Abstract: DC signals for controlling variably controlled filters for input audio frequency signals are derived by filtering the input signal to reduce its level in response to variations in intensity and frequency and then logarithmically compressing the filtered signal. An absolute value signal proportionately corresponding to the compressed signal is then derived and combined with an externally generated reference signal of variably selectable level. The combined signal is then converted to a dc variable filter control signal which is blocked when its level falls below the level corresponding to the preselected reference signal level.