GB1469058A - Noise reduction circuit - Google Patents

Abstract

1469058 Volume expansion R M GRODINSKY 14 May 1975 [20 May 1974] 20403/75 Heading H4R In a volume expander for noise reduction of uncompressed or randomly compressed audio signals, volume expansion is applied over the full frequency range of the audio signal controlled by a signal derived only from the higher frequency region of the audio signals, i.e. the region of the audio signal containing appreciable energy content of the harmonics of the signal. As described the audio signal can be applied to a resistive potentiometer (2) Fig. 9 (not shown) one element of which is a photosensitive resistor illuminated by a light emitting diode fed from the output of a high pass filter connected to receive the audio signal being expanded so that the attenuation of the potentiometer decreases with the increase in level of the high frequency components of the signal. In alternative embodiments the filter can be an RC circuit, (3) Fig. 5 (not shown) or an LC band pass filter 3<SP>1</SP>, Fig. 6. The filtered signal is rectified and applied to a ripple filter capacitor 16, Fig. 6, having a time constant of 50-200 m.secs., and a circuit 5<SP>1</SP> which has two paths to the control input of amplifier 2. One path via resistor 17 exercises a relatively rapid control while a second path, including resistor 18 and capacitor 19 provides a longer time constant circuit which provides a response to slow variations in the dynamic range of the audio signal. In addition a further stage of filtering, having a rapid charge and slow discharge, can be provided between the rectifier and controlled amplifier by diode 28, resistor 27, and capacitor 29. Transistor 30 provides isolation between the further filter and the controlled amplifier 2.