[FFmpeg-devel] [PATCH] decklink: support all valid numbers of audio channels

Matthias Hunstock atze at fem.tu-ilmenau.de
Sun Dec 20 15:09:42 CET 2015

Am 20.12.2015 um 14:53 schrieb Nicolas George:
> Le decadi 30 frimaire, an CCXXIV, Matthias Hunstock a écrit :
>> SDI just transports 16 mono channels of audio, without any implied
>> semantic of what is in there. So the closest matching layouts are 2.0,
>> 8.0 and 16.0, if they existed.
> That does not mean anything. The channel layout called 5.1 "exists" because
> the convention has made it the short name for "front left + front right +
> front center + rear left + rear right + subwoofer". It is just a convenient
> name.
> "16.0" does not "exist" because there is not, at this time, an audio
> configuration with 16 channels that is widely used.

Correct, it is unusual to have 16 channels in a track. And in SDI there
is no concept of a track, just channels.

> I have absolutely no idea what decklinks cards actually do, and in
> particular where they take their audio. 

Decklink cards actually do just some bitbanging: take digital input in
"raw" format (AFAIK UYVY422 10 bit, PCM 16bit) and copy that into your RAM.

> If there is a layout that is way
> more common than anything else, then it makes sense to have it the
> default. Otherwise, it is better to leave the layout unspecified to
> avoid leaking false information in output files.

IMO the only common layout is 2ch stereo for the 2 channel case, though
even that is a guess. In the case of 8 oder 16 channels, the layout
should remain unspecified, if there is no problem inside FFmpeg with that.

Another possibility is to demux each channel as a mono track. Would this
make more sense?


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