Patent GB-1031687-A (GB)
Title: A synchronising signal detector
IPC: H04L7/04
Filed: 1964-03-12
Abstract: 1,031,687. Multiplex pulse code signalling. NIPPON ELECTRIC CO. Ltd. March 12, 1964 [March 18, 1963], No. 10471/64. Heading H4L. [Also in Divisions G1 and G4] An arrangement for detecting timing points given by a synchronizing signal contained in an input signal received at a receiver, for use in synchronizing the receiver, comprises a matched filter which receives the input signal and produces a correlation signal indicative of the correlation between the input signal and a waveform similar to the synchronizing signal, a timing signal input terminal for receiving a timing signal determinent of the reference time point for the a priori probability distribution of a synchronizing timing point in the synchronizing signal, a waveform generator for generating in response to the timing signal an output waveform indicative of the a priori probability distribution of the synchronizing time point, an adding circuit which combines the correlation signal and the said waveform and a detector which produces an output signal when the combined signal exceeds a preset value, thus detecting the timing points. The relative detection level for a time point in the correlation signal is varied in accordance with the a priori probability distribution for the position of a synchronizing time point. The input signal y(t) consists of the synchronizing signal u(t) and the information plus noise signal n(t), i.e. y(t) = u(t) + n(t), and passes from the source 11A (Fig. 1) to the receiver input 11 and hence to, the matched filter 12. This includes a tapped delay line 21, and each tapped output from the delay line is connected to a corresponding coefficient circuit a 0 , a 1  &c. These circuits give outputs which have a predetermined relationship to the received signals and all these outputs are added on the line 23. It is shown in the Specification that with the correct choice of parameters the arrangement 12 will provide as an output the correlation function defined by where No/ 2  is the mean power of the information plus noise signal n(t) and T is the synchronizing time point. To ensure that the correlation function thus derived will yield the correct information concerning the position of the synchronizing time point (T), a function p(T) corresponding to the a priori probability of occurrence of the point (T) is generated in the receiver and combined with the correlation function to give a function representing the a posteriori probability that the time point T is the true synchronizing point. This probability function is given by the relation To detect the time which represents the correct synchronization with maximum probability it is necessary to detect when q(T) + log p(T) reaches a maximum. In the apparatus a generator 100 generates timing signals corresponding to the anticipated positions of synchronizing timing points and the signals are fed to a waveform generator 17. This generator consists of a delay line 26 the tapped outputs from which are fed through coefficient circuits b 0 , b 1  &c., which produce outputs in predetermined relationship to the timing signals. These outputs are summed on the line 28, and parameters are so chosen that the summed output signal, g(T)=log p(T). The adder 18 forms the sum g(T) +q(T). The detector 19 determines when this sum exceeds a threshold value by limiting at this threshold and determining when the time derivative of the limited combined output becomes zero. The detection level is arranged to be adjusted in accordance with the a priori probability curve p(T), so that when maximum probability occurs the detection level is at its lowest. By setting the level higher when the occurrence of synchronizing signals is less probable, the possibility of obtaining erroneous synchronizing signals (due for example to peaks in the correlation output of the filter 12 due to noise) is reduced. The output obtained from the detector 19 may be used as the timing input to the waveform generator 17, supplied via a timing circuit which may consist of a tuned filter or phaselock oscillator (Fig. 3, not shown). Furthermore, the detection level in the detector 15 may be adjusted to allow for any variation in noise level in the input signal to the system (Fig. 4, not shown). The output from the waveform generator 17 is controlled in this case so that the function g(T) is modified according to the noise power No/ 2 . An embodiment wherein synchronizing signals consisting of coded digital signals contained in digital information are detected is described (Fig. 6). It may be necessary with such information to overcome a shift of synchronizm of several bits, which may prevent detection of a synchronizing time point within a detection interval. The device described provides for the detection of such a time point by extending the synchronizing signal detection interval by one bit until the synchronizing time is eventually determined. To this end the system includes in addition to the matched filter 12, combining circuit 18, detector 19 and waveform generator 17, similar to those of the embodiment of Fig. 1, a clock pulse generator 52, energized by the input signals. The clock pulses B are supplied to a distributer 54, e.g. a ring counter or rotary switch, which provides pulses successively at N step positions. The distributer is arranged so that a series of pulses arranged about the anticipated time point T are delivered and are fed to the waveform generator 17 where they are each attenuated so that their overall amplitude time configuration corresponds to the required a priori probability distribution. An output C from the detector 19 enables the distributer to be correctly controlled, and switches off a flip-flop 66, which was previously turned on by a pulse Sm from the distributer. If no synchronizing time pulse is detected at 19, the flip-flop 66 is not turned off, and since its output F is applied to an AND gate 68, a pulse from the distributer Sp is allowed to pass through the gate to the input D of the distributer. This resets the distributer such that the detection interval is extended, and the process continues until any shift in synchronizm is compensated for.