'Vorpal (later) analysis'
Author:Lord Crass (guest: search)
Date: Mon, Mar 28th, 2011 @ 00:01 ( . )

What process are you using to determine the start of the track and then load the track in?

I was looking at some original disks on a real 1541 through Maverick's track editor. Here's what I noticed:

1. The track lengths for all these no-sync tracks always seems to be the same ($1E28/$1E29).

2. If you read across the track gap, the data always shifts, but does so predictably.

3. Since there's no sync, there's no framing reference, so any track read can come back in 1 of 8 different ways. This is irrelevant to the loader and the Maverick copier. You could force it to come back with a specific pattern by using the header search feature, but that would occasionally hang for a very long time as not every pattern seemed to come up due to a timing issue. I had to lift the drive latch and drop it back down a few times before it would result in that particular arrangement and read the track.

4. The first byte after the end of track pattern (the repeating single byte) is maybe 40% of the time a bad GCR byte. This would indicate to me that this is the write splice and is the actual end of track. The repeating 20 bit pattern after this is inert gap bytes.

As long as you start reading after the gap and stop when reaching the repeating single-byte pattern, you should get a track that Vorpal can read. It will be framed in 1 of 8 ways, but it doesn't matter since the Vorpal loader will handle it. The sync mark that Maverick adds simply makes it easier to make another copy of in the future with a generic 8k nibbler or parallel copier. This mark isn't necessary for the copy to load properly, and I'm guessing the SuperCard nibbler does the same thing as Maverick, but doesn't add the bonus sync mark. Maybe I should disassemble that copier to verify.

If you blindly read in the whole track just once without waiting for the actual start of track, you'll wind up with the write splice somewhere in the middle and the track data on one side shifted one way, and the data on the other side shifted a different way. If you just try to paste this together, it's ruined.

It's interesting that there are sync marks on the odd numbered tracks at all, since analysis of the loader indicates they aren't necessary. The same load routine is used for every track, sync or not. Perhaps a technical requirement for the mastering process?

I didn't have time to try it, but you should be able to copy these syncless tracks with the Maverick track editor as well. Read in the track blind, scroll through the buffer to find the track start, then put the first 4 bytes or so in the search header and read the track in again. Scroll down to where the gap is, enter edit mode and put a $00 byte after the first byte of the repeating single-byte pattern to mark the end of track (Maverick will now show the length of the track). Ensure your drive is slowed down and write the track to your target disk. This should work as it's basically the same thing the custom copier does.


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--* Vorpal (later) analysis
3/28/2011 @ 21:25--hyper active
4/02/2011 @ 01:53--Lord Crass

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