audio sequencer with swing (shuffle) Obj-C - objective-c

Question

I'm working on a drum computer with sequencer for the iPad. The drum computer is working just fine and writing the sequencer wasn't that much of a problem either. However, the sequencer is currently only capable of a straight beat (each step has equal duration). I would like to add a swing (or shuffle as some seem to call it) option, but I'm having trouble figuring out how.
'Swing' according to Wikipedia
Straight beat (midi, low volume)
Beat with Swing (midi, low volume)
If I understand correctly, swing is pretty much achieved by offsetting the eights notes between the 1-2-3-4 with a configurable amount. So instead of
1 + 2 + 3 + 4 +
it becomes something like
1 +2 +3 +4 +
The linked midi files illustrate this better...
However, the sequencer works with 1/16th or even 1/32th steps, so if the 2/8th (4/16th) note is offset, how would that affect the 5/16th note.
I'm probably not approaching this the correct way. Any pointers?
Sequencer code
This is the basics of how I implemented the sequencer. I figured altering the stepDuration at certain points should give me the swing effect I want, but how?
#define STEPS_PER_BAR 32
// thread
- (void) sequencerLoop
{
while(isRunning)
{
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
// prepare for step
currentStep++;
if(currentStep >= STEPS_PER_BAR * activePatternNumBars)
currentStep = 0;
// handle the step/tick
...
//calculate the time to sleep until the next step
NSTimeInterval stepDuration = (60.0f / (float)bpm) / (STEPS_PER_BAR / 4);
nextStepStartTime = nextStepStartTime + stepDuration;
NSTimeInterval now = [NSDate timeIntervalSinceReferenceDate];
// sleep if there is time left
if(nextStepStartTime > now)
[NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceReferenceDate:nextStepStartTime]];
else {
NSLog(#"WARNING: sequencer loop is lagging behind");
}
[pool release];
}
}
Edit: added code

Answer 1

I'm not familiar with the sequencer on iOS, but usually sequencers subdivide steps or beats into "ticks", so the way to do this would be to shift the notes that don't fall right on a beat back by a few "ticks" durring playback. So if the user programmed:
1 + 2 + 3 + 4 +
Instead of playing it back like that, you shift any notes falling on the "and" back by however many ticks (depending on exactly where it falls, how much "swing" was used, and how many "ticks" per beat)
1 + 2 + 3 + 4 +
Sorry if that's not much help, or if I'm not much more than restating the question, but the point is you should be able to do this, probably using something called "ticks". You may need to access another layer of the API to do this.
Update:
So say there are 32 ticks per beat. That means the "+" in the diagram above is tick # 16 -- that's what needs to be shifted. (that's not really a lot of resolution, so having more ticks is better).
Lets call the amount we move it, the "swing factor", s. For no swing, s = 1, for "infinite" swing, s=2. You probably want to use a value like 1.1 or 1.2. For simplicity, we'll use linear interpolation to determine the new position. (As a side note, for more on linear interpolation and how it pertains to audio, I wrote a little tutorial) we need to break the time before and after 16 into two sections, since the time before is going to be stretched and the time after is going to be compressed.
if( tick <= 16 )
tick *= s; //stretch
else
tick = (2-s)*tick + 32*(s-1) //compress
How you deal with rounding is up to you. Obviously, you'll want to do this on playback only and not store the new values, since you won't be able to recover the original value exactly due to rounding.

Answer 2

Change the number of steps to 12 instead of 16. Then each beat has 3 steps instead of 4. Triplets instead of 16th notes. Put sounds on the first and third triplet and it swings. Musicians playing swing use the second triplet also.
Offsetting the notes to create a shuffle does not give you access to the middle triplet.