Posted by Richard Crowley on 07/13/06 20:09

"Rick Merrill" wrote ...
> Excuse me for reminiscing, but the early IBM drives had something like 9
> read heads - the extra track was a parity track - and analog reads on each
> track could trigger the parity correction if one track was too much
> worse/lower a signal level than all the others. However, if that track
> were consistently bad it would literally be ignore for the whole tape -
> the consequence was that any drop outs on other tracks would be ignored
> completely!

Don't forget that parity checking was 2-dimensional.
Not only was there parity checking for each byte
(8 bits + parity bit = 9 tracks), but there was also
more sophisticated ECC for each block of data on
the tape. Enough to 100% accurately reconstruct
2-3 bad bytes of data in a block.

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