[MEncoder-users] Dolby downmix

[Rolf Ernst]

On 12/30/2010 6:34 AM, Reimar Döffinger wrote:
> <snip>
> With all the FFmpeg discussion (yes, suggesting it is ok, but please try
> to keep the discussion a bit separate, hopping between programs makes it
> a pain to follow the discussion) I haven't managed to actually
> understand what the goal is.
> Do you just want downmixing to two channels? Just use -channels 2
> instead of -channels 6.
> If you want to have a very specific downmixing matrix (instead of
> something that should produce a reasonable 2-channel representation
> from all 6 channels), use -channels 6 and -af pan with the precise
> matrix coefficients.

Reimar,

what I want to is not to simply discard center, lfe, and back speakers 
but downmix so I can decode at least the Surround via Dolby II from the 
resulting channels (if possible).

Carl's response seemed to suggest that specifying -channels 2 would 
simply discard instead of mixing in information from the remaining four 
channels. Is this not so?

Now that you mention it I checked the doc and there is indeed an -af 
pan. The Nero aac encoder claims it is doing a Dolby II surround mix and 
so is Handbrake. Now, I listen to the Handbrake output through my HTS 
amp on Dolby II decode settings and I don't quite buy it but there is 
something coming out of the rear speakers so I don't quite know how well 
this can be done at all.

So the question again: My input file has AC3 5.1 Dolby audio and I would 
like to create a compressed output file that downmixes this information 
to Dolby II (Dolby II carries info for five speakers and the decoder can 
somehow pull this out of a dual channel signal).

Did I clarify this sufficiently?

At present I have found a somewhat cumbersome workaround:

Remember that my initial problem of simply processing the streams 
separately was defeated by the fact that my input stream occasionally 
exposes a glitch that shows the sync off. It's unnoticeable but as I 
could demonstrate to Carl with an example it is definitely there. The 
input signal is also 59.94 fps and I convert this to 23.976 with some 
tricky mencoder filter combination. If I push audio and video through 
mencoder the code is very smart, does the pulldown correctly and keeps 
a/v in sync. (Say what you will but mencoder is superior to ffmpeg in 
terms of doing things like this). So the separate processing creates a/v 
sync problems but when I take the whole file with -oac copy (getting out 
the same ac 5.1 in the video track) I can demux it and it apparently has 
the a/v glitch patched up (mencoder issues some sort of message 
'Concealing DC whatever' so I think it does some sort of triage). When I 
take this audio and postprocess it using Nero and multiplex it in, 
replacing the 5.1 with Nero aac I have perfect sync and Dolby downmix, 
at least it sounds right through my HTS.

My question was thus: Can mencoder be made to downmix a 5.1 ac3 signal 
into Dolby II Prologic decodeable discrete channels during the a/v 
encoding process?

-- 
/re

"There's a fine line between audacity and idiocy." — Jim Butcher