[mythtv] A prelude to transcoding MPEG2->MPEG2

[Geoffrey Hausheer]

On Mon, 8 Dec 2003 13:40:19 -1000, " James L. Paul james-at-mauibay.net
|mythtv/1.0-Allow|" <0ambhpgqea0t at sneakemail.com> said:
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> On Monday 08 December 2003 13:17, Derek Atkins wrote:
> > "Geoffrey Hausheer" <ou401cru02 at sneakemail.com> writes:
> > > Note that I have yet to see a PVR250 stream which is DVD compliant.  DVD
> > > requires either an AC3 or PCM stream to be present, and I'm not aware of
> > > any way to get such a thing from the PVR250.  I've yet to find an AC3
> > > encoder which is really compliant (the one that comes with ffmpeg is not
> > > compatible with my DVD player, and I've had to use commercial encoders in
> > > Windows to get compliant streams).  Anyhow, that isn't really relevant as
> > > it has nothing to do with the issue at hand, I just thought I'd mention
> > > it.
> >
> > Not quite true..  A DVD can also use MP2 audio, which the PVR will do.
> > I was able to make a video DVD without any audio transcoding, and
> > it works just fine in my set-top DVD player.
> 
> I just posted to this effect as well. Here's some specifics on DVD
> support for 
> MPEG audio, from the excellent dvddemystified.com website:
> 
> MPEG audio is multi-channel digital audio, using lossy compression from 
> original PCM format with sample rate of 48 kHz at 16 or 20 bits. Both
> MPEG-1 
> and MPEG-2 formats are supported. The variable bit rate is 32 kbps to 912 
> kbps, with 384 being the normal average rate. MPEG-1 is limited to 384
> kbps. 
> Channel combinations are (front/surround): 1/0, 2/0, 2/1, 2/2, 3/0, 3/1,
> 3/2, 
> and 5/2. The LFE channel is optional with all combinations. The 7.1
> channel 
> format adds left-center and right-center channels, but is rare for home
> use. 
> MPEG-2 surround channels are in an extension stream matrixed onto the
> MPEG-1 
> stereo channels, which makes MPEG-2 audio backwards compatible with
> MPEG-1 
> hardware (an MPEG-1 system will only see the two stereo channels.) MPEG
> Layer 
> 3 (MP3) and MPEG-2 AAC (also known as NBC or unmatrix) are not supported
> by 
> the DVD-Video standard. MPEG audio is not used much on DVDs, although
> some 
> inexpensive DVD recording software programs use MPEG audio, even on NTSC 
> discs, which goes against the DVD standard and is not supported by all
> NTSC 
> players.
> 
I looked at this some time ago, and it seems that MP2 must be accompanied
by either AC3 or PCM to gaurantee a compatible disc.  (For instance, you
can have an AC3 primary audio, and MP2 alternate-audio, and be
compliant).  Note that SVCD only supports MP2 audio, and most players
that support SVCD will also support MP2-only DVDs, (effectively a DVD
with MP2 audio will be idnetical to SVCD at a higher bitrate) however,
this is not gauranteed.

Sorry, I don't have a reference for the above statement.  I believe I
originally had documentation to back it up, but I can't find it now. 
However, at one time I spent an awful lot of effort making the most
compatible DVDs I could for a video project, and found that the above
matched well with empirical tests.  That said, if your player will play
an mp2 encoded DVD, and that is what you need, then that should be good
enough.

.Geoff