Iphone iOS：降低包含iPod音乐的MPMediaItem的比特率

我正在制作一个将主题音乐添加到视频的应用程序

一些用户抱怨说，如果他们的音乐是苹果的无损格式，视频就会太大

我发现这是因为我使用的

AVMutableComposition

只是将原始音乐格式放入我生成的视频中

有没有办法降低MPMediaItem中音乐的比特率，或者改变它的编码格式

这是我用来将音乐添加到视频中的代码片段

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];
        }
    }
}

静态NSString*常量kWriterInputIsReadyForMoreData=@“readyForMoreMediaData”；
#进口
@实现AudioUtil
{
Avassetrader*_assetrader；
AVAssetWriter*_assetWriter；
AVAssetWriterInput*_assetWriterInput；
AvassetraderTrackOutput*\u读取器输出；
无效(BOOL);；
CMSampleBufferRef_sampleBufferToAppend；
}
-（void）downSamplingAudioWithSourceURL:（NSURL*）sourceURL destinationURL:（NSURL*）destURL timeRange:（CMTimeRange）timeRange回调：（void（^）（BOOL））回调
{
n错误*错误=nil；
_回调=回调；
[[NSFileManager defaultManager]删除属性：destURL错误：nil]；
//初始化读取器
AVURLAsset*inputAsset=[AvurlAssetWithUrl:sourceURL]；
_AssetTreader=[[AvAssetTreader alloc]initWithAsset:inputAsset错误：&错误]；
_AssetTreader.timeRange=时间范围；
AVAssetTrack*track=[[inputAsset tracksWithMediaType:AVMediaTypeAudio]对象索引：0]；
NSMutableDictionary*audioReadSettings=[NSMutableDictionary]；
audioReadSettings[AVFormatIDKey]=@（kAudioFormatLinearPCM）；
audioReadSettings[AVNumberOfChannelsKey]=@（[QLVeoFormatProvider audioChannelCount]）；
_readerOutput=[AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track outputSettings:audioReadSettings]；
NSAssert（[[u assetReader CanadOutput:[u readerOutput]，@“reader无法添加输出”）；
[\u assetReader addOutput:\u readerOutput]；
//初始化写入程序
_assetWriter=[[AVAssetWriter alloc]initWithURL:destURL文件类型：[QLVideoFormatProvider audioFileType]错误：nil]；
NSMutableDictionary*音频输出设置=[NSMutableDictionary]；
audioOutputSettings[AVFormatIDKey]=[QLVideoFormatProvider audioFormatKeyForEncoder]；
audioOutputSettings[AVNumberOfChannelsKey]=@（[QLVideoFormatProvider audioChannelCount]）；
audioOutputSettings[AVSampleRateKey]=@（[QLVideoFormatProvider audioSampleRate]）；
audioOutputSettings[AvencoderBiteKey]=@（[QLVideoFormatProvider audioBitrate]）；
_assetWriterInput=[AVAssetWriterInput assetWriterInputWithMediaType:AvMediaType音频输出设置：音频输出设置]；
[_assetWriter附加输入：_assetWriterInput]；
//开始
[_AssetWriterStartWriting]；
[_AssetWriterStartSessionatSourceTime:kCMTimeZero]；
BOOL canStartReading=[u assetrader startReading]；
NSLog（@“可以开始读取%d”，可以开始读取）；
如果（！canstartreding）{
回调（否）；
返回；
}
[_AssetWriterInputAddObserver:self-forKeyPath:KwriterInputIsReadyForMore数据选项：NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew-context:NULL]；
_sampleBufferToAppend=[\u readerOutput copyNextSampleBuffer]；
[自行出现]；
}
-（无效）附录
{
如果（[\u assetWriterInput已准备好接收更多MediaData]）{
如果（_sampleBufferToAppend）{
[\u assetWriterInput appendSampleBuffer:\u sampleBufferToAppend]；
CFRelease（_sampleBufferToAppend）；
}
_sampleBufferToAppend=[\u readerOutput copyNextSampleBuffer]；
如果（_sampleBufferToAppend）{
[自行出现]；
}
否则{
[\u assetWriter使用CompletionHandler完成写入：^（）{
如果（_回调）{
_回调（_assetWriter.status==AVAssetWriterStatusCompleted）；
};
}];
}
}
否则{
}
}
-（void）observeValueForKeyPath:（NSString*）对象的键路径：（id）对象更改：（NSDictionary*）更改上下文：（void*）上下文
{
if（[keyPath isEqualToString:kWriterInputIsReadyForMoreData]）{
如果（[change[NSKeyValueChangeNewKey]布尔值]==是）{
[自行出现]；
}
}
}