Patent GB-941704-A (GB)
Title: Improvements in or relating to electric circuit arrangements for transmitting electrical signals representative of sound signals
Inventor: BATCHELOR MICHAEL
IPC: H03K7/00
Filed: 1959-02-20
Abstract: 941,704. Automatic volume control. ELECTRIC & MUSICAL INDUSTRIES Ltd. Feb. 22, 1960 [Feb. 20, 1959], No. 5832/59. Headings H4J and H4R. In a circuit for reducing impulsive interference, such as that derived from unwanted displacements of a gramophone stylus, a blocking pulse whose duration is dependent upon the duration of the interference is applied to a gate arranged so that at least high frequency components of electrical signals applied to the circuit are attenuated during the application of the blocking pulse. A signal from an input 1, Fig. 1, is separated into low and high frequency components by a low-pass filter 2 in channel 3 and a high-pass filter 4 in a channel 5. Channel 5 also includes a gate 6 which is normally open but is closed by blocking pulses from a channel 7. The channel 7 includes a threshold control 8, in the form of a potentiometer, and an interfering pulse identification unit 9. A delay line 10 ensures that the gate 6 is closed before an interfering pulse arrives at the gate. Outputs from channels 3, 5 are combined and applied to an output gain control 11. The common cut-off frequency of filters 2 and 4 may be adjustable to 1,000 or 500 c/s. The low-pass filter may be omitted and the high-pass filter by-passed. The threshold control 8 and interfering pulse identification unit 9 includes a pentode amplifier V1, Fig. 2, to which a portion of the signal is applied, and a high-pass filter F which transmits frequencies above 16 kc/s to a variable potentiometer VR1 acting as the threshold control and controlling the level of the signal applied to an amplifier V2a. The output of amplifier V2a is fed to a monostable multivibrator V3 which generates a square wave output whenever the voltage of the signal applied thereto exceeds the triggering voltage. The square wave output is fed through a cathode follower V2b and rectifier V5a to a monostable multivibrator V4. This derives by means of capacitor C1 output pulses X and Y having a duration equal to that of the triggering signals plus about one millisecond. The pulses X are negative and are applied through a cathode follower V8a, Fig. 3, to a gate V7 in the form of a double triode acting as a push-pull class A amplifier. They are fed to the centre tap of an input transformer T1 so as to bias the control electrodes of amplifier V7 well beyond cut-off, a diode V9a being employed to clamp the quiescent control electrode potential. Controls VR3 and VR4 equalize the two sections of gate V7 and are adjusted for minimum hum after injecting a 50 c/s. signal at switch S1 (see Fig. 2). A by-pass capacitor C3 reduces the slope of the leading edge of the pulse X, and also to a certain extent its trailing edge, to cause the gate V7 to close and open slowly, in approximately ¢ millisecond. Positive pulses 7 from multivibrator V4 are applied to terminal 13 and after rectification by rectifier V9b are fed to the control electrode of triode V8b. Whenever a pulse Y exists a needle of metre M is deflected. Discriminator.-The output from the threshold control VR1, Fig. 2, is also applied by a control VR2 to an amplifying stage V6 and then by a rectifier V5b to a capacitor C2. The capacitor charges slowly and the voltage across it is applied to the suppressor electrode of pentode V1 to reduce the gain of this stage. A single interfering pulse or short train of pulses will not reduce the gain of pentode V1 significantly but signals due to the programme and lasting more than a few milliseconds reduce the gain to cause the multivibrator V3 to cease. Thus the duration of a short pulse or short train of pulses will determine the length of time the gate is closed. A continuous signal of more than a few milliseconds duration which charges capacitor C2 will cause the gate to open after the capacitor C2 has charged.