change channel function

[CNelias]

Selecting the instrument of a track is usually done by adding a program change event to the track which describes when to change and what instrument to use.

This, however, is not the case when dealing with drums. The MIDI convention is that notes to be rendered as drums have to be on the channel 10.
I propose to add an internal attribute to the MIDITrack structure that describes the channel to be used when calling the addnotes! function. That way the notes would be directly send to the drum channel without having to modify them.

Then, the constructor of MIDITrack could be modified to accept an instrument argument in order to add the correct program change event or update the channel attribute of the track so that the instrument of the MIDI track is correct upon synthesis.

What do you think ?

[Datseris]

We have programchange as a function implemented here, it is just untested. The first step is to test it and see how well it works, because of #96

The only difficulty I see here is that this field that you suggest adding to the MIDITrack should also be somehow recovered when reading a MIDI file from disk. How do you think this could be done?

[CNelias]

programmchange should of course be used since it has already been written, the only issue is that the current implementation will not be applicable to drums since drums sounds works differently than other instruments in MIDI.

Concerning your second point : after thinking about it in more detail I don't think the modification I suggested is necessary. We can modify the programmchange function to treat the drum notes themselves directly by assigning them to the correct channel. I'll see if I can implement it myself soon.

[Datseris]

The function is here : https://github.com/JuliaMusic/MIDI.jl/blob/master/src/miditrack.jl#L312-L317 (and it should also be changed to accept the channel as a standard UInt8, i.e. from 0 to 127 instead of 1 to 128).

But I wonder, isn't it easier to simply write the drum notes directly on channel 10? You can specify the channel of each note. This is how I do it when I create drums exercises for myself, as e.g. here https://juliamusic.github.io/JuliaMusic_documentation.jl/dev/blog/garibaldi_dragadiddle/

[CNelias]

If you are creating a MIDI file from scratch you can directly assign the notes to channel 10 yes.
In some situations however, like when you extract a MIDI file from a project (cubase or reaper) the channel information gets lost. In those cases it would be nice if we could just do programmchange(REAPERnotes, instrument = "drums") or something equivalent.

[Datseris]

Ah now I finally see. Okay, what you want is solved by defining a dictionary that maps instrument strings (e.g. "drums") to channel numbers. If the second argument of programmchange is Integer, just use that number. If second argument is string use the result of the dictionary (via multiple dispatch).

[Datseris]

programmchange(REAPERnotes, instrument = "drums")

What is asked here is as simple as

CHANNEL = 9
newnotes = [Note(n.pos, n.vel, ..., CHANNEL) for n in oldnotes]

perhaps it can be added as a convenience function changechannel and changechannel!, but other than that, no reason to do anything more complicated.