[mythtv-users] Video quality?

R. G. Newbury newbury at mandamus.org
Sun Sep 30 21:53:56 UTC 2007

Piers Kittel wrote:
> 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:
> XvMC
> Storage optimization
> Anything else?
> Thanks very much for your time!
> Regards - Piers

XvMC is now quite stable on the chipsets which support it (Intel with 
i810, nvidia and via (various chips but not all tv output encoders..yet.)

One thought from left field. Somewhere, probably here, someone mentioned 
  that re-compiling your kernel to use CONFIG_HZ=300 instead of 
CONFIG_HZ=1000 is helpful. The exact reasons I do not remember, beyond 
the fact that 300 is divisible by both 50 and 60 (PAL and NTSC frame 
rates. But I could guess that if the frame-rate does not match the 
kernel's internal rate, video frames probably get folded, spindled and 
mutilated along the way through because the kernel controls the battlespace.


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