Patent GB-443243-A (GB)
Title: Improvements in or relating to recording sound on films by the variable density method and to reproducing sound therefrom
IPC: G11B7/00
Filed: 1934-08-24
Abstract: 443,243. Optical sound recording and reproducing apparatus. WESTERN ELECTRIC CO., Ltd., Bush House, Aldwych, London.-(Bell Telephone Laboratories, Inc. ; 463, West Street, and Electrical Research Products, Inc. ; 195, Brodway, both in New York, U.S.A.) Aug. 24, 1934, No. 24492. [Class 40 (ii)] [See also Group XL] Relates to a method of recording sound on films by the variable density method or of reproducing or re-recording sound from variable density records in which the exposure of the records is restricted to the straight line portion of the H and D curve. A record so produced when reproduced directly gives distorted reproduction owing to the exponential relationship between the exposure and transmission thereof. According to the invention one or more devices having an exponential or non-linear characteristic, such as variable mu thermionic valves, is or are inserted in the recording channel in order to produce a record which can be reproduced directly without distortion or, alternatively, such a device or devices are inserted in the reproducing channel so as to produce undistorted reproduction. Fig. 1 shows a recording system in which currents from a microphone 1 amplified at 2 are fed to a valve 4 having an exponential characteristic, this valve being coupled by a linear resistance 9 to an amplifier 10 in the output circuit of which is a light valve 14 arranged between a light source 17 and film 18. According to a modification, Fig. 2 (not shown), the resistance 9 is replaced by a non-linear resistance, whilst according to a further modification, Fig. 3 (not shown), a further amplifier is provided between the resistance 9, Fig. 1, and the amplifier 10 this further amplifier being coupled to amplifier 10 by a non-linear resistance. Fig. 4 shows a reproducing system in which the varying potential difference produced across the resistance 36 in the output of a photoelectric cell 34 is applied to the control electrode and cathode of the amplifying tetrode 37 which is controllably biassed from battery 38 and potentiometer 39. The valve 37 is coupled by resistance 41 to a variable mu tube 42, the output of which is supplied through transformer 44 to further amplifiers 45 and thence to a reproducer 46. Adjustment of potentiometer 39 ensures that the biassing potential applied to the control electrode of valve 42 will be of the correct value to fix the operating point in the correct part of the characteristic curve of valve 42. With records in which the average transmission of the record is continually changing, such as those made by the "noise reduction " recording systems, manual adjustment of the biassing potential is unsatisfactory and in such case a system as shown in Fig. 5 is employed. In this arrangement, the fluctuating voltage across the resistance 36 is amplified in the multi-stage resistance capacity coupled amplifier 47, 48 and applied to the grid resistance 59 of the variable mu valve 49. A resistance 50, condenser 51 and battery 52 arranged as shown compensate for changes in the steady biassing potential applied to valve 49 corresponding to changes due to the variation in the average transmission of the record. Fig. 6 shows a further reproducing arrangement in which the potential difference developed across resistance 104 is applied to a variable mu screen grid tetrode 101, the output current of which produces a varying potential difference across the resistance 106 which is fed back through resistance 105 and condenser 160 to resistance 104. The potential difference developed across resistance 104 as modified by the potential fed back from the valve 101 is applied to the input circuit of an amplifier 102 where it is linearly amplified and produces a potential difference across resistance 108 which is applied through condenser 124 and produces a potential difference across the resistance 109. Valve 103, which has the same exponential characteristic as valve 101, then exponentially amplifies the voltages developed across resistance 109 and supplies them through transformer 125 to the reproducing circuit. The tapping 109 is adjusted until the output of valve 103 is an undistorted sinusoid. Fig. 7 shows a modified arrangement of Fig. 6, the valves 132, 133 and 134 corresponding to the valves 101, 102 and 103 and the resistances 137, 138 replacing the resistance 104. The potential difference developed across the resistances 137, 138 is applied to the valve 131 and an amplified potential difference is produced across the resistance 141 and applied to the floating cathode of the valve 132. The output circuit of this valve includes the resistance 138 so that the input voltage applied to the valve 131 is modified, this modified voltage appearing in an amplified form across resistance 141 from which it is applied across the variable resistance 145. A controllable portion of the potential difference developed across resistance 145 is applied to valve 133 which is direct-coupled across resistance 150 to the cathode of the valve 134, the control grid of which is earthed through battery 151. The output of valve 134 is applied through transformer 152 to the reproducing circuit. The tapping 145 is adjusted until the output of valve 134 is an undistorted sinusoid. If in addition to correcting all the amplitudes for the distortion due to the character of the record, a discrimination is desired between different frequencies, the resistances 104, Fig. 6, and 137, 138, Fig. 7, may be replaced by frequency discriminative impedances.