在AVMutableComposition上设置跟踪级别

Question

我试图将4个音轨合并为一个合成，然后将此合成导出到文件。到目前为止，我的文件已成功创建，但所有音轨都以全音量播放，而不是我试图设置的音量。这里就是我现在做的：

AVMutableComposition *trackComposition = [AVMutableComposition composition]; 

AVAsset *asset1 = ... 
AVAsset *asset2 = ... 
AVAsset *asset3 = ... 
AVAsset *asset4 = ... 

NSMutableArray *inputParams = [NSMutableArray arrayWithCapacity:4]; 

// Add 4 tracks to composition (but only if there are no errors and the track isn't muted 
NSError *err; 
if(asset1 && ![self trackIsMuted:1]){ 
    AVAssetTrack *rawTrack = [[asset1 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]; 
    AVMutableCompositionTrack *compositionAudioTrack = [trackComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid]; 
    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [asset1 duration]) ofTrack:rawTrack atTime:kCMTimeZero error:&err]; 
    AVAudioMixInputParameters *audioParams = [self audioParamsForTrack:compositionAudioTrack volume:[self gainForTrack:1]]; 
    [inputParams addObject:audioParams]; 
} 
if(asset2 && !err && ![self trackIsMuted:2]){ 
    AVAssetTrack *rawTrack = [[asset2 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]; 
    AVMutableCompositionTrack *compositionAudioTrack = [trackComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid]; 
    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [asset2 duration]) ofTrack:rawTrack atTime:kCMTimeZero error:&err]; 
    AVAudioMixInputParameters *audioParams = [self audioParamsForTrack:compositionAudioTrack volume:[self gainForTrack:2]]; 
    [inputParams addObject:audioParams]; 
} 
if(asset3 && !err && ![self trackIsMuted:3]){ 
    AVAssetTrack *rawTrack = [[asset3 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]; 
    AVMutableCompositionTrack *compositionAudioTrack = [trackComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid]; 
    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [asset3 duration]) ofTrack:rawTrack atTime:kCMTimeZero error:&err]; 
    AVAudioMixInputParameters *audioParams = [self audioParamsForTrack:compositionAudioTrack volume:[self gainForTrack:3]]; 
    [inputParams addObject:audioParams]; 
} 
if(asset4 && !err && ![self trackIsMuted:4]){ 
    AVAssetTrack *rawTrack = [[asset4 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]; 
    AVMutableCompositionTrack *compositionAudioTrack = [trackComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid]; 
    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [asset4 duration]) ofTrack:rawTrack atTime:kCMTimeZero error:&err]; 
    AVAudioMixInputParameters *audioParams = [self audioParamsForTrack:compositionAudioTrack volume:[self gainForTrack:4]]; 
    [inputParams addObject:audioParams]; 
} 

AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix]; 
audioMix.inputParameters = inputParams; 

// Export the composition to a file 
AVAssetExportSession *export = [AVAssetExportSession exportSessionWithAsset:trackComposition presetName:AVAssetExportPresetAppleM4A]; 

NSURL *outputURL = [NSURL fileURLWithPath:[[NSTemporaryDirectory() stringByAppendingPathComponent:[NSString guidString]] stringByAppendingPathExtension:@"m4a"]]; 

[export setOutputURL:outputURL]; 
[export setOutputFileType:@"com.apple.m4a-audio"]; 

[export setAudioMix:audioMix]; 

[export exportAsynchronouslyWithCompletionHandler:^{ ... }]; 

唯一的其他有趣的事情是audioParamsForTrack方法，它是在这里：

- (AVAudioMixInputParameters *)audioParamsForTrack:(AVAssetTrack *)track volume:(float)vol{ 
    AVMutableAudioMixInputParameters *audioInputParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:track]; 
    [audioInputParams setVolume:vol atTime:kCMTimeZero]; 

    return [audioInputParams copy]; 
} 

任何人能发现我在做什么错？我试过通过各种不同的音轨来创建音频参数，但它似乎没有什么区别。我看到一些关于音轨首选音量的东西 - 这可能对我有帮助吗？我有点卡住在这一点上，任何反馈意见是赞赏！

Answer 1

调试时，我可以看到问题，但我不知道为什么会发生。

当您的PARAMS添加到数组，像这样：

[inputParams addObject:audioParams]; 

道ID被设置为0，因此从未接到任何曲目：

从控制台：

<AVMutableAudioMixInputParameters: 0x16eab300, track ID = 0, volume mix: volume 0.010 at time 0.000> 

我解决了这个问题，不使用audioParamsForTrack方法，而是在数组之前完成这项工作，如下所示：

NSMutableArray *inputParams = [NSMutableArray arrayWithCapacity:4]; 

      AVMutableAudioMixInputParameters *audioInputParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:AudioTrack]; 
      [audioInputParams setVolume:.01f atTime:kCMTimeZero]; 

      AVAudioMixInputParameters *audioParams = audioInputParams; 
[inputParams addObject:audioParams]; 

      audioMix.inputParameters = inputParams; 

Answer 2

我也有类似的问题，但什么工作对我来说是明确设置输入参数的轨道ID：

[audioInputParams setTrackID:compositionAudioTrack.trackID];