PCM audio bit depth, ffmpeg, Decklink Quad HDMI

Got a strange problem.

We have a Windows 11 NUC device, feeding into Quad HDMI recorder.

Video working fine, but not audio.

On the NUC side of the fence we are outputting PCM (well at least we believe we are) at 48kHz. The Window settings for the HDMI audio device only have two options for the output format:
24 bit, 48000 Hz and
24 bit, 44100 Hz.

We have it set to 48kHz.

I assume this is all that Decklink is "offering", or alternatively, there is a problem with the negotiation of HDMI audio formats, because in the "Supported Formats" tab for the device, it lists "Maximum Number of Channels = 0", and "Bit Depths = None".

On the Decklink PC side, running ffprobe or ffplay against the input is picking up the audio format as pcm16le, which is 16 bit, not 24 bit. (And it sounds totally abnormal, so i would say it's getting 24 and treating it as 16).

It looks to me though that the decklink implementation in ffmpeg only supports 16 or 32 bit depth pcm, but neither of these options are selectable on the source PC.

Thoughts?

Ok We looked into this some more. It turns out the EDID that the DeckLink HDMI inputs are offering in relation to the audio specs do not seem to be correct in terms of "Bit Depth" options. It only offers 24 bit, however 24 bits isn't actually supported by the board. At least, the Decklink API doesn't support it.

There doesn't seem to be a way on Windows PC's to force the audio output to select anything other than what the EDID offers when using PCM. (i.e. as opposed to audio passthrough where the app constructs the audio rather than Windows doing a conversion to PCM).

We did find that by inserting an HDMI splitter in the link, the source PC picked up the EDID of the splitter itself and was thus able to send 16bit audio, which the decklink successfully accepts.

So this looks to me like a bug in the Decklink drivers... I have reported it to tech support but have yet to receive a reply. Just weird that others don't experience this as a common problem.

OK - a few red herrings it would seem.

Yes Decklink does want 24bit PCM audio only to be presented at input, but it down or up-scales this onboard to 16 or 32 bits.

The real reason we were having problems: it was only occurring specifically with an input resolution of 2560 x 1440 @ 60. This resolution sits in a different part of the Decklink EDID from where all the TV resolutions are stored. My supposition is that Windows PC therefore perceives it more like a Desktop Monitor a la DVI interface, i.e. with no audio capability. So whatever audio it puts out over HDMI (if anything) is borked from the point of view of Decklink.

The reason introducing an HDMI splitter appeared to solve the problem was simply that the splitter does not support 2560x1440 at all, so Windows was picking the next highest rez above that which was 4Kp60. (Hence a case of desktop resolution being 2560x1440 but signal resolution 3840x2160). And since that resolution appears in the extended resolution table, Windows thinks it's a TV and the audio works fine.