DSP tutorial: RTTY decoder using FFT filtering

This example is a slightly better RTTY decoder, it uses FFT only for filtering, still uses chunks but doesn’t make bit decisions by calculating the average power of them.

Instead it uses the signal flow discussed in this paper:

Note that this example only implements the demodulator, UART emulator and Baudot terminal blocks from the diagram. For further explanation of the mechanism, continue to the next section of the tutorial.

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// reads a chunk from the sound device
    public int getChunk(double[] chunkBuffer) {
        byte[] abBuffer = new byte[oneChunkSampleCount*2];
        int samplesRead = 0;

        // waiting for the buffer to get filled
        try {
            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
            samplesRead = bytesRead / 2;
            for (int i = 0; i < samplesRead; i++) {
                chunkBuffer[i] = ((abBuffer[i * 2] & 0xFF) | (abBuffer[i * 2 + 1] << 8)) / 32768.0;
            }
        } catch (InterruptedException e) {
        }
        return samplesRead;
    }

    // analyzes a chunk with FFT and returns the bit value
    public int demodulator() {
        int samplesRead = getChunk(signalLine1Buffer);

        // copying to the other line
        for (int i = 0; i < signalLine1Buffer.length; i++) {
            signalLine2Buffer[i] = signalLine1Buffer[i];
        }

        // emptying fft buffer in case of samples read != fft buffer length
        for (int i = 0; i < signalLine1FFTData.length; i++)
            signalLine1FFTData[i] = signalLine2FFTData[i] = 0;
        for (int i = 0; i < samplesRead; i++) {
            // copying audio data to the fft data buffer, imaginary part is 0
            signalLine1FFTData[2 * i] = signalLine1Buffer[i];
            signalLine1FFTData[2 * i + 1] = 0;

            signalLine2FFTData[2 * i] = signalLine2Buffer[i];
            signalLine2FFTData[2 * i + 1] = 0;
        }
        fft.complexForward(signalLine1FFTData);
        fft.complexForward(signalLine2FFTData);

        int binOfFreq0 = (int)Math.round((FREQ0*(signalLine1FFTData.length/2)/(double)SAMPLERATE)*2);
        int binOfFreq1 = (int)Math.round((FREQ1*(signalLine2FFTData.length/2)/(double)SAMPLERATE)*2);

        // zeroing out all freqs except the carrier on signal line 1
        for (int i = 0; i < signalLine1FFTData.length; i += 2) {
            if (i != binOfFreq0)
                signalLine1FFTData[i] = signalLine1FFTData[i + 1] = 0;
        }
        // zeroing out all freqs except the carrier on signal line 2
        for (int i = 0; i < signalLine2FFTData.length; i += 2) {
            if (i != binOfFreq1)
                signalLine2FFTData[i] = signalLine2FFTData[i + 1] = 0;
        }
        fft.complexInverse(signalLine1FFTData, false);
        fft.complexInverse(signalLine2FFTData, false);

        for (int i = 0; i < samplesRead; i++) {
            signalLine1Buffer[i] = signalLine1FFTData[2 * i];
            signalLine2Buffer[i] = signalLine2FFTData[2 * i];
            signalLine1Buffer[i] *= signalLine1Buffer[i]; // squaring
            signalLine2Buffer[i] *= signalLine2Buffer[i]; // squaring
            signalLine2Buffer[i] *= -1; // inverting signal line 2

            // summing the two signal lines
            signalLine1Buffer[i] += signalLine2Buffer[i];
        }

        int lowPassCutFreq = (int)Math.ceil(BITSPERSEC);
        int binOfLowPassCutFreq = (int)Math.round(((lowPassCutFreq * (signalLine1FFTData.length/2)) / (double)SAMPLERATE)*2);

        // lowpass filtering
        for (int i = 0; i < signalLine1FFTData.length; i++)
            signalLine1FFTData[i] = 0;
        for (int i = 0; i < samplesRead; i++) {
            // copying audio data to the fft data buffer, imaginary part is 0
            signalLine1FFTData[2 * i] = signalLine1Buffer[i];
            signalLine1FFTData[2 * i + 1] = 0;
        }
        fft.complexForward(signalLine1FFTData);

        for (int i = 0; i < signalLine1FFTData.length; i += 2) {
            if (i > binOfLowPassCutFreq)
                signalLine1FFTData[i] = signalLine1FFTData[i + 1] = 0;
        }
        fft.complexInverse(signalLine1FFTData, false);

        for (int i = 0; i < samplesRead; i++) {
            signalLine1Buffer[i] = signalLine1FFTData[2 * i] / 70;
        }
        baos.write(getBytesFromDoubles(signalLine1Buffer, samplesRead), 0, samplesRead * 2); // writing to the output wav for debugging purposes

        double average = 0;
        for (int i = 0; i < samplesRead; i++) {
            average += signalLine1Buffer[i];
        }
        average /= samplesRead;

        if (average > 0)
            return 0;
        else
            return 1;
    }

    // this function returns at the half of a bit with the bit's value
    public int getBitDPLL() {
        boolean chunkPhaseChanged = false;
        int chunkVal = -1;
        int chunkPhase = 0;

        while (chunkPhase < CHUNKCOUNTPERBIT) {
            chunkVal = demodulator();
            if (chunkVal == -1)
                break;

            if (!chunkPhaseChanged && chunkVal != oldChunkVal) {
                if (chunkPhase < CHUNKCOUNTPERBIT/2)
                    chunkPhase++; // early
                else
                    chunkPhase--; // late
                chunkPhaseChanged = true;
            }
            oldChunkVal = chunkVal;
            chunkPhase++;
        }

        // putting a tick to the output wav signing the moment when the DPLL returned
        /*baos.write(100);
        baos.write(100);
        baos.write(100);
        baos.write(100);
        baos.write(100);
        baos.write(100);*/

        return chunkVal;
    }

    // this function returns only when the start bit is successfully received
    public void waitForStartBit() {
        int bitResult;

        while (!Thread.interrupted()) {
            do {
                bitResult = demodulator();
            } while ((bitResult == 0 || bitResult == -1) && !Thread.interrupted());
           
            //System.out.println("sb0: 1");
           
            do {
                bitResult = demodulator();
            } while ((bitResult == 1 || bitResult == -1) && !Thread.interrupted());
           
            //System.out.println("sb1: 0");

            // waiting half bit time
            for (int i = 0; i < CHUNKCOUNTPERBIT/2; i++)
                bitResult = demodulator();

            //System.out.println("sb2: " + bitResult);

            if (bitResult == 0)
                break;
        }
        //System.out.println("start bit ok");
    }
   
    @Override
    public void run() {
        tdl.start();

        int byteResult = 0;
        int byteResultp = 0;
        int bitResult;
       
        while (!Thread.interrupted()) {
            waitForStartBit();
           
            System.out.print("0 "); // first bit is the start bit, it's zero
           
            // reading 7 more bits
            for (byteResultp = 1, byteResult = 0; byteResultp < 8; byteResultp++) {
                bitResult = getBitDPLL();
                if (bitResult == -1) {
                    byteResult = -1;
                    break;
                }

                switch (byteResultp) {
                    case 6: // stop bit 1
                        System.out.print(" " + bitResult);
                        break;
                    case 7: // stop bit 2
                        System.out.print(bitResult);
                        break;
                    default:
                        System.out.print(bitResult);
                        byteResult += bitResult << (byteResultp-1);
                }
            }

            if (byteResult == -1)
                continue;

            switch (byteResult) {
                case 31:
                    mode = RTTYMode.letters;
                    System.out.println(" ^L^");
                    break;
                case 27:
                    mode = RTTYMode.symbols;
                    System.out.println(" ^F^");
                    break;
                default:
                    switch (mode) {
                    case letters:
                        System.out.println(" *** " + RTTYLetters[byteResult] + "(" + byteResult + ")");
                        break;
                    case symbols:
                        System.out.println(" *** " + RTTYSymbols[byteResult] + "(" + byteResult + ")");
                        break;
                    }
            }
        }

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

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