Patent GB-1460033-A (GB)
Title: Telephone systems
IPC: H04M11/06
Filed: 1973-12-11
Abstract: 1460033 Multiplex systems WESTERN UNION INTERNATIONAL Inc 11 Dec 1973 [13 Dec 1972 1 Feb 1973] 57424/73 Heading H4M A voice communication channel of given bandwidth is utilized for two duplex conversations by restricting the bandwidth of each conversation to less than half the given bandwidth. Fig. 2B depicts the normal use of the channel for a 300-2400 Hz voice signal, a 2600 Hz off-hook and dialling signal, and possibly a data signal in the remaining band up to 3000 Hz. Fig. 2A depicts the bandwidth limited case for two speed signals and an additional 1300 Hz signalling frequency between the speech bands. The higher band is obtained by limiting the second speech signal to the band 0-1100 Hz, modulating it on to a 2600 Hz carrier and then passing it through a 2400 Hz low pass filter. Fig. 1 depicts the general arrangement for doubling the capacity between two PBX's over a limited number of 4-wire trunks 22. Data may be multiplexed on to these trunks using modems. The details of the multiplexers are found in Figs. 3A, 3B. At rest, the signalling frequencies 1300 Hz and 2600 Hz, obtained by clocking gates 48 and 88 respectively, are transmitted via a resistive linear combining network 232 to the respective Go wires of the two multiplexors. If a PBX "A" subscriber calls, an off-hook signal is conveyed over his PBX's M lead 276 to a routing circuit which causes a gate inhibiting signal to be applied to lead 64. The 1300 Hz tone thus ceases and after a suitable checking period, e.g. 200 m.sec. an off-hook detector in PBX "B"s multiplexor responds by pulling-up relay 136 and thus alerting PBX "B" of an incoming call over the E lead 280. Dialling impulses are repeated on bursts of 1300 Hz oscillation. Speech is subsequently effected over leads 54, low pass filter 42 and the linear combiner 232 at the Go end, and via leads 138, filter 118 and lead 146 at the Receive end. A subsequent call takes into use the "transmit two" and "receive two" apparatus in an analogous manner. It may be of interest to note that the 1100 and 1300 Hz filters are respectively 4 and 5 stage, 7th and 9th order CAUER filters incorporating operational amplifiers; the tone detectors use integrated circuit modules while the pulse width discriminators use JK flip-flops. Details of these and the other circuit blocks are provided in Figs. 5-10, not shown. In order to use the full band for a single conversation, a low band connection is set up as mentioned above whereafter both parties actuate a key in their substations. The signal on lead 274 prepares the routing circuit 38 and actuates relays 220, 222 (Fig. 3B). The latter have the effect of connecting the full bandwidth signal at the output of a 600 ohm interface 40 in "transmit one" apparatus to the input of a 2400 Hz filter 82 in "transmit two" apparatus thus bypathing all the equipment used when narrow band speech is to be transmitted. At the Receive end, the incoming signal on lead 186 passes directly from LPF 162 in the "Receive two" apparatus to the output amplifier 120 and interface 122 of the "receive one" apparatus. When the call ends a relay in steering circuit 38 comes up to inhibit lead 64 thus cancelling the 1300 Hz signalling tone and to connect the local PBX's low band E and M leads to the 2600 Hz signalling leads. Restoration of the multiplexor to two ¢-channel working is not discussed.