DSP tutorial: RTTY decoder using FFT at dreamport

DSP tutorial: RTTY decoder using FFT

Last modified: March 4th, 2012

This example is a very basic RTTY decoder, similar to the FSK decoder explained previously. It doesn’t look for a start byte, uses no bit synchronization. It oversamples 3 times (divides one bit length count of samples into 3 pieces called chunks) and decides between 0 and 1 by calculating the dominant frequency in these chunks. After the loop gathered 8 bits it reconstructs the sent ASCII byte using the Baudot code and displays it. Operates poorly even with a little noise on the channel.

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public void run() {
ByteArrayOutputStream baos = new ByteArrayOutputStream(); // this will store sound data

int oneBitSampleCount = (int)Math.round(SAMPLERATE/BITSPERSEC);
int oneChunkSampleCount = (int)Math.round(oneBitSampleCount/3);
System.out.println("One bit length: " + 1/BITSPERSEC + " seconds, " + oneBitSampleCount + " samples");
System.out.println("One chunk length: " + (1/BITSPERSEC)/3 + " seconds, " + oneChunkSampleCount + " samples");
byte[] abBuffer = new byte[oneChunkSampleCount*2];
double[] abBufferDouble = new double[oneChunkSampleCount];
int[] chunkResults = new int[3];
int chunkResultp = 0;
int byteResult = 0;
int byteResultp = 0;
int numberOfInvalidResults = 0;

fft = new DoubleFFT_1D(abBufferDouble.length);
// we need to initialize a buffer where we store our samples as complex numbers. first value is the real part, second is the imaginary.
double[] fftData = new double[abBuffer.length]; // size == samples * 2
int binOfFreq0 = (int)Math.round((FREQ0/(double)SAMPLERATE)*fftData.length);
int binOfFreq1 = (int)Math.round((FREQ1/(double)SAMPLERATE)*fftData.length);

System.out.println("FFT bin of freq. 0 (" + FREQ0 + "Hz): " + binOfFreq0);
System.out.println("FFT bin of freq. 1 (" + FREQ1 + "Hz): " + binOfFreq1);

tdl.start();

try {
while (!Thread.interrupted()) {
// waiting for the buffer to get filled
while (tdl.available() < abBuffer.length)
Thread.sleep(0, 1); // without this, the audio will be choppy

int bytesRead = tdl.read(abBuffer, 0, abBuffer.length);

// converting frames stored as bytes to double values
int samplesRead = bytesRead / 2;
for (int i = 0; i < samplesRead; i++) {
abBufferDouble[i] = ((abBuffer[i * 2] & 0xFF) | (abBuffer[i * 2 + 1] << 8)) / 32768.0;
}

for (int i = 0; i < fftData.length; i++)
fftData[i] = 0;
for (int i = 0; i < samplesRead; i++) {
// copying audio data to the fft data buffer, imaginary part is 0
fftData[2 * i] = abBufferDouble[i];
fftData[2 * i + 1] = 0;
}

// calculating the fft of the data, so we will have spectral power of each frequency component
// fft resolution (number of bins) is samplesNum, because we initialized with that value
fft.complexForward(fftData);

double PWRFreq0 = Math.sqrt(fftData[binOfFreq0] * fftData[binOfFreq0] + fftData[binOfFreq0+1] * fftData[binOfFreq0+1]);
double PWRFreq1 = Math.sqrt(fftData[binOfFreq1] * fftData[binOfFreq1] + fftData[binOfFreq1+1] * fftData[binOfFreq1+1]);

if (PWRFreq0 > PWRTHRESHOLD || PWRFreq1 > PWRTHRESHOLD) {
if (PWRFreq0*PWRFreq0-PWRFreq1*PWRFreq1 < 0)
chunkResults[chunkResultp] = 1;
else
chunkResults[chunkResultp] = 0;

//System.out.print("PWRFreq0: " + PWRFreq0 + " PWRFreq1: " + PWRFreq1 + " chunk[" + chunkResultp + "]=" + chunkResults[chunkResultp]);

chunkResultp++;
if (chunkResultp == 3) {
chunkResultp = 0;

int bitResult;
if (chunkResults[0] + chunkResults[1] + chunkResults[2] >= 2)
bitResult = 0;
else
bitResult = 1;
//System.out.print(" resulting bit: " + bitResult);

if ((byteResultp == 0 && bitResult != 1) || (byteResultp == 6 && bitResult != 0) || (byteResultp == 7 && bitResult != 0)) {
byteResultp = byteResult = chunkResultp = 0;
System.out.println("rst");
} else {
switch (byteResultp) {
case 0:
case 6:
case 7:
break;
default:
System.out.print(bitResult);
byteResult += (bitResult == 0 ? 1 : 0) << (byteResultp-1);
}

byteResultp++;
if (byteResultp == 8) {
switch (byteResult) {
case 31:
mode = RTTYMode.letters;
System.out.println();
break;
case 27:
mode = RTTYMode.symbols;
System.out.println();
break;
default:
switch (mode) {
case letters:
System.out.println(" *** " + RTTYLetters[byteResult] + "(" + byteResult + ")");
break;
case symbols:
System.out.println(" *** " + RTTYSymbols[byteResult] + "(" + byteResult + ")");
break;
}
}
byteResultp = byteResult = 0;
}
}
}
//System.out.println();
numberOfInvalidResults = 0;
} else {
//System.out.println("invalid!");
numberOfInvalidResults++;
}

if (numberOfInvalidResults > 0) {
//System.out.println("reset");
numberOfInvalidResults = byteResultp = byteResult = chunkResultp = 0;
}

//System.out.println("Samples read: " + samplesRead + " sampleAfterSamples: " + bitChunkLengthInSamples);
abBufferDouble[0] = 1;
baos.write(getBytesFromDoubles(abBufferDouble, samplesRead), 0, samplesRead * 2);
}
} catch (InterruptedException e) {
}

tdl.stop();
tdl.close();

writeWavFile(baos.toByteArray(), baos.size() / 2, "output.wav");
}

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