Patent GB-487966-A (GB)
Title: Improvements in or relating to circuits for sound reproducing or like apparatus
IPC: G11B7/00
Filed: 1937-02-22
Abstract: 487,966. Reproducing and re-recording sound. WESTERN ELECTRIC CO., Ltd. (Electrical Research Products, Inc.) Feb. 22, 1937, No. 5238. [Class 40 (ii)] [Also in Group XL] In a thermionic valve apparatus for reproducing or re-recording from original or negative straight-line variable-density sound records, the output of a photo-electric cell is fed through a logarithmic valve arrangement to an exponential valve arrangement and thence to a load, the valve arrangements being coupled by purely resistive means and being supplied with operating voltages from a common source. The logarithmic valve arrangement preferably comprises a pentode valve 31 with a linear characteristic and a pentode valve 35 with an exponential characteristic, the photo-electric cell being connected to the control grid of the linear valve, the output of which is connected to the control grid of the exponential valve, the output of the exponential valve being fed back in a negative sense to the control grid of the linear valve, and the potential applied to the control grid of the exponential valve being fed to the exponential valve arrangement. A linear pentode valve arrangement 40 may be provided between the logarithmic valve arrangement and the exponential valve arrangement which may comprise a pentode valve 42 having an exponential characteristic. The operating voltages may be obtained by means of parallel resistance chains supplied with direct current obtained by rectification of alternating current. The circuits include feed backs and filters where necessary. As shown, the apparatus comprises four valves, a linear valve 31, an exponential valve 35, a. linear valve 40, and an exponential valve 42. The anodes of these valves derive their voltages from a direct current source 17 by means of three parallel sets of resistances, one 21, 22, 23, 24, a second one 15, 18, 19, 20, 29, and a third one 25, 26, 27, 28. The cathodes are indirectly heated from a source 51 bridged by resistances 52, 53, the middle point of which is connected to the cathode of the first valve 31 and to an earth terminal 30. The input from the photo-electric cells is received at terminals 10, 11. The terminal 10 is connected to the control grid of the first valve 31 through a shielded conductor, the shield 12 of which is connected to the earth terminal 30. The other terminal 11 of the cell is connected to earth through a condenser 32. Moreover these terminals 10, 11 are bridged by resistances 13, 14, 15, 16 for coupling and filtering purposes and for supplying bias voltage to the cell. The various valves are intercoupled by the resistances and potentiometers shown. The output of the apparatus is connected to the primary 54 of a transformer, which may be shunted by a resistance 55 in conjunction with condensers 56, 57, 58 for attenuating and equalizing the output of the transformer. A key 50 is provided to permit the output transformer to be coupled to the last valve 42 when a negative sound record is being reproduced, or to the valve 35 when a positive sound record is being reproduced. Keys 33, 36, 43, 71 are provided in the anode circuits of the valves to enable a meter 70 to be inserted when required for adjustment or monitoring purposes. Specification 443,243 is referred to. When the apparatus is employed for the reproduction of both positive and negative films, the emulsion surfaces of these films being inserted on opposite sides in the apparatus, an optical correction is required and is made by the insertion of a thin plane optical glass of approximately 13 to 15 mils thickness in the light path between the objective lens and the scanning system.