[hatari-devel] Videl : Some questions

[Anders Eriksson]

On Wed, 30 Mar 2011, Laurent Sallafranque wrote:

> 1) Are VBL, HBL and TimerB related to Videl or to the monitor attached to the 
> computer ?
>
> if the answear is "to the monitor", does it mean that there are the following 
> static tables to implement ?
>  TimerB, HBL, VBL cycles for 50 hz , 60 Hz, 70 Hz  on a TV/monitor and 50 Hz 
> and 100 Hz for VGA monitor ?
>  (what about monochrome on a VGA monitor ?)
>  How can one select a 100 Hz VGA monotor (is it the monitor that drives the 
> VBL or can I parameter it into the Videl ?

Hi,

the Videl is driving the monitor, not the other way around. However there 
are pins in the monitor port that enables the Falcon to detect what kind 
of monitor is attached and then adjust the Videl parameters to that.

If you connect a VGA screen, TOS will setup the Videl to 60Hz/31kHz in 
all resolutions (including ST-High).

If you connect a SM124/144, TOS will setup the classic ST-High mode 
(71Hz/35kHz or something, I don't remember exactly).

If neither of these are detected, TOS will setup a 50Hz/15.6kHz screen in 
PAL mode and 60Hz/15.6kHz in NTSC (approx values). PAL/NTSC can be 
selected as a parameter in the NVRAM. These are the classic 
SC124/144/RF-modulator setups.


However, this is just what TOS supports. Writing your own video code or 
using a third party resolution configuration tool you can more or less get 
any frequency that is doable within the bandwidth limit of the Videl.

So if you want to make VBL/HBL/Timer B emulation, I think it needs to be 
very flexible in terms of video frequency.


Oh and about HBL vs Timer B, I don't know the inner technicals, but Timer 
B's interupt occours much earlier than the HBL. HBL begins at the first 
visible pixels of the scanline, while HBL begins long before that, thus 
much more suitable for rasters.

--
Anders Eriksson
ae at dhs.nu     http://www.dhs.nu/
ae at atari.org  http://www.atari.org/