Re: Weak Bits

From: Pete Rittwage (
Date: 2007-12-05 01:57:11

Armin Vogl wrote:
> Hi Pete,
> I read the infos on your page. I still have a few questions:
>> We mentioned before that if the drive gets more than two '0' bits in 
>> a row, it clocks in a random '1' bit occasionally.
> Is it really random ? I mean the 74LS193 is not reset by the BIT_SYNC 
> line in case there are more than two "0" in a row. Is it the variation 
> in rotation speed together with variation in the decoder clock which 
> creates different bytes for each rotation ? The decoder clock defines 
> the "width" of the bit in time and clocks the 74LS193. Would it be 
> enough to have a number of bits on the track that can not be divided 
> by 8 to generate a pattern that changes with each rotation ? This 
> would still not be random, because after a "1" bit is read, the byte 
> that will be read only depends on how long you get "0" bits. But this 
> patternis not random. I think it would look like random if the amount 
> of bits on the track can not be divided by 8.
Hi Armin,

I do not know much about the low-level electronics- I've only studied 
what the drive sees with varying patterns of missing '1' bits.  On old 
1541 hardware, patterns emerge based on the density setting and rotation 
speed that look like this:

11 11 11 12  22 22 22 24 44 44 44 44 48 88 88 88 88 11 11 11

The pattern is longer with each set, but it looks like this when there 
are *no* 1 bits on the track at all, like in a unformatted track.    
Some 1571 and some 1541-II drives see actual "00" bytes and some of them 
see very random garbage data, like the drive is randomly generating it 
on purpose to be "compatible" with the old behavior.  Womo was doing 
some research on this at one point recently.

> So the question is: does G64 support an amount of bits on a track 
> rather than bytes ?
It is only the bytes that the 15x1 board would see appearing after 
decoded, always framed to the syncs.  This is why syncless tracks do not 
work properly in emulation currently.  This is changing, though.  Hoxs 
may do it, and VICE is being rewritten slowly.  CCS is closed source so 
I don't know.

> I still don't get what is "weak" about the bits. I don't understand 
> the term "weak bit". It is not that the bits themselves are not stable 
> on the magnetic medium, but it is more that the drive electronics 
> reads a different byte for each rotation, or ?
> Sorry if I ask questions that have already been answered elsewhere, 
> but I couldn't find a detailed explanation on the web.
No problem at all!  I adopted the term 'weak bits' from CAPS/SPS, but it 
is probably a bad term for our purposes.  On the Amiga disk controller, 
it is described as a weak magnetic flux, with the effect that sometimes 
it's seen as a transition and sometimes it's not- so you get random 
data.  That is the same outcome on the 1541, but not the same cause, but 
the name still stuck at some point.  Technically, it's a nonsense 
bitstream to the 1541.  Unformatted is a better term.

Pete Rittwage

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