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

[Justin Hornsby]

Keith Packard wrote:
> 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.
>
>   

Right.  I tested the system as soon as I could when I got home and the 
flickering was still there.  Much head scratching ensued but then I 
found a doc about the specifics of PAL & it got me wondering...

Anyway the fix (my my TVs at least) is as follows: change nbr_end to 288 
from 286 in i830_tv.c    .  The patch I emailed previously can be 
disregarded.

Regards,
Justin