[mythtv-users] Video quality?

Brian Wood beww at beww.org
Sun Sep 30 14:17:46 UTC 2007

Dan Harper wrote:
> On 30/09/2007, at 12:52 AM, Brian Wood wrote:
>> Dan Harper wrote:
>>> On 29/09/2007, at 10:05 PM, Piers Kittel wrote:
>>>> What I would see is very very slight
>>>> jerking in between frames - it's not totally smooth, I can see very
>>>> slight jumps when it transitions in between frames.
>>> I get this also, so I'd love to know how to fix it.
>> The first question would be what sort of hardware are you using? Both
>> CPU and Video?
> I have a IBM Thinkpad R51.  ATI Radeon 9200M video using standard ati  
> X.org drivers.  The binary ATI driver no longer supports this card,  
> so I'm SOL on that one (*sarcasm* yeah, like, it's an ancient 3 years  
> old! */sarcasm*.  The IT world sucks sometimes).

The open source drivers are less than optimum, not the fault of the
authors but of their lack of data on the chips. The fact that AMD has
announced they will make the ATI information available to the Open
Source Community may help this situation in the long run.

> I have a 20" Viewsonic LCD on the external VGA port.  Xinerama to get  
> the MythTV GUI on the external screen.

> The CPU is a Pentium M 1.6Ghz
> I too notice a slight improvement when switching off deinterlacing,  
> so is the problem due to the deinterlacing algorithms?

De-interlacing takes up CPU cycles, so you may just be seeing the effect
of that.

> I also see some nasty artifacts after transcoding and having kernel  
> deinterlacing switched on, particularly for high contrast movement on  
> the screen (Futurama and The Simpsons look particularly nasty due to  
> the high contrast movement caused by the black outlines of the  
> characters).

Adjusting your transcoding parameters might help this. One main reason
for transcoding is to reduce file size, remember that you never get
something for nothing.

(Well, theoretically you can transcode from MPEG2 to MPEG4 and get a
smaller file with no perceived impairment in visual quality, especially
if you use two-pass encoding, but somehow this doesn't often work out in
the real world, at least not without very expensive encoders from
outfits like Tandberg or Rhode and Schwartz)

> CPU is cruising when playing back, probably max at around 50-60%.

If you're measuring  CPU usage with top remember that this is an
average, taken over what is to the video signal many frames. You may
well be hitting peaks significantly over that average.


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