6 May
1999
6 May
'99
10:51 p.m.
Hi,
I need some information about the Commodore cassette storage format.
I want to salvage something from a damaged cassette, and I have a (rather
crummy) audio digitizer. I've spent a couple of hours staring at the signals
from stored programs but I just don't 'get
it'.
In the data portions, it appears as if every 20th wave is special, with
maximum amplitude and a smaller frequency. If these are synchronization
points, why does it appear to be at every 20th wave? Why not every 9th? Does
the format not care about bytes? Or is there some kind of error detection or
correction scheme taking up more bits?
There are about 200 of these 'synchronization waves' just in the header. The
machine-readable file I was looking at only has a three-character filename.
What data is normally stored in the header? Filename, load address, checksum
or CRC, etc?
It sounds like there are two identical copies of the header information. I
know that there are two copies of the actual file data.
How is the data actually stored? I expected to see square waves with
frequency modulation. IIRC, that's what the Apple does. The PET appears to
use sine waves (non-flattened) and I can't visually discern any frequency
differences in the data (at a sample rate of approx. 25,500 per second). A
lot of the data portion of the file I'm looking at looks like it is stored
with amplitude modulation, but I don't think that would make sense with such a
flaky thing as cassette tape. There are large amplitude "gulleys" here and
there in the file, yet the file I'm looking at loads properly.
(Of course, it sure SOUNDS like it's using frequency modulation. Perhaps I
should sample at a higher rate, and get out a ruler to measure the width of
the waves...)
Those 20 waves don't represent a single bit, do they? (Lessee... approx. 60
waves per 600 samples, 25,500 samples per second... that's 2,550 Hz, and 20
waves/bit = 127.5 bps, which isn't enough but is getting somewhat closer to
the numbers I'd expect. Maybe I'll answer some of my own questions with
another look at the samples.)
The damaged file that I want to salvage has had its header wiped out (someone
momentarily hit "RECORD"), but the data appears to be untouched. Is there a
way of getting the PET (or any other machine) to read the data without the
file header?
--
Doug Spence
ds_spenc(a)alcor.concordia.ca
http://alcor.concordia.ca/~ds_spenc/
7 May
7 May
2:51 a.m.
At 05:51 AM 5/7/99 -0500, Doug Spence wrote:
I want to salvage something from a damaged cassette, and I have a (rather
crummy) audio digitizer. I've spent a couple of hours staring at the signals
from stored programs but I just don't 'get it'.
A number of Commodore emulators and tools will process a sampled .WAV file
into runnable data.
My web site at <http://www.threedee.com/jcm/> (winner of the "Geek Site
Of The Day" Award on October 16, 1996) has some of my aspirations to
recover data this way, too.
After looking at the source code of several tools that do this, I was
disheartened to see they use techniques as simple as measuring the
time-distances between zero-crossings, and no fancy FFT is involved.
When you think about it, the hardware in many of these early machines
did no more than watch the timing on the square waves, too. A signal
processing approach might be able to rescue more data, or remove
print-through on old tapes, of course.
- John
11 May
11 May
1:47 a.m.
On 07-May-99, John Foust wrote:
My web site at <http://www.threedee.com/jcm/>
(winner of the "Geek Site
Of The Day" Award on October 16, 1996) has some of
my aspirations to
recover data this way, too.
Nice site. You say that you would like to see more samples... what
kind and what for? :)
I was thinking of adding a page to my own site with the sounds of
various cassette formats. Not complete programs (not enough room!) but
just enough to get a feel of what the storage format sounds like.
I find that there's a certain character to cassette formats, and most
people don't even seem to know that cassette tape was used for home
computer storage at one time.
(I was also thinking of collecting character set information and having
a page just for the different character sets from historical machines.
I have the time now, so I guess I should do it!)
BTW, what is a "Hurkle" tape? :) And I saw that funny shape (the wave
on your page) in my PET program sample, and thought it was messed up,
even though that particular program loaded fine. So I guess that mess
is supposed to be there. :)
- John
--
Doug Spence
ds_spenc(a)alcor.concordia.ca
http://alcor.concordia.ca/~ds_spenc/
9358
days inactive
9362
days old
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ds_spenc＠alcor.concordia.ca -
jfoust＠threedee.com