How to combine 2 raw dumps from the same disk to patch errors?

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Alex
Posts: 33
Joined: Wed May 26, 2021 11:44 pm

How to combine 2 raw dumps from the same disk to patch errors?

Post by Alex »

I have two 5.25" drives one 1.2 MB and one 360 KB drive. For the same 360 KB floppy on the 1.2 MB I get a bad sector but on the 360 KB drive all is OK.

Why sometime some drive read OK and some not? Also is there a way to combine the OK .raw sector from the 360 KB drive in the 1.2 MB .raw files?
brightcaster
Posts: 448
Joined: Fri Nov 08, 2013 10:48 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by brightcaster »

It's just "copy and paste" as discribed here:

viewtopic.php?f=3&t=520

viewtopic.php?f=3&t=637

If you mix a 40 track dump with a 80 track dump you have to rename the files...

David
SomeGuy
Posts: 337
Joined: Wed Feb 18, 2015 8:18 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by SomeGuy »

There are a variaty of reasons that one drive might read a disk better or worse than another drive.

A 360k drive has a wider read/write track width so picking up good data at the "edges" that a 1.2mb would not touch can result in a better read. Conversely, sometimes the "edges" may be bad but the center is still good, and in that case a 1.2mb drive might have better results.

Also things like head alignment, spindle speed and stability, the amount of pressure the head puts on the disk, analog signal filtering, automatic gain control, all kinds of little things can make a difference.
Alex
Posts: 33
Joined: Wed May 26, 2021 11:44 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by Alex »

I know we can mix and match good raw track files from a dump into another, but what I don't understand is that how 2 different "good" track read on 2 different drives yield very different files (binary wise)?
brightcaster
Posts: 448
Joined: Fri Nov 08, 2013 10:48 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by brightcaster »

That should only happen in very rare cases (half track copy protection for example on c64 floppies). In general it can't happen If the data was written with a normal 360k (48tpi) drive with correct alignment).

Feel free to upload a sample if you wish (would be interesting to look at) ...

David
Alex
Posts: 33
Joined: Wed May 26, 2021 11:44 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by Alex »

Here is an example. The same disk imaged on a Teac FD-55GFR (1.2 MB drive) and on a Teac FD-55BR (360 K drive). On the GFR dump the track 72.1 is "bad" while on the BR everything is OK. This would correspond to the 36.1 track on the BR dump, but what I don't understand is that all other (good track) are not binary identical...

Here are the disks.
brightcaster
Posts: 448
Joined: Fri Nov 08, 2013 10:48 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by brightcaster »

I decoded both dumps to an mfm sector image and I can't see any differences in the binary (except those on track 36). What makes me confusing is the fact that both dumps decode perfectly with the HxC-Tool...

David
Alex
Posts: 33
Joined: Wed May 26, 2021 11:44 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by Alex »

Oh yeah, when both dumps converted to IMG everything is fine (the bad read is maybe used in a unused sector?). I was wondering when I compare RAW files for good sectors almost everything is different (for the corresponding sector in both dumps).
brightcaster
Posts: 448
Joined: Fri Nov 08, 2013 10:48 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by brightcaster »

There must be something on the floppies surface (maybe mold) starting at track 36 up to 39 (at sector 5), which leads to a flutter in speed. You can see it clearly in the scatter window. For whatever reason the dump from the 40track drive is a bit better and decodes fine, whereas the 80track drives dump can't decode side 1, track 36, sector 5 correct.

Maybe you did the 80track dump first and the surface was already cleaned a bit by the head when you did the 40track dump after that. Can you visually inspect the surface to see if there is anything on the it (side 0 or side1, as the flutter is on both sides in the scatter)?

Anyway, this has nothing to do with differences in the binary data of both dumps. They are perfectly equal (except sector 5 at track 36 side 1).

David
Last edited by brightcaster on Thu Aug 05, 2021 6:05 pm, edited 1 time in total.
brightcaster
Posts: 448
Joined: Fri Nov 08, 2013 10:48 pm

Re: How to combine 2 raw dumps from the same disk to patch errors?

Post by brightcaster »

Uhhps, sorry our postings have overlapped each other! Yes, you can't compare the raw files. Every dump is a bit different in timing especially if you use different drives with different speeds (360 rpm vs 300 rpm)!

And yes, the bad sector is unused...

David
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