[clug] Re: MythTV Transcoding

Paul Wayper paul.wayper at anu.edu.au
Mon Aug 28 02:53:07 GMT 2006

Paul wrote:
> Can you give me a example of the transcoding for say DivX or something
> that MythVideo can read
> Preferably something that doesn't reduce the quality too much.
> BTW how long does it take to run?
I usually use these parameters with the current version of nuvexport:

Export type: xvid
Use MythTV cut list: yes (set one up beforehand using 'e' during playback)
Use noise reduction: no (if you're using DVB, you've got top quality)
Deinterlace: yes (better playback later)
Crop the picture: no (likewise, you've got the picture exactly)
Audio bitrate: 128 (in kbps - standard 'CD quality' MP3)
Use variable bit rate compression: yes
Use two-pass compression: yes
Bit rate: 600 (usually ends up around 200MB per hour)
Picture size: 720x576 (means no resize on playback)

If you're capturing 16:9 pictures, set the picture to 720 x 400.

This does the first pass in around 25 minutes, and the second pass in
around the same time, for a half hour show - thus meaning it takes about
twice as long to compress as to record.  My experience is that this
gives a pretty good quality of final product.  The most significant
factor in the compression is really the picture size - reduce that to
640x480 or 400x300 (640x360 or 384x216 for 16:9) and you'll still be
getting reasonable pictures thanks to antialiased scaling on your video
card (if you're playing back through OpenGL using mplayer, for example)
but getting more quality per pixel, so to speak.

Why 384x216?  Because most compression methods often object to having
frame sizes which are not multiples of 8 or 16, and 400 * 9 / 16 = 225,
which is not a multiple of 16.  nuvexport will warn you if it doesn't
like the frame size you've chosen.



P.S. I spent a bit of time working on Nick Cherbuin's MythTV machine -
the one we upgraded to Fedora Core 5 during the night and that delayed
us getting out of there until midnight.  I got the backend and frontend
working, but the screen configuration no longer uses 720x576 for correct
PAL TV output and for some reason the remote control doesn't work any
more.  Oh, cruel irony...  If anyone has more experience with mode lines
for PAL output, using either the ATI fglrx drivers or the Open Source
radeon drivers, then we'd both be glad to hear it.

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