GB624522A - Improvements in or relating to radio receivers - Google Patents

Abstract

624,522. Automatic noise suppression. HUGHES, D. H., CLARKE, D., and PYE, Ltd. Oct. 5, 1946, No. 29741. [Class 40 (v)] A radio receiver is muted by means of a rectifier which forms a low impedance shunt across the audio amplifier, but which is so biassed from a D.C. amplifier as to become non- conductive when the signal input exceeds a predetermined level. The audio amplifier (not shown) includes an attenuator comprising series resistance 8, Fig. 3, and shunt elements formed by diodes 4, 4a in series with audio byepass condensers 6, 6a. The diodes are connected in series between an adjustable source of positive potential 5 and the anode of a D.C. amplifier 2, resistances 11, 11a maintaining the correct distribution of D.C. potential. The amplifier 2 is connected to the signal detector, or in a frequency-modulation receiver to the limiter so that the anode potential rises as the signal input increases, causing the diodes to cut off and the muting to be removed when the predetermined level is reached. The arrangement may be simplified by omitting the diode 4, and further by using the valve 2 as the audio amplifier (Figs. 1, 2, not shown). In an arrangement in which the muting level depends upon the signal to noise ratio, the D.C. amplifier 2 as described above derives a first D.C. potential from the detector 12, Fig. 4, over a filter network 25, 26, 27. A noise amplifier 16 is coupled to the detector over a high-pass filter 14, 17 which attenuates audio signals but passes noise of higher frequencies, and is biassed over a network 18, 19, 20 so that the gain is reduced as the signal increases. The output of the valve 16 is rectified by a diode 21 and is applied to the valve 2 in opposition to the first bias.