Patent GB-1090941-A (GB)
Title: Improvements relating to methods and apparatus for modulated groove sound recording and modulated groove sound records so produced
Inventor: BATCHELOR MICHAEL
IPC: G11B3/64
Filed: 1964-01-29
Abstract: 1,090,941. Modulated groove sound recording; automatic volume control. ELECTRIC & MUSICAL INDUSTRIES Ltd. Jan. 21, 1965 [Jan. 29, 1964; Oct. 10, 1964], No. 3729/64 and 41437/64. Headings H4J and H4R. In producing a modulated groove record by means of a record cutter, a signal used to effect the recording in one or more frequency bands is compressed in dependence upon at least one of the velocity and acceleration of the cutter if threshold levels are exceeded. In one embodiment (Fig. 6) an input terminal 1 is connected to nine filters 3A, 3B ... 3I, of which filters 3A, 3B ... 3F are arranged to have controllable gain and filters 3G, 3H, 3I have fixed gain. Filters 3A, 3B ... 3I have centre frequencies of 16 kc/s., 8 kc/s., 4 kc/s., 2 kc/s., 1 kc/s., 500 c/s., 250 c/s., 120 c/s., 51 c/s. respectively. Their outputs are applied to a combining amplifier 4 from which an output signal is transmitted via terminal 6. A portion of the output signal is applied to a bridged-T attenuator 8 having two adjustable resistors 9, 10. The signal from attenuator 8 is applied to six control channels A, B, C, D, E, F. In channel A is a filter 11A having the same centre frequency as filter 3A. An equalizing network 13A has a frequency response equal to the recording characteristic. Its output is amplified and applied to a rectifier 18A and via a differentiating circuit to an attenuator 15A and thence to a rectifier 17A. The gain of the control channel is varied by attenuator 8 and attenuator 15A by adjustment of resistors 9, 10 and resistor 20, by means of a servo mechanism coupled to the cutting lathe or by manual operation. The output signal of amplifier 14A is proportional to recorded slope. The output signal of amplifier 16A is proportional to the curvature of the recorded modulation. The output signals of amplifiers 14A and 16A are rectified by rectifiers 18A and 17A which are arranged to charge a capacitor 29 rapidly, the voltage on capacitor 29 being proportional to recorded slope or curvature, whichever is the greater. Capacitor 29 is part of a control circuit 19A which determines the impedance appearing at the secondary winding of a transformer 23 connected in series with a capacitor 24 across the base-emitter path of the transistor in filter 3A. The gain of filter 3A is thereby controlled in dependance upon recorded slope or curvature. Control channels B, C, D are similar to control channel A, but control channels E, F lack components 15, 16, 17 responsive to recorded curvature, because recorded slope overload occurs before recorded curvature overload at the frequencies covered by these channels. As curvature overload occurs before slope overload in channel A, slope signal rectifier 18A may be omitted from this channel. Overload resulting from simultaneous occurrence of signal components producing high slope or curvature loadings in two frequency bands may be reduced or overcome by providing signals representative of the slope and curvature which, on exceeding the threshold levels, are applied via gates controlled by slope and curvature functions to control the gains in the respective channel or channels. Only signal components contributing greatly to the slope and curvature loadings may be reduced in amplitude. These last arrangements may be additional or alternative to the previously described examples. In another arrangement (Fig. 1) an input terminal IN is connected to one input of a network 1 and to the inputs of six separate gain-controlled band-pass filters 2A, 2B ... 2F The outputs of the filters 2 are applied to another input of the network 1 which includes a subtracting circuit so that any signal transmitted through the filters 2 is subtracted from that applied from the IN terminal to network 1. The filters 2 introduce considerable attenuation in the absence of a control signal so that their output has negligible effect on the signal applied to the circuit 1. As the control signal rises, so the attenuation of the filters 2 is reduced. The output of the network 1 is connected to the terminal OUT for application to the recording head amplifier. A fraction of the output of the network 1 is applied to six band-pass filters 3A, 3B ... 3F having the same centre frequencies as filters 2A, 2B ... 2F respectively and then through six identical equalizing amplifiers 4A, 4B ... 4F having frequency responses equal to the recording characteristic and next through six identical equalizing amplifiers 5A, SB ... 5F having frequency responses rising at 6 dB/octave. Signals are applied to rectifiers 6 and 7 from amplifiers 4 and 5 and are proportional to recorded velocity and acceleration respectively. Two different direct potentials, both functions of the record diameter, are derived from variable resistors mounted on the recording lathe and driven by the lathe carriage. These potentials are added to the signals applied to the rectifiers 6 and 7 respectively, the functions being chosen so that the combinde signals fed to the rectifiers 6, 7 are approximately proportional to groove modulation slope and curvature respectively. All the rectifiers are biased by a constant third potential and it is arranged that one or more of the rectifiers 6 or 7 will conduct when either the signals proportional to groove modulation slope or curvature have approached overload values, taking into account both frequency and groove diameter. An alternative method of controlling the rectifier thresholds in accordance with record groove diameter is to control the gain of amplifiers 4, 5. In another example (Fig. 4, not shown), the functions of filters 2 and 3 are combined together and performed by filters 11. The gain of the filters 11 is reduced by the control signals instead of being increased as in Fig. 1. The outputs of all the filters are combined in a mixer 13 and in the absence of a control signal the combined frequency response of the filters 11 and a filter 12 between the input IN and mixer 13 is flat over the frequency range required for the recording.