[Denemo-devel] MIDI out

[Richard Shann]

There has been some chat on freenode#denemo with Nils and others about
what the MIDI out (that has so many votes in the denemo poll) actually
means.
For those not there and to summarize what I have understood here are
some thoughts:

Just putting out a MIDI note to a libjack output connection in place of
the playnotes which currently goes to the /dev/sequencer or to portaudio
is not what is being talked about.

What is meant is (at a minimum) to be able to point to a place in the
score and have denemo promptly start outputting MIDI to libjack. The
MIDI events it would output would correspond to the notes from the
pointed place onwards in the score, including the prevailing
dynamic/crescendo, midi instrument/channel (anything else? - embedded
MIDI commands I guess). Notes that are hanging on at the given point
would not be sounded (so if the given point just contained notes tied
from previous notes, nothing would be heard until a new note started).
The tempo would be a prevailing tempo at the note concerned (so we would
allow an embedded MIDI command that would change the tempo).

The actual jackmidi connection is trivial, of course, and someone on
#denemo came up with a sample
http://trac.jackaudio.org/browser/trunk/jack/example-clients/midiseq.c
which shows how to do that.
It is not intended that the user should be allowed to edit the score
while it is being played. So the value of the gui->changecount variable
is fixed for any given playback.

What would require a little design is how to create the MIDI events from
the denemo data. I think I have a good idea how to do this:
An initialization stage would be good, partly to avoid excessive
computation while playing & also for convenience of coding. So if this
MIDI output system has not been initialized since the last edit to the
score the first thing that happens is INITIALIZATION: this creates a
list of MIDI events, with links from the notes in the score to first
event that that note triggers. (As mentioned notes that are tied to
earlier notes have no MIDI event).
On requesting playback from a given note, the MIDI events are simply
shovelled out to jackmidi.
There are some extras that are easily incorporated - silencing staffs
etc. but the real work is the INITIALIZATION task.

I think it can be done as follows:
        INITIALIZATION: run a special version of exportmidi.c which instead of
writing to a file creates a list (GList* midi_events in the DenemoScore
structure) of MIDI events, putting a link from each note (etc) to the
MIDI event which it causes.

To determine if INTIALIZATION is needed, we need to set a variable
(gui->midichangecount) that is set to the gui->changecount when
INITIALIZATION is run, so that if the comparison
midichangecount==changecount fails when the play request is received we
know INITIALIZATION is needed again.

As Nils noticed, exportmidi has at least one bug (dynamics) which I have
put in the bug tracker.

I hope I have convinced someone that they could afford to devote an
afternoon to implementing this.

Richard