Jameson February 2016
### Changing block size causes FFT analysis to fail

I'm trying to record audio and get the frequencies. I can successfully do so with a sampling rate of 44100 and a block size of 2048. The bin size is around 20, I believe. However, if I try to increase the block size to 4096, then rather than get accurate frequencies, I just get the same inaccurate frequency back, with no magnitude/decibels.

My recording task is as follows:

```
private class RecordAudio extends AsyncTask<Void, double[], Boolean> {
@Override
protected Boolean doInBackground(Void... params) {
int bufferSize = AudioRecord.getMinBufferSize(frequency,
channelConfiguration, audioEncoding);
audioRecord = new AudioRecord(
MediaRecorder.AudioSource.DEFAULT, frequency,
channelConfiguration, audioEncoding, bufferSize);
int bufferReadResult;
short[] buffer = new short[blockSize];
double[] toTransform = new double[blockSize];
try {
audioRecord.startRecording();
} catch (IllegalStateException e) {
Log.e("Recording failed", e.toString());
}
while (started) {
if (isCancelled() || (CANCELLED_FLAG == true)) {
started = false;
//publishProgress(cancelledResult);
Log.d("doInBackground", "Cancelling the RecordTask");
break;
} else {
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
}
transformer.ft(toTransform);
publishProgress(toTransform);
}
}
return true;
}
@Override
protected void onProgressUpdate(double[]...progress) {
int mPeakPos = 0;
double mMaxFFTSample = 150.0;
for (int i = 100;
```

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```### Answers

jaket February 2016
You'll need to look closer at the documentation of the `RealDoubleFft.ft`

function. The values going into the function are real but the values coming out are complex FFT coefficients such that `toTransform[0]`

is the real part of the first coefficient, `toTransform[1]`

is the imaginary part of the first coefficient and so on. The final array size is the same but since complex numbers each take up 2 doubles then there are a total of N/2 coefficients where the last one is the coefficient for sampleRate/2.

Next since you are interested in the magnitude you need to compute the magnitude of the complex numbers. For a complex number `x = a + bj`

, the magnitude `|x| = sqrt(a*a + b*b)`

```
double maxMag = 0;
int peakIndex = 0;
for (int i = 0; i < progress[0].length/2; i++)
{
double re = progress[i*2];
double im = progress[i*2+1];
double mag = Math.sqrt(re*re + im*im);
if (mag > maxMag)
{
peakIndex = i;
maxMag = mag;
}
}
double peakFreq = sampleRate/fftLen * i/2; // might need a bit of tweaking.
double magInDb = 20*Math.log10(mag);
```

```
```#### Post Status

Asked in February 2016

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