Patent GB-967642-A (GB)
Title: Improvements in or relating to pulse code modulation systems
IPC: H04Q11/04
Filed: 1961-09-27
Abstract: 967,642. Pulse code modulation systems; automatic exchange systems. WESTERN ELECTRIC CO. Inc. Sept. 27, 1961 [Oct. 20, 1960], No. 34658/61. Headings H4K and H4L. In a pulse code modulation system, particularly a time division multiplex system for use between telephone central offices in an exchange area, which employs a predetermined number of digit spaces for message transmission in each pulse group, a two-state direct current signal for supervising and control is transmitted from the originating office by adding a further digit space exclusive of the message digit spaces and transmitting a binary " 1 " in the added space in response to the one signalling condition and a binary " 0 " in response to the other state, the original D.C. signalling state being reconstructed at the terminating office in the corresponding channel in response to the content of the added digit space. In the reverse direction the additional space is used for transmitting " revertive impulses " during the setting of selectors a further additional channel being provided by using the least significant message digit space of the message channel concerned for indicating the off-hook condition of the called line. The arrangement is described as applied to a conventional telephone system which uses a seven digit code, the seventh digit being the least significant, and an eighth digit being added for signalling purposes. The closed-loop (busy) or open-loop (idle) conditions on a two-wire line at the originating office are monitored by a diode and converted into binary 1 or binary 0 respectively, Fig. 4, and combined, via a gate circuit controlled by a timing pulse coincident with the eighth digit period D8, with the outgoing signals. At the terminating office a gate operated during the eighth digit period selects the signalling information which is reconstituted into the appropriate D.C. condition on a twowire line by means of relay-operated contacts in the line. The D.C. signalling information (onhook or off-hook) supplied by this line at the terminating office is similarly monitored by a diode detecting polarity and transmitted as the appropriate binary signals to the originating office to operate a relay and reconstitute the signalling conditions on the two-wire line at the originating office. A further diode detects ground pulses at the terminating office and thus detects revertive pulses. As shown in Fig. 5, a binary " 0 " is caused to be transmitted in the seventh digit space and a " 1 " as the eight digit, to indicate " on-hook." For an " offhook " condition if the channel is in use a random binary " 1 " appears in the seventh digit space D7 due to the message signal and a " 1 " is sent as the eighth digit D8 and it is stated that when the channel is not in use there is enough random noise on the line to cause the random presence of binary " 1." When a revertive pulse is applied to the two-wire line it is transmitted as a binary "0" in the seventh digit space and binary " 0 " is transmitted as the eighth digit. At the originating office the random " 1 " signals appearing as the seventh digit are selected and integrated and supplied to a slow-release relay operating polarity reversing contacts in the two-wire line. The pulses appearing as the eighth digit are also selected and integrated and operate a further relay controlling contacts which ground the tip side of the line and put a negative potential on the ring side or vice versa-or short circuit the ring and tip, thus reproducing the conditions detected at the terminating office. Specification 855,609 is referred to.