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johncl
Registered: Aug 2007
Posts: 37 |
Turbo Tape, how does it work?
I have always been curious about how turbo tape really worked. I know that the tape drive originally could only read 50 bytes per second which would take a whopping 21 minutes to fill up the whole memory. I was always under the impression that games used compression to speed up tape loading (Just did a test with Giana Sisters which took 4 minutes to load).
Did Turbo Tape fiddle with the speed that data was written and read to the tape? I didnt think that was possible (only thought that was how they sped up disk reading/writing). If anyone got a good resource at how e.g. the well known Turbo Tape works, please post a linky here. |
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doynax
Account closed
Registered: Oct 2004
Posts: 212 |
Interesting. I never did have a datassette to play with and only vaguely remember a friend and I clocking a load at 15+ minutes for one of his C64 games. Still, if the speeds discussed here are accurate then you could squeeze an astonishing two megabytes onto a 90 minute tape.
Anyone care to explain how the physical encoding works and what the low-level limitations are, as opposed to, say, the magnetic coding on a floppy drive?
I mean why is a pulse scheme with flexible lengths preferred/required as opposed to something like GCR or MFM coding? And if differing tape speeds is the main obstacle then why isn't it possible to dynamically calibrate the loader? |
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Mace
Registered: May 2002
Posts: 1799 |
To add to Doynax's questions: what's the differences between those proprietary loaders like Cyberload? |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: Interesting. I never did have a datassette to play with and only vaguely remember a friend and I clocking a load at 15+ minutes for one of his C64 games. Still, if the speeds discussed here are accurate then you could squeeze an astonishing two megabytes onto a 90 minute tape.
Anyone care to explain how the physical encoding works and what the low-level limitations are, as opposed to, say, the magnetic coding on a floppy drive?
I mean why is a pulse scheme with flexible lengths preferred/required as opposed to something like GCR or MFM coding? And if differing tape speeds is the main obstacle then why isn't it possible to dynamically calibrate the loader?
I guess pulse widths are prefered because every bit is a transition then, i.e safer/simpler.
Calibration should be possible, enthusi mentions it is done by the kernal cbm loader.
Also using GCR should be possible. That could be used to make a byte the length of 10 '0'-bits.
I'm not sure how safe it would be. Depends on how much/fast the tape speed varies _during_ loading.
Perhaps calibration could be made on the fly? |
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doynax
Account closed
Registered: Oct 2004
Posts: 212 |
Come to think of it, if the cycle counts for a turbo-tape one-bit is half again as long as a zero then a GCR (5-to-4 bits) wouldn't really win anything unless you had more than fifty percent ones (which you shouldn't, since you can always invert all the data to get less than 50%.)
You'd be better off interleaving a Huffman-coder weighted towards zeroes with the loader instead. |
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tlr
Registered: Sep 2003
Posts: 1790 |
In my GCR suggestion I assumed '0' was the shorter bit. For GCR only short bits are required.
In GCR a missing bit means 0, and a bit (=transition) means '1'. |
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doynax
Account closed
Registered: Oct 2004
Posts: 212 |
Quote: In my GCR suggestion I assumed '0' was the shorter bit. For GCR only short bits are required.
In GCR a missing bit means 0, and a bit (=transition) means '1'.
As did I. It's just that a GCR encoded byte needs two extra bits, so a worst-case turbo-tape with 50% ones should just about break even with it.
At least, I think that's how it works out.. |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: As did I. It's just that a GCR encoded byte needs two extra bits, so a worst-case turbo-tape with 50% ones should just about break even with it.
At least, I think that's how it works out..
Hmm, yes you are right.
I assumed a '1'-bit was twice as long as a '0'-bit, but now I realize the difference is much less.
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enthusi
Registered: May 2004
Posts: 677 |
check neoload for a error-correction loader.
The most complex there is so far.
Neoload |
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tlr
Registered: Sep 2003
Posts: 1790 |
Yes, noted that before. Very cool!
The discussion about GCR was an attempt to propose a faster loader than turbo tape though.
Perhaps it is already close to max? Very cool in that case as it is from 1983. :) |
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Frantic
Registered: Mar 2003
Posts: 1648 |
The loader in Deep Throat by Durex uses four pulse lenghts corresponding to 00, 01, 10, and 11. This minimizes the constant minimal length you always need to have in a pulse. I think there was also a fifth pulse length, signalling "syncrhonization point", since you can ffwd and rewind the tape while the demo is playing, but still resync at regular intervalls...
Greetings too Krill! (...and greetings to everyone from the Polish/German border..;) |
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