channel

Think of a "channel" as a "guy playing an instrument".
 - At any given time, a guy can pick up only ONE instrument.
 - He can play tons of notes at once on the instrument.
 - He can swap instruments any time he wants.
But only one instrument at a time.

ProgramChanges:
   A channel has an assigned sound.  Defaulting to piano.
   Assigning an instrument (sound) to a channel is called a "program change".

   Midi defines 128 standard melodic instruments and about 61 standard drums.
   These are only the standard ones.
   In practice, this is WAY too few available instruments for everyone in the
      whole world to limit themselves to.
   So you usually end up using "custom" ones.
   And mapping these "custom" instruments back to the closest "standard" one
      when giving the song to somebody else.

Midi Maximums:
   You can have as many melodic and percussive instruments as you want,
   but per device, only 15 melodic instruments can be used at a given instant
                                (on the melodic channels)
      Plus one channel of percussive sounds.  (on channel 10)

   If you want more simultaneous sounds, you need more sound module devices.
   Most of the time, you've got a couple of em.
   So you can have 30 (plus 2 drumsets) or 45 sounds (plus 3 drumsets) at once.
   That's usually plenty.  If not, just get more devices.

   Plus, you can switch off a channel to another instrument at ANY time.
   (With one of them thar "program changes".)
   So within a bar of music, you could have a zillion instruments playing.

   However, doing program changes can sometimes cause an audible bzzzt.
   And lots of program change events can make it hard to keep your music
                                                             easy to understand.

WHY all these wierd numbered limits??
   Well, MIDI was designed in the 80s when computers were WEAK!!
   (and MUCH simpler.  and I was a younger man...)
   That's all the explanation you get for now.  Sorry.


Next up, note events