Playing Percussion instruments with midi on iOS - ios

Question

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!

Answer 1

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.

Answer 2

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;
}