Patent GB-1177934-A (GB)
Title: Improvements in or relating to apparatus for Monitoring Grooved Sound Records
Inventor: BATCHELOR MICHAEL
IPC: G11B27/00
Filed: 1966-02-15
Abstract: 1,177,934. Monitoring grooved sound records. ELECTRIC & MUSICAL INDUSTRIES Ltd. 15 Feb., 1967 [15 Feb., 1966], No. 6479/66. Heading H4J. In apparatus for monitoring a grooved sound record, the record is played backwards and two output signals are derived from the groove along substantially orthogonal directions. A difference signal is derived from the output signals and is applied to a detector of noise impulses. A stereophonic pick-up 1 (Fig. 1) is associated with a turntable 2 which rotates such that the record is normally played backwards (but can be reversed such that the record can be played intelligibly in the vicinity of portions suspected of containing defects). The left and right signals are applied to non-equalized preamplifiers 3 and 4. Their outputs are applied to equalized amplifiers 5 and 6 and to earphones or loud-speakers 7 and 8 so that a record can be monitored by ear. L and R signals from preamplifiers 3 and 4 are applied to a differencing circuit 9, providing signal L-R, and to a summing circuit 28, producing signal L + R. The signal L-R is taken as the main signal to the monitoring components because it contains most of a defect signal, yet little of a programme signal. When monitoring a monaural record the programme content L-R is sensibly zero. Output of differencing circuit 9 is applied by high pass filter 10 (cut-off 500 c/s.) to a gain controlled amplifier 11. The output of amplifier 11 is applied by band-pass filter 32 (500 c/s.- 10 kc/s.) to fault detectors 41, 42, 43 and to a further amplifier 15, the output of which is rectified by a diode 16. The output of diode 16, which corresponds to the envelope of output of amplifier 15, is smoothed by time constant circuit 17, 18 and the voltage across capacitor 18 is applied to amplifier 11 via diode 19. Time constant 17, 18 is such that amplifier 11 is switched to low gain for a signal having a selected rise time, of value between ¢ ms. to 5 ms. Thus, if the attack time is longer than the rise time, the gain of the amplifier is reduced in sufficient time to prevent that signal producing a fault signal. The output from the summing circuit 28 is, after passage through a network 29 and high-pass filter 30, applied to an amplitude compressor 31. The output of amplifier 31 is rectified by a diode 36, smoothed by circuit 37, 38 and the voltage across capacitor 38 controls the gain of amplifier 31. The control signal is selected to have a rise time of between 10 and 20 ms. If the signal across capacitor 38 is greater than the signal across capacitor 18 the diode 39 will conduct, turning off diode 19. The gain of amplifier 11 will then be controlled by the signal across capacitor 38 so that a relatively large amplitude programme signal in the sum signal will reduce the gain and therefore effectively increase the threshold of the defect detection channel. In another embodiment (Fig. 3, not shown) the gain of amplifier 11 is controlled by the signal across capacitor 18, the diodes 19, 39 being omitted. This signal is derived from amplifier 15, diode 16 and resistor 17. The input to the amplifier 15 is produced by a summing circuit (54) which adds together the output of amplifier 11, fed via the band-pass filter 32, and the output of amplifier 31. The gain of amplifier 31 is controlled by soft diode clipping (51, 52) together with a resistor (53) in the negative feedback path of amplifier 31. The circuit of Fig. 2 is simpler than that of Fig. 1, which results in partial cancellation of the effect of R signals and reinforcement of L signals in the control of the gain of the amplifier 15.