Swift 3 AVAudioEngine set microphone input format - ios

Question

I want to process the bytes read from the microphone using Swift 3 on my iOS. I currently use AVAudioEngine.
print(inputNode.inputFormat(forBus: bus).settings)
print(inputNode.inputFormat(forBus: bus).formatDescription)
This gives me the following output:
["AVNumberOfChannelsKey": 1, "AVLinearPCMBitDepthKey": 32, "AVSampleRateKey": 16000, "AVLinearPCMIsNonInterleaved": 1, "AVLinearPCMIsBigEndianKey": 0, "AVFormatIDKey": 1819304813, "AVLinearPCMIsFloatKey": 1]
<CMAudioFormatDescription 0x14d5bbb0 [0x3a5fb7d8]> {
mediaType:'soun'
mediaSubType:'lpcm'
mediaSpecific: {
ASBD: {
mSampleRate: 16000.000000
mFormatID: 'lpcm'
mFormatFlags: 0x29
mBytesPerPacket: 4
mFramesPerPacket: 1
mBytesPerFrame: 4
mChannelsPerFrame: 1
mBitsPerChannel: 32 }
cookie: {(null)}
ACL: {(null)}
FormatList Array: {(null)}
}
extensions: {(null)}
}
The problem is that the server I want to send the data to does not expect 32 bit floats but 16 bit unsigned ints. I think I have to change the mFormatFlags. Does anybody know how I can do this and what value would be the right one?
The resulting byte stream should be equivalent to the one I get on android using
AudioRecord recorder = new AudioRecord(MediaRecorder.AudioSource.MIC, SAMPLES_PER_SECOND,
AudioFormat.CHANNEL_IN_MONO, AudioFormat.ENCODING_PCM_16BIT,
recordSegmentSizeBytes);
I tried this:
let cfmt = AVAudioCommonFormat.pcmFormatInt16
inputNode.inputFormat(forBus: bus) = AVAudioFormat(commonFormat: cfmt, sampleRate: 16000.0, channels: 1, interleaved: false)
but got this error
Cannot assign to value: function call returns immutable value
Any ideas?

Answer 1

Oh my god, I think I got it. I was too blind to see that you can specify the format of the installTap callback. This seems to work
let audioEngine = AVAudioEngine()
func startRecording() {
let inputNode = audioEngine.inputNode!
let bus = 0
let format = AVAudioFormat(commonFormat: AVAudioCommonFormat.pcmFormatInt16, sampleRate: 16000.0, channels: 1, interleaved: false)
inputNode.installTap(onBus: bus, bufferSize: 2048, format: format) { // inputNode.inputFormat(forBus: bus)
(buffer: AVAudioPCMBuffer!, time: AVAudioTime!) -> Void in
let values = UnsafeBufferPointer(start: buffer.int16ChannelData![0], count: Int(buffer.frameLength))
let arr = Array(values)
print(arr)
}
audioEngine.prepare()
do {
try audioEngine.start()
} catch {
print("Error info: \(error)")
}
}