使用AVAssetExportSession将MP4转换为WAV

Question

I'm working with AVAssetExportSession and am looking to convert the audio from a MP4 video to WAV. I suspect it may not be possible with this tool, but wanted to check and see if I was missing anything. Here is my code that converts it to M4A.

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
        }
    }
}

This code works, but changing my outputFileType to AVFileTypeWAVE , my output filename extension to .wav, and the session's preset to "passthrough" gives me an error:

Error Domain=AVFoundationErrorDomain Code=-11838 "Operation Stopped" UserInfo={NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The operation is not supported for this media.}

Any idea where I'm going wrong? Or is conversion-to-wav just not possible with AVAssetExportSession ? If AVAssetExportSession isn't the right tool for this job, is AVAssetReader/AVAssetWriter or Extended Audio File Services the approach to use? Thanks for reading.

Answer 1

Yes, as I understand it you need to use CoreAudio for this. Look at the function convertToPCM in this class:

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