Thanks again, @Fahri . The diagrams are especially helpful.
Unfortunately I cannot play the backing track from the phone as you describe - the Pixel 5 does not have a 3.5 mm headphone jack. I guess they figure that everybody is using bluetooth headsets these days. A wired headset can be used with a dongle to the USB-C port, but of course that is already in use with the DAI.
No problem - I have all my music related files on Dropbox so I can access them from a phone, tablet or pc, so I’m using another phone to play the backing track.
Prior to reading your post I realized that my Zoom B1Four could be useful here - great minds think alike.
Right now the B1Four is connected to the UMC204HD through an “insert” which is an fX loop (yet another use for the versatile TRS connector). I plugged the 2nd phone into the aux input on the B1Four and I have my backing track.
Everything is now working exactly as desired except that all the audio (both bass and backing track) is on the left channel only, just as we discussed.
Since I’m not using the #2 channel for anything my plan is to move the B1Four to ahead of the DAI and feed the output into both channels of the DAI using a splitter like this:
I would, however, like some reassurance that essentially bridging the two channels isn’t going to damage the DAI.
I’ve also approached the issue from other angles. If I view the video on my Linux PC using the VLC Media Player I can set the audio output to mono and problem solved. This is fine as long as I’m only making videos for my own critiquing.
How about converting the file? This could probably be done in a video editor, but I’ve looked at several videos and concluded that they all have very steep learning curves, at least for someone like me who has no experience with video editing. If anyone knows of a basic, intuitive easy to use video editor, I’d love to hear about it.
Fortunately, I found a simpler solution.
[WARNING: Serious old school computer geek stuff ahead!]
ffmpeg is a program installed by default on most Linux distributions that is a back end for many video related programs. It can be run from the command line.
I discovered that if I open a terminal window, navigate to the video file and run the following:
ffmpeg -i myvideo.mp4 -codec:v copy -ac 1 myvideomono.mp4
where “myvideo.mp4” is the video file with the left channel audio and "myvideomono.mp4 will be whatever I want to call the new file to be produced which will leave the video unchanged and convert the audio to mono.
This only takes a few minutes to run and seems to work just fine.