Patent GB-628348-A (GB)
Title: Improvements in or relating to control circuits for signal transmission systems
IPC: H04B3/20
Filed: 1947-07-18
Abstract: 628,348. Two-way transmission. WESTERN ELECTRIC CO., Inc. July 18, 1947, No. 19261. Convention date, Oct. 4, 1939. [Class 40 (iv)] In a two-way transmission system including a volume-operated gain-adjusting device (VOGAD) in the transmit path and a variableloss device in the receive path, a control tone is transmitted first through the variable-loss device and then through the VOGAD, and means automatically responsive to the control-tone output of one or other of the devices controls the signal volume in the receive path. General lay-out and mode of operation. The embodiment described is a 4-wire terminating circuit for a radio-telephone system, which as shown in Fig. 1 comprises a transmitting circuit TC leading to a radio transmitter RT and a receiving circuit RC leading from a radio receiver RR associated with a toll switchboard by the hybrid transformer H. Each single line in this figure represents a two-wire transmission path and the apparatus is represented by suitably labelled boxes. Contacting or separated arrowheads in a path denote that the path is normally made or disabled respectively at that point. Arrows directed at made or disabled points from a box indicate that the path will be disabled or enabled at that point by operation of the apparatus in the box. The box 4 represents a two-way privacy apparatus (not described). The normally disabled transmitting circuit TC is enabled at 5 and 6 ; and RC is simultaneously disabled at 7 and 8 under the control of outgoing telephonic signals which operate the transmitting vodas TV connected by hybrid transformer HI to TC. The delay circuit 11 delays the transmitted signals to prevent their loss during the operating time of TV. A receiving vodas RV connected to RC by the pad 15 is operated by received speech currents and disables TV (at point 16) and the vogad 17. False operation of TV by echoes, and change of gain of the vogad during receive periods are thus prevented. The gain in TC is automatically adjusted by the vogad 17 to provide a predetermined output level. The received volume is kept at such a level as will just avoid operation of the vogad gain increaser or of the transmitting vodas by echoes of received signals too weak to have produced operation of the receiving vodas and consequent disablement of the vogad and of the transmitting vodas. This is achieved by feeding the output of a local 6 K/c. pilot tone oscillator 19 into the receiving circuit over the ladder potentiometer 12 in the input circuit of RV and the circuit dividing pad 15. This tone is transmitted through the variable-loss element 18 and the receive amplifier 25 and thence via byepass circuit 24. Part of the tone current is applied over network 23 to a first amplifier-detector 20 and the remainder passes into the transmitting circuit, through the vogad 17 and is-applied to a second amplifier-detector 21. These two amplifier-detectors are arranged so that the one having the greater tone output exercises sole control over the loss inserted by the loss element 18. The controlling amplifier-detector controls the element 18 so as to keep the tone input to that amplifier-detector constant for any given setting of transmitting vodas detector or vogad detector sensitivity. Suitable adjustments of pilot oscillator output and amplifier-detector sensitivity result in the received level being kept at the desired value. Variations in the return loss at the hybrid H which affect the strength of echoes are compensated by manual adjustment of the loss pads 27 and 29 in the input circuits of the amplifier-detectors 20, and 21, respectively. Detailed description, Figs. 2, 3. The transmitting vodas TV includes in its input circuit variable-loss pads 47 and 48 which under the control of key 49 provide a total loss of 0, 5, or 10 d.b. to control the sensitivity of the amplifierdetector 10. This detector, which is designed to be unoperated by the maximum expected noise level on TC but to be operated quickly by speech currents, comprises a voltage amplifier-detector 50 controlling the switching relays of the transmitting vodas, and a syllabic-type amplifier-detector 51 operating as an enabler for 50. the two amplifier-detectors being coupled to a common input circuit by transformer 52, a portion of the secondary winding of which is shunted by a voltage-dividing resistance VD. The detector 50 comprises one stage of amplification fed from the upper tap and the bottom terminal of VD and coupled by a transformer VIS whose secondary is tuned to 1300 cps. to a full wave rectifier VR. The last stage, to which the output of VR is fed, provides direct current amplification and controls the master transmitting relay TM which operates switching relays 38 to 41. The other detector 51 comprises a similar amplifying stage which is fed from the secondary winding of 52 and is coupled by a transformer SIS tuned to 1800 cps. to a rectifier SR consisting of the diode portion of the diode-triode SDA. A bandpass filter SBF comprising resistances 66 to 70 and condensers 71 to 74, and having a passrange of 1-15 cps., is connected in the output circuit of SR and feeds the direct-current amplification stage provided by the triode portion of SDA. The output of detector 51 controls relays TMI and TMF. With no speech waves applied to detector-amplifier 10 the lower portion of VD is short-circuited by the back contacts TMI and TMF in series and the voltage input to detector 50 is reduced by approximately 10 d.b. This drop is sufficient to prevent false operation of 50 to normal line noise, while false operation of 51 is prevented by SBF which passes only the syllabic frequencies of speech and therefore rejects the relatively 'steady noise. As soon as speech is transmitted over TC a portion of the signals passes through the inter-stage transformer of detector 51 which has a pass-range of 1500-3000 cps. and.makes the detector substantially consonant-operated. The filter SBF passes the low frequencies produced by the syllabic nature of the speech and the resulting anode current of the SDA triode is applied to TMI and TMF. These relays are poled and biassed so that TMI operates at the beginning of a word or syllable and TMF at the end Operation of either removes the shortcircuit from the lower portion of VD and steps up the input level to detector 50 by 10 d.b. approx. Detector 50, which is essentially vowel-operated, the pass-range of its inter-stage transformer being 800-1500 cps., operates to the resulting higher level signals and brings up TM followed by 38 to 41 which unblock TC and disable RC to suppress echoes and prevent singing. In the absence of outgoing speech signals in TC, RC is operative and speech from RR passes through the noise reducer 42 which may be of the type disclosed in Specification 452,084. Part of the signals is applied to the receiving vodas RV. If the level is high enough the output of the vodas amplifier-detector 14 operates RM which opens the operating circuit of TM in the transmitting vodas and brings up 37 which disables the transmitting vogad 17. The variable-loss device used in the receiving circuit is the vario-repeater 18 which consists of a pentode amplifier RL, the gain of which is controlled by a direct current voltage fed to the control grid through the filter TF. The voice signals and the pilot tone output of the amplifier tube RL pass through the dividing network 43 to the receive amplifier 25. Part of the voice signals passes through the low-pass filter 44 to a monitoring circuit (not shown). The output of 25 splits, the voice signals passing via low-pass filter 26 and pad 32 to the hybrid H, while the 6 K/c. pilot tone is transmitted through the byepass circuit 24 which is tuned by condensers 96, 98 and transformer 97 to pass 6 K/c. tone but to offer high impedance to currents in the voice band. Part of the tone signal is impressed by transformer 97 on the control grid of the pentode RA forming the amplifier stage of amplifier-detector 20. This stage is coupled by a transformer tuned to 6 K/c. to one diode RD of the double-diode 101, through the sensitivityadjusting series resistance 102. The rectified direct current output of the diode RD appears across the load resistance 103 and is applied to the control grid of pentode RL. The part of the pilot signal introduced into the transmitting circuit through byepass 24 passes via pads 31, 36 and the vogad 17 to transformer 105 which is tuned to 6 K/c. This transformer applies it to the control grid of the pentode TA of amplifier-detector 21 which is similar to 20. The rectified output of diode TD thus appears across resistance 103 in like manner to that of RD. Each of the diode anodes of tube 101 is negatively biassed to about 20 volts by voltage drops produced by battery 107 so that no rectified output appears until voltages in excess of 20 volts are applied to either RD or TD. As soon as such a voltage is applied to one of them the other is back biassed and only that diode having the greater signal applied to it is effective to develop a voltage across 103, and thus to control the gain of the pentode RL. This control acts to reduce the gain of RL and at the critical point changes the normal gain of the tube to a large loss, so that at the critical input level to one of the amplifier-detectors a rise in level of 1 d.b. reduces the amplifier gain by about 40 d.b. The sensitivity of amplifier 20 is initially adjusted by resistances 117 to 121 switched into the cathode-ground path of tube TA by relays M, G and H, L under the control of the switchboard operator and the sensitivity control key 49 of the transmitting vodas TV. That of 21 is similarly adjusted by resistances 122 to 128 switched in by relays V, NS, G, and M controlled from the switchboard. For a given detector sensitivity the amplifier-detector controlling the pentode RL holds the input level of the 6 K/c. tone at that amplifierdetector constant i