[mythtv-users] Video quality?

Brian Wood beww at beww.org
Sun Sep 30 15:07:31 UTC 2007

Piers Kittel wrote:
> 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. 

It always amazes me that some people can look at what is to me
horrendous digital artifacts and not see them at all. Sports programming
is a good example of this. That's why I used the term "uneducated
viewer", I didn't mean they are lacking in education, they just don't
know what to look for.

Sports can be particularly bad because often it is backhauled as MPEG,
decoded by the local station so they can switch it, then re-encoded,
often with a cheap encoder (if in a small market). Also, sports has to
be encoded in real time, while some prime-time shows can be pre-encoded,
often with multiple passes.

 Freeview (UK DVB-T) does have digital artifacts, but
> not as bad as Sky TV.

Bandwidth may be at more of a premium on a satellite, causing them to
compress more ? Is the resolution the same?
> 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.

That would certainly point to the Myth playback software, but when
running Myth many more things are happening than when just running VLC.

> 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.
> 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?

Yes, the GeForce 5200 series nVidia chip, such cards are made by several

>> 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:
> XvMC
> Storage optimization
> Anything else?

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