How to edit the default instrument of an AUGraph? - iphone

Question

I'm working with the MusicPlayer API. I understand that when you load in a .mid as a sequence, the API creates a default AUGraph for you that includes an AUSampler. This AUSampler uses a simple sine-wave based instrument to synthesize the notes in the .mid
My question is, how does one change the default instrument in the AUSampler? I understand that you can use SoundFont2 files (.sf2) and add them using the AudioUnitSetProperty method. But, how does one access this default AUGraph? Do you have to open the graph before you can edit the AudioUnit or is opening a graph only for editing connections between nodes?
Thanks :)

Answer 1

I've written a tutorial on this but here but here's an outline of the process:
Function to load a Sound Font file (taken from the Apple documentation):
-(OSStatus) loadFromDLSOrSoundFont: (NSURL *)bankURL withPatch: (int)presetNumber {
OSStatus result = noErr;
// fill out a bank preset data structure
AUSamplerBankPresetData bpdata;
bpdata.bankURL = (__bridge CFURLRef) bankURL;
bpdata.bankMSB = kAUSampler_DefaultMelodicBankMSB;
bpdata.bankLSB = kAUSampler_DefaultBankLSB;
bpdata.presetID = (UInt8) presetNumber;
// set the kAUSamplerProperty_LoadPresetFromBank property
result = AudioUnitSetProperty([pointer to your AUSampler node here],
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; }
Then you need to load the Sound Font from your Resources folder:
NSURL *presetURL = [[NSURL alloc] initFileURLWithPath:[[NSBundle mainBundle] pathForResource:#"Name of sound font" ofType:#"sf2"]];
// Initialise the sound font
[self loadFromDLSOrSoundFont: (NSURL *)presetURL withPatch: (int)10];
Hope this helps!

Answer 2

You might take a look at the Audiograph example. It doesn't use soundFonts but should give you an idea of how to set up a graph.
When I use the MusicPlayer I always generate the midi note data from code/GUI and create the AUGraph (with a mixer) from scratch. There are ways to derive/extract the default generated AUGraph & AUSampler resulting from loading a midi file (example code below) but I never had success setting a new soundFont this way. On the other hand, creating the AUGraph from scratch and then loading an .sf2 file works great.
AUGraph graph;
result = MusicSequenceGetAUGraph (sequence, &graph);
MusicTrack firstTrack;
result = MusicSequenceGetIndTrack (sequence, 0, &firstTrack);
AUNode myNode;
result = MusicTrackGetDestNode(firstTrack,&myNode);
AudioUnit mySamplerUnit;
result = AUGraphNodeInfo(graph, myNode, 0, &mySamplerUnit);