[mythtv-users] Video quality?

Piers Kittel mailing at biased.org
Sun Sep 30 09:10:10 UTC 2007

On 30 Sep 2007, at 02:22, Brian Wood wrote:

> I'm not saying the PCs you used aren't capable of good playback  
> (should
> be indistinguishable from live to an uneducated viewer), but they  
> might
> well have to be "optimized", at least to the point of eliminating
> unnecessary processes (which a lot of distros start by default),
> possibly going for a lightweight desktop (or none) and making sure you
> are using nVidia's binary drivers (ATI drivers I don't know  
> anything about).

I always use nVidia's own drivers, and when I install Debian, when it  
shows me what I want to install, I de-select everything and finish  
the installation.  Not even "less" is installed for example, not a  
single X package installed, etc.  When I log in, I install things as  
I go along, making for the most minimal possible system.  I use  
WindowMaker as a desktop manager.

> A "slight jump as it transitions between frames" would mean 30  
> jumps per
> second. I suspect you mean scene changes?

It's hard to explain.  It doesn't matter what's on the screen - it's  
just very very faint gaps I am able to see.  Maybe some people can't  
see it?  I don't know.  For example, on one of my old frontends, the  
subtitles caused the video to jump (dropped a frame or two I'd  
suspect - probably due to lack of CPU power), my girlfriend couldn't  
see it, but I could.  I hate watching Sky TV as the picture is full  
of digital artifacts, but most people always say "What digital  
artifacts?  Look good quality to me" and they don't see it even if I  
point it out.  Freeview (UK DVB-T) does have digital artifacts, but  
not as bad as Sky TV.

Lack of CPU power would explain it, but the PowerMac is more than  
powerful enough to handle the video, and I see it.  Also, like I  
mentioned before, I can see it when playing back using MythTV, but  
using VLC, I don't see it, and that's on the same computer, same  
video file located on the same hard drive, etc.

My partner wants me to reinstall XP on her P4 2.8GHz laptop soon, and  
I will be installing Ubuntu on it as well.  I'll be trying MythTV on  
that as well.

> Also make sure your resolution and bitrate are set for decent quality,
> the defaults are not anywhere near DVD quality (that is, if you are
> encoding, if you're receiving a digital stream you have no control  
> over
> this).

I never transcode because MythTV's transcoding deletes the subtitles,  
and I need subtitles to watch TV.  The video is raw DVB MPEG-2 with  
no processing done on it apart from commercial flagging.  I've got  
some video left over from my PVR-250 days which was connected to an  
external Freeview box, but the signal between the box and computer  
was analogue, so the PVR card had to encode the video.  The problem  
was there.

> And, make sure your storage system is optimized (using DMA etc.).

Should be, I hope!

> A 5200 series is pretty much the baseline for Myth systems today.  
> Peple
> have made earlier cards work but it seems to be some effort.

5200 series?  You mean some sort of video card?

> Have you tried XvMC?

think I have done it once a long time ago, didn't seem to work well.   
I'll try that again, but only one of my current frontends can use XvMC.

I'll make a list of things I should try and I'll do it when I get  
back from holidays.  So I'll try:

Storage optimization

Anything else?

Thanks very much for your time!

Regards - Piers

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