[FFmpeg-user] DeckLink Output Controls

[Reuben Martin]

On Friday, October 7, 2016 5:25:10 PM CDT Matthias, Thomas wrote:
> Hi There,
> 
> I’m looking into using FFMPEG for its DeckLink device control.  The capture
> part of the API seems fairly fully featured, however the output/playback
> controls seems to have some strange limitations.  Specifically, on output,
> there is a hard-coded limit of only 8 channels of audio.  

HDMI is limited to 8 channels. Sombody somewhere probably didn’t want to 
implement the logic required to make channel limits depend on the output 
interface used.

> What’s strange,
> is that the capture partition can handle up to 16, which makes sense
> considering most DeckLink cards have SDI input/output, and 16 channels is a
> standard workflow for SDI.  Also, is there a reason why only 2 and 8
> channels are supported, instead of just “n” channels (4 or 6 channels, for
> example)?

HDMI offers stereo mode, and 8 channel mode when using PCM streams. There is 
no a’la-carte audio mode. Dolby / DTS has things like 5.1, but that is an 
entirely differnt stream type separate from PCM.
 
> Would it be possible to output 16 channels of audio?  Is there a better way
> to do this (via the command line?)?  Could a custom version of the
> decklink_enc files be built to handle 16 channels or is there some other
> limitation I’m not aware of?
 
Probably. Depends on what the API supports.

> Thanks in advance, I really appreciate any advice/ideas here.  Also, is
> there a specific reason that input/output are limited to only 48khz for
> audio?

SDI is 20bit 48k audio only. There’s really no good reason for anything else. 
Technically SDI also supports 48048 and 47952 to allow for pull up/down, but I 
think that is done by manipulating the clock rather than explicitly changing 
the sample rate.

-Reuben