GB160223A - Improvements in telephonic and like apparatus - Google Patents

Abstract

160,223. Sykes, A. F. Nov. 18, 1919. Thermionic amplifiers. - For the production of gramophone or phonograph records, sounds are converted into electric currents bv a non- resonant detector, and the currents, after being amplified by thermionic valves and distorted to give the higher frequencies their proper proportionate effect, are recorded bv a non-resonant electromagneticallyoperated stylus. Alternatively, the recorder may possess mechanical resonance, this being corrected by circuit arrangements which correspondingly attenuate the currents in the resonance range of frequencies. In most of the forms described, the non-resonant detector or recorder consists of an oscillatory .coil mounted with negligible elastic restraint (e.g. on a freely-suspended diaphragm) in a magnetic field. The potential induced in a detector of this kind is a true copy of the displacement-variations of the actuating sound-wave, but for a given amplitude of pressure-variation of the sound-wave the potential induced is inversely proportional to the frequency. Moreover, for a given amplitude of current-variation the movement of the recorder is inversely proportional to the square of the frequency. Thus, accordingly as it is desired to produce a record agreeing with the displacement or pressure curve of the original sound, it is necessary to amplify the currents in the ratio of the second or third power of their frequency. This is effected by suitable arrangements of the thermionic va.lve circuits. Similar methods may be used to correct for other factors, such as the resistance of the wax to the cutting- too]. In the arrangement shown in Fig. II (31210/19), the detecting-coil 6 delivers currents to a condenser 18, of relatively large impedance, through a resistance 17, which is bridged across the input terminals 19 of a cascade of amplifying-valves. For given amplitudes of induced potential-variation, the currents taken by the condenser (and therefore the potentials across the terminals 19) are proportional to the frequency, so that this arrangement amplifies each harmonic component in proportion to its frequency, and thus gives a current which is a true copy of the sound pressure-wave. In the arrangement shown in Fig. III (3053/20), the same principle is made use of in two stages; the potential across a resistance 24 in series with the second condenser 23 controls a valve-amplifier 25 which supplies current to the recording-coil 1 through a transformer 26. This arrangement amplifies each harmonic component in proportion to the square of its frequency. A resistance shunt 27 transmits a fraction of the microphone current in undistorted form to compensate for the resistance of the wax blank. In the arrangement shown in Fig. IV (3053/20), currents from the detecting-coil 32 are magnified by a valve 37 (or by a cascade of valves) and supplied to a valve 42, the output circuit of which includes an inductance 43 of small impedance relative to that of the plate circuit. For given strength of current, the potential across this inductance is proportional to the frequency, and this potential is supplied to the next valve 44, which controls the valve 46 in the same way. Consequently, in the current supplied to the valve 46, each harmonic component is amplified in proportion to the square of its frequency. In the arrangement shown in Fig. II (28528/20), currents from the detecting-coil 29, 32 are amplified by valves 42, 44, 46 before being supplied to the recorder, which in this case possesses a. mechanical resonance frequency of its own. To correct for this resonance, the primary of the transformer 57, which supplies the valve 46, is shunted by a tuned circuit 45, 55, 56. which is arranged to have the same resonance frequency and the same degree of damping as the recorder. The same method may be used to correct for several distinct resonance frequencies, or to compensate a record for the resonance peculiarities of a reproducing gramophone &c. The sound-conveying currents may modulate carrier waves and may be transmitted wirelessly.