Finding nearest zero-crossing in an AVURLAsset - ios

Question

I'm trying to find a way to fade in an audio track held locally without an audio glitch. I'm using AVPlayer and referencing mp3s from the iPodLibrary using AVURLAsset. The following method works most of the time but not all the time so I'm thinking I need to scan through the audio data looking for the nearest volume zero-crossing and do my fade from there. Any pointers would be much appreciated.
float duration = 0.5;
AVAsset *asset = [self.av_Player.currentItem asset];
NSArray *keys = [NSArray arrayWithObject:#"tracks"];
[asset loadValuesAsynchronouslyForKeys:keys completionHandler:^(void) {
NSError *error = nil;
NSTimeInterval now = [self currentPlaybackTime];
CMTime mainFadeIn = CMTimeMakeWithSeconds(now, 6000);
CMTime mainFadeDuration = CMTimeMakeWithSeconds(duration, 6000);
CMTimeRange timerange = CMTimeRangeMake(mainFadeIn, mainFadeDuration);
AVKeyValueStatus trackStatus = [asset statusOfValueForKey:#"tracks" error:&error];
switch (trackStatus) {
case AVKeyValueStatusLoaded:
if( self.av_Player ) {
NSArray *tracks = [asset tracksWithMediaType:AVMediaTypeAudio];
AVMutableAudioMixInputParameters *volumeMixInput = [AVMutableAudioMixInputParameters audioMixInputParameters];
[volumeMixInput setVolumeRampFromStartVolume:0.0 toEndVolume:tovolume timeRange:timerange];
[volumeMixInput setTrackID:[[tracks objectAtIndex:0] trackID]];
AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
[audioMix setInputParameters:[NSArray arrayWithObject:volumeMixInput]];
[self.av_Player.currentItem setAudioMix:audioMix];
}
break;
default:
break;
}
}];

Answer 1

No idea why I was getting glitches with the above method but assume it might have something to do with using blocks. Anyway I used the code below and it now works smoothly for fade in and fade outs - hope this helps others.
NSTimeInterval now = [self currentPlaybackTime];
AVPlayerItem *playerItem = self.av_Player.currentItem;
AVAsset *asset = playerItem.asset;
NSArray *audioTracks = [asset tracksWithMediaType:AVMediaTypeAudio];
NSMutableArray *allAudioParams = [NSMutableArray array];
for (AVAssetTrack *track in audioTracks) {
AVMutableAudioMixInputParameters *audioInputParams = [AVMutableAudioMixInputParameters audioMixInputParameters];
[audioInputParams setVolume:fromvolume atTime:CMTimeMakeWithSeconds(now-0.1, 6000)];
[audioInputParams setVolume:fromvolume atTime:CMTimeMakeWithSeconds(now, 6000)];
[audioInputParams setVolume:tovolume atTime:CMTimeMakeWithSeconds(now+duration, 6000)];
[audioInputParams setTrackID:[track trackID]];
[allAudioParams addObject:audioInputParams];
}
AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
[audioMix setInputParameters:allAudioParams];
[playerItem setAudioMix:audioMix];