Sometimes no audio on Capture, sometimes there is.

[BartZLederman]

I've been seeing a problem that seems to be different than the other "no audio" issues posted here.
I'm using an HTML capture device: it's been working with VirtualDub for several years now. I've also
set this up for a friend of mine. We have Windows 7 and Windows 10. The audio device shows up
fine in VirtualDub, I can hear audio and / or see the Volume meter bars moving when in capture mode.
However: when I start an actual capture, either directly through the menu or by using VDTimer,
video is always captured, but sometimes not audio. There don't seem to be any error messages,
the log file says "Starting capture to <filename>" and "Capture was completed successfully" for every
capture, regardless of whether audio was included or not. I can't find any rhyme or reason for why
audio is sometimes ignored. Stopping and re-starting capture usually fixes it, but if I have my timer
set to record something and can't be there the moment it starts, it can be extremely frustrating. This
all happens without changing the video connection, or source material, or USB port. Successive
captures without making any changes whatsoever to anything on the system will usually capture
audio, but sometimes will not.

Changing the audio compression setting between raw and AC3 doesn't have any effect on whether
audio is captured or not.

Can anyone suggest a place to look? The problem does not seem to be in the audio device or drivers
or connection, as the device always shows up and the audio stream can be seen in the "Volume meter"
right up to the moment the actual capture starts. It also happens on more than one system, and with
different versions of Windows, so it can't be the specific PC or operating system, or the actual capture
device.

Any suggestions as to what to do next will be appreciated.

[jagabo]

Many HDMI capture devices can only capture stereo uncompressed PCM sent from the HDMI source. If the source is sending AC3 or some other compressed audio the devices ignore the audio.

[BartZLederman]

I believe I mentioned in my post that I have tried uncompressed PCM ("raw"), and it made no difference.

The source is not sending AC3, it's sending PCM. And it has worked for years with VirtualDub
doing the conversion from PCM to AC3.