[mythtv-users] mpeg2 playback performance issue makes no sense

Michael J. Lynch mlynch at gcom.com
Tue Oct 12 14:27:27 UTC 2004

Ah ha....See comment below.

Joseph A. Caputo wrote:

>On Tuesday 12 October 2004 08:54, Michael J. Lynch wrote:
>>I ran into a strange problem that makes absolutely no sense to me and
>>was hoping someone could shed some
>>light on it.  Here's what I did.  First I ripped a DVD and used
>>mplayer (via "watch videos") to playback the
>>ripped VOB(s).  I checked CPU idle time and it was about 70% idle.
>>Just for grins and giggles I wanted to see how performance of Myth's
>>internal playback compared to playing
>>a file using mplayer.  Since VOB, and NUV files are simply MPG files
>>with non-standard extenstions, I figured
>>I could simply rename or copy one of the VOB files to NUV (with
>>proper basename format), inform Myth of the
>>NUV files existance, and play it back using Myth's internal playback.
>>First I used Myth to record something.  Then I deleted the resulting
>>NUV file and renamed one of the VOB files
>>from my DVD rip to the file I had deleted.  This effectively informed
>>myth of the existence of the recording.  I
>>probably could have used the rebuilddb script too, but this seemed to
>>get the job done.  I went to "watch recordings"
>>and sure enough the VOB file was watchable under the name of what I
>>had recorded.
>>Now this is where it get's weird.  The playback was skipping and
>>generally acting like there was not enough
>>processor to do the playback, so I checked it out.  Sure enough,
>>playback using Myth's internal playback was
>>pegging the processor.
>>First, is the test methodology I described valid?  If so, what could
>>account for the large difference in processor
>>usage between "watch videos" and "watch recordings" playing the same
>>My system is 733 PIII with 384M, FC1, and Myth 0.16.
>Well, the first thing that accounts for the difference is that "Watch 
>Videos" uses mplayer (at least, that's what you told us), and "Watch 
>Recordings" uses Myth's internal player.  The second thing is that you 
>didn't need to go through all those steps to test Myth's internal 
>player; indeed, that may be the cause of the problem.
>When you record something in Myth, it writes a seektable to the 
>database.  When you copied a different file over top of the one you 
>recorded, you may have confused Myth.  Was the file Myth recorded an 
>MPEG-2 recording (i.e., made with a PVR-x50 or similar card)?  Even if 
>it was, the seektable in the database no longer matches the recording.  
>This could be a source of playback problems.
>Also, I'm not sure it's really valid to simply treat a VOB file as a 
>standard MPEG-2 file.  Mplayer may handle it OK because it has support 
>for VOBs, but Myth's internal player may have problems.  Someone with 
>more knowledge in this area may want to chime in...
>If you really want to see how Myth's internal player performs, instead 
>of trying to trick Myth into playing it as a recorded TV program, 
>simply use MythVideo ("Watch Videos").  There is a setup option that 
>lets you configure the player command to use for this; apparently you 
>have yours configured for mplayer (the default, IIRC).  If you change 
>this to "Internal", it will use the internal player without the need to 
>fool Myth.
This is what I've been looking to do (use the internal player instead of 
mplayer) to
playback an externally generated video.  I'm not really so much 
interested in the
difference in performance between the internal player and mplayer but am 
interested in two things.

First, whether or not I have enough CPU crank to play back high quality 
Once I know this, I can determine whether or not I need a 250 or a 350 
to be able
to "watch live tv" in reasonbly high quality.

Second, I want an objective method by which the playback settings can be 
indepentdent of backend recording settings.  That is, if I have a "known 
good recording"
I can make adjustmets to playback knowing that none of the recording 
settings are
affecting the quality, etc. of the video.

Actually, even though the video skipped, etc. I was able to do the 
second thing I was
interested in getting done.  Now knowing what you've written here, I 
think I can
probably safely get away with a PVR-250.

I've searched the archives and asked the question several times and this 
is the first
response from any of my posts that tells me how to do this.  Thank you.

>Finally, (and I'm not sure if this still holds true) you could simply 
>launch the internal player from the command line with "mythtv 
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Michael J. Lynch

What if the hokey pokey IS what it's all about -- author unknown

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