Intel PAL TV out on Aopen i945GMm-HL - slight chroma flicker on svideo output

[Keith Packard]

On Tue, 2007-12-04 at 23:08 +0000, Justin Hornsby wrote:
> Hi folks.
> 
> I'm experiencing a slight problem with svideo out using the 'intel' 
> driver version 2.1.
> 
> I eventually managed to find out how to use xrandr to set the TV 
> standard to PAL and how to get the picture nicely overscanned with the 
> margin setting options of xrandr but...
> 
> I've noticed that the svideo connection on my Aopen motherboard is 
> flickering slightly near the top of the screen.  Turning the colour 
> control on the TV down makes the problem go away which seems to indicate 
> an issue with the chroma signal on the svideo port.  I quickly hacked a 
> spare svideo cable up a little and found (by connecting the Y output to 
> a composite input on the TV) that the chroma signal is also present on 
> the Y pin.  It's my understanding that for svideo, only luminance should 
> be present.  Depending on how the TV treats svideo internally, having 
> chrominance on Y & C could cause problems.

If you hook only the Y pin up, the encoder will be configured for
composite video; it automatically detects the kind of cable purely based
on which wires are connected. It shouldn't change after you've turned it
on though, so hooking it up to svideo, starting the output, then
switching should keep it in svideo mode.

> I don't know anything about the TV encoder used on this board (yet) but 
> I have access to video test gear at work where I plan to have a real 
> good look at the signals coming out of the svideo port.  We also have 
> good monitors which I know for sure don't do any quick & dirty tricks 
> with Y&C signals so if the issue is one of 'beating' of the 'Y chroma' 
> and C signals it should look nice & clean on there.
> 
> Are there any quick things I can try?  It seems (from a quick look in 
> the driver code) that xrandr supports only TV_FORMAT & margin 
> adjustments for the TV encoder.  All the timing values look sane to me 
> but as I've already said I plan to examine the signals very closely at work.

Oh, cool! I looked at getting some TV test equipment to figure out
whether I was generating legal signals, but couldn't really justify the
price.

There are a million values you can frob to play with the video signals,
but most of them are not exposed through the protocol. Check out
i830_tv.c and i810_reg.h which documents the relevant registers
reasonably completely. In particular, using one of the test patterns may
help isolate the particular issues you're seeing with the image.

Let me know if you have specific signal problems and I'll read through
the documentation to see what I can figure out.

-- 
keith.packard at intel.com
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