使用AVAssetExportSession将MP4转换为WAV

Question

我正在使用AVAssetExportSession并希望将音频从MP4视频转换为WAV。 我怀疑使用此工具可能无法实现，但想检查一下是否丢失了任何东西。 这是将其转换为M4A的代码。

func saveAudioFromVideo(withURL url:NSURL){

   // Create composition of all audio tracks in asset
    print("Creating composition...")
    let sourceAsset = AVURLAsset(URL: url)
    let sourceTracks = sourceAsset.tracksWithMediaType(AVMediaTypeAudio)
    let audioComposition = AVMutableComposition()
    for track in sourceTracks{
        let compositionTrack = audioComposition.addMutableTrackWithMediaType(AVMediaTypeAudio,
                                                                             preferredTrackID: kCMPersistentTrackID_Invalid)
        do{
            try compositionTrack.insertTimeRange(track.timeRange, ofTrack: track, atTime: kCMTimeZero)
        }catch{
            print("Track insert failed")
        }
    }

    // Get output URL
    print("Getting output URL...",terminator:" ")
    let documentsPath = NSFileManager.defaultManager().URLsForDirectory(.CachesDirectory,
                                                                        inDomains: .UserDomainMask)[0]
    let outputURL = documentsPath.URLByAppendingPathComponent("ConvertedAudio.m4a")
    print(outputURL)

    // Removing previous instance
    print("Checking for previous file...", terminator:" ")
    if NSFileManager.defaultManager().fileExistsAtPath(outputURL.path!){
        print("found!");
        do{
            try NSFileManager.defaultManager().removeItemAtURL(outputURL)
            print("File removed.")
        }catch{
            print("Could not remove previous file")
            return
        }

    }

    // Setup export session
    print("Setting up export session...")
    guard let exportSession = AVAssetExportSession(asset: audioComposition,
                                                   presetName: AVAssetExportPresetAppleM4A) else{
                                                    print("Export session could not be created")
                                                    return
    }
    exportSession.outputFileType = AVFileTypeAppleM4A
    exportSession.outputURL = outputURL

    // Export data to new format
    print("Exporting data...",terminator:" ")
    exportSession.exportAsynchronouslyWithCompletionHandler{
        let status = exportSession.status
        print("complete")
        switch status{
        case .Completed:
            print("Success.")
            self.delegate?.audioConverted(withURL: exportSession.outputURL!)
        case .Failed:
            print("Fail: \(exportSession.error)")
        default:
            break
        }
    }
}

这段代码有效，但是将我的outputFileType更改为AVFileTypeWAVE ，将我的输出文件扩展名更改为.wav，并将会话的预设更改为“ passthrough”给我一个错误：

错误域= AVFoundationErrorDomain代码= -11838“操作已停止” UserInfo = {NSLocalizedDescription =操作已停止，NSLocalizedFailureReason =此媒体不支持该操作。}

知道我哪里出错了吗？ 还是使用AVAssetExportSession无法转换为波形？ 如果AVAssetExportSession不是适合此工作的工具，则使用AVAssetReader / AVAssetWriter或扩展音频文件服务吗？ 谢谢阅读。

Answer 1

是的，据我了解，您需要为此使用CoreAudio。 在此类中查看函数convertToPCM：

https://github.com/AudioKit/AudioKit/blob/master/AudioKit/Common/Internals/AKConverter.swift