iOS：降低包含iPod音樂的MPMediaItem的比特率

Question

我正在製作將主題音樂添加到視頻的應用程序。

某些用戶抱怨說，如果他們的音樂是蘋果無損格式，則視頻會過大。

我發現這是因爲我使用的AVMutableComposition只是將原始音樂格式放入我生成的視頻中。

那麼有什麼辦法可以降低MPMediaItem中的音樂比特率，或者改變它被編碼的格式？

這是我用來將音樂添加到視頻的代碼的代碼片段。

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) {} 

Answer 1

我終於明白了，這是我使用的代碼：

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]; 
     } 
    } 
}