GB646218A - Audio frequency reproduction system - Google Patents

Abstract

646,218. Sound reproducing circuits. RADIO CORPORATION OF AMERICA. Dec. 22, 1947, No. 33851. Convention date, Jan. 9. [Class 40 (iv)] The signal-to-noise ratio of a sound reproduction system is improved by dividing the audio signals from a source 10 such as a pick-up, into at least two frequency bands, one or more of the higher frequency bands being transmitted over a path or paths each containing an element with a non-linear characteristic before the high and low frequency bands are mixed again for reproduction at 5. A non-linear characteristic enables a channel to be blocked until the signal-tonoise ratio for the channel attains a minimum acceptable value. The frequency bands are divided by means of a low-pass filter 6 (0-4000 c.p.s.) and a band-pass filter 8 (4000-8000 c.p.s.). A second band-pass filter 9 covering the same range as 8 blocks any harmonics produced by the non-linear element 7. A non-linear element with the characteristic shown in Fig. 2 may comprise biassed diodes as shown, or biassed rectifiers, Fig. 5 (part). The two diodes 22, 23 provide separate paths for the positive and negative half-cycles of currents. The output is amplified at 28. The characteristic shown in Fig. 7 is provided by the element shown in Fig. 8 (part), in which further diodes 106, 107 are biassed so as to shunt the primary of transformer 81 only when the amplitude of the output signal from 102, 103 reaches a value beyond the threshold value of the diodes 106, 107.