AVAssetWriter recorded audio no sound

Question

My app can record the audio from the chat and save it in file. I recorded some music on the app screen but when I playback the audio.m4a file there is no sound coming out. The file show as "Apple MPEG-4 audio" and has 12KB size. Did I config the setting wrong? Thanks in advence.

edit: I added the stop recording function.

var assetWriter: AVAssetWriter?
var input: AVAssetWriterInput?
var channelLayout = AudioChannelLayout()

func record() {
        guard let doc = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
            return
        }
      
        let inputURL = docURL.appendingPathComponent("audio.m4a")
   
        do {
            try assetWriter = AVAssetWriter(outputURL: inputURL, fileType: .m4a)
        } catch  {
            print("error: \(error)")
            assetWriter = nil
            return
        }
        
        guard let assetWriter = assetWriter else {
            return
        }

        channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_MPEG_5_1_D

    let audioSettings: [String : Any] = [
        AVNumberOfChannelsKey: 6,
        AVFormatIDKey: kAudioFormatMPEG4AAC_HE,
        AVSampleRateKey: 44100,
        AVEncoderBitRateKey: 128000,
        AVChannelLayoutKey: NSData(bytes: &channelLayout, length: MemoryLayout.size(ofValue: channelLayout)),
    ]

        input = AVAssetWriterInput(mediaType: .audio, outputSettings: settings)
        
        guard let audioInput = input else {
            print("Failed to find input.")
            return
        }
        
        audioInput.expectsMediaDataInRealTime = true
        
        if ((assetWriter.canAdd(audioInput)) != nil) {
            assetWriter.add(audioInput)
        }

        RPScreenRecorder.shared().startCapture(handler: { (sample, bufferType, error) in
            guard error == nil else {
                print("Failed to capture with error: \(String(describing: error))")
                return
            }
            
            if bufferType == .audioApp {
                if assetWriter.status == AVAssetWriter.Status.unknown {
                    if ((assetWriter.startWriting()) != nil) {
                        assetWriter.startSession(atSourceTime: CMSampleBufferGetPresentationTimeStamp(sample))
                    }
                }
                
                if assetWriter.status == AVAssetWriter.Status.writing {
                    if audioInput.isReadyForMoreMediaData == true {
                        if audioInput.append(sample) == false {
                        }
                    }
                }
            }
        })
    }

    func stopRecord() {
    RPScreenRecorder.shared().stopCapture{ (error) in
        self.audioInput.markAsFinished()
        
        if error == nil{
            self.assetWriter.finishWriting {
                print("finish writing")
            }
        } else {
            print(error as Any)
        }
    }
}

Rhythmic Fistman · Answer 1 · 2021-02-16T09:44:17.300

2

In light of your comments, you definitely don't need six channel audio. Try these simpler mono audio settings.

let audioSettings: [String : Any] = [
    AVNumberOfChannelsKey: 1,
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100,
    AVEncoderBitRateKey: 128000,
]

You don't say whether this is on iOS or macOS. You have a problem on macOS because as of 11.2.1 no .audioApp buffers are captured. If you still want microphone, you can configure that:

let recorder = RPScreenRecorder.shared()
recorder.isMicrophoneEnabled = true
    
recorder.startCapture(handler: { (sample, bufferType, error) in
    if bufferType == .audioMic { 
       // etc
    }
})

Don't bother checking for writer status, just append buffers when you can

if audioInput.isReadyForMoreMediaData {
    if !audioInput.append(sample) {
        // do something
    }
}

PREVIOUSLY

You need to call assetWriter.finishWriting at some point.

It's interesting that you have 6 channel input. Are you using a special device or some kind of virtual device?

edited Feb 16 '21 at 09:44

answered Feb 15 '21 at 16:09

Rhythmic Fistman

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I'm not sure what to put in the setting part. The tutorials I found has 6 channels. I do have assetWriter.finishWriting in stop record function. I just edit the code. – 14079_Z Feb 15 '21 at 20:05
in that case you don't want 6 channel. updated answer – Rhythmic Fistman Feb 16 '21 at 09:44
Thank you for the setting config codes. It still didn't work in my project no audio was recorded. I'm thinking maybe there is some limitation for recording full song on Spotify or Apple Music? I'm just going to use the preview they provided instead. Thank you. – 14079_Z Feb 17 '21 at 16:17

AVAssetWriter recorded audio no sound

1 Answers1