Patent GB-1375955-A (GB)
Title: System for correcting detected errors in a high-speed digital data transmission system
IPC: H03M5/20
Filed: 1973-04-19
Abstract: 1375955 Digital transmission; error correction ROCKWELL INTERNATIONAL CORP 19 April 1973 [12 May 1972] 18945/73 Heading H4P In a partial response digital data transmission system, a decision device 16 derives from the received signal an estimate #D i  of its correct   value, and an estimate #Y i  of the residual error, the latter being used together with the output of an error detector 18 operating on the estimate   #D i  to further correct this estimate. The error detector 18 operates upon the signal #D i  to provide an output signal indication within a few baud times after an error occurs in the estimate of a transmitted digit while also providing an output signal indicating the polarity of the detected error. The error corrector 20 responds to signals from the error detector 18 to scan a set number of previous residual signals,   #D i , to select the largest residual signal having a polarity opposite to that of the indicated error. The error corrector then assumes that the digit associated with the largest estimated residual signal, with a polarity opposite to the indicated error, is the digit that is in error. The error corrector then changes the estimated value of the digit corresponding to that residual signal by one level in the direction opposite to the error polarity indication obtained from the error detector and polarity indicator 19. Usually this one change corrects the digit error. The output of the error corrector 20 is then the final digit decision #D i . This digit decision is then fed to a partial response decoder 30 which transforms   the #D signals into the output digits, b, which digits ideally correspond to the input digits, b, fed into the partial response encoder 9. For a 4- level transmitted signal, the decision circuit must discriminate between 7 different levels arising from the overlapping of adjacent pulses inherent in a partial response system. The decoder 30 extracts the 4-level data by comparing signal samples taken in successive symbol periods (l 0 , l 1 , Fig. 2, not shown). The decision circuit 16 (Fig. 3, not shown) compares the input with successively increased reference levels (90, 92) and when the nearest is found a corresponding stored decision (91) is gated out (93). The error detector 18 (and Fig. 4A, not shown)   combines #D i  with a fed-back signal (d<SP>1</SP> i -2) to   give a "digit estimate" #d i , which is limited (at 23) to the maximum value the 4-level signal is permitted to attain (Œ3) and delayed by 2 bands (at 22) to provide the feedback signal for a   later input. An error in the estimate #D i  produces a corresponding error in #d i  which is propagated around the loop until the limit (Œ3) is reached, whereupon a + or - error is indicated (at 22, 23). Limiter 22 prevents any further propagation of this error. The error corrector 20 (and Figs. 4B, 4C, not shown) scans a number N of the residual digits  Y i , selectively odd and even, where N is fixed or can be reduced if the last error occurred less than N digits earlier. N may also be reduced to just include the last suitable polarity maximum digit which could be an error, if this is known. Multilevel signals other than 4- and 7-level signals may be used.