From writing utilities MIDI2TXT and TXT2MIDI I got some knowledge of midi file format. Once a person on net searched for help on reviving a defect midi.

I had a look at it and found that the note data seemed to be ok but the structure of the midi file was lost. The number of midi tracks and the length field of most tracks were 0.

I measured the number of tracks and the length of each track and edited the correct values with a binary hexadecimal editor into the midifile. The file was totally rescued by very simple changes.

I found that this simple operations could be done very quickly by a small computer program.