Search code examples
javascripthtmlmidiweb-audio-api

Understanding how midi file is mapped to object in javascript

I'm using this library I found to read a midi file

As there is very little documentation, I have no idea how to read the output object?

question: What does: Channel, data, deltaTime and type keys mean?

In the end I would love to map this js object to some kind of visualization. enter image description here

Solution

  • Channel: The MIDI format uses the concept of channels to allow different MIDI devices to only listen to specific MIDI events by listening to such a channel. This makes it possible to use a single MIDI file for multiple instruments that should play different notes, etc. So when you have a note on event you should check the channel of the event and only play the instruments that are interested in events that happen in this channel.

    Data: Data is a bit arbitrary, but in your example we have an event of type 255 (0xFF) which is a meta event. It has a meta type of 3 (0x03) which means it's a Sequence/Track-name. This was probably assigned by the program that created the MIDI file you use. There's a pretty nifty and concise list of events here: http://www.ccarh.org/courses/253/handout/smf/

    deltaTime: Since events in the MIDI file is tempo agnostic it uses the concept of ticks. It's basically a resolution expressed as ticks per quarter note. I think 480 ticks per quarter note is pretty standard, though that is purely based on my own experience, so YMMV. Events can then either be expressed in absolute time (ie. this note on events happens 4800 ticks from the start of the track) or delta time. Delta time is the number of ticks since the last MIDI event happened.

    Type: Each MIDI event in a MIDI file has a type to identify what kind of an event it is. This matters since different types of events has different formats (and thus changes the way we decode it, since MIDI is a binary format), where some have a fixed length and others include information on how long the event is (the number of bytes that make up the event).

    It's been a couple of years since I last worked with the MIDI format, but I think the above is accurate.