DSP tutorial: RTTY decoder using FFT

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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