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Re: [fluid-dev] Patch for handling midi note events

From: Pedro Lopez-Cabanillas
Subject: Re: [fluid-dev] Patch for handling midi note events
Date: Fri, 6 May 2011 20:03:20 +0200
User-agent: KMail/1.13.5 (Linux/; KDE/4.4.4; i686; ; )


Sorry for coming so late to the discussion.

On Friday 06 May 2011, Jonathan Slenders wrote:
> One thing, I moved player->user_callback(...)
> before fluid_synth_handle_midi_event. Because I sometimes also wanted to
> adjust the pan, the volume, and instruments on the channel played for a
> note.
> I can also image that someone (maybe me too) wants to alter the midi events,
> supress notes or change pitch an octave up at a certain point.

Me too :-)

Seriously. This is something that I was planning for using in one of my pet 
projects: http://www.kde.org/applications/multimedia/kmid . This program is a 
MIDI/karaoke player with features like pitch transpose, tempo control, channel 
solo/mute, synchronized lyrics display and integrated pianola. It plays to 
hardware MIDI ports or soft synths. The program has three MIDI backends right 
now: Linux/ALSA, Windows and MacOSX/CoreMIDI. My plan was to develop a fourth 
backend based on FluidSynth only. It would require exactly the same feature you 
are trying: a callback just before delivering the MIDI event to the synth, 
allowing the client application to change event's parameters. But for my 
program this is only one of the required additions to FluidSynth. Another one 
would be a callback while FS is parsing the MIDI file, reporting to the client 
application all the meta-events like titles, track names, copyright notices, 
texts and lyrics, time/key signatures, that are ignored right now in FluidSynth 
but would be required by kmid. Talking about text and lyrics, these events 
would be reported by the player callback at playing time as well, although they 
would be ignored by the synthesizer. 

Considering the two proposed callbacks, instead of new_fluid_player2() perhaps 
I would prefer two functions: fluid_player_register_playback_callback() and 
fluid_player_register_parser_callback() with their corresponding unregister() 

The parser callback would be quite innocuous in process time constraints, and 
will be called in the same thread as fluid_player_add(), so there is little 
concern about it. The playback callback is another issue: first, it will be 
potentially called from a realtime thread, with small time constraints. If the 
callback allows event modifications it should be called synchronously, ahead of 
the delivering time to the synth, and the client application would need to 
return within this time frame. It would be easier to implement a pure 
notification callback, called asynchronously from FluidSynth but without 
allowing event modifications.

Talking about events. The structure fluid_midi_event_t has been opaque for a 
long time in FluidSynth, and I think it should remain opaque. There are API 
functions to access some event properties, and more API functions would be 
welcome if needed.


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