Patent GB-1210037-A (GB)
Title: Improvements in or relating to multichannel delta modulation message transmission systems
Inventor: DAVIS CLAUDE GENE
IPC: H04Q11/04
Filed: 1969-08-27
Abstract: 1,210,037. Automatic exchange systems; Companders. WESTERN ELECTRIC CO. Inc. 27 Aug., 1969 [28 Aug., 1968], No. 42608/69. Headings H4K and H4R. Widely distributed subscribers, as in a rural situation, have short lines which are tee-connected to points along the length of a timedivision-multiplex bus which is looped through their area from an origination at a main exchange to a termination at that exchange, the bus providing fewer channels than there are subscribers and employing delta-modulation, a control circuit at the main exchange allocating a time slot on the bus for a connection and sending the identity of the slot to a subscriber's circuit providing the tee-connection with the bus and such as to store the slot identity so as to govern extraction of the appropriate bit train from the bit stream sent from the main exchange and to substitute a bit train for transmission to the main exchange. As described, 80 subscribers are concentrated on to 14 channels provided by the bus, the subscribers being arranged in small groups so that the subscribers of each group can be tee-connected to the bus at a common point, Fig. 1 not shown. The main exchange is equipped with a line circuit for each of the 80 subscribers and, as shown in Fig. 2, each line is connectible to channel units CH1 to CH14 by way of a 4-wire cross-bar switch 201, each channel unit corresponding to a time slot on the bus. Such connection employs the first two wires over the switch and is made to a channel unit preallotted by a scanner which steps over busy conditions present on the third wires of the switch outlets and comes to rest on the first idle outlet to operate the associated select magnet, Fig. 3, not shown. If all channels of the bus are busy the 15th and 16th outlets of the switch are preallotted to connect busy tone. The fourth wires of the switch, Fig. 4, are used for operation of the bridge magnets H1 to H80 and to generate codes identifying the subscriber to a connection and identifying the allotted time slot. A binary counter 404, stepping at the frame rate of the bus multiplex, by way of a 1-out-of-80 converter 406, scans the bridge magnets one-byone and, if a calling condition is present on a fourth wire SD1 to SD80 from the main exchange, the bridge magnet operates to connect the associated inlet to the preallotted outlet channel unit. The position of counter 404 identifies the subscriber's line appearance, and a so-called trunk translator 402, activated by the fourth wire of the seized outlet, produces the identity of the time slot constituting the channel. These identities are converted to serial form for transmission over the bus. At the subscriber' s  tee-connection circuit to the bus the own line appearance code is recognized as it appears in the bit stream and effects a gating action to extract the immediately following time slot code for registration in the circuit so that the circuit may be suitably cycled to function in that time slot. Each time the counter 404 scans the switch inlet the line and slot codes are regenerated and transmitted to confirm the connection. The line code is generated at each scan invariably, whether a connection exists or not, and the transmission of this code effects a gating at the corresponding distant subscriber's teecircuit so as to return an indication of on-hook/ off-hook conditions. If an off-hook signal is received back from round the loop of the bus it occurs with the counter 404 still in its position marking the corresponding bridge-magnet and this causes the bridge-magnet to operate and set up a connection in the same way as if the connection had been initiated from the main exchange. On-hook/off-hook supervision also takes place over another supervisory channel to effect switching of the speech channel. Supervisory channels in the multiplex.-As seen in Fig. 5 the bit stream on the bus is composed of the repeated bit sequence TSS1, TSS2, TS1 to TS14, of which TSS1, TSS2, are supervisory bits and TS1 to TS14 are delta modulation channel bits. Each such sequence forms a group of which 16, GRS1, GRS2, and GR1 to GR14, comprises a cycle. Two cycles form a subframe and four cycles comprise a frame. The supervisory bits TSS1, TSS2, in any group relate to the correspondingly numbered TS speech channel and these bits in the groups during even cycles of the first subframes relate to two-digit codes transferred between syllabic compressors and expanders to indicate the step size at which the delta-modulators and demodulators should operate. The bits TSS2 of groups in odd cycles are used to relay ringing from the main exchange by controlling a ringing generator at a subscriber's distant teeconnection circuit and are used on the return portion of the bus to effect, at the channel units at the exchange, loop supervision of the distant subscribers. The 7-bit code identifying a subscriber's line appearance is sent from counter 404 and converter 405 as bits TSS1 in groups GRS1, GRS2, and GR1 to GR5 in odd cycles of the first sub-frame of a frame. In the same sub-framed odd cycles the bits TSS1 of groups GR6 to GR10 are used to transmit the code identifying the time slot of a connection as supplied from converter 403 and translator 402. Failure to detect a time slot code accompanying the line appearance code is interpreted as release by the distant tee-connection circuit, as failure to detect loop closure at the channel unit is interpreted as release at the exchange. Companding.-The syllabic compressors associated with each delta-modulator produce two-bit codes which are sent periodically to govern the step size of the distant expander controlled demodulators.