Question

I have created a simple app that triggers 3 different sounds based on the x, y, z axis of the accelorometer, like an instrument. At the moment, if I set the frequency update interval of the accelometer too low, it plays the sound to much, and if I set it too high it it isn’t responsive enough. I am a complete beginner to objective c and iphone development, can you tell by the code!..

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional 

    UIAccelerometer* accelerometer = [UIAccelerometer sharedAccelerometer];
    [accelerometer setUpdateInterval: 25.0 / 10.0f];

    [[AVAudioSession sharedInstance] setDelegate: self];
    [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error: nil];
   [accelerometer setDelegate:self];

    UInt32 audioRouteOverride = kAudioSessionOverrideAudioRoute_Speaker;
    AudioSessionSetProperty (kAudioSessionProperty_OverrideAudioRoute,sizeof (audioRouteOverride),&audioRouteOverride);


    player.volume = 0.5;
    player.numberOfLoops = 0;
    player.delegate = self;
}

- (void)accelerometer:(UIAccelerometer *)acel didAccelerate:(UIAcceleration *)aceler 
{


    if (aceler.x > 0.5) {
        NSString *fileName = [NSString stringWithFormat:@"snare"];
        NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:fileName ofType:@"mp3"];
        NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
        player = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL error:nil];
        NSLog(@"acceleration.x = %+.6f greater", aceler.x);
        [player play];

    }
    else if (aceler.y > 0.5) {
        NSString *fileName = [NSString stringWithFormat:@"kick2"];
        NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:fileName ofType:@"mp3"];
        NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
        player = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL error:nil];
        NSLog(@"acceleration.y = %+.6f greater", aceler.y);
        [player play];

    }
    else if (aceler.z > 0.5) {
        NSString *fileName = [NSString stringWithFormat:@"hat"];
        NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:fileName ofType:@"mp3"];
        NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
        player = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL error:nil];
        NSLog(@"acceleration.y = %+.6f greater", aceler.z);
        [player play];
    }

    else  {
        [player stop];
    };

}

Answer 1

Setting the accelerometer’s update frequency to a low value wont help here. Imagine the following situation:

time:                          ------------------------>
real world acceleration event: ___X_____X_____X_____X___ 
acceleration update:           X_____X_____X_____X_____X
sound output started:          _________________________

This draft represents a user shaking the device with the same frequency as the accelerometer is updated. But the shake event occurs just in the middle between two accelerometer updates. As a result, the shake events won’t be registered and no sound is played.

Consider a high accelerometer update frequency in contrast to the prior approach:

real world acceleration event: ___X_____X_____X_____X___ 
acceleration update:           XXXXXXXXXXXXXXXXXXXXXXXXX
sound output started:          ___X_____X_____X_____X___

Basically all real world events result in a sound play within this situation.

The amended code is the following:

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSLog(@"viewDidLoad");

    UIAccelerometer* accelerometer = [UIAccelerometer sharedAccelerometer];
    //[accelerometer setUpdateInterval: 25.0 / 10.0f];
    [accelerometer setUpdateInterval: 0.01f];

    [[AVAudioSession sharedInstance] setDelegate: self];
    [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error: nil];
    [accelerometer setDelegate:self];

    UInt32 audioRouteOverride = kAudioSessionOverrideAudioRoute_Speaker;
    AudioSessionSetProperty (kAudioSessionProperty_OverrideAudioRoute,sizeof (audioRouteOverride),&audioRouteOverride);


    //player.volume = 0.5;
    //player.numberOfLoops = 0;
    //player.delegate = self;

}

- (void)accelerometer:(UIAccelerometer *)acel didAccelerate:(UIAcceleration *)aceler 
{


    if ((aceler.x > ACC_THRESHOLD)&&((playerX == nil)||(playerX.isPlaying == NO))) {
        NSString *fileName = [NSString stringWithFormat:@"snare"];
        NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:fileName ofType:@"mp3"];
        NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
        playerX = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL error:nil];
        NSLog(@"acceleration.x = %+.6f greater", aceler.x);
        playerX.delegate = self;
        [playerX play];

    }

    if ((aceler.y > ACC_THRESHOLD)&&((playerY == nil)||(playerY.isPlaying == NO))) {
        NSString *fileName = [NSString stringWithFormat:@"kick2"];
        NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:fileName ofType:@"mp3"];
        NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
        playerY = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL error:nil];
        NSLog(@"acceleration.y = %+.6f greater", aceler.y);
        playerY.delegate = self;
        [playerY play];

    }

    if ((aceler.z > ACC_THRESHOLD)&&((playerZ== nil)||(playerZ.isPlaying == NO))) {
        NSString *fileName = [NSString stringWithFormat:@"hat"];
        NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:fileName ofType:@"mp3"];
        NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
        playerZ = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL error:nil];
        NSLog(@"acceleration.z = %+.6f greater", aceler.z);
        playerZ.delegate = self;
        [playerZ play];
    }

    //else  {
    //    [player stop];
    //};

}

Please note, that multiple sound plays may be triggered by just one event as each dimension is evaluated separately now. A constant has been introduced by #define ACC_THRESHOLD 0.5f. A new sound play for one dimension is started only, after a previous play finished.

After these general change of event handling you may start optimizations with a signal filter.

Additionally you may use AVAudioPlayer’s delegate methods for more detailed sound handling:

#pragma mark -
#pragma mark AVAudioPlayerDelegate methods

- (void)audioPlayerDidFinishPlaying:(AVAudioPlayer *)player successfully:(BOOL)flag{

    NSLog(@"audioPlayerDidFinishPlaying: %i", flag);

}

- (void)audioPlayerDecodeErrorDidOccur:(AVAudioPlayer *)player error:(NSError *)error{

    NSLog(@"audioPlayerDecodeErrorDidOccur: %@", error.description);
}