> On Oct 12, 2015, at 9:16 PM, Rich Alderson <[email protected]>
> wrote:
>
> ...
> The M tracks are longitudinally encoded (6-bit values chosen such that they
> read the same as NRZ backwards and forwards for DECtape, 4-bit values for
> LINCtape) to predefine blocks (cf. disk sectors) for data.
More precisely: it's Manchester encoding, not NRZ. The result is that mark
track codes are complemented and reversed end for end if you read them in the
opposite order.
The code choices are such that this process (obverse complement) produces
another code word with the right meaning for this spot of the tape in that
direction. So "in the data field of the block" reads the same in both
directions. But "block start" in one direction reads as "block end" in the
other, which is just the result you want.
The DECtape patent (3,387,293 -- on bitsavers among other places) describes
this very nicely.
paul
