Patent GB-945163-A (GB)
Title: Improvements in or relating to electrical signalling systems
Inventor: PARRETT GEOFFREY ERIC
IPC: H04B3/44
Filed: 1959-12-31
Abstract: 945,163. Telephone transmission systems. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. Dec. 20, 1960 [Dec. 31, 1959 ; Feb. 15, 1960], Nos. 44430/59 and 5343/60. Heading H4R. A carrier current telecommunication system including a plurality of repeater stations having both-way amplifiers is divided into sections and each section is controlled by a different pilot current which serves to vary a gain regulator in its section to produce variations of feed current therein and thereby control gain-regulating networks in the individual repeater stations of the section. A signalling cable extends between terminal stations T1, T2, Fig. 1. It passes through intermediate repeaters A1, A2, A3 &c. of which repeater A3 is near the middle of the line and serves as a " turn-around point " in addition to its normal repeating function. Direct current source B1 energizes the section of the line between terminal T1 and repeater A3 while direct current source B2 energizes the section between terminal T2 and repeater A3, the two sections being separated by capacitors C1, C2. Pilot generator G1 energized by source B1 produces a signal of frequency fl which passes through high pass filters HP1, HP2 in the intermediate repeaters and is extracted by filter PF1 and supplied to dynamic gain regulator DR1 which adjusts the feed current by means of a slide on a resistor R1. Similarly, a signal of frequency f2 derived from generator G2 passes through low-pass filters in intermediate repeaters (not shown) to operate dynamic gain regulator DR2 and adjust feed current from source B2. Distributed correction is obtained by gain regulating networks in the repeaters, each such network comprising a Zener diode Z1 and thermistor H shunted across the repeater amplifier, the thermistor being controlled by the feed current. A second Zener diode Z2 having a slightly different characteristic acts as a stabilizer in case of failure of thermistor H. In the event of failure of one pilot generator a generator of the same frequency may be employed at the terminal station of the other section. In a second arrangement, Fig. 2, a single pilot generator G3 at station T2 supplies a pilot signal of frequency 1364 kc/s. to turn-around station A3 and a local oscillator LO changes the pilot frequency to 50 kc/s. for transmission back through filter F2 to station T2 for control of dynamic gain regulator DR2. The signal of frequency 1364 kc/s. is transmitted by filter F1 to station T1 to control dynamic gain regulator DR1. In the event of failure of the frequency changer or local oscillator the first half of the route is put on manual gain, the gain being adjusted at intervals in accordance with information relayed from station T1 over the speaker channel. In the second arrangement several intermediate feeding stations may be employed, a single pilot current passing from end to end of the line and the return frequency employed being different at each turn-around point. Alternatively, the return frequency may be the same if an appropriate filter is located at each intermediate feeding station.