iOS：降低包含iPod音乐的MPMediaItem的比特率
我正在制作一个应用程序，将主题音乐添加到video中。
有些用户抱怨说，如果他们的音乐是苹果无损格式，那么video将会太大。
我发现这是因为我使用的AVMutableComposition只是把原来的音乐格式放入我生成的video中。
那么有什么办法可以降低MPMediaItem中的音乐比特率，或者改变它被编码的格式？
这是我用来添加音乐到video的代码的代码片段。
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 ) {}
我终于明白了，这是我使用的代码：
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]; } } }