Most MIDI tunes don't work in my Nokia 3510i. They seem to download OK but trying to play them produces an error message saying "Unable to open file".

The problem doesn't seem to be the size or polyphony, because very small and even 3-channeled tones sometimes cause this error. The file format isn't likely to be the problem either because some other MIDI files produced with the same program work fine.

Downloading and playing the very same MIDI files with the 3510i emulator works without any trouble. Does the emulator handle MIDI files differently from an actual phone?

Are there any freely available specs about the SP-MIDI format supported by Nokia phones?