Patent GB-1299226-A (GB)
Title: Method for synchronizing digital signals and an arrangement for carrying out the method
IPC: H04J3/17
Filed: 1970-03-25
Abstract: 1299226 Digital transmission systems TELEFONAKTIEBOLAGET L M ERICSSON 25 March 1970 [26 March 1969] 14610/70 Heading H4P In a method for synchronizing blocks of digital signals, a cyclically repeated synchronizing word is generated the length of which is such that it can be contained in a block in at least twice, the words being superposed on the information signals transmitted. At the receiver the same synchronizing word is again superposed on the combined signals in order to restore the original information signal. The synchronizing words are inserted in any intervals which may occur between the information signals. Continuous speech is assumed to have at least 30% intervals exceeding 30 m./sec. during which intervals synchronizing signals are sent. In the transmitter a vocoder converts amplitude speech signals into digital form which are combined in an exclusive OR circuit EES with a series of binary pulses from a code generator KGS which generates a cyclically repeated pulse train i.e. a synchronizing word having a combination to decrease the probability that a corresponding series of bits can appear at random in the vocoder signals. The code generator may consist of a number of JK bi-stable circuits. The first fifteen bit spaces are not used and the second fifteen in a 60 bit word may be used for synchronization. In the receiver a code generator KGR controlled by counting chain WR generates a sequence identical to that produced KGS controlled by chain WS, which is applied together with the received signal to an exclusive OR circuit EER which may be formed by NAND circuits K-N, Fig. 2. Counting chains WS, WR must be set simultaneously to zero and synchronizing words must appear synchronously which is determined by developing a 1 signal to set WR from a determined number of synchronizing words i.e. three words in the case of a 60 bit block. Serial input signals comprising vocoder signals, synchronizing words or both are applied to a shift register SKR having 15 stages which are each connected to a resistor matrix R1-R15 and a threshold detector T which is activated each time the shift register contains a synchronizing word; a counter at SM records how many times a synchronizing word has been received. When the appropriate number has been recorded, a zero setting signal is sent to WR and a new block is started. When WR reaches say 16, KGR receives a starting signal and generates synchronizing words until stopped by zero setting of WR. 2400 baud signals and block length (L) of 60 bits.-When counter RK reaches position 30 indicating that a number of synchronizing words i.e. 3, have been received, signals from RK, T are received by AND gate LC which sets bi-stable circuit C and hence via logic LG sets monostable circuit EV. Monostable circuit EV delivers a 0 setting pulse to WR hence this starts its counting period of 60 bits. When RK reaches position 32, bi-stable circuit A-C are set to zero via AND gate LA the inputs to which are connected to RK via OR gate EA and RK is stopped in the 0 position by bistable circuit A. If only two synchronizing words have been received no signal will emanate from AND gate LC and bi-stable circuit C is not set to Fig. 1 hence EV is not activated and bi-stable circuits A, B are zero set when RK reaches 32 hence RK is stopped in its 0 position. A corresponding chain of events stops RK in the 0 position if only one synchronizing word is received. Operation at 1800 bd and L=40 bits also 1200 bd and L = 30 bits is described in the Specification.