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