Android AudioRecord frequency filter step by step

Android: I am new to voice process, I have created AudioRecord Object with sampleRate 8000Hz using JTransform library i am trying to filter frequency there are couple of things in the following code which i do not understand my questions as follows Q.1 Why we are converting "toTransform[i] = (double) buffer[i] / 32768.0; // signed 16 bit" buffer into 16bit value ? Q.2 Rite now audioRecord read data array is short array if i read byte array how will i convert it into singed 16 bit ? Q.3 I want to display sound frequency in Hz with double array how can i calculate sound frequency ? Q.4 I wrote filter method filterAudio() but it is not filtering range of frequency ? Please help me out i have lot of questions in my mind /* Code as follows */ ``` private final int[] mSampleRates = new int[] { 8000, 11025, 22050, 44100 }; final AudioRecord audioRecord = findAudioRecord(); if(audioRecord == null){ return null; } final short[] buffer = new short[blockSize]; final double[] toTransform = new double[blockSize]; audioRecord.startRecording(); while (started) { Thread.sleep(100); final int bufferReadResult = audioRecord.read(buffer, 0, blockSize); for (int i = 0; i < blockSize && i < bufferReadResult; i++) { toTransform[i] = (double) buffer[i] / 32768.0; // signed 16 bit } //Audio Filter passing frequency of mSampleRates[3] filterAudio(bufferReadResult, toTransform, mSampleRates[3]); transformer.realForward(toTransform); publishProgress(toTransform); } audioRecord.stop(); audioRecord.release(); public static void filterAudio(int bufferSize, double[] audioBuffer, float sampleRate ){ //it is assumed that a float array audioBuffer exists with even length = to //the capture size of your audio buffer //float frequency=0F; //The size of the FFT will be the size of your audioBuffer / 2 int FFT_SIZE = bufferSize / 2; //RealDoubleFFT mFFT = new RealDoubleFFT(FFT_SIZE); DoubleFFT_1D mFFT = new DoubleFFT_1D(FFT_SIZE); //this is a jTransforms type //Take the FFT mFFT.realForward(audioBuffer); //mFFT.ft(audioBuffer); //The first 1/2 of audioBuffer now contains bins that represent the frequency //of your wave, in a way. To get the actual frequency from the bin: //frequency_of_bin = bin_index * sample_rate / FFT_SIZE //assuming the length of audioBuffer is even, the real and imaginary parts will be //stored as follows //audioBuffer[2*k] = Re[k], 0<=k<n/2 //audioBuffer[2*k+1] = Im[k], 0<k<n/2 //Define the frequencies of interest float freqMin = 14400; float freqMax = 14500; //Loop through the fft bins and filter frequencies for(int fftBin = 0; fftBin < FFT_SIZE; fftBin++){ //Calculate the frequency of this bin assuming a sampling rate of 44,100 Hz float frequency = (float)fftBin * sampleRate / (float)FFT_SIZE; //Now filter the audio, I'm assuming you wanted to keep the //frequencies of interest rather than discard them. if(frequency < freqMin || frequency > freqMax){ //Calculate the index where the real and imaginary parts are stored int real = 2 * fftBin; int imaginary = 2 * fftBin + 1; //zero out this frequency audioBuffer[real] = 0; audioBuffer[imaginary] = 0; } } //Take the inverse FFT to convert signal from frequency to time domain mFFT.realInverse(audioBuffer, false); } final AudioRecord findAudioRecord() { for (int rate : mSampleRates) { for (short audioFormat : new short[] { AudioFormat.ENCODING_PCM_8BIT, AudioFormat.ENCODING_PCM_16BIT }) { for (short channelConfig : new short[] { AudioFormat.CHANNEL_CONFIGURATION_MONO , AudioFormat.CHANNEL_CONFIGURATION_STEREO }) { try { bufferSize = AudioRecord.getMinBufferSize(rate, channelConfig, audioFormat); if (bufferSize != AudioRecord.ERROR_BAD_VALUE) { // check if we can instantiate and have a success AudioRecord recorder = new AudioRecord(AudioSource.DEFAULT, rate, channelConfig, audioFormat, bufferSize); if (recorder.getState() == AudioRecord.STATE_INITIALIZED){ Log.d(TAG, "Attempting rate " + rate + "Hz, bits: " + audioFormat + ", channel: " + channelConfig); return recorder; } } } catch (Exception e) { Log.e(TAG, rate + "Exception, keep trying.",e); } } } } return null; } ```