Interleaving into AC3

[Tim Müller]

On Thu, 2016-09-15 at 23:48 +0000, John P Poet wrote:

Hi John,

> While working on adding multi-channel audio support to
> gstdecklinkaudiosrc, I tried to come up with a pipeline which throws
> away all but the first six channels, and encodes them into AC3.  The
> following pipeline works:
>  (snip)
> 
> I would like to understand why the `wavenc ! wavparse` is necessary,
> though.  Without that extra conversion step, I get (an error).

I haven't tried any of your pipelines, but my guess is that it's got to
do with channel positioning ("channel-mask" field in the raw audio
caps). decklinkaudiosrc probably outputs channel-mask=0 meaning all
channels are unpositioned, but the AC3 encoder likely wants positioned
channels. wavenc ! wavparse might work around this by accident more
than by design by assigning a default channel mask for 6 channels when
none is set in the headers, thus making the encoder happy.

You should be able to achieve the same effect either

 - by setting the "channel-positions" property on interleave from code
(this won't work with gst-launch-1.0)

 - by setting a channel position (channel-mask in the caps) on each
input channel to the interleave, then it will/should pick up the
positions and interleave them appropriately (hopefully).

Cheers
 -Tim

-- 
Tim Müller, Centricular Ltd - http://www.centricular.com

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