Patent GB-1187688-A (GB)
Title: Digital Data Receiver
Inventor: NUSSBAUMER HENRI JEAN
IPC: H04L7/027
Filed: 1968-10-10
Abstract: 1,187,688. Digital transmission systems. INTERNATIONAL BUSINESS MACHINES CORP. 10 Oct., 1968 [8 Nov., 1967; 9 Nov., 1967; 8 Dec., 1967], No. 47975/68. Heading H4P. Synchronizing signals are derived from a signal which has digital data signals at times tn, t<SP>1</SP>n and non-significant signals at all other times. Whenever the signal has a preset amplitude a sync. pulse is generated. This amplitude occurs at tn, t<SP>1</SP>n and may also occur at other times. The receiver prevents sync. pulses occurring at times other than tn, t<SP>1</SP>n from effecting the oscillator. In one embodiment, the received signal contains binary data and passes to a subtraction unit S, Fig. 3, via rectifiers 31, 32 and delay 33 whose period t = t<SP>1</SP>n-tn. The output of S is a signal which has zero amplitude whenever the preset amplitude is detected, and is passed via monostables SS1, SS2 and inverter I1 to OR gate 01 which gives pulses p whenever the output of S is zero. Gate 01 feeds oscillator Plo which clock pulses of period T, T being the recurrence period of both tn and t<SP>1</SP>n. Gate A1 is inhibited by the output of monostable SS3, fed by the clock pulses, so that any sync. pulses occurring at times other than tn, t<SP>1</SP>n cannot reach Plo. During unitial synchronization, when a special start signal is received, a signal on Dem maintains A1 enabled. In a second embodiment, Fig. 5 (not shown), the digital signals may take any of four preset valves. Rectifiers 31, 32, Fig. 3, are additionally connected to two weighted subtraction units whose outputs pass to an EXCL-OR gate which feeds a second EXCL-OR gate inserted between S and SQ: Another arrangement, Fig. 6, in which there is no initial sync. signal, uses a series of delay units, each of period T, to which the rectified received signal is fed. An AND gate, A3 is fed as shown and emits a pulse about the times tn, t<SP>1</SP>n, Fig. 2b (not shown), since at other times at least one input to the gate is likely to be zero. The output of the gate is used to gate appropriate pulses of signal p to the oscillator. Alternatively, Fig. 7 (not shown), the delays may be replaced by a single delay T fed via an AND gate with the rectified signal, the delay giving synchronizing pulses which are also fed back to the gate. The delay may comprise a shift register, and a threshold unit may precede the AND gate, Fig. 8 (not shown). In another embodiment, Fig. 11, there is a unit 111 generally similar to Fig. 3, and in addition a weighted adder 112 and a unit 113 containing an adder, a delay, and a subtractor. The outputs of both 112 and 113 are zero only about tn, t<SP>1</SP>n, and are fed to analog adder AD3 together with the clock signal from PL0. The resulting signal c is zero at times t<SP>1</SP>n and attenuates the unwanted pulses from 111, the wanted pulses synchronizing the oscillator PL0. Where 4-level signals are received, Fig. 13 (not shown), units 112, 113, Fig. 11, are replaced by weighted adders Ad4, Ad5, the lesser of the two outputs passing to the gate Ad3. In a final embodiment, Fig. 15, the received signal is fed direct and via a delay t to a unit 151, which outputs the lesser of the two signals. This output passes to capacitor C1 to Cn, switched in sequentially for a period T/n by unit Seq. A capacitor charges slowly when switched in unless the output of 151 is zero, when the capacitor rapidly discharges. Since 151 gives zero output at random, but never at the times t<SP>1</SP>n, after several periods T one capacitor will have a charge higher than the others, so identifying the time t<SP>1</SP>n.