[mythtv-users] RE: MPEG-2 to MPEG-4 Transcoding comparisons

[Joseph A. Caputo]

On Wednesday 19 November 2003 18:19, Augustin Chan wrote:
> Variable bit rate seems to be the way to go in terms of getting the
> highest quality recordings with the smallest file size.  From the
> MythTV menus, it seems that it supports it (since there's a
> mpeg2bitrate and mpeg2maxbitrate setting when specifying encoding
> parameters).
>
> However, from what I've seen empirically (by looking at file sizes),
> only the mpeg2bitrate setting is used.  Is this because of the
> hardware I'm using (PVR-250, natch) or is this a bug?
>
> The other question then becomes, if VBR is supported, why aren't
> there matching mpeg4bitrate and mpeg4maxbitrate settings when
> transcoding to MPEG4?
>
> Also, does anyone know of a utility that can do VBR transcodings of
> MythTV format .nuv files :)?

The MPEG-2 bitrate parameters are passed to the ivtv driver to control 
the bitrate of the hardware MPEG-2 encoding.  AFAIK recordings made 
with software MPEG-4 encoding use CBR.  I don't think you can transcode 
*to* MPEG-2, so it's not an issue there, but in transcoding MPEG-2 -> 
MPEG-4, since it's 'offline' (i.e., not encoding a live stream), it 
should be possible, theoretically, to do a 2-pass VBR encoding, but I 
don't think Myth supports that at the moment.

Mythtranscode has the ability to output raw (decoded) audio & video, 
though, so you can use that facility (as nuvexport does) to use any 
kind of encoding scheme you want.  If you're recording with a PVR-250, 
though, you don't need mythtranscode to do that, as you can use any 
transcoding tool on Linux, Windows or Mac that can handle an MPEG-2 
source.

-JAC