Help wanted: Imaging a protected disk

A forum about Atari protected floppy disks analysis, preservation, emulation, tools

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jvas
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Re: Help wanted: Imaging a protected disk

Postby jvas » Tue Feb 09, 2016 6:49 am

dlfrsilver wrote:this Atari ST disk mag must have been written by a trace machine, industrially duplicated, hence the reliability :)


I don't think so (I'm from Hungary)

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Re: Help wanted: Imaging a protected disk

Postby AtariZoll » Tue Feb 09, 2016 7:56 am

There were diverse formats used - only one # is with 12 s/tr, some are with 9 s/tr. Silver is right about that it was written with some duplicator.
But was it industry machine, or just some HW guru hacked Atari ST FDC clock ?
We need to engage detective Columbo to resolve this, for future of humanity crucial dilemma :D
P.S. Doing SPC image seems more realistic than Columbo :idea:
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Re: Help wanted: Imaging a protected disk

Postby JimDrew » Tue Feb 09, 2016 4:36 pm

ijor wrote:
JimDrew wrote:You can get 12 to fit at a higher data rate and basically no track gap, but that is the max with a 1772 (it won't read 13 sectors at a data rate high enough to fit).


Yes, it is possible, I said so in my post already. The question is how reliable this is. Especially when you read the disk in a different drive than the one you recorded it.


It's perfectly reliable with the WD1772. You can not fit 13 sectors though. With SuperCard Pro I have the luxary of changing the data rate to be anything I like. I can take a disk image and increase the data rate so that 10 sectors fits in the space of 5 sectors, and a high density drive/FDC will read those sectors just fine. I experimented with just how far the WD1772 would allow using various data rates and Panzer. I found that you could put as few as 7 sectors and as many as 12 sectors. It's pretty wide window, and I have to admit I was surprised.
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Re: Help wanted: Imaging a protected disk

Postby ijor » Wed Feb 17, 2016 3:55 pm

I forgot … One more thing. :D

Perhaps, the best way to pack more data per track is to use no sectors at all. Each sector has some overhead, gaps and header. Without sectors, just raw track, you save all the overhead. It is a bit less reliable, but as long as you implement some custom checksum (FDC won’t help you without sectors), is not that bad.

There are two problems though. First problem is that the FDC can't write arbitrary data outside sectors, which must be done when formatting the track. Some bytes are interpreted by the FDC as special formatting commands. But let's assume that we use a custom controller to overcome this limitation. The FDC still can't even read arbitrary data outside sectors, using the read track command. Some bit patterns will trigger the sync mark detector, forcing the FDC to skip some bits and even worse, start reading from the clock pattern instead of the data pattern.

But because the bit patterns that provoke this are known, you can use some encoding to avoid them. For instance, escape some bytes to avoid the patterns. This is not just an idea, and it is not even mine. Some games do implement this scheme.

Of course, the encoding, whatever one you use, will add some overhead. Then it might depend on the actual data if you gain something (vs using sectors) or not. It might be an issue of perform some statistics to calculate the average. Anyway, just wanted to comment on the nice idea (which again, is not mine).

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Re: Help wanted: Imaging a protected disk

Postby AtariZoll » Wed Feb 17, 2016 5:25 pm

Yes, that's used in many games - Lankhor liked it. Some called such tracks Amiga tracks :D
But for TOS compatibility you need regular 512 byte sectors.
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Re: Help wanted: Imaging a protected disk

Postby dlfrsilver » Thu Feb 18, 2016 1:39 pm

..........Or Obitus Ijor :)
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