How is Transcoding triggered?

[marxjohnson]

I am currently listening to 2 different audiobooks, streaming from my server (running on Docker, behind an nginx reverse proxy) to my Android phone via the official app.

Both audiobooks have their chapters in Mp3 format.

One audiobook always plays "Direct", and works very smoothly.

The other mostly plays using "Transcode", which mostly works smoothly. However, sometimes when switching back to this book, it starts playing "Direct" and is in the wrong position. Seeking within a chapter doesn't work well, and after a few seeks it switches to "Transcode" after which I can seek to the correct position and it's smooth again.

The only significant difference I can think of between the two audiobooks is the length. The "Direct" one is 14 hours with chapters between 2 and 60 minutes long. The "Transcode" one is 528 with 3-4 hour chapters.

My question is, how does the app/server decide whether it's going to play via Direct or Transcode, and can I force it to do one or the other for a particular audiobook?