Here's maybe one way this could be done for magnetic tape -- and think of
the ramifications for badly-deteriorated tapes and even sticky but not
stuck-together tapes. Tape is fed into a machine that coats the oxide with a
magnetic developer, then onto a scanning bed that scans X inches per pass
and produces a linear scan for the decoder. Decoder would have do be able to
decode developed magnetic tracks, which I do not know enough about to know
if they contain readable amplitude and frequency information -- or would the
decoder need to simulate what happens when a given magnetic pattern passes
over a given type of head? If it works for mag-tape, it would of course
work for mag-film, mag discs, mag anything that can be developed and
scanned.
This might be too Buck Rogers -- like I said I don't know exactly what the
developer reveals beyond the tracks' physical location on the tape.
-- Tom Fine