AVPlayer/AVAudioMix fade-in effect clicks in the beginning

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Created Jul ’15

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I'm trying to implement a fade-in effect based on AVPlayer + AVAudioMix + AVAudioMixInputParameters. It basically works except when playing the audio for the first time after starting my app there is a click in the beginning. Subsequent plays work perfect though, but the first-time glitch is pretty stable and reproducible.

My Play button is enabled only after the AVPlayerItem's `status` is set to ready, so it's impossible to fire a play method while the player is not ready. In fact it doesn't matter how long I wait after loading the audio file and constructing all the objects.

This happens on OS X (10.10), I haven't tested it on iOS yet.

Note that for this test you need an audio file that starts with sound and not silence. Here is my stripped down code without the GUI part (`testFadeIn` is the entry point):

static AVPlayer* player;
static void* PlayerItemStatusObserverContext = &PlayerItemStatusObserverContext;

- (void)testFadeIn
{
  AVURLAsset* asset = [AVURLAsset.alloc initWithURL:[NSURL fileURLWithPath:@"Helicopter.m4a"] options:@{AVURLAssetPreferPreciseDurationAndTimingKey: @YES}];
  AVPlayerItem* item = [AVPlayerItem playerItemWithAsset:asset];
  player = [AVPlayer playerWithPlayerItem:item];
  [item addObserver:self forKeyPath:@"status" options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew context:PlayerItemStatusObserverContext];
}

- (void)applyFadeIn
{
  assert(player.currentItem.tracks.firstObject);
  AVMutableAudioMixInputParameters* fadeIn = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:player.currentItem.tracks.firstObject];
  [fadeIn setVolume:0 atTime:kCMTimeZero];
  [fadeIn setVolume:1 atTime:CMTimeMake(2, 1)];
  NSMutableArray* paramsArray = [NSMutableArray new];
  [paramsArray addObject:fadeIn];
  AVMutableAudioMix* audioMix = [AVMutableAudioMix audioMix];
  audioMix.inputParameters = paramsArray;
  player.currentItem.audioMix = audioMix;
}

- (void)observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(NSDictionary*)change context:(void*)context
{
  if (context == PlayerItemStatusObserverContext)
  {
    AVPlayerStatus status = (AVPlayerStatus)[[change objectForKey:NSKeyValueChangeNewKey] integerValue];
    if (status == AVPlayerStatusReadyToPlay)
    {
      [self applyFadeIn];
      [self performSelector:@selector(play:) withObject:nil afterDelay:1.0];
    }
  }
}

- (void)play:(id)unused
{
  [player play];
}

Click! What is wrong with this?

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Answer 1

hmel OP

Jul ’15

An obvious workaround that I use at the moment is: when the player reports it's ready, I do a short 50ms playback with volume=0, then restore currentTime and volume and only then I report to the main app that the player is ready. This way there are no clicks. Interestingly, anything less than 50ms still gives the click.

This seems like an issue with something that's being cached by AVFoundation after the first playback. It's neither the tracks, as they are available when I set the fade in params, nor the seek status.

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