Patent GB-390812-A (GB)
Title: Improvements in or relating to telephonic and like transmission systems
IPC: H04B3/06
Filed: 1931-10-08
Abstract: 390,812. Long - distance transmission. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Mathes', R. C., 171, Sagamore Road; Wyoming, New Jersey, U.S.A.) Oct. 8, 1931; No. 27944. Convention date, Oct. 8, 1930. [Class 40 (iv).] As the energy spectrum of speech is high at lower and middle frequencies and low at high frequencies, an automatic volume control operated directly by normal speech waves tends to emphasize the high frequencies. To obviate this, speech waves, before being used for control purposes, are passed through an equalizer to make their characteristics more uniform. The arrangement is particularly advantageous for long-distance submarine cable transmissions where the control station A is connected to the cable terminal B by a long land line. In Fig. 1A, an equalizer 119 is used in the transmission line itself so that the volume control 6 does not emphasize the high frequencies, while in Fig. 1B, normal speech is put into the control 6 and an equalizer 24 is used at the distant cable terminal. In both cases, high frequencies are present in the land line above their normal level relatively to the low frequencies so that cross-talk (greater at high frequencies) and high frequency components of noise in the input to A are increased. In Fig. 1C, an equalizer 19 is placed in the input of the control 6. so that the relative levels of the frequencies in the land line are normal. The equalizer 19 preferably has the same characteristics as an equalizer 24 at the distant end of the land line. Fig. 3 shows a long four-wire circuit connecting the control station A and cable terminal B. Transmission east from L passes over hybrid coil H to an automatic volume control 6 similar to that described in Specification 367,616, but modified as described below, a voice-controlled switching device comprising amplifier-rectifier 11, relay 12 and electrical, mechanical or acoustic relay device 13 being provided at station A. The transmitting path at terminal B comprises an equalizer 24, and a further voicecontrolled switching-device 21-23 which includes a relay 45 to disable the receiving path. The receiving path comprises at B, equalizers 25, 26 from which a tapping is taken to a control device 27, 28 which disables switching-device 21-23, and at station A the signals operate a further control device 29, 30 which disables the switching-device 11-13. The receiving path at A may contain a volume regulator. The volume control 6 comprises a pair of valves with their anode-cathode paths connected in series in the transmitting path, their impedances being controlled by grid bias due to a charge on condenser 35 derived from speech currents entering the control circuit at 33, the condenser being discharged or not in accordance with the condition of relays 48, 50. as described in Specification 367,616. In the present case, the speech currents determining the charge on the condenser 35 are passed through equalizer 19 before entering amplifier-rectifier 39. Specification 268,906, [Class 40 (v.), Wireless signalling &c.], also is referred to.