How to program a real-time accurate audio sequencer on the iphone?
- by Walchy
Hi...
I want to program a simple audio sequencer on the iphone but I can't get accurate timing. The last days I tried all possible audio techniques on the iphone, starting from AudioServicesPlaySystemSound and AVAudioPlayer and OpenAL to AudioQueues.
In my last attempt I tried the CocosDenshion sound engine which uses openAL and allows to load sounds into multiple buffers and then play them whenever needed. Here is the basic code:
init:
int channelGroups[1];
channelGroups[0] = 8;
soundEngine = [[CDSoundEngine alloc] init:channelGroups channelGroupTotal:1];
int i=0;
for(NSString *soundName in [NSArray arrayWithObjects:@"base1", @"snare1", @"hihat1", @"dit", @"snare", nil])
{
[soundEngine loadBuffer:i fileName:soundName fileType:@"wav"];
i++;
}
[NSTimer scheduledTimerWithTimeInterval:0.14 target:self selector:@selector(drumLoop:) userInfo:nil repeats:YES];
In the initialisation I create the sound engine, load some sounds to different buffers and then establish the sequencer loop with NSTimer.
audio loop:
- (void)drumLoop:(NSTimer *)timer
{
for(int track=0; track<4; track++)
{
unsigned char note=pattern[track][step];
if(note)
[soundEngine playSound:note-1 channelGroupId:0 pitch:1.0f pan:.5 gain:1.0 loop:NO];
}
if(++step>=16)
step=0;
}
Thats it and it works as it should BUT the timing is shaky and instable. As soon as something else happens (i.g. drawing in a view) it goes out of sync.
As I understand the sound engine and openAL the buffers are loaded (in the init code) and then are ready to start immediately with alSourcePlay(source); - so the problem may be with NSTimer?
Now there are dozens of sound sequencer apps in the appstore and they have accurate timing. I.g. "idrum" has a perfect stable beat even in 180 bpm when zooming and drawing is done. So there must be a solution.
Does anybody has any idea?
Thanks for any help in advance!
Best regards,
Walchy
