Patent GB-553290-A (GB)
Title: Multichannel signal transmission
IPC: H04L25/04
Filed: 1942-06-15
Abstract: 553,290. Multiplex transmission. STANDARD TELEPHONES & CABLES, Ltd. June 15, 1942, No. 8146. Convention date,'June 14, 1941. [Class 40 (iv)] In a multi-channel system of the kind described in Specification 537,893 in which the signals are accompanied by a pilot wave of non-signal frequency which varies in amplitude so that the total power applied to the line is constant and the gain at each repeater is dependent on the total power received thereat, arrangements are described for diverting and adding signals at intermediate repeater stations. The system shown in Fig. 1 caters for 12 channels between 12 and 60 kilocycles, and the terminal station transmits a pilot frequency of 61 kilocycles, for example, modulated in accordance with the envelope of the total signal power applied to the line. It is assumed that channels Nos. 11, 12 provide programme circuits to intermediate stations Al, A2, B and other more distant stations, and that channel No. 3 provides a conference circuit between the terminal and A1, A2, B, this channel being used also for a short connection between B and A3 and again between C and a more distant station, while channels Nos. 4, 8, 9, 10 are diverted and re-introduced as indicated. At station Al, Fig. 2, the output of the amplifier is applied to a hybrid coil 5 having two sets of output terminals, one set connected to the next section of line and the other set connected to parallel band-pass filters 6 for the diverted channels Nos. 3, 11, 12. All the channels pass to the outgoing line section, which is in conjugate relation to the filters 6, and the loss of power due to absorption in the diverting equipment is uniform over the frequency range and is automatically compensated at the next repeater. Each of the filters 6 is designed to present a uniform input impedance over the whole frequency range so that any reflection does not disturb the relative power content of the transmitted channels. Any diverting circuit may be disconnected as by a switch 9 without affecting the operation of the system. Stations A2, A3 are of similar type. At station B, Fig, 3, after amplification the diverted channels are tapped off through a resistance 10 and filter 11, while the pilot and the unwanted channels (including channel No. 4 which was diverted at the preceding station A2) are eliminated from the through path by filters 13, 14. The remaining channels are applied through a resistance pad 15 to an amplifier 17 of the kind described in Specification 537,893 which has a positive feed back path for generating the pilot frequency at such amplitude that the total output is constant. This amplifier is also fed with the added channels Nos. 3, 4 from a modulator 16. At station C, Fig. 4, after amplification, channel No. 3 (which was diverted at A3) and the pilot are suppressed by filters 21, 22, and the remaining channels are combined with the added channels from a modulator 24 in a hybrid coil 23 which feeds an amplifier 27 similar to 17, Fig. 3. In a system including intermediate stations such as B, C where the pilot wave is generated afresh, the effects of inter-modulation between the pilot and the signals will be reduced if the new pilot is of the same frequency but reversed in phase relative to the suppressed pilot, and a similar result may be obtained without phase-reversal if the pilots in successive line sections differ slightly in frequency.