Question

我正在制作一个简单的视频编辑应用。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)];
    }

有人能就这个问题给我一些建议吗？谢谢！

Answer 1

为了能够播放带有声音的慢动作，只需为AVPlayerItem设置合适的audioTimePitchAlgorithm

playerItem.audioTimePitchAlgorithm = AVAudioTimePitchAlgorithmVarispeed;

Answer 2

由于apple api中的错误，ScaleTimeRange不适用于音频（错误号：14616144）。但是，可以使用重新采样来完成时间缩放音频。如果以48 kHz采样音频，重新采样至96 kHz并以48 kHz播放音频，则播放时间将延长两倍。一般为：

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

如果不希望，音高会降低。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，我猜您可以使用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];

Answer 3

如果您正在使用scaleTimeRange，并且想知道为什么它不起作用或播放任何音频，那么您应该将其添加到playerItem。

playerItem.audioTimePitchAlgorithm = AVAudioTimePitchAlgorithm.varispeed

scaleTimeRange对音频类型AVMutableCompositionTrack没有影响

3 个答案: