GB513309A - Improvements in public address stabilizing systems - Google Patents

Abstract

513,309. Public address systems. RADIO CORPORATION OF AMERICA. Dec. 30, 1937, No. 36107. Void. Convention date, Dec. 31, 1936. [Class 40 (iv)] [Also in Groups XXXV and XL] Acoustic reaction in a public address system is reduced by continuously varying the air path between the microphone and loudspeaker, by introducing a continuously variable phase change in the circuit between the microphone and loudspeaker or by slightly reducing all the component frequencies in that circuit. In Figs. 1, 2, the loudspeaker 1 is carried by a plate 2 which is supported on arms 3 by universal joints 6 and which is cyclically rotated by a crank pin 8 coupled thereto by links 9, 10. In Fig. 3 a variable phase change in the circuit between the microphone 12 and loudspeaker 1 is introduced by passing sound through an air tube 15, 17 of which the U-shaped portion 17 slides to and fro over the straight portions 15 under the control of a crank 19. A similar result is obtained electrically in Fig 4 by means of an artificial line 24 suitably terminated by a resistance 25 and an exploring coil 26 which moves to and fro over the artificial line. In Fig. 5, the currents arrive at the loudspeaker 1 slightly reduced in frequency. The currents from the microphone 12 pass to a modulator 33 and modulate a carrier frequency, e.g., 20,000, supplied by an oscillator 34 and the upper side band is selected by a filter 31. This is mixed at 36 with a second carrier frequency from an oscillator 35 differing slightly from the first carrier, e.g., 20,000, and the resulting currents pass through a detector 37 and amplifier 38. The audio frequencies are therefore reduced by 2 cycles per second. Some methods of obtaining two carrier frequencies with a constant small difference between them are shown in Figs. 6, 7. In Fig. 6, a frequency f1 is supplied to two stator coils 43, 44 disposed at right angles, the currents in one being 90‹ out of phase with respect to those in the other. The voltage induced in a rotor coil 47 driven at f2 revolutions per second is of frequency f1 Œ f2. In Fig. 7, a carrier frequency f1 is modulated at a low frequency f2 in balanced modulators 50, 51, the voltages supplied to 50 being in quadrature with those supplied to 51. By combining the outputs a frequency f1 Œ f2 is obtained. Since a quadrature phase shift in a very low frequency is difficult to obtain, such a frequency may be derived (Fig. 8, not shown) as the beat frequency of two higher frequencies, the phase shift being introduced in both these frequencies to feed one of the modulators. Specifications 486,290 and 486,291 are referred to.