Re: Fast GCR decoding?
Date: 2005-11-12 03:17:29

On 2005-11-11, at 21:22, David Wood wrote:

>>> No, there aren't enough cycles to decode to nybbles in realtime,
>> Could you remind me how many of them are there in the worst case?

> I know its offtopic and we all love to hate apple,

;-) In EU we hated Spectrums... :-)

> but I read some
> interesting information in a hardware reference book.  There's a  
> GCR nybble
> exactly every 32 cycles.  There's no byte ready line or other hint  
> that a
> nybble was read.  Just read, and -exactly- 32 cycles later, read  
> again or
> miss a nybble. :)

Interesting. And how the first one was synchronised?

> The 1541 uses a variable bit clock, so it can vary, but the  
> smallest sector
> count matches the evil system at 16/track, so it's probably not far  
> off.  ;-)

It is probably not far off the "evil system" but I'd like to be more  

I understand that I have as much time as it takes for the next byte  
to get shifted-in. Now this depends on bitrate so with the fastest  
bitrate there will be the least time to do anything. Adding few  
percents of contingency would give me safe result.

I recall I did those calculations some ages ago and I _think_ it was  
something like

INT( (cpuclock/fastestbitrate) * 8 * (1 - 5%) )

As long as I didn't screw something, it should be enough to know the  
fastestbitrate and the cpuclock numbers, which I don't remember ATM.  
AFAIR the 1541 had slightly faster clock than the (PAL) 64 but the  
bitrate? 320kb/s?

We do not inherit the earth from our ancestors, we borrow it from our  

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