[mythtv-users] nVidia tv-out quality improvement

Steve Davies steve at one47.co.uk
Wed Jan 14 12:46:13 EST 2004

Simon, this post interested me mightily as I've been attempting a similar

Simon at the Threshold said:
> Well I appear to have made some inroads into my nVidia MX440 tv-out
> quality.. Having read a few of the postings I figured I'd have a go at
> attempting to match my incoming tv-stream with my tv's resolution.

I have a full 4200Ti card, but the principle should apply.

> I figured I'd have a go at one of the DVB-t streams which comes in at
> 720x576 (widescreen) and thus I created a new 60Hz XFree86 mode..
> ModeLine    "720x576" 32.7 720 744 816 912 576 577 580 597
Why 60Hz? If you are in the UK, television signals are clocked at
50Hz/25fps. Which X driver/version are you using? I tried to configure a
50Hz modeline for 720x576 on the Nvidia binary driver, and it tells me it
is not a valid resolution (Perhaps I should just binary-edit the driver
:-) )

> I then re-configured mythfrontend to use the same resoution for its
> interface and for the tv playback.

If you just make 720x576 the highest resolution in your X config, this
should "just happen" as if by magic - What configuration did you do here?
Perhaps there is something I am missing.

> The playback just using plain-out XVideo was better than before but
> still suffered from motion sickness. However the best was in the XV
> NVIDIA mode where the image was spot on..

The "motion sickness" is probably caused by the difference between 50Hz
and 60Hz. In fact UK DVB-T transmissions are all AFAIK at 25fps, which
does not divide into 60Hz very nicely at-all.

> However since in the UK DVB-T channels can be broadcase in diffirent
> resoutions this doesn't solve the problem for all of them.

True, but I have found that the vertical resolution is always 576 (perhaps
I've simply not come across the exception yet) and in the exceptional
cases, Myth does a good job of re-scaling them. Vertical scaling may
introduce motion artifacts, but horizontal scaling should be (and on my
system is) fine.

> Does anyone know if or how difficult it would be to plug in a xvidtune
> (or equivilent) into the playback code so I can change the playback
> resolution to match the feeds?

Ouch! Nasty.

FYI, I am currently running my TV out at 800x600 at 50Hz, and capture using
DVB-T. My experiments showed some pretty good results. The colour balance
is a bit poor, and I cannot use the newest Nvidia X driver (which has
tools to correct this AFAIK) because it panics my kernel :-(

Oh, I just realised. I am probably benefitting from using a 16:9
television - This means that the vertical image is virtually un-scaled,
regardless of the aspect ratio of the source stream, only the horizontal
size will be modified.


More information about the mythtv-users mailing list