Playing Percussion instruments with midi on iOS

1
votes

I have an app that handles playing multiple midi instruments. Everything works great except for playing percussion instruments. I understand that in order to play percussion in General MIDI you must send the events to channel 10. I've tried a bunch of different things, and I can't figure out how to get it to work, here's an example of how I'm doing it for melodic instruments vs percussion.

    // Melodic instrument
    MusicDeviceMIDIEvent(self.samplerUnit, 0x90, (UInt8)pitch, 127, 0);

    // Percussion Instruments
    MusicDeviceMIDIEvent(self.samplerUnit, 0x99, (UInt8)pitch, 127, 0);

The sampler Unit is an AudioUnit and the pitch is given as an int through my UI.

Thanks in advance!

2
iosobjective-cswiftmidi
Those two lines of code look fine. Are you using AUSampler or AUMIDISynth? Is the problem that the percussion sounds are silent, or do you see an error message somewhere? - Andrew Madsen
@AndrewMadsen Using AUSampler, and the output when I set the channel to 10 is just the same as channel 1 - CodyMace
Have you loaded a soundfont? As far as I know, AUSampler just uses sine wave tones on iOS unless you load a soundfont yourself. - Andrew Madsen
Oh, yeah. I'm using a soundfont, Fluid. The instruments all sound great, it just doesn't seem like the reserved channel 10 is doing what it should. - CodyMace
Are you sending any program change or bank select messages? - CL.

2 Answers

1
votes

Assuming you have some sort of a General MIDI sound font or similar loaded, you need to set the correct status byte before sending pitch/velocity information. So in case of a Standard MIDI Drum Kit (channel 9), you'd do something like this in Swift:

var status  = OSStatus(noErr)
let drumCommand = UInt32( 0xC9 | 0 )
let noteOnCommand = UInt32(0x90 | channel)
status = MusicDeviceMIDIEvent(self._samplerUnit, drumCommand, 0, 0, 0) // set device
status = MusicDeviceMIDIEvent(self._samplerUnit, noteOnCommand, noteNum, velocity, 0)  // sends note ON message

No need to undertake anything special for MIDI note off messages.

0
votes

Ok, so I got it working. I guess the way I load the sound font makes it so the channel stuff doesn't do anything. Instead I had to set the bankMSB property on AUSamplerBankPresetData to kAUSampler_DefaultPercussionBankMSB instead of kAUSampler_DefaultMelodicBankMSB

I added a different font loading method specifically for percussion:

- (OSStatus) loadPercussionWithSoundFont: (NSURL *)bankURL {
    OSStatus result = noErr;

    // fill out a bank preset data structure
    AUSamplerBankPresetData bpdata;
    bpdata.bankURL  = (__bridge CFURLRef) bankURL;
    bpdata.bankMSB  = kAUSampler_DefaultPercussionBankMSB;
    bpdata.bankLSB  = kAUSampler_DefaultBankLSB;
    bpdata.presetID = (UInt8) 32;

    // set the kAUSamplerProperty_LoadPresetFromBank property
    result = AudioUnitSetProperty(self.samplerUnit,
                                  kAUSamplerProperty_LoadPresetFromBank,
                                  kAudioUnitScope_Global,
                                  0,
                                  &bpdata,
                                  sizeof(bpdata));

    // check for errors
    NSCAssert (result == noErr,
               @"Unable to set the preset property on the Sampler. Error code:%d '%.4s'",
               (int) result,
               (const char *)&result);

    return result;
}