Patent GB-724302-A (GB)
Title: Compandor control arrangement for electric communication systems
IPC: H04B14/02
Filed: 1953-03-13
Abstract: 724,302. Automatic waveform-control systems. STANDARD TELEPHONES & CABLES, Ltd. March 13, 1953 [April 23, 1952], No. 6974/53. Class 38 (4). [Also in Groups XXXIX and XL (c)] Distortion in a special signal transmitted over a communication system incorporating compandor means is detected and utilized to correct distortion in the intelligence signals transmitted over the system. As described, one of the channels of a pulse code multiplex system carries a sine-wave modulation and the phase of odd and even harmonics thereof in this channel at the output of the expandor is used to determine the correction required and its degree. At the transmitter the modulator 7, Fig. 1, produces interleaved trains of amplitude-modulated positive and negative channel pulses from the sources 6 of audio signals and the source 18 of the sine-wave control signal. These pulses are fed to the compressor 1, which has the characteristic curve 5, and includes means for adding a constant positive amplitude to all the compressed pulses so that all the output pulses are positive. The pulses are then coded in circuit 8, which feeds the radio transmitter 9, 10. At the receiver, the pulses recovered from the radio circuit 11, 13 are decoded in circuit 12, the resulting amplitude-modulated pulses being restored to positive and negative form in the modulator 14 and applied to the expandor 3, which has a characteristic 15, which is the inverse of the compressor characteristic 5. The channel pulse trains are then distributed by circuit 16 to the individual audio channel detector circuits 17 and to the control channel detector 19. The output from the latter is fed to a filter 20 tuned to a selected odd harmonic of the control wave frequency and a filter 23 tuned to an even harmonic. A phase discriminator 21 produces a control voltage depending upon the relative phases of the odd harmonic from filter 20 and a current of the same frequency derived locally from the fundamental frequency, this voltage being used to control the gain of an amplifier preceding the expandor in circuit 3. A similar phase discriminator 24, fed from the even harmonic filter 23, produces a control voltage which is applied to the modulator 14 to control the subtraction process by which the pulses are restored to the positive and negative form. By this means, the input to the expandor is correctly "centred" with respect to its characteristic curve. The expandor 3 may comprise an amplifier 44, Fig. 4, the gain of which is controlled by a voltage at terminal 22 from the discriminator 21, followed by a wide-band amplifier 45 having a negative feedback path including a comprssor 46 having the same characteristic as compressor 1, Fig. 1. The modulator 14, in one embodiment, comprises two oppositely-directed diodes 32, 33, Fig. 2, fed, in parallel with the decoded pulses from terminal 29 through resistors 30, 31 respectively, the common output terminal being connected also to a positive biasing source 36, through resistor 35. The diodes are opened at the appropriate time to pass the decoded pulses, by gating pulses 28, applied thereto and the " centring " control voltage operates a motor 26 to vary the position of the earth tapping on a potentiometer 27 connected across the resistances 30, 31. In a modification the control voltage is applied to a triode to control the current in its cathode resistor which is connected to the junction of resistors 30, 31, Fig. 3 (not shown).