Patent GB-1069628-A (GB)
Title: Multichannel communication system
IPC: H04L5/26
Filed: 1963-12-19
Abstract: 1,069,628. Multichannel data transmission. WESTERN ELECTRIC CO. Inc. Dec. 19, 1963 [Dec. 28, 1962], No. 50123/63. Headings H4K and H4R. A number of trunk circuits TR1 to TR6, Fig. 1, each carrying narrow band data signals, are frequency multiplexed on to a four-wire voice transmission circuit 26 together with a further channel over which is transmitted supervisory signals derived from the trunk circuits and timo division multiplexed, in multiplexer 14 on to a narrow band adjacent to the data channels in the voice band. Data in each of the trunk circuits TR1 to TR6 is transmitted from the call originating station as frequency shift modulation of Œ 100 c.p.s. about a frequency of 1170 c.p.s. and received as frequency shift modulation of Œ 100 c.p.s. about 2125 c.p.s., the frequencies being reversed for the terminating station. The data channels are frequency multiplexed on to the four-wire line 23 via a modulator arrangement as shown in Fig. 4. The duplex data signals on line 40 are applied via a hybrid 44 to a limiter 45, modulator 46, filter 47 to remove the upper sideband of the modulation product, an A.G.C. circuit 49, and a band filter 52, to be combined with the output signals from similar modulators fed with different carrier frequencies and fed to the voice channel via the line circuit 22. Received signals are filtered in filter 53, to extract the relevant frequency channel, demodulated in demodulator 54, and the appropriate signal selected by filter 55 and applied via amplifier 56 and hybrid 44 to the trunk circuit 40. Since a terminal station interchanges its frequencies for transmission and reception when it changes from an originating office to a terminating office the carrier supplies for the modulator and demodulator have to be interchanged. This is accomplished by relay contacts K1-2 and Kl-3 of relay K1 under the control of the supervisory signals transmitted with the data channels as described later. In. addition, an attenuator 42 is switched out of circuit by the relay when the station acts as an originating station in order to equalize the signals in the lines. The A.G.C. circuit has an inhibiting gate 51 in its feedback path which provides the maximum gain condition when the on-hook signal appears on either line CE or CM, indicating that one station is calling the other. Since the channels in the frequency multiplexed signal are evenly spaced at 477À5 c.p.s. intervals the various carrier frequencies for the modulators and demodulators of the six data channels are derived from one oscillator of 3820 c.p.s. divided down by a factor of 8 to form a frequency of 477À5 which controls a pulse generator to form a harmonically rich signal of fundamental frequency 477À5 c.p.s., the various carrier frequencies being obtained via narrow band filters connected to the pulse output, Fig. 3 (not shown). Supervisory signals appear on the lines E and M in Fig. 1, the M leads giving an indication of the switch hook position in the trunks at the home station and the E leads the switch hook condition relative to the trunks at the distant station and received over the line in TDM form. Fig. 6 shows the apparatus used to time division multiplex the signal on the M leads in the trunks on to a single channel to be transmitted over the same voice channel as the data signals in which a 477À5 c.p.s. signal, derived from the carrier supply circuits for the data channel modulators; is divided by five and the resulting 95À5 c.p.s. signal applied to a threestage binary counter 72, 73 and 74, which would normally cycle once every eight pulses of the input signal. A further binary stage 76 is, however, connected to be operated from the output of binary C (or 74) on the eighth pulse and the output of stage N (or 76) inhibits gate 71 and prevents the ninth pulse reaching the counter. Gate 76 is enabled by binary N however so the ninth pulse passes through gate 86 to reset binary N to open gate 71 for the tenth and succeeding pulses, the cycle of the counter chain is thus extended to nine pulses. The appropriate outputs of the binary stages A, B, C, and N are connected to the gates 1 to 6, S1, S2, and T to enable each of these gates in sequence. Gates 1 to 6 also have the signals from the M leads of the trunk circuits connected to them so that the output from the gates may be combined in a mixer 87 to supply the multiplexed supervisory signal. In addition gates S1 and S2 are enabled by the counter ABC and N at the appropriate point in each frame but due to their connection with binary D they are alternately enabled in succeeding cycles to provide a synchronizing signal which may be used at the receiving station to allocate the switch hook appearances to the appropriate trunk. In addition a further gate T is enabled once per cycle and may be utilized for indicating a fault condition. The multiplexed output is fed from mixer 87 to a frequency shift modulator to provide a signal of Œ35 c.p.s. about a centre frequency of 350 c.p.s. which is transmitted over the voice line with the data channel signals. Supervisory signal demultiplexer, Fig. 7.- The binary signal recovered after demodulation of the frequency shift supervisory signal from the voice band is applied in opposite phases to gates 91 and 92 and the outputs of these gates is applied to the inputs of gates 114-125 pairs of which are opened sequentially in accordance with the outputs of binary stages AB and C, the counting cycle of which is extended to nine by gating out, at gate 95, the drive pulses to the binary stages for the duration of one signal pulse at the end of the counters nominal count of eight. In addition the gate 95 may be blocked by the logic circuit if the synchronizing pulses, occurring in each frame and consisting of a pair of pulses alternately 1, 0 and 0, 1 in successive frames; does not occur in the correct position, i.e. at the end of the binary chains count of eight. The serial supervisory signal data is thus fed through the appropriate gates to set the registers 1 to 5 so that the supervisory signals E1 to E5 may be applied to the appropriate trunk circuit.