iPhone：减慢录制的视频速度

Question

任何人都可以解释为什么这不会减慢我的视频到5秒？ secondAsset.duration等于15秒。导出后，它只播放视频5秒，然后显示剩余10秒的最后一帧..

AVMutableCompositionTrack *secondTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    [secondTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, secondAsset.duration) ofTrack:[[secondAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:firstAsset.duration error:nil];

    [secondTrack scaleTimeRange:CMTimeRangeMake(kCMTimeZero, secondAsset.duration) toDuration:CMTimeMake(5,1)];