GB594077A - Frequency modulating and demodulating arrangement for use as an amplifier for sound reproducing and the like - Google Patents

Abstract

594,077. Electrostatic instruments and circuits therefor. RADIO CORPORATION OF AMERICA. July 7, 1944, No. 13051. Convention date, June 12, 1943. [Class 40 (iv)] [Also in Group XL] A frequency-modulated transducer device such as a gramophone pick-up or a microphone consists of a variable differential condenser, one half of which varies the frequency of an oscillator and the other half of which varies the frequency of a discriminator circuit in the opposite direction so increasing the effective response. Differential inductive means may alternatively be used. In Fig. 5a, movement of the stylus causes variation of the capacities 14<1>, 15<1> and 15<1>, 16<1> in opposite directions. Capacity 14<1>, 15<1> tunes coil 11<1> to form with the first two grids of valve 10<1> an oscillating circuit electron-coupled to the discriminating circuit comprising inductance 23<1> tuned by the capacity 15<1>, 16<1>, its resonant frequency being such that the oscillator frequency corresponds to a straight slope on its response curve. The resulting amplitude modulated signal is rectified by the diode portion of valve 10<1> to produce audio-frequencies via the R.F. choke L. In a modification, Fig. 5 (not shown), a pentode is used for the oscillator-discriminator and a diodetriode for subsequent rectification and amplification. Figs. 6 and 7 show a pick-up which comprises a sapphire point 60 mounted on a stylus wire 61 of rectangular cross-section fixed to the brass block 62 which is screwed to the block 63. Condenser plates 65, 66 (66 removed in Fig. 6) are mounted on the block 63 parallel to the stylus wire 61. Damping member 67 suppresses torsional resonance of the wire. Block 63 is secured to the tone arm 70 by shock absorbing flexible plates 80, 90. The transmission lines 92, 93 from the condenser plates run in separate channels in the tone arm in which they are supported centrally on plastic blocks to minimize their capacity and increase shielding. The stylus wire is connected to the main body of the tone arm which is earthed. A push-pull form of condenser microphone for use with the above circuit has a diaphragm consisting of the centre plate of a differential condenser, a fixed back plate, and a fixed front plate perforated with conical holes to permit the passage of sound to the diaphragm.