Is NSTimer precise enough for BPM sounds ?

[PeteX]

Hi,

I'm building a simple bpm / metronome app.
Most of it is working fine, but I'm having a hard time getting my click sounds to play in time.

I am using an NSTimer to trigger my sounds at the BPM interval.
I even put the NSTimer on a separate GCD queue, but still the sounds are sometimes (not always) out of sync.

I think I have it narrowed down to the interval value of my NSTimer.
Could it be that NSTimer is having a hard time rounding my interval value ?
Specifically, when I set my interval at 0.441 (which is 136 BPM), then playback is not consistent.

dispatch_async(myQueue, ^(void) {
		myTimer = [[NSTimer alloc] initWithFireDate:[NSDate date]
                                                  interval:frequency
                                                    target:self
                                                  selector:@selector(timerTask:)
                                                  userInfo:nil
                                                   repeats:YES];
		NSRunLoop *runloop = [NSRunLoop currentRunLoop];
		[runloop addTimer:myTimer forMode: NSDefaultRunLoopMode];
		[myTimer release];
        [runloop run];

- (void)timerTask:(NSTimer *)aTimer
{
    
    
    if(!soundIsMuted) {
        
        //play clickSound
 AudioServicesPlaySystemSound (self.oneFileObject);
        
        

    }
    
    // set button to highlight for blinking in tempo
    [tapButton setImage:[UIImage imageNamed:@"button-highlight.png"] forState:UIControlStateNormal];
    [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(changeButtonBack:) userInfo:nil repeats:NO];
    
    

}

The above is what I'm using now, the interval frequency is set by buttontaps (but at the moment I'm hardcoding the value to test).
The NSTimer is calling my audio and button flashing code.

I have already gone from AVFoundation to AudioToolbox as I thought I didn't need the overhead to play very small sound fragments.

To be clear, everything works, but the sounds are sometimes not in tempo.

It is also not the case that the NSTimer might not keep up, if I set it's interval to 0.3 (= 200 BPM) the clicksounds are perfectly in tempo, and this is faster than 0.441, so speed seems not to be an issue.
This is why I believe it might be a rounding factor ?

I also need this level of precision (3 digits after comma) because :
134 BPM = 0.447 s
135 BPM = 0.444 s
136 BPM = 0.441 s

Is there a better way to tackle this ?

[nobre84]

0.441 cannot be represented as a binary number. Hence it is rounded/truncated and you lose precision. However 0.3 can't either and you say its timing is acceptable.
Maybe you could try with smaller fractions that are perfectly representable, lets say 0.125 , 0.0625, and work up from there until you reach the desired tempo.

[PeteX]

Thanks for your reply.

I figured something like this would be the case.
I'll see where I get trying to round my intervals to acceptable values.

As you noticed, 0.3 does perform perfect.

What is strange about all this, is that I use the NSTimer to trigger a loop at a given interval.

Inside this loop, I play a sound, and I flash my button image in tempo.

At certain intervals (like 0.441 or anything in that area) the sounds are skewed out of tempo, but the button flash looks to be perfectly in tempo.

This is leading me to believe that some delays are occurring in PlaySystemSound... but why not at all tempo's ?

This is really giving me a headache.

Is there maybe a better way to trigger (loop) soundfiles at an exact tempo ?

[PeteX]

I'll have to be more precise.

The problem that I am hearing is that my sounds are not equally apart at certain interval times, where as the flashing button is.

so at interval 0.441 I hear two short sounds followed by a delay, instead of a steady rhythm.

The steady rhythm is working fine though in the button flashes.

[nobre84]

Hmmm... So it seems the timer is not really the culprit ? Have you tried other means of playing the sound to see if it helps ?

[PeteX]

Initially I was using AVFoundation, but read in the documentation that playback was not instant.

I thought for my application I would be better with Sound System services so changed it.

In both instances I experienced the strange audio timing.


So I ended up putting the NSTimer on a separate queue with GCD, but it still behaved the same way.

I've now put my code full of breakpoints to see if something funky comes up...but I'm really scratching my head.
It certainly is something stupid I did, I just can't see it.

[PeteX]

just to follow up.

I have now gone back to AVAudioPlayer which is performing better than AudioToolbox, and have also switched my code from GCD to NSThread, just like the Apple metronome sample code, and this is working very well.