Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!ames!purdue!tut.cis.ohio-state.edu!ucbvax!POSTGRES.BERKELEY.EDU!dillon
From: dillon@POSTGRES.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga.tech
Subject: Re: amiga disk drives ...
Message-ID: <8907051954.AA04521@postgres.Berkeley.EDU>
Date: 5 Jul 89 19:54:30 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 57


	E-Gad!  Talk about fine tuning.  These deviations are due to
    minute deviations in motor speed.  You *must* allow for the worst
    case which is why there is padded gap at the end of the track when
    it gets written out (i.e. no gaps in between the sectors but a gap
    at the end of the track).  

	I'm not sure exactly what C-A does but this is the right way
    to do it (numbers picked at random) for writing a track:

    buffer: <padding ~200 bytes> <data ~12K> <padding ~200 bytes>

    Proper operation is where the padding at the end partially overwrites
    padding at the beginning.  The amount of padding must allow for the
    worst case and best case motor speed deviation.  I think if you 
    look at the spec'd allowed deviation for the drives and convert it
    to bytes you will find the numbers fit within the spec.

    You can easily *read* data written on a slightly faster or slower drive 
    because the drive uses a phase-locked loop to clock the data
    and the PLL 'locks' onto the frequency of the data on the track.

    This is the whole reason for MFM encoding in the first place... you
    must *somehow* encode a clock onto the track so the drive's PLL 
    stays locked.

	To encode more actual data onto the drive you can try a different
    encoding method.  The old commodore PET/CBM drives used a form of
    GCR encoding.

				-Matt


:The amiga disk drive as any other drive can only write a given number
:of WORDS (2 bytes) on any single track. On my drive I can write and can only
:write about $31d8 (+/- 2) bytes. (they are composing the raw MFM bytes of the
:sectors, headers and gap .)
:Now I have encountered disks written by other amiga computers which had 
:more bytes on them (the abacus disk had 320a bytes on track) or less bytes
:on disk (like Thexder which had 30ca or something ... (forgot)).
:
:now my question : how can I slow/speed up the motor (?) write spead (?) 
:of my disk drive so that I can have more/less bytes storable on a track
:
:I have tried to play with the PRECOMP stuff but that didnt help, I was still
:stuck with being able to only write 31d8 bytes on a track. 
:I even tried to alter the data written by the DMA (like writting only
:00 on a full track , then writing less than 31d8 bytes on that same track.
:(lets say 3000 bytes) but I still got a track of 31d8 bytes storable on it..
:
:any Hardware modification is definitely out of the question. 
:I have read and reread the HArdware manual but could find any lead.
:
:anyway ... I am waiting for any replies ... 
:
:	
:			8-#