having their intermediate frequency components removed, are mixed with the signal from microphone 6 and amplified by amplifiers 3La, 3Ra. Howling is unlikely to occur, but can be removed by making microphone 6 inoperative momentarily or stopping supply of sound momentarily. A band pass filter 17a, may be inserted. Clicks are heard from the loud-speakers 4L, 4R at the time of change-over of switching circuit 23 or if the sound input to the microphone 6 is momentarily interrupted. These clicks are eliminated in a circuit, Fig. 6 comprising a capacitor 46 having a time of charge or discharge sufficient to delay changeover of amplifiers 3La, 3Ra or 3Lb, 3Rb until. the Schmidt trigger 22 and switching circuit 23 are operated.