Re: CD's, C64's and error-correction

From: Marko Mäkelä (
Date: 1999-10-15 12:40:32

On Mon, 11 Oct 1999, Martijn van Buul wrote:

> It might be worthwhile to contact Markus Brenner then -- He wrote
> this tap2voc program, and can doubtlessly tell us how the **** that 
> tape format works.

Indeed a good idea.  BTW, when I met Daniel Dallmann last Sunday in
Frankfurt, we discussed this tape thing a little.  It occurred to me that
there's absolutely no need for that GCR coding.  On the floppy drive it is
required, because the bit rate may vary.  On the CD it is constant.
Therefore, we can use 8 samples for data and 1 or 2 samples for "stop
bits", to allow for storing the byte and for resynchronization.  The raw
speed would still be 4410 or even 4900 bytes per second, although I think
that the latter (doing only with one stop bit) would require scattering
the pointer incrementation and comparison operations in the unrolled
byte-receive loop, which is a somewhat non-trivial task.  Had these
conferences I'm participating in been more boring, I'd probably already
have such a routine. :-)

A word on the hardware.  I think it could be done with 3 or 2 ICs.  One
amplifier-limiter (for converting the line-level signal, 1 volt
peak-to-peak or something like that, to TTL, maybe with some sort of a
Schmitt trigger), and something to convert the rising and falling edges of
the signal to pulses between 1 and 21 microseconds (those are the limits
for the pulse width; the actual width should be constant).  Maybe you can
combine these two steps.  Power is available from the cassette connector.


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