How do I convert audio format from mp3 to wav in iOS using Swift?

Question

I am using cmusphinx in iOS using Swift, and cmusphinx uses audio files in wav format, while the files I need to use are in mp3 format.

How do I convert audio files of mp3 format to wav format in iOS using Swift?

So far I have the following code using AVAssetExportSession from the Apple AVFoundation framework, but I keep getting an error saying I used an invalid output file type, no matter what I set the outputFileType property of the AVAssetExportSession object.

I am open to using other frameworks other than from Apple.

The error occurs where I mark it with a comment that says "error here", where the AVAssetExportSession.outputFileType instance property is set.

override func viewDidLoad() {
    super.viewDidLoad()

    let assetURL: URL = Bundle.main.url(forResource: "audio16000", withExtension: "mp3")!

    let asset = AVAsset(url: assetURL)

    let presets: [String] = AVAssetExportSession.allExportPresets()

    for preset in presets {
        print(preset)
    }

    let localDocumentsURL: URL = try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)

    let outputURL: URL = localDocumentsURL.appendingPathComponent("audio16000").appendingPathExtension("wav")

    if FileManager.default.fileExists(atPath: outputURL.path) {

        do {

            try FileManager.default.removeItem(at: outputURL)

        } catch {

            print(error)

        }

    }

    let exporter = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetHEVC1920x1080)

    exporter?.outputURL = outputURL
    exporter?.outputFileType = AVFileType.wav // error here

    exporter?.exportAsynchronously {

        print("exporter status =", exporter?.status as Any)

        switch exporter!.status {
        case .unknown:
            print("status unknown")
        case .waiting:
            print("status waiting")
        case .exporting:
            print("status exporting")
        case .completed:
            print("status completed")
        case .failed:
            print("status failed")
        case .cancelled:
            print("status cancelled")
        @unknown default:
            print("@unknown default:")
        }

    }

}

Here's what the debug window says:

2019-09-17 11:42:14.766703-0500 TrialAVAssetExportSession[5620:2016827] [Accessibility] ****************** Loading GAX Client Bundle ****************
AVAssetExportPreset1920x1080
AVAssetExportPresetLowQuality
AVAssetExportPresetAppleM4A
AVAssetExportPresetHEVCHighestQuality
AVAssetExportPreset640x480
AVAssetExportPreset3840x2160
AVAssetExportPresetHEVC3840x2160
AVAssetExportPresetHighestQuality
AVAssetExportPresetMediumQuality
AVAssetExportPreset1280x720
AVAssetExportPreset960x540
AVAssetExportPresetHEVC1920x1080
2019-09-17 11:42:25.574394-0500 TrialAVAssetExportSession[5620:2016827] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[AVAssetExportSession setOutputFileType:] Invalid output file type'
*** First throw call stack:
(0x233d4a98c 0x232f239f8 0x239d527f4 0x1004d2f58 0x1004d40f0 0x2601fe224 0x2601fe628 0x2607dce64 0x2607dd40c 0x253f82620 0x2607edce8 0x2607a0908 0x2607a5fe0 0x2600692a4 0x26007183c 0x260068f28 0x260069818 0x260067b64 0x26006782c 0x26006c36c 0x26006d150 0x26006c224 0x260070f24 0x2607a45e8 0x2603a0e04 0x2366c69fc 0x2366d040c 0x2366cfc14 0x100584c78 0x100588840 0x236701040 0x236700cdc 0x236701294 0x233cdc728 0x233cdc6a8 0x233cdbf90 0x233cd6ecc 0x233cd67c0 0x235ed779c 0x2607a7c38 0x1004d5594 0x23379a8e0)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 

I've tried all the presets I received from the AVAssetExportSession.exportPresets(compatibleWith:) method. I receive the same exact array of strings when I use the AVAssetExportSession.allExportPresets().

Answer 1

WAV and contain PCM, look at Core Audio, specifically:

1. Audio File Services (to read the MP3 format and write AIFF or WAV)
2. Audio File Conversion Services (to convert the MP3 data to PCM, and/or to encode from PCM to some other codec if you were to write a file).Note that a given converter cannot convert between two encoded formats. you can do MP3-to-PCM or PCM-to-AAC, but to do MP3-to-AAC, you'd need two converters.
3. Extended Audio File Services, which combine both of the above.

Also, be sure to understand the difference between codecs and file formats, and what codec/format combinations are legal. I was surprised the first time I found out that PCM must be little-endian in a WAV, big-endian in an AIFF.

Look at the nice example here taken from v4 AudioKit