IOS record audio and split it into files in real time

Question

I am making an app which needs to stream audio to a server. What I want to do is to divide the recorded audio into chunks and upload them while recording.

I used two recorders to do that, but it didn't work well; I can hear the difference between the chunks (stops for couple of milliseconds).

How can I do this?

Answer 1

Your problem can be broken into two pieces: recording and chunking (and uploading, but who cares).

For recording from the microphone and writing to the file, you can get started quickly with AVAudioEngine and AVAudioFile . See below for a sample, which records chunks at the device's default input sampling rate (you will probably want to rate convert that).

When you talk about the "difference between the chunks" you are referring to the ability to divide your audio data into pieces in such a way that when you concatenate them you don't hear discontinuities. eg LPCM audio data can be divided into chunks at the sample level, but the LPCM bitrate is high, so you're more likely to use a packetised format like adpcm (called ima4 on iOS?), or mp3 or aac. These formats can only be divided on packet boundaries, eg 64, 576 or 1024 samples, say. If your chunks are written without a header (usual for mp3 and aac, not sure about ima4), then concatenation is trivial: simply lay the chunks end to end, exactly as the cat command line tool would. Sadly, on iOS there is no mp3 encoder, so that leaves aac as a likely format for you, but that depends on your playback requirements. iOS devices and macs can definitely play it back.

import AVFoundation

class ViewController: UIViewController {

    let engine = AVAudioEngine()

    struct K {
        static let secondsPerChunk: Float64 = 10
    }

    var chunkFile: AVAudioFile! = nil
    var outputFramesPerSecond: Float64 = 0  // aka input sample rate
    var chunkFrames: AVAudioFrameCount = 0
    var chunkFileNumber: Int = 0

    func writeBuffer(_ buffer: AVAudioPCMBuffer) {
        let samplesPerSecond = buffer.format.sampleRate

        if chunkFile == nil {
            createNewChunkFile(numChannels: buffer.format.channelCount, samplesPerSecond: samplesPerSecond)
        }

        try! chunkFile.write(from: buffer)
        chunkFrames += buffer.frameLength

        if chunkFrames > AVAudioFrameCount(K.secondsPerChunk * samplesPerSecond) {
            chunkFile = nil // close file
        }
    }

    func createNewChunkFile(numChannels: AVAudioChannelCount, samplesPerSecond: Float64) {
        let fileUrl = NSURL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent("chunk-\(chunkFileNumber).aac")!
        print("writing chunk to \(fileUrl)")

        let settings: [String: Any] = [
            AVFormatIDKey: kAudioFormatMPEG4AAC,
            AVEncoderBitRateKey: 64000,
            AVNumberOfChannelsKey: numChannels,
            AVSampleRateKey: samplesPerSecond
        ]

        chunkFile = try! AVAudioFile(forWriting: fileUrl, settings: settings)

        chunkFileNumber += 1
        chunkFrames = 0
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        let input = engine.inputNode!

        let bus = 0
        let inputFormat = input.inputFormat(forBus: bus)

        input.installTap(onBus: bus, bufferSize: 512, format: inputFormat) { (buffer, time) -> Void in
            DispatchQueue.main.async {
                self.writeBuffer(buffer)
            }
        }

        try! engine.start()
    }
}