Patent GB-548786-A (GB)
Title: Improvements in or relating to electrical communication systems
IPC: H04B3/20
Filed: 1941-08-25
Abstract: 548,786. Two-way transmission. ELECTRICAL RESEARCH PRODUCTS, Inc. Aug. 25, 1941, No. 10825. Convention date, Aug. 31, 1940. [Class 40 (iv)] [Also in Group XL] A transmission channel normally blocked against the transmission of speech is conditioned at the transmitting station by voiceoperated switching devices which also control the transmission of pilot current to condition the 'channel at the receiving station, the relative times of arrival of the pilot and speech currents being controlled by an adjustable delay network at the transmitting station. The adjustment may be made manually in response to verbal reports of "clipping," or, manually or automatically in response to automatically controlled backward signals. The invention is particularly useful for channels including wireless or other links subject to fading resulting in variation' of the time required for the conditioning of the receiving station. The application described comprises a two-way system in which telegraphy is interpolated in each direction during absence of speech and is similar to the system described in Specification 531,394. The backward signal may be a tone or a telegraph signal. In the terminal station shown in Fig. 3, the transmitting path comprises an automatic volume control 311, delay network 315, and two-stage secrecy devices 318, 320, the telegraph transmitter being connected at hybrid coil 321 and the pilot current source 340 at hybrid coil 322. When speech takes charge, detector 313, responsive to syllables but not noise, operates a relay set 314 which conditions the suppressors 317, 319 and the secrecy device 318. The relay set 314, subject to certain conditions in the telegraph transmitter, Fig. 1, to be described later, controls the pilot current suppressor 341 in order that current from source 340 may be transmitted. When speech ultimately arrives at the point 317-318 it controls a device 342 to reduce the pilot current output. The receiving path comprises suppressors 331, 333, 334 and secrecy devices 330, 332. Suppressor 334 is a gradually operating one to reduce clicks. Received pilot current passes from hybrid coil 325 to filter 326, amplifier 346 and detector 327 to operate the relay set 328 which conditions the receiving path and when speech arrives at the point 335-336 it also passes over filter 344 and amplifier to detector 327 to maintain operative the relay set, pilot current at the transmitter having by this time been reduced in level. Relay set 328 also disables the voice detector 313 and controls the volume regulator 311 in the transmitting path. No further equipment is required if the transmitting operator is to effect manual adjustment in response to verbal reports of clipping by a listening operator. Fig. 3, however, shows equipment for determining automatically the occurrence of clipping. Pilot and speech currents are fed respectively to detectors 349 and 350 controlling relays 351, 352 or a single relay with opposed windings so that, if speech arrives before the pilot current thereby giving rise to clipping, alarm 355 is operated to enable the matter to be reported (switch 354 in lower position) or a particular telegraph code. is transmitted (354 in upper position) as described later. The transmitting operator adjusts the delay network accordingly. The alarm or code transmission control circuit is opened at 353 by the relay set so as to prevent operation thereof due to the relapse of the pilot current relay 351 when the voice currents have taken charge of the receiving circuit. In a modification, Fig. 4 (not shown), for full automatic control of the delay, a relay arrangement similar to 351, 352 is used to control the transmission of tone to the distant end where it is separated out to supply an adjusting device for the delay network. Adjustable delay networks.-Fig. 7 shows an automatically adjusted network, but the manually adjusted arrangement, Fig. 6 (not shown), is similar with the relays 701-704 replaced by keys. The relays, controlled over switch 714, connect up networks 705-708 of varying delay periods or these networks may be of equal value and each relay causes each preceding relay to be energized. When a network is out of circuit it is replaced by an equivalent H-type resistance network 709. The switch 714 is driven by a reversible motor 715 which tends to drive the switch to a position providing minimum delay. When tone arrives, it operates relay 716 which completes a circuit for the winding of a slow thermal relay 717 in series with the forward drive winding of the motor. After a delay, the reverse drive winding (shown in operation) is disconnected and the switch is moved forward. When tone ceases the reverse drive comes into operation again after a delay and a sort of hunting action ensues. If the switch is driven to its last contact (maximum delay), thermal relay 720 brings in an alarm which may be transmitted to the distant end by a telegraphic code. Telegraphic apparatus.-The transmitter, Fig. 1, is similar to that described in Specification 531,394 and comprises a pair of tape transmitters and a diplex distributer, disabled when the voice currents take command but finishing a code if a single element remains to be sent, the pilot current suppressor 341, Fig. 3, in the telephone circuit being dependent on the condition of relay 104 operated by distributer segments 133, 134 just prior to transmission of the last element. The actual transmitter 131 is disabled by the operation of relay 103 which connects spacing potential to relay 128. Taut tape switches 142 operate relays 138, 145 which respectively cause the transmission of codes comprising all marks and all spaces. Relay 150 operated by the " clipping " signal referred to above operates relay 138 to transmit an all-mark code to call for delay adjustment at the distant end: The receiver, Fig. 5, is also similar to that described in Specification 531,394. It is enabled to receive by control of a " telegrad " (Specification 538,006) 500 from relay set 328, Fig. 3, which when the voice currents take charge, releases relay 539 (operated when the circuit is receiving) to move relay 508 or 509 of the two printing mechanisms to the position opposite to that shown, thereby disconnecting the corresponding printing magnet.