[mythtv-users] How to transcode to SD within mythtv - system too slow to play HD

A. F. Cano afc at shibaya.lonestar.org
Wed Dec 11 01:42:52 UTC 2013


On Mon, Dec 09, 2013 at 04:40:29PM +0000, John Pilkington wrote:
> ...
> First; why are you recording HD?  Is that the only version that is
> available to you?

As other people in another part of this thread have pointed out, that
is the only choice I have.  I use an antenna and the material comes
in HD.

> Second; it looks as if your transcode isn't working.  Here in the UK

Interestingly, when I tried to transcode manually with the command you 
gave below, mythffmpeg was not installed.  I installed it via aptitude.
We'll see if this has any effect next time I record a show.

> a good-quality SD recording stripped to just the video and one audio
> channel comes at around 1.4 GB per hour; I can usually get 3
> one-hour shows onto a DVD, with no re-encoding.

It looks like the default parameters cause extreme lossy compression.
>From 5.2G to 254M just by specifying 960 x 512 (1/2 the height and 1/2
the width).  Obviously this is not the optimal setup.  I can watch other
videos full screen (with mplayer) and they look much better even though
they are of lower resolution.  I'll have to try with the -b:v and -b:a
parameters.

254M Dec 10 17:45 2131_20131127030000.mpg
5.2G Nov 26 23:00 2131_20131127030000HD.mpg

> I have done conversion from HD to SD like this:
> 
> mythffmpeg -threads 2 -v verbose -i infile.mpg -target pal-dvd -b:v
> 6000k -s 720x576 -acodec mp2 -b:a 256k -ac 2 -aspect 16:9
> outfile.mpg

This is what I tried:

mythffmpeg -threads auto -v verbose -i 2131_20131127030000HD.mpg -s
960x512 -acodec mp2 -ac 2 -aspect 16:9 2131_20131127030000.mpg

Also ran mythcommflag manually, but it finished quickly with an
error.  It might be because the file was quite corrupted because
of reception problems.  When I tried to watch it within myth, it
gave a long list of errors and exited back to the "watch recordings"
window, which incidentally still says that it's 1080i, so something
didn't get updated properly.

> and on my 2006-vintage laptop throughput is around 30 fps.  Input
> filesize (all streams) around 3 GB per hour, output filesize around
> 2 GB per hour.

The above transcode took about 2:45 for the 1 hour show.

> Obviously this may need modifying for location, and maybe omit
> --threads or set it to 1, but it should give you a start if no-one
> comes up with a Myth-based solution.  Rename the input file to
> xxxHD.mpg, use xxx.mpg as output and when done use mythcommflag
> --rebuild --file xxx.mpg to fix the database.

Thanks a bunch.  At least now I can watch the highly pixelated shows
with mplayer and the slow cpu can keep up.

One other question.  I deleted a test recording from within myth.  It
says that deleting the show from the list does not remove the file.
Is there a way to manually remove recordings completely? the mpg file,
the data that gets stored in the database, etc...  I'm aware that there
is some expiration time, but since the disk is not full, myth is
apparently not removing shows.

Thanks.

Augustine



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