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PostPosted: Wed Aug 06, 2008 1:42 pm 
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Joined: Fri May 21, 2004 11:55 pm
Posts: 1206
Location: Silicon Valley, CA
I've been playing around with the 'transcode' feature of MythTv and I have come to the conclusion there is a fundamental flaw in the concept -- at least for HDTV watchers. Transcoding an MPEG2-TS file into MPEG4 results in a file larger than the original.

I have tried playing with the transcoding parameters, but I have not come up with any that (a) don't fail, (b) look nearly as good as the original, and (c) save space. There seems to be a huge failure in the whole concept of transcoding when the transcoded result is 10% to 200% larger than the file it replaces.

Has anyone else found the magic numbers that make transcoding worthwhile? It seems to me that, for the present version at least, transcoding is worthless from the outset.

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PostPosted: Wed Aug 06, 2008 2:55 pm 
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Joined: Fri Apr 02, 2004 10:08 am
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Location: Virginia, USA
First question is how are you trying to transcode? There's a number of ways: selecting Transcode from the on-screen menu when watching a recording, setting a recording to transcode automatically as a job, transcoding to iPod or Xvid via MythWeb.

Secondly, did you change any of the default transcoding settings and/or add new ones?

Third, are you trying to get HD-resolution MPEG4 files? 960x540 or 1280x720?

(I haven't played with it since all my MPEG2 HD transcoding is just lossless to remove commercials; I usually move recordings off to other computers if I want to try transcoding to other codecs)


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PostPosted: Wed Aug 06, 2008 3:41 pm 
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Location: Silicon Valley, CA
I'm trying to transcode HDTV into a form that stores in something less than 8GB/hour. Most of my source material is 1080i, but some programs are broadcast in 720p. At this point I'm trying to create a magic box that I put an HDTV show into and it comes out the same resolution and quality, only smaller. I thought MPEG4 could do that, but so far I have not been able to achieve it.

Of the transcoding options, so far I have tried only the built-in "high," "medium," and "low" options. I found some settings on the forum here and tried several variations. Most resulted in a transcoder error, but the few that worked gave me files that were larger than the original MPEG2-TS source files.

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PostPosted: Wed Aug 06, 2008 3:55 pm 
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Joined: Fri Apr 02, 2004 10:08 am
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Location: Virginia, USA
This seems like a good thread:

http://readlist.com/lists/mythtv.org/my ... 56600.html

In another thread it was claimed that you would get much more savings (or better quality at the same bitrate) with x264 instead of MPEG4.


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PostPosted: Wed Aug 06, 2008 9:20 pm 
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Joined: Fri Feb 03, 2006 11:36 pm
Posts: 63
Location: Melbourne
To get transcoding of hd working, I had to add a transcoder called something like "Autodetect from 1080i". You need to see what transcoder it's looking for in the mythbackend log. When I did it it was looking for a transcoder for "Autodetect from 1080i" and not finding it. It would then use the "Autodetect from MPEG" transcoder, which didn't result in a smaller file.


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