scaleTimeRange对音频类型AVMutableCompositionTrack没有影响

Question

我正在制作一个简单的视频编辑应用程序.我想为我的应用添加慢动作.我注意到AVMutableCompositionTrack类中有一个scaleTimeRange方法,所以我用它来实现我的目的.我发现scaleTimeRange在视频轨道上运行良好,但对音轨没有任何影响.这意味着音轨仍以原始速度播放.

关注是我的代码:

    CMTime insertionPoint = kCMTimeZero;

    double wholeDuration = CMTimeGetSeconds([asset duration]);
    double doubleDuration = CMTimeGetSeconds([asset duration])*2.0;
    CMTime trimmedDuration = CMTimeMakeWithSeconds(wholeDuration, 600.0);

    // Create a new composition
    self.mutableComposition = [AVMutableComposition composition];

    // Insert video and audio tracks from AVAsset
    if(assetVideoTrack != nil) {
        AVMutableCompositionTrack *compositionVideoTrack = [self.mutableComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
        [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, trimmedDuration) ofTrack:assetVideoTrack atTime:insertionPoint error:&error];

        [compositionVideoTrack scaleTimeRange:CMTimeRangeMake(kCMTimeZero, trimmedDuration) toDuration:CMTimeMakeWithSeconds(doubleDuration, 600.0)];
    }
    if(assetAudioTrack != nil) {
        AVMutableCompositionTrack *compositionAudioTrack = [self.mutableComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
        [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, trimmedDuration) ofTrack:assetAudioTrack atTime:insertionPoint error:&error];

        [compositionAudioTrack scaleTimeRange:CMTimeRangeMake(kCMTimeZero, trimmedDuration) toDuration:CMTimeMakeWithSeconds(doubleDuration, 600.0)];
    }

谁能给我一些关于这个问题的建议？谢谢!

Answer 1

为了能够用声音播放慢动作,只需为AVPlayerItem设置适当的audioTimePitchAlgorithm

playerItem.audioTimePitchAlgorithm = AVAudioTimePitchAlgorithmVarispeed;

Answer 2

由于 Apple api 中的错误（错误 ID：14616144），ScaleTimeRange 不适用于音频。然而，可以使用重采样来完成时间缩放音频。如果音频以 48 kHz 采样，则重新采样到 96 kHz 并以 48 kHz 播放音频，这将需要两倍的播放时间。一般来说：

scaledSampleRate = (orignalSampleRate / playRate);
playRate = (originalSampleRate / scaledSampleRate);

如果不希望的话，音高将会降低。解决方案可能是第三方 api。如果您可以将音频提取到单独的播放器，我想您可以使用 AVAudioPlayer，您可以在其中设置播放速率：

player = [[AVAudioPlayer alloc] initWithContentsOfURL:
                  [NSURL fileURLWithPath:path] error:&err];
        player.volume = 0.4f;
        player.enableRate=YES;
        [player prepareToPlay];
        [player setNumberOfLoops:0];
        player.rate=2.0f;
        [player play];