Calibration oscillators for noise reduction systems

Claim 1

In a signal compressor for compressing the dynamic range of an input signal, comprising a signal channel, a variable gain means arranged to vary the gain of the signal channel at least within a restricted frequency band, so as to effect compression of the dynamic range of the input signal amplifying means and rectifying and smoothing means coupled to the signal channel downstream of the variable gain means to derive a control signal, and means applying the control signal to the variable gain means to reduce the said gain in response to increase of input signal level, the improvement comprising a selectively operable positive feedback circuit, and a first switch having a first connection mode for normal operation of the compressor and a second connection mode operable to render ineffective the input signal to the compressor and to place the circuit in an oscillator mode by coupling the positive feedback circuit from the output to the input of the amplifying means to establish a closed positive feedback loop, and an oscillator output terminal coupled to a point in the positive feedback loop, the parameters of the positive feedback circuit being such that a signal is provided at the output terminal, in the oscillator mode, with a stable amplitude such as to provide a calibration signal at a predetermined level within the operating range of the compressor, for feeding to a recording or transmission channel for the purpose of adjusting the channel gain, for proper complementary expansion of signals compressed by said compressor when said switch is in said first connection mode, the rectifying and smoothing means including a circuit causing the rectifying and smoothing means normally to respond more rapidly to abrupt changes in signal level than to non-abrupt changes, and comprising a further switch operative with the first switch to render the last said circuit inoperative in the oscillator mode, thereby to render the speed of response uniform for all signal changes.