Patent GB-1251878-A (GB)
IPC: H04L25/48
Filed: 1969-10-09
Abstract: 1,251,878. Digital transmission systems. HASLER A.G. 9 Oct., 1969 [15 Nov., 1968], No. 49715/69. Heading H4P. Binary data is converted to a form in which a 0-bit is represented by the bit-pair 01, and a 1-bit by 11 and 00 alternately. A service signal may be transmitted by replacing three bitpairs (representing three data bits) with a bitpair 10, which indicates the start of the signal, followed by two bit-pairs which uniquely represent the three data bits.  In a first embodiment, Fig. 1 (not shown), the binary data and a bit-rate clock signal are passed to a gating arrangement which includes a bi-stable whose state indicates whether the last data 1-bit was odd or even. The arrangement selectively presents the bit-pairs 01, 11, or 00 to the output. At the receiver, Fig. 2 (not shown), the bit-pairs pass to a 2-bit shift register (15) which is stepped by clock pulses from a source (13) synchronized by the received bitpairs. The parallel output from the register passes to a decoder (16) also fed with bit-rate clock pulses obtained from the clock pulse source via a divider (14). The decoder gives outputs corresponding to the bit-pairs 00, 11 and 01, and operates a bi-stable accordingly to give data identical to that at the transmitter input. Should the decoder detect a bit-pair 10, which cannot be generated at the transmitter, either a noise pulse is present or the bit-rate clock pulses are 180 degrees displaced from their correct phase. If the rate of occurrence of such bitpairs 10 exceeds a certain level, as determined by a counter (19), the divider (14) is adjusted 180 degrees. The counter prevents adjustment occurring in response to isolated noise pulses. In a second embodiment, Fig. 4 (not shown), in which service signals may be transmitted, the binary data is passed via a 3-bit shift register (25) and gates (26, 27) to a code unit (23) which converts it to bit-pairs 01 representing 0, and 00, 11 alternately representing 1. When a service signal is to be transmitted a bi-stable (30) is set, preventing further data from being passed from the register to the gates. The three bits currently in the register are now passed to a code converter (36) which presents signals to the code unit such that it generates a bit-pair 10, to represent the start of the service signal, and two bit-pairs to represent the three data bits, Fig. 3b (not shown). At the receiver, Fig. 5 (not shown), the bitpairs pass to a shift register (45) which presents two successive bit-pairs at its outputs and feeds two decoders (46, 51). In the absence of a bitpair 10 one of these decoders (46) presents binary data to a second shift register (47) which feeds the receiver output. If the bit-pair 10 is detected the other decoder (51) impresses a 3-bit signal, corresponding to the said three bits at the transmitter, on to the second register (47). Consequently the output of the second register is identical to the data at the transmitter input. A counter (49) set in operation when the bit-pair 10 is detected, gives an output indicating the reception of a service signal provided that no further 10 is received during a preset period, i.e. provided the receiver clock is in phase. If a second 10 is received in this period the clock phase is altered by 180 degrees.