Method and apparatus for preventing errors in PCM signal processing apparatus

PCM signal processing apparatus adapted to receive successive transmission blocks, each comprised of time-interleaved PCM, error-correction and error detection words, from either of first or second selectable data sources, means for changing over the selection of said data sources from one to the other to establish an error interval during the transition period that said selection of data sources is changed over, said apparatus comprising detecting means responsive to said error detection words for detecting if a received transmission block contains an error;

• error identifying means for identifying as being erroneous each of the time-interleaved words included in the received transmission block which has been detected as containing an error;
• de-interleaving means for time de-interleaving each received transmission block to recover a de-interleaved block comprised of de-interleaved PCM and error-correction words, erroneous ones of said de-interleaved words being respectively identified;
• error correcting means coupled to said de-interleaving means for correcting an erroneous PCM word in said de-interleaved block as a function of the remaining non-erroneous PCM and error-correction words in that de-interleaved block;
• means for indicating when the de-interleaved block contains at least one word derived from said first data source and another word derived from said second data source;
• and inhibit means for inhibiting said error correcting means if said de-interleaved block supplied thereto contains at least one word derived from said first data source and another word derived from said second data source.

PCM signal processing apparatus adapted to receive successive transmission blocks, each comprised of time-interleaved PCM, error-correction and error detection words, from either of first or second selectable data sources, means for changing over the selection of said data sources from one to the other to establish an error interval during the transition period that said selection of data sources is changed over, said apparatus comprising detecting means responsive to said error detection words for detecting if a received transmission block contains an error;

• error identifying means for identifying as being erroneous each of the time-interleaved words included in the received transmission block which has been detected as containing an error;
• de-interleaving means for time de-interleaving each received transmission block to recover a de-interleaved block comprised of de-interleaved PCM and error-correction words, erroneous ones of said de-interleaved words being respectively identified;
• error correcting means coupled to said de-interleaving means and operative to correct an erroneous PCM word in said de-interleaved block as a function of the remaining PCM and error-correction words in that de-interleaved block, provided that the total number of erroneous words in that block is less than a predetermined amount;
• syndrome generating means for generating a syndrome from said PCM and error-correction words in said de-interleaved block;
• means for producing a syndrome signal if said syndrome differs from a predetermined value;
• means coupled to said de-interleaving means for producing a no-error signal if none of the words in said de-interleaved blocks are erroneous;
• means for supplying an inhibit signal of predetermined duration to said error correcting means in response to the concurrence of said syndrome and no-error signals, whereby the operation of said error correcting means is inhibited for the duration of said inhibit signal;
• counting means for counting the number of successive transmission blocks which contain errors;
• means for detecting if said count exceeds a predetermined value representing a predetermined duration of said error interval;
• and error designating means responsive to said means for detecting for designating an error-correction word in a predetermined number of the next received transmission blocks as being erroneous, whereby the number of words identified as erroneous in each de-interleaved block that contains words which are derived from said first and second data source is at least equal to said predetermined amount.