Sound carrier detecting circuit

Abstract

A sound carrier detecting circuit for use in a television or a video tape recorder is disclosed. During the sound carrier detection, the carrier frequency of the sound signals is corrected by utilizing pulse width modulation signals output from a microprocessor, so that the noise due to shifting of the detected frequency can be eliminated. The circuit includes a key input part provided with one or more keys for selecting a tuning state, and a microprocessor for outputting pulse width modulating signals and manual sound tuning signals in response to appropriate settings of the key input means. A variable oscillating part for converting the pulse width modulating signals into dc component signals and, in turn, outputting variable frequency signals in accordance with the level of the dc component signals is provided alongside a frequency modulating and oscillating part which oscillates and outputs set frequency signals set to a specific frequency in accordance with the relevant broadcasting method. A switching part selects between the variable oscillating part and the frequency modulating and oscillating part in accordance with the state selected by the key input part. The detecting circuit eliminates the noise due to the shifting of the frequency, and can be applied to all types of video signal receiving apparatuses.