Patent GB-1481849-A (GB)
Title: Digital code transmission systems
IPC: H04L1/00
Filed: 1974-07-24
Abstract: 1481849 Telegraphy NIPPON TELEGRAPH & TELEPHONE PUBLIC CORP 24 July 1974 [2 Aug 1973 22 Aug 1973] 32601/74 Heading H4P In a method for monitoring the operation of a digital code transmission system which incorporates bit rate converters operating such that a data bit pulse train is divided into blocks and the bits in each block are time compressed to provide vacant bit positions between successive blocks, a check code related to each block of data bits before bit rate time compression is formed at the transmitter and inserted into the vacant bit positions available after compression. The transmitted check code is extracted from these positions at the receiver before bit rate reconversion and is compared with a check code formed from the reconverted bits in a similar manner to the way in which the check code at the transmitter was originally formed. The comparison between the two check codes at the receiver enables any defect in the overall system to be detected. In one embodiment data to be transmitted 1 is passed via a timing recovery circuit 2 to a memory 4, operating in accordance with a timing pulse generator 5 controlled by the recovery circuit 2. The memory is read out at a greater bit rate by a read pulse generator 6, which is controlled from a timing generator 9 whose output pulses are locked by a phase lock oscillator 8 in turn controlled by both generators 5 and 6 to establish the length of a block. The memory is read into a multiplexer 4 and the bit rate is such that there are three vacant bit positions per block. The original data bits are also passed to a counter 10, which is controlled by a pulse generator 11 in turn controlled by generators 5 and 9 such that bits in a block are summed. The sum is converted into a check code which is inserted into the vacant bit positions at the multiplexer together with a synchronizing pulse from the generator 9. The multiplexer output is transmitted. At the receiver, the check code is extracted at the demultiplexer passes to a bit rate converter 19 operating in inverse manner to that at the transmitter and in which control pulses related to the same block as those produced by generator 11 at the transmitter are produced by generator 26. These are used to control the counter 27 which accordingly counts the output pulses on line 24 during the time and encodes the count. The code is passed to a second input of the error detector 28. An output from this detector indicates a fault somewhere in the system. The embodiment described above requires that the number of bits per block should be an integral multiple of the memory capacity (and the counter control 11 is synchronized with the store timing generator). This constraint may be avoided, and the count carried out over any arbitrary interval if the error detected output is connected to an integrator which provides an output when the integrated value exceeds a set amount such that the counter control generator at the receiver is inhibited. The system then hunts until the receiver counter control generator synchronizes with that at the transmitter (Figs. 4-6, not shown). This operation can be hastened by controlling the counter at the receiver from the phase lock oscillator via a divider circuit (Fig. 8, not shown).