Movie night from a laptop

The house TVs are fed by streaming boxes through a video switch and an AV receiver. I wanted to play a movie file from my laptop on one of them, and people were already waiting on the couch. The first run took about 15 minutes. With the steps in the right order it's about 6.

The steps

1. Find the box. The house AV controller knows which streaming box each room is watching, so I ask it instead of guessing.
2. Check nobody's using it. The box reports what app is open and whether anything is playing.
3. Fix the audio first. Convert it to stereo AAC and leave the video alone. More on why below.
4. Serve the file over DLNA from the laptop with rclone serve dlna, with one file in the folder.
5. Drive the box over its local control API. Launch the media player app, then send keypresses: media type, server, file.
6. Check it without seeing the screen. The DLNA log shows the box asking for byte ranges of the file, and the box reports state="play" and audio="aac". If it says audio="none", the room is silent.

ffmpeg -i IN -map 0:v:0 -map 0:a:0 -c:v copy \
  -c:a aac -ac 2 -b:a 192k -movflags +faststart OUT.mp4

What went wrong

• Surround sound played silent. 5.1 audio, both AAC and Dolby Digital, came out as nothing through the switch and receiver. I lost two rounds to that before converting to stereo up front. I never worked out why.
• Deep links do nothing. Launching the player with a file URL returns 200 OK and then just sits there, so it has to be keypresses.
• The player caches folder listings. A renamed file gives an error on screen. Swap the new file in under the same name.
• rclone caches the folder too, for 5 minutes by default. I set it to 5 seconds.

It all lives in one command now: pick a room, and it asks the AV controller which box to use. Once the movie is playing, the rule is that nothing touches the TV.