Can MP3 cards play music in background?

At least SymbOS can do it because of it's multitasking. But only the sunrise compatible MP3 cards are supported.

Indeed. I was thinking more about the device reading the MP3 by its own.

the se-one is a great MP3 cartridge and plays from a USB stick on its own

Any MP3 player requires the disk interface to be fast enough to be able to provide it with data, even for standalone playback. It will have a circular or double buffer, but it will likely be a few kB in size and won’t be large enough to fit an entire MP3. So it relies on the MSX to pump data to it before it runs empty. It must work like this since any MP3 is easily several megabytes in size and that much memory is not typical for MSX-es.

Of course it will likely consume a good chunk of CPU, probably several interrupts worth, but that’s not a problem in principle, just everything outside the TSR will run slower. And if the disk somehow stops it will glitch of course, though you can of course also have an additional memory buffer in the MSX large enough to absorb such an event if the MP3 cart’s internal buffer isn’t large enough. I think normally that wouldn’t happen with the SD interfaces and such we have nowadays anyways.

I don’t think you can count on a fixed amount of vsynced time. Streaming from disk out of principle will be unreliable timing-wise given the various amounts of disk interfaces out there. And of course floppy disks are out of the question, they are most definitely too slow. Run a disk speed testing tool, the transfer speed should be greater than the bitrate of the MP3 for it to work.

But just look at SymbOS videos on Youtube about what’s possible in principle. There’s several demos using the MP3 player in parallel with other tasks.

It is possible to program with GR8NET, but there're several things must be sorted out beforehand:
1. access sharing. If MP3 player will use SD-card for background playback, then there must be a mechanism to ensure there is no conflict between it and, for example, Nextor accessing the SD-card. Same for network - it will occupy one of the sockets, which then will not be available for BASIC and other applications.
2. buffering. While not that critical when playing from SD-card, it is critical when accessing network. MP3 player is currently using 128KB buffer size inside the GR8NET. This means this space will not be available for user/applications any more.
3. data source selection. How player, in background, will select file to play? Must it be provided with the list of the files (where to store it then?) or it is provided directory name and then plays all files from it in some pattern? In case of network it is not applicable.

Is this feature is generally useful? You can have another MSX nearby playing MP3 (even without monitor connected), or any of your contemporary gadgets doing it.