Noise floor indicative circuit

Abstract

An improved circuit for producing a signal indicative of an input signal floor level is disclosed for use in determining the noise level of an audio signal in a receiver voting system. An input audio signal which contains noise is amplified by variable gain amplifier circuitry consisting of a constant gain amplifier and a variable attenuator. Detector circuitry then develops a signal which is indicative of the noise level, this signal substantially corresponding to the minimum peak magnitude of the amplified input audio signal. During pauses between audio tones in the input audio signal, this minimum signal magnitude coincides the residual background noise level of the input audio signal. Control circuitry couples the noise level indicative signal to the variable attenuator and constitutes a negative feedback loop for inversely and nonlinearly varying the amplification of the input signal in response to the magnitude of the noise indicative signal. In essence, the signal floor level circuit functions as an AGC circuit where the AGC control voltage is minimum level dependent and also corresponds to the noise floor indicative signal. The use of a negative feedback loop readily provides an accurate, nonlinearly varying, temperature stable noise level indicative signal for input audio signals having noise levels which vary over a large dynamic range.