I'm trying to take a local m4a file and compress/down-sample this file (For the purposes of making a smaller file).

Now I stuck with error when I try to append sample buffer.

Error description:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[AVAssetWriterInput appendSampleBuffer:] Cannot append sample buffer: Input buffer must be in an uncompressed format when outputSettings is not nil'

Code where I try compress original audio file into same file format but with lower bitrate:

@objc func nextTapped() {
        let audioURL = RecordWhistleViewController.getWhistleURL()

        var asset = AVAsset.init(url: audioURL)
        let exportPath = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent("out.m4a").path
        print("export PATH IS \(exportPath)")

        let exportURL = URL(fileURLWithPath: exportPath)

        var readerError: Error? = nil
        var reader: AVAssetReader? = nil
        do {
            reader = try AVAssetReader(asset: asset)
        } catch {
            print("error in reader \(error)")
        }

        let track = asset.tracks(withMediaType: .audio)[0]
        let readerOutput = AVAssetReaderTrackOutput(track: track, outputSettings: nil)
        reader?.add(readerOutput)

        var writerError: Error? = nil
        var writer: AVAssetWriter? = nil
        do {
            writer = try AVAssetWriter(outputURL: exportURL, fileType: .m4a)
        } catch {
            print("ERROR IN writer \(error)")
        }

        var channelLayout = AudioChannelLayout()
        memset(&channelLayout, 0, MemoryLayout<AudioChannelLayout>.size)
        channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo

        let outputSettings = [
            AVFormatIDKey: Int(kAudioFormatMPEG4AAC),
            AVSampleRateKey: 12000,
            AVNumberOfChannelsKey: 2,
            AVEncoderBitRateKey: 128000,
            AVChannelLayoutKey: Data(bytes: &channelLayout, count: MemoryLayout<AudioChannelLayout>.size)

        ] as [String : Any]

        let writerInput = AVAssetWriterInput(mediaType: .audio, outputSettings: outputSettings as? [String: Any])
        writerInput.expectsMediaDataInRealTime = false
        writer?.add(writerInput)

        writer?.startWriting()
        writer?.startSession(atSourceTime: .zero)

        reader?.startReading()
        let mediaInputQueue = DispatchQueue(label: "mediaInputQueue")
        writerInput.requestMediaDataWhenReady(on: mediaInputQueue) {
            print("Asset writer ready: \(writerInput.isReadyForMoreMediaData)")
            while writerInput.isReadyForMoreMediaData {
                var nextBuffer: CMSampleBuffer?
                nextBuffer = readerOutput.copyNextSampleBuffer()
                if nextBuffer != nil {
                    if let nextBuffer = nextBuffer {
                        print("adding buffer")
                        writerInput.append(nextBuffer)
                    }
                } else {
                    writerInput.markAsFinished()
                    reader?.cancelReading()
                    writer?.finishWriting {
                        print("Asset writer finished writing")
                    }
                    break
                }
            }
        }

    }

The original audio have settings:

let settings = [
            AVFormatIDKey: Int(kAudioFormatMPEG4AAC),
            AVSampleRateKey: 12000,
            AVNumberOfChannelsKey: 1,
            AVEncoderAudioQualityKey: AVAudioQuality.high.rawValue
        ]

Code for URL to original file:

class func getDocumentsDirectory() -> URL {
        let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
        let documentsDirectory = paths[0]
        return documentsDirectory
    }

here is full description of Error:

assetWriter_finishBuildingAudioTrackWithSourceFormatDescription signalled err=-12413 (kFigAssetWriterError_InappropriateSourceFormat) (AssetWriter can only compress LPCM audio) at /Library/Caches/com.apple.xbs/Sources/EmbeddedCoreMedia_Sim/EmbeddedCoreMedia-2765.6/Prototypes/Export/FigAssetWriter.c:636

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[AVAssetWriterInput appendSampleBuffer:] Cannot append sample buffer: Input buffer must be in an uncompressed format when outputSettings is not nil'