Patent GB-1357905-A (GB)
Title: Time sharing subscriber communications system
Inventor: MOSES D W
IPC: H04N7/16
Filed: 1971-04-19
Abstract: 1357905 Telecommunications systems MINNESOTA MINING & MFG CO 19 April 1971 [9 March 1970] 23435/71 Headings H4K H4R and H4L In a TDM system in which subscribers terminals are connected in series with a wideband channel, the position of each time slot within a frame uniquely identifies a subscriber and the duration of each time slot corresponds to the amplitude of the analogue signal sample relating to the subscriber. The wideband channel may comprise a coaxial cable of a wired TV distribution system in which the first, e.g. 50 MHz band is reserved for telephone use. The cable may be looped (Fig. 2) between a transmitter 10 and a receiver 18 of a central exchange associated with the distribution network. The latter's receiving terminals 33 and repeaters 32 are shunted by low pass filters while the telephone subscriber terminals 16 and pulse regenerators 26 are shunted by high pass filters so as to segregate telephone and TV traffic. Alternatively the cable may be unlooped (Fig. 3) in which case the output of the transmitter 10 is modulated at 43 to a frequency band of, e.g. 108-174 MHz, lying between two TV bands, transmitted over the high frequency circuits of the cable to a distantly located demodulator 50 where the signal is converted back to the 50 MHz band and injected into the cable for distribution via the low frequency circuits. The subscriber terminals may serve up to six subscribers each of whom may be a computer. Transmitter Fig. 5 comprises a control board carrying a 10 KHz clock 88 whose output is applied to a pulse generator 61 as a synch signal and to the first modulator 64 of say 1000 modulators pertaining to the channels served by the system. The modulators are arranged in groups of ten and are arranged in tandem such that they operate in succession to provide a pulse width modulated pulse to the generator 83 so as to cause this to switch its output level across a zero axis thus providing one identifiable zero-axis crossing. Each modulator (Fig. 6, not shown) consists of an operational amplifier having one input fed by a capacitor that is caused to charge by a flip-flop which is set on the occurrence of a pulse from the previous modulator and which is reset on the occurrence of a pulse from the considered modulator, the latter pulse occurring when the analogue signal applied to the other input of the amplifier and the charge on the capacitor match. Channels which are not currently allocated are maintained in reservations boards 72 whose outputs are stepped over at a uniform rate since they carry no modulation. The output of the pulse generator is applied to coaxial line 14 via a transistorized voltage and current driver 160 (Fig. 17, not shown). Receiving terminal (Fig. 7) comprises an equalizing circuit 162 including surge limiters (Fig. 15, not shown) whose output is applied via a D.C. comparator 166 on the one hand to a ring counter 170 and on the other hand to an inverter 210. The former (Fig. 18, not shown) comprises three decade counters whose outputs are connected to gates so as to pick off any one or more particular time slots allocated to the terminal. In the case shown this is slot 1, output A. The length of the slot is measured by integrator 180 and the result sampled and stored by circuit 182. Depending on whether the resultant signal represents a voice sample or a D.C. signal, e.g. on/off hook, ringing, dialling it is passed by comparator 184 or operational amplifier Chebychev filter 186 to a hybrid circuit 190. The latter (Figs. 11 and 20, not shown) consists of a Go and a Return operational amplifier which are coupled to prevent singing and a pair of relays for transferring the D.C. signals past a speed transformer. The output is applied to Tip, Ring and Sleeve wires 20 extending to a subscriber. Signals from the latter are used to modulate the same time slot as that used for the incoming signals by means of D.C. comparators 222 (V.F.) or 224 (signalling) and operational amplifier 218. This has one input fed by a capacitor which is caused to charge by the output from a flip-flop 216 set by the occurrence of the trailing edge of the last time slot of the frame as detected by the ring counter output B. The modulated time slot as transmitted onwards by this terminal is always the last in any frame (see below "modulation") and is added thereto at gate 202 following inverter 210. Receiver 18 is similar to the demodulator portion of a receiving terminal but of course contains 1000 such demodulators and has no provision for adding information to any time slots of the frame. Modulation and synchronization. Fig. 4A indicates a 999 channel pulse width modulated frame defined between synch. pulses 54. At the first receiving terminal, the first slot is demodulated and all the pulses are inverted, Fig. 4B (as described above). The synch. pulse is effectively extended so as to eliminate the first time slot, by making use of output C (Fig. 7). Signals for onward transmission from the terminal are added to the frame by modulating a time slot occurring at the end thereof prior to the following synch. pulse, Fig. 4C.