Patent GB-426488-A (GB)
Title: Improvements in or relating to apparatus for electrically recording signals
IPC: G11B7/00
Filed: 1933-10-04
Abstract: 426,488. Valve amplifying circuits. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Scoville, R. R., and Harley, J. B.) Oct. 4, 1933, Nos. 27327 and 27328. Convention dates, Oct. 20, 1932 and Feb. 16, 1933. [Class 40 (v)] [See also Group XXXVIII] Relates to apparatus for recording sound or signals, of the kind in which in addition to a current or voltage representing the sound &c., a control or bias voltage or current representing a characteristic such as amplitude or volume level is utilized in making the record. According to the invention, the apparatus comprises a source of alternating current, preferably of constant and high frequency, and such current is modulated in amplitude in a manner dependent upon a characteristic of the signals being recorded and is then rectified to produce the required bias current or voltage. In the sound-recording system shown in Fig. 1, microphone currents amplified at 14 and 17 are supplied to the ribbons 10 of a light valve, and the light beam transmitted from a source 1 to a film 8 is thereby controlled in accordance with the instantaneous variations in the currents. The ribbons 10 are so adjusted that normally the light transmitted is such that the resultant exposure of the film is in the centre of the selected range of the characteristic of the photographic emulsion. High-frequency oscillations are supplied by a generator O to a transformer 33, by which they are applied through a biassing battery 34 to the grid of a valve 35, and through a condenser 36 to the filament of the valve; the amplitude of the applied voltage is variable by a resistance 37. The output of this valve is transmitted through a transformer 39 to a full-wave rectifier R. The undirectional pulsating current thus produced is smoothed by a filter 45, 46, and passes through the ribbons 10 of the light valve. This current draws the ribbons together, thus reducing the exposure of the film 8. By adjustment of the magnitude of the pulsating current (through resistances 32, 37, 47), the time of exposure is reduced until, in the absence of signal currents, it is at the lower end of the selected range of the photographic characteristic. A portion of the amplified microphone currents flows through wires 61 to a transformer 50, under control by a constant impedance potentiometer 51 ; the voltage induced in the secondary of the transformer is rectified by a rectifier 54, the output of which charges the condenser 36. This impresses a negative potential on the grid of the valve 35, reducing its amplifying power and thus reducing the rectified current supplied to the ribbons 10, so that the average height of the light-transmitting slot is increased. By this means, when a wave of signal current of large amplitude is suddenly applied to the circuit, the height of the light slot is suddenly increased. In order that when the signal wave decreases or ceases the restoring action should not occur too rapidly, the value of the capacity 36 and of a resistance 59 are so selected or adjusted that the time of discharge of the condenser is sufficiently long. Thus the load-carrying capacity of the light valve is varied approximately with the envelope of the variations in the signal current. In a modification, (Fig. 2 not shown), the general arrangement is similar to that shown in Fig. 1, but a battery is provided to supply current to the light-valve ribbons through a variable resistance, an inductance, and a unilaterally conducting device connected in the lead 48. This current reduces the height of the light slot to a small value in the absence of signal currents. The connections of the rectifier 54 to the condenser 36 are modified, and the biassing potential supplied by the battery 34 to the grid of the valve 35 is such that in the absence of signal currents the H.F. currents transmitted to the transformer 39 are substantially zero. With this arrangement, rectified signal current from the rectifier 54 causes a positive potential to be applied to the grid of the valve 35, and the resulting H.F. currents supplied to the transformer 39 bring about an increase in the spacing of the light valve ribbons. Fig. 3 shows a different arrangement, in which a generator of high-frequency oscillations O<1> is coupled symmetrically through a transformer 91 to the input circuit of a pair of push-pullconnected valves 92, 93, the output of which is rectified at 100, smoothed by an inductance 101 and condenser 104, and supplied to the ribbons 10 of the light valve. The connections to the ribbons through the secondary of the transformer 18 through which signal currents are transmitted to the light valve are such that magnetic saturation is avoided. The output of rectified H.F. current is adjusted to such a strength that in the absence of signal currents the spacing of the ribbons is reduced from that determined by mechanical adjustment. A controllable portion of the signal currents is utilized, through an amplifying valve 73, a rectifier 75, and a condenser 79 to apply a voltage, which varies with the volume of the sound being recorded, to a resistance 77 in the common portion of the grid-cathode circuits of the valves 92, 93, so as to tend to bias the grids positively. Thus, when signal currents are supplied, the impedance of the input circuit of the valves 92, 93 is decreased, and, through the transformer coupling 91, a decrease in the voltage developed by the source O<1> is effected. This results in a diminution of the unidirectional current delivered through the rectifier 100, so that the spacing of the light valve ribbons increases. The effect of the signal currents in charging the condenser 79 is rapid, whilst the discharge occurs more slowly when the signal currents diminish, thus producing an asymmetric timing of the action on the light valve. Specifications 362,915 and 362,993 are referred to.