[mythtv-users] minimize the amount of disk space used by recordings

[Tom Metro]

This seems like this should be an FAQ, but I didn't see it addressed in 
any of these places:

http://www.mythtv.org/wiki/index.php/Frequently_Asked_Questions
http://www.mythtv.org/modules.php?name=MythInstall
http://www.mythtv.org/wiki/index.php/User_Manual:Daily_Use
http://www.mythtv.org/wiki/index.php/Recording_Parameters

The question is, what settings should I use in order to minimize the 
amount of disk space used by my recordings?

Of course you need to add qualifiers to that. As this section on 
transcoding points out:
http://www.mythtv.org/docs/mythtv-HOWTO-23.html#ss23.14

any encoding scheme is a compromise between storage space and image 
quality, but there are still some general principles involved, such as 
MPEG4 encoding usually takes up less space than MPEG2 encoding, for the 
same apparent quality.

On of the links above:
http://www.mythtv.org/wiki/index.php/Recording_Parameters

was helpful when I made a first pass at this problem and wanted to 
simply optimize the MPEG2 recordings coming from my PVR-500 so I could 
choose a low quality recording profile if I wanted to save space. But 
where is the equivalent for MPEG4 transcoder settings? It's not here:

http://www.mythtv.org/wiki/index.php/Transcoding

or at the places that page links to.

It also strikes me that it would be more user-friendly if MythTV came 
out of the box configured with some reasonable defaults for minimizing 
disk space usage. Obviously no one-site-fits-all solution is possible, 
and not everyone wants to endure the overhead of transcoding, but I 
wonder how many "install-and-forget" users there are who live with the 
default MPEG2 recording parameters and never realize that they could at 
least double their storage capacity by adjusting some settings.

On a related note, before I set up MythTV, I operated under the illusion 
that if Xvid was among the most space efficient encoding schemes 
available, and if there were open source libraries for producing Xvid, 
then presumably that's what MythTV would use as its storage format (or 
at least that'd be among the choices). The next bit of info I saw was 
that MythTV supported MPEG4, and I understood that Xvid was a type of 
MPEG4 encoding, so it still seemed reasonable that it should support 
Xvid, or at least have comparable compression.

But the reality seems to be quite different. While an hour (44 minutes) 
of 1 Mbps Xvid encoded HD video is about 350 MB, an hour (44 minutes) of 
1 Mbps MythTV MPEG4 encoded SD video runs nearly 1.3 GB. Is the 
difference just a matter of tweaking parameters?

  -Tom