Date   : Wed, 06 Feb 2002 18:33:36 GMT
From   : Thomas Harte <thomasharte@...>
Subject: Re[1]: Firetrack

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> I don't believe there's any reason to emulate any of the 
horizontal stuff
> precisely, as I don't know of anything which hacks it in a 
really nasty way
> (as that would be pure evil!), but the vertical stuff is worth 
getting
> right I think.

There is a single Electron demo that does two mode changes 
per scanline, so it may not be entirely safe to assume that 
such tricks are unheard of in the BBC/Electron realm.

Boast moment : ElectrEm can change display mode mid 
scanline.

> The only purpose of the VSync signal is
> to *initiate* the vertical flyback in the VDU, at which point 
it'll just
> get on with it! It's not that the flyback lasts for the duration 
of the
> VSync pulse.

>From my internet based reading on the topic just recently :

On most TVs, the vsync signal initiates the vertical flyback only 
if the TV feels it should be flying back 'soon' - i.e. in most 
cases with a TV the flyback is done if the CRT is beyond the 
bottom of the visible area and has not already 'timed out' and 
flown back itself, however with a genuine monitor in some 
cases there is no such 'time out' mechanism. Also, some 
monitors will initiate the flyback immediately upon vertical 
sync regardless of where they are in the display. So mileage 
does vary.

> The exact breakdown of the number of scanlines of the total 
312 

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.

-Thomas
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