Patent GB-1025378-A (GB)
Title: Error-correcting data transfer system
IPC: H03M13/17
Filed: 1964-10-09
Abstract: 1,025,378. Error correction. TELEFUNKEN PATENTVERWERTUNGSGESELLSCHAFT G.m.b.H. Oct. 9, 1964 [Nov. 28, 1963], No. 41290/64. Heading G4A. In an error-correcting data transmission system, an n-digit check word is formed for each data word by adding without carries into an accumulator a characteristic n-digit word for each binary " 1 " in the data word, the receiver unit forming from the received data and check words an n-digit correction word which is all " 0 "s only if the transmission is error-free. The system is applicable to " group codes", reference being made to the book " Error Correcting Codes " by W. W. Peterson. As described, a 4-bit data word to be transmitted is provided with a 7-bit check word capable of correcting either single or double bit errors. As shown in Fig. 2, a counter 6 and associated recoder (not shown) provides inputs to AND gates 11-14 which inputs are arranged to represent four successive 7-bit words in synchronism with the four data bits to be transmitted which are applied to open the AND gates 11-14 when the data bit to be transmitted is a " 1 ", thereby causing a 7-bit word to be added without carries in EXCLUSIVE- OR gates 7-10 to the contents of a register 33, which at the conclusion of the addition stores a 7-bit check word transmitted after the data word to the receiver. Single error correction.-The receiver, Fig. 3 (not shown), is effective to correct a single bit error in the received data or check words, since a single error results in a 7-bit correction word which corresponds exactly with a 7-bit word whose position in a code table (given in the Specification) corresponds to the position in error. The receiver operates in two steps in the first of which a correction word is formed and in the second of which the received information is corrected. In the first step, a multiple contact 19 is in the position shown and the eleven incoming bits received at 17 are effective as in the transmitter to accumulate in a register 33 a 7-bit word formed by gating the contents of a counter 6, corresponding to the counter 6 of Fig 1 (not shown), to EXCLUSIVE-OR gates 7-10 if the incoming bit is "1", thereby forming a 7- bit correction word in the register 33. In the second step, the multiple contact 19 is changed over and the received information, which has been delayed in a shift register 18 is transmitted to an output 22 via an EXCLUSIVE-OR gate 23 whose other input is connected to a NOR gate 20 and, as the counter 6 is stepped through its eleven coded values, these are compared with the contents of the register 33. If a single error has occurred, the recognition of a successful comparison causes the NOR gate 20 to produce an output which corrects the erroneous bit at the EXCLUSIVE-OR gate 23. If the received data is correct, the contents of the register 33 is zero and the correctly received data is passed unchanged through the gate 23. If more than one error occurs, this is detected since the contents of the register 33 is non-zero, but correction is not possible. Double error correction.-Due to the chosen code, the correction word formed if a double error occurs is equal to the sum without carries of two code words whose position in the code table defines the positions in error. By providing an auxiliary register and a further multiple contact the receiver can be adapted (Fig. 4, not shown) to ascertain the positions in error.