iOS: lowering bitrate of MPMediaItem containing an iPod music - iphone

Question

I'm making an app which add a theme music to an video.
Some user complains that if their music is in apple lossless format, the video will be too large.
I found that is because the AVMutableComposition I use just put the original music format in to the video I generated.
So is there any way I can lower the bitrate of the music in MPMediaItem, or change the format it is encoded?
This is a code snippet of the code I use to add music to video.
AVMutableComposition* mixComposition = [AVMutableComposition composition];
AVMutableCompositionTrack *compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
atTime:kCMTimeZero error:nil];
AVMutableCompositionTrack *compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
atTime:kCMTimeZero error:nil];
AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition
presetName:AVAssetExportPresetPassthrough];
NSURL *exportUrl = [NSURL fileURLWithPath:_videoOutputPath];
if ([[NSFileManager defaultManager] fileExistsAtPath:_videoOutputPath]){
[[NSFileManager defaultManager] removeItemAtPath:_videoOutputPath error:nil];
}
_assetExport.outputFileType = #"com.apple.quicktime-movie";
_assetExport.outputURL = exportUrl;
_assetExport.shouldOptimizeForNetworkUse = YES;
[_assetExport exportAsynchronouslyWithCompletionHandler:^(void ) {}

Answer 1

I finally got it, this is the code I use:
static NSString * const kWriterInputIsReadyForMoreData = #"readyForMoreMediaData";
#import <AVFoundation/AVFoundation.h>
#implementation AudioUtil
{
AVAssetReader *_assetReader;
AVAssetWriter *_assetWriter;
AVAssetWriterInput *_assetWriterInput;
AVAssetReaderTrackOutput *_readerOutput;
void (^_callback)(BOOL);
CMSampleBufferRef _sampleBufferToAppend;
}
-(void)downSamplingAudioWithSourceURL:(NSURL *)sourceURL destinationURL:(NSURL *)destURL timeRange:(CMTimeRange)timeRange callBack:(void (^)(BOOL))callback
{
NSError *error = nil;
_callback = callback;
[[NSFileManager defaultManager] removeItemAtURL:destURL error:nil];
//initialize reader
AVURLAsset *inputAsset = [AVURLAsset assetWithURL:sourceURL];
_assetReader = [[AVAssetReader alloc] initWithAsset:inputAsset error:&error];
_assetReader.timeRange = timeRange;
AVAssetTrack* track = [[inputAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
NSMutableDictionary* audioReadSettings = [NSMutableDictionary dictionary];
audioReadSettings[AVFormatIDKey] = #(kAudioFormatLinearPCM);
audioReadSettings[AVNumberOfChannelsKey] = #([QLVideoFormatProvider audioChannelCount]);
_readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track outputSettings:audioReadSettings];
NSAssert([_assetReader canAddOutput:_readerOutput], #"reader can't add output");
[_assetReader addOutput:_readerOutput];
//initialize writer
_assetWriter = [[AVAssetWriter alloc] initWithURL:destURL fileType:[QLVideoFormatProvider audioFileType] error:nil];
NSMutableDictionary *audioOutputSettings = [NSMutableDictionary dictionary];
audioOutputSettings[AVFormatIDKey] = [QLVideoFormatProvider audioFormatKeyForEncoder];
audioOutputSettings[AVNumberOfChannelsKey] = #([QLVideoFormatProvider audioChannelCount]);
audioOutputSettings[AVSampleRateKey] = #([QLVideoFormatProvider audioSampleRate]);
audioOutputSettings[AVEncoderBitRateKey] = #([QLVideoFormatProvider audioBitrate]);
_assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:audioOutputSettings];
[_assetWriter addInput:_assetWriterInput];
//start
[_assetWriter startWriting];
[_assetWriter startSessionAtSourceTime:kCMTimeZero];
BOOL canStartReading = [_assetReader startReading];
NSLog(#"can start reading %d",canStartReading);
if (!canStartReading) {
callback(NO);
return;
}
[_assetWriterInput addObserver:self forKeyPath:kWriterInputIsReadyForMoreData options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:NULL];
_sampleBufferToAppend = [_readerOutput copyNextSampleBuffer];
[self appendBufferToAppend];
}
-(void)appendBufferToAppend
{
if ([_assetWriterInput isReadyForMoreMediaData]) {
if (_sampleBufferToAppend) {
[_assetWriterInput appendSampleBuffer:_sampleBufferToAppend];
CFRelease(_sampleBufferToAppend);
}
_sampleBufferToAppend = [_readerOutput copyNextSampleBuffer];
if (_sampleBufferToAppend) {
[self appendBufferToAppend];
}
else {
[_assetWriter finishWritingWithCompletionHandler:^(){
if (_callback) {
_callback(_assetWriter.status == AVAssetWriterStatusCompleted);
};
}];
}
}
else {
}
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
if ([keyPath isEqualToString:kWriterInputIsReadyForMoreData]) {
if ([change[NSKeyValueChangeNewKey] boolValue] == YES) {
[self appendBufferToAppend];
}
}
}