GB425256A - Recording and reproduction of electrical impulses - Google Patents

Abstract

425,256. Recording and reproducing music &c. RADIO CORPORATION OF AMERICA, 30, Rockefeller Plaza, New York, U.S.A. Feb. 1, 1934, No. 3366. Convention date, Feb. 1, 1933. [Class 40 (iv)] The volume range of sounds is compressed during recording, the amount of control being represented on the record by the amplitude of a pilot frequency outside the audible range superposed on the audio-frequencies. During reproduction, the pilot frequency is separated out and is used, in accordance with its amplitude, to expand the volume range to its original extent. The volume of sound and pilot currents during recording may be controlled manually by potentiometers as in Fig. 1, (not shown), but as shown in Fig. 3 the control is automatic. The recording channel comprises transformer 35, amplifier 36, filter 41, amplifier 14 and recorder 11 which may be of mechanical, optical or other type. A control frequency say of 20 cycles is injected from 15 into the channel over an amplifier 43 and low-pass filter 48. Voltages according to the sound level at transformer 35 are applied to amplifiers 51, 53 and then rectified at 55, smoothed at 57 and used to control the grid bias of the amplifiers of both the sound channel and control frequency channel so that low volume levels corresponding to say below 30 per cent of the maximum recording level are boosted up above the noise level and high volume levels corresponding to say above 90 per cent of the maximum recording level are reduced, 10 per cent of the record track being reserved for the pilot frequency which is controlled simultaneously with the sound levels. In reproducing, Fig. 2, the sound and pilot frequencies are separated by filters 23, 29, the sound currents being amplified at 25 and fed to reproducer 28. The pilot current is rectified at 30, smoothed and applied to a bias resistance 281 in amplifier 25 so that the volume range is restored. The separation of the sound and pilot frequencies may be effected electromechanically as shown in Figs. 7 and 8. The pick-up 70 operates in the usual manner in response to the recorded sound and a tuned vibrator responding to pilot frequency is mounted on the tone arm 71. The vibrator comprises an adjustably weighted armature 77 mounted on resilient supports 78 carried by a magnet 76 secured to the tone arm, control frequencies being generated in coil 80 carried by armature 77.