Date   : Thu, 07 Feb 2002 09:52:36 +0000
From   : Richard_Talbot-Watkins@...
Subject: Re: Re[1]: Firetrack

Thomas Harte wrote:

> Small point : most TVs are happy with several fewer, and the
> 128kb ZX Spectrum's certainly only generate 311. So don't
> think 312 is completely set in stone.

Yep, this seems true - most tellys appear to be have a degree of tolerance
with the refresh frequency, which is why it's possible to do something like
?&FE00=5:?&FE01=7 in (say) mode 1, thereby giving a 319 scanline mode, and
have it displayed fine.  I remember at school though there were certain
monitors that used to roll the picture when playing Spy Hunter in hi-res,
so i assume this was due to them not being quite so tolerant!

On the other hand, you get a noticeable screen judder when varying the
scanline count (e.g. in a bit of code that increments R5 from 0 through to
7, then does a hardware scroll up 8 pixels and reloops), which I assume is
due to some kind of VDU recalibration to the new refresh frequency when the
total scanline count changes suddenly from 319 back to 312 again.

I'm interested in the moment in the CRTC cycle that the total scanline
adjust (R5) happens.  I'd always assumed it was after the main character
rows had been iterated through.  Thomas, your document suggested (and I
quote):

> When vertical sync exits, the scanline adjust counter (SAC) is
> zero'd and subsequently incremented every horizontal sync.
> Until it is found to be equal to register 5 (vertical total adjust),
> all vertical activities are resumed.

but this suggested to me that it required a vertical sync in order to kick
it off, but this doesn't seem to be the case as the Firetrack scrolling
method requires two settings of R5 for each half of its screen, but only
one of these allows a VSync.  When I tried some code last night, I found
that if I placed the setting of R5 *before* the VSync happened (in the
'second' half of the screen) I didn't get the correct results, but placing
it *after* the VSync worked fine.  As this is in the same CRTC cycle, I'm
obviously missing something.  In the 'first' half of the screen, the R5
setting just worked as expected, despite it not kicking a VSync at all.
Any more thoughts?

--
Rich Talbot-Watkins,
SCEE Cambridge.
Richard_Talbot-Watkins@...     
All views expressed are mine, not my employer's... blah blah...




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