Re: A/V cable

Christer Palm wrote:

> Perhaps it's just me who is not up to date on the VGA interfacing
> discussion, but I was under the impression that the current belief 
> is that that Xbox can not provide RGB output - at least not by 
> simple strapping at the A/V connector, and definitely not with a
> separate sync. 
> 
> This sure suggests otherwise, as SCART RGB uses a separate composite
> sync. Also, given the size and price of the cable, I doubt that it
> contains any electronics.

The Scart RGB mode is a special case in the Conexant video encoder.
There is a specific "Scart-RGB mode" in the chip for this. Also, 
the signal levels for Scart RGB are different from the VGA ones. 

The specs indicate that also VGA-style RGB modes are possible (even
with color format conversion on the chip), but there is apparently
an upper limit to what kind of modes the chip can process.

Yet another possibility is setting the chip into mode where it only
acts as a DAC for the nVidia GPU (disabling all the special processing
and filtering the chip is capable of for tv-out modes), but the jury
is still out on whether this it feasible. At least it would require
in-depth knowledge and configuration of the registers in both chips.

The sync is a problem in itself, unless there is some special sync
routing and processing elsewhere on the XBox motherboard - straight
to the A/V connector pins - or unless the monitor you use can grasp 
composite sync or sync-on-green.

Also note that when some people talk about it being _impossible_ to
get VGA-style RGB out from the XBox, they generally mean that the
_native_ XBox BIOS/kernel does not offer this kind of video mode for
games. What does this mean for XBox-Linux? Not much. You probably 
cannot ever get a standard VGA RGB signal (with a straight cable) 
out from a normal XBox game. However, on the Linux side, things are 
a bit different as Linux kernel/applications do not have to rely on 
the XBox BIOS, and can tweak the chip registers directly, generating 
just the modes you want.

My personal educated belief is that using a straight VGA cable with
XBox-Linux project will become possible. But it is still a bit too
early to tell. The current situation with XBox-Linux is that the
video modes and the necessary register-tweaking have not yet been
extensively researched. For example, the stand-alone boot ROM 
project for XBox cannot yet initialize video modes at all, and when
Linux is booted from the dashboard using the XBE bootloader, it 
relies on the XBox BIOS to initially set up the chip registers
correctly for it. 

At this stage, it is probably more important to get the standard 
TV out modes to work correctly (with overscan compensation etc.
for various video standards and output signal modes), and only
after this has been done, it is time to start tweaking with more
exotic and esoteric modes.

-- 
 Jukka Aho             +358  6 317 4282
 Konepajakatu 9 Q 48   +358 50 349 8812    IRCNet:znark
 FIN-65100 Vaasa       jukka.aho@xxxxxx    http://www.iki.fi/znark/



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