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Re-read bad track step up/down/random

Posted: Wed Oct 10, 2012 9:27 am
by spags
I believe currently when Kryoflux reads a bad track (say track x), it steps back to the previous track (x-1) and steps forward to the bad track (x) again to try and re-read it.

One possibility might be to have an option to step to the next track (x+1) then back to the bad track (x) in an attempt to see if a different result is obtained. Another might be to step in a random direction and then back. It shouldn't make any difference, but it may help drives that appear to work fine but have a very slight alignment issue.

Re: Re-read bad track step up/down/random

Posted: Wed Oct 10, 2012 11:28 am
by IFW
DTC steps in the opposite direction before each retry attempt, ie -1/+1 alternating.
The only exception is the minimum and maximum tracks allowed for the drive as obviouly there is only one possible direction in those positions.

Re: Re-read bad track step up/down/random

Posted: Tue Oct 30, 2012 8:31 pm
by hawui1
I dumped hundreds of disks now.. and I continue to think that the best solution would be to move at least 5 tracks away and then back on the unreadable track and then reread.. the algorithm could be very simple.. just step 5 tracks forward and then back while reading in the first half of the disk, instead step 5 tracks backward and then back while reading in the second half of the disk..
The reason why this method works better than just +/-1 track skip is that the movement of the read head on the disk surface while the disk is rotating makes the disk stabilize..
many times the read error happens just because the disk slightly moves vertically.. you can easily spot this because you hear a kind of tick tick tick while the disk rotates.. or a sound like something is slightly rubbing at periodic intervals.. this noise usually disappears as the read head moves away 4-5 tracks .. resetting the drive with the one track skip usually doesn't help in these (many) cases..
I don't pretend you to believe me but please give it a try.. on 3,5" drives I think you will easily spot the difference..

Re: Re-read bad track step up/down/random

Posted: Tue Oct 30, 2012 9:56 pm
by IFW
The next DTC version re-calibrates the drive after the specified amount of failed retries, then retires again - up to the user selectable complete retry cycles.
However keep in mind, that stepper motor errors are not cumulative, therefore the net effect of this is mostly fixing mechnical problems - like unbalanced disk rotation -, rather than fixing seeking issues, as for those a single step is enough.

Re: Re-read bad track step up/down/random

Posted: Thu Nov 27, 2014 10:01 am
by mr.vince
We think that microstepping is the way to go... Everything else usually is pure luck.

Re: Re-read bad track step up/down/random

Posted: Thu Nov 27, 2014 10:24 pm
by brightcaster
I remember a discusion about 25 years ago, where the conclusion was that the moving of the head does nothing else but removing fine partikels out of the heads slot. So cleaning the head does the same without further damage of the disks surface!

Re: Re-read bad track step up/down/random

Posted: Fri Nov 28, 2014 5:14 pm
by brightcaster
Talked to an old mate today who could also remember the disussion during the late eighties. To his memory it was not a problem with the heads slot but fine partikels wich summ up in the front of the head and lift the head for some micrometers. Turning the head right or left throws this partikels out of the way and enables the head to rely closer to the sutface again.

Nevertheless we both kept in mind that it only happend to floppies wich where used.verry hard like system-disks in early macs who had no harddisk.

Regards, David

Re: Re-read bad track step up/down/random

Posted: Fri Nov 28, 2014 6:30 pm
by mr.vince
Since we seek to 00 from time to Tim because of this, that should take care of the particle issue.

Not saying that cleaning a suspicious disk every ten tracks would not make more sense...