AudioKit Tap to convert microphone data for Google Speech-to-Text API

GoogleSpeechToTextStreamingTap.swift

open class GoogleSpeechToTextStreamingTap {

    internal var converter: AVAudioConverter!
    
    @objc public init(_ input: AKNode?, sampleRate: Double = 16000.0) {
        
        let format = AVAudioFormat(commonFormat: AVAudioCommonFormat.pcmFormatInt16, sampleRate: sampleRate, channels: 1, interleaved: false)!
        
        self.converter = AVAudioConverter(from: AudioKit.format, to: format)
        self.converter?.sampleRateConverterAlgorithm = AVSampleRateConverterAlgorithm_Normal
        self.converter?.sampleRateConverterQuality = .max
    
        let sampleRateRatio = AKSettings.sampleRate / sampleRate
        let sampleLength = 0.1 // 100ms
        let inputSampleRate = AudioKit.format.sampleRate
        let inputBufferSize = sampleLength * inputSampleRate // i.e 100ms of 44.1K = 4410 samples.
        
        input?.avAudioNode.installTap(onBus: 0, bufferSize: AVAudioFrameCount(inputBufferSize), format: nil) { buffer, time in
            
            let capacity = Int(Double(buffer.frameCapacity) / sampleRateRatio)
            let bufferPCM16 = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: AVAudioFrameCount(capacity))!

            var error: NSError? = nil
            self.converter?.convert(to: bufferPCM16, error: &error) { inNumPackets, outStatus in
                outStatus.pointee = AVAudioConverterInputStatus.haveData
                return buffer
            }
            
            let channel = UnsafeBufferPointer(start: bufferPCM16.int16ChannelData!, count: 1)
            let data = Data(bytes: channel[0], count: capacity * 2)
            
            SpeechRecognitionService
                .sharedInstance
                .streamAudioData(data,
                                 completion: { response, error in
                                    if let error = error {
                                        print("ERROR: \(error.localizedDescription)")
                                        SpeechRecognitionService.sharedInstance.stopStreaming()
                                    } else if let response = response {
                                        print(response)
                                    }
                })
        }
    }
}