[Coco] Cassette data format

[Gene Heskett]

On Sunday 25 August 2013 14:19:25 Mark J. Blair did opine:

> On Aug 25, 2013, at 03:32 , Ciaran Anscomb <cocomalt at 6809.org.uk> wrote:
> > The cycle durations actually vary quite a bit depending on where in
> > the output it is - the first bit of any byte will be slightly longer,
> > as there's a bit of setup involved, etc.  Conversely, the "ideal"
> > cycle durations to fall exactly in the middle of the recognition
> > window during reading vary depending on which bit is being read...  I
> > have some notes from old code:
> > 
> > /* The "ideal" pulse widths vary according to where they are:
> > *
> > * Leader cycle:            68 + n0 * 28, 5 + n1 * 28
> > * First byte cycle:        81 + n0 * 28, 5 + n1 * 28
> > * Subsequent byte cycles: 120 + n0 * 28, 5 + n1 * 28
> > * Other bit cycles:        67 + n0 * 28, 5 + n1 * 28
> > */
> 
> Cool! Thanks!
> 
> 
> I wonder why they bothered to use a DAC for cassette output instead of
> just wiggling a digital output. Since the reader only looks at edge
> crossings, I'd think that sending a square wave to the cassette
> recorder should work just fine... assuming that the cassette recorder
> doesn't misbehave when it sees a square wave, i.e. ringing heavily on
> the edges.

The square wave, if sharp enough, would alias with the bias frequency, if 
the deck used one, and the CCR-81 doesn't, and create all sorts of 
unpleasant side tones.  The CCR-81 used a permanent magnet for both erasure 
and recording bias, and because this raises the backgound noise level 20 or 
more DB, is responsible for 99% of the problems encountered with any such 
machine.  Back in the 80's I had (still do but a different one) a 4 track 
stereo deck that was rated pretty good, and someone wanted a copy of Jake 
Commanders Chrmakey, but this deck didn't have a remote start ability, so I 
had to start the deck & then the csavem.  I recorded it on a single track 
to minimize any azimuth problems on the other fellows deck, made two 
csavem's of it and mailed it.  He, and I don't recall the name, emailed me 
back at princeton & said it was the best recorded tape he had ever 
received, loaded first time every time.

At the time, a tape or cassette deck that had a good 100khz erase and bias 
in it, was about a 100$ premium.  Unforch, most folks simply didn't 
understand that the lack of it was 100% of the problems they had.

That, FWIW, was a patched version of Jakes code.  He saved one byte by 
attempting to bypass the shack recommended way to access the roms, and when 
basic went from 1.1 to 1.2, one of his jump addresses was wrong.  I fixed 
it to use the correct and indirect jump & made it at least 20x faster on 
the coco2's.  As I soon found os9 & never looked back, I have no clue if it 
may be on and old rsdos floppy or not.  But it was indeed the basic editor 
to use at the time, blowing everything else into the next drainage if not 
farther.  The original code, broken on coco2's, was in either the Rainbow 
or Hot CoCo in the middle 80's.

Cheers, Gene
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