On Tue, Apr 7, 2009 at 2:17 PM, Marko Mäkelä <msmakela@gmail.com> wrote:
> On Tue, Apr 07, 2009 at 09:47:06AM -0400, Ethan Dicks wrote:
>> ... I probably have a small stack of tapes that had read
>> errors, at least one of which is commercial (and occasionally had read
>> problems back in the day).
>
> Can you archive the problematic tapes in the *.tap format?  That should be
> enough for decoding.

I'll give that a try first.  I last tried this long ago with a PC
parallel-port interface to a real C2N drive and some, IIRC, DOS
software on a 486.

> If the *.tap files are too corrupted, then you could
> try playing with the azimuth settings or sample it in PCM format (*.wav)
> and use an appropriate digital filter.

That should be easy enough to do on modern machines.

>  A few years ago, I made some
> experiments with TAPir <http://tapir.sourceforge.net/>, but I can't remember
> if it was of any use to me.  I didn't try to mess around with the stereo
> head of the analog tape deck that I used.

OK.  I might consider that, too.

>> ... I do
>> have a number of "PET Rabbit"-format tapes.

> There are four decoder functions implemented so far: decode and decode264
> in decode.c, decode_kim in kim_d.c and decode_oric in oric_d.c.  You could
> start by studying the decoders in decode.c.

Thanks for the pointer.

> Hmm, I seem to remember that I once encountered VIC Rabbit.  Sure enough,
> I even documented the format:
>
> http://www.zimmers.net/anonftp/pub/cbm/vic20/roms/tools/4k/Rabbit.txt

Nice.

> I probably wrote some throw-away Perl one-liners to decode the samples
> that I produced with the cartridge image.  I must have run the "save"
> command on the VIC-20 and sampled the pulses with the C2N232 directly
> from the tape port, without going through analog tape.

OK.

>  If the PET Rabbit is anything like Vic Rabbit, I wish you luck.

That sounds like you put some time into it and gave up.

Maybe I should go the route of digitizing these tapes and cleaning the
audio up with filters.

> Basically, the decoding can be divided to four steps:
>
> (1) sample the tape
> (2) detect edges or pulses from the samples
> (3) quantize the pulse widths to symbols B, C, D (using the c2n232 notation)
> (4) decode the string of symbols to binary

Got it.

> The decoder functions in the "c2n" program implement step (4).
>
> I hope that this helps.

It does.  Very much so.

Thanks!

-ethan


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