DSP tutorial: SSTV decoder
Last modified: March 18th, 2012To decode an SSTV signal, you have several options:
- Decode the VIS code and start image decoding according to the mode read from the VIS code
- Ignore the VIS code and decode an image according to a previously set mode
- Ignore the VIS code, start decoding when enough number of sync pulses have been received using a previously set mode
- Autodetect the mode, start decoding when enough sync pulses have been received for the autodetection
This example uses the second approach, although it reads and processes the VIS code correctly. The previously set mode is Martin M1. After startup,a window opens and the app waits for an SSTV transmission header, and starts decoding line by line (it displays the decoding results in real time). The sound source can be a .wav file or the sound card.
Note that I’ve tried to implement SSTV decoding using FFT, but the resulting pictures were noisier than the recursive filter method (used in this example) when the SNR was low. You can download the source code for the example with the FFT method at the bottom of this page.
Resources
SSTV demodulation using recursive filters
VIS code and image data timings and format
Matlab code for SSTV modulation, demodulation
FFT demodulation, improvements, slant correction
Technical reference for SSTV
SSTV on Wikipedia
Open source SSTV apps:
Source code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 | // this function returns the bit value of the current sample // possible results: // 1900 Hz -> 4 (leader tone) // 1500 Hz -> 3 (sync porch, separator pulse) // 1300 Hz -> 2 (VIS bit 0) // 1200 Hz -> 1 (break, VIS start&stop bit) // 1100 Hz -> 0 (VIS bit 1) public int demodulator(double sample) { double[] lines = new double[5]; lines[0] = bandPassFreq0(sample); lines[1] = bandPassFreq1(sample); lines[2] = bandPassFreq2(sample); lines[3] = bandPassFreq3(sample); lines[4] = bandPassFreq4(sample); // calculating the RMS of the lines (squaring them) lines[0] *= lines[0]; lines[1] *= lines[1]; lines[2] *= lines[2]; lines[3] *= lines[3]; lines[4] *= lines[4]; // lowpass filtering the lines lines[0] = lowPass0(lines[0]); lines[1] = lowPass1(lines[1]); lines[2] = lowPass2(lines[2]); lines[3] = lowPass3(lines[3]); lines[4] = lowPass4(lines[4]); // writing output wav for debugging purposes wav0.write(getBytesFromDouble(lines[0]), 0, 2); wav1.write(getBytesFromDouble(lines[1]), 0, 2); wav2.write(getBytesFromDouble(lines[2]), 0, 2); wav3.write(getBytesFromDouble(lines[3]), 0, 2); wav4.write(getBytesFromDouble(lines[4]), 0, 2); // deciding which line is the highest and returning it's index int maxIndex = 0; double maxVal = lines[0]; for (int i = 1; i < lines.length; i++) { if (lines[i] > maxVal) { maxVal = lines[i]; maxIndex = i; } } return maxIndex; } // this function returns at the half of a bit with the bit's value public int getVisBit() { int val = 0; for (int i = 0; i < oneBitSampleCount; i++) { val = demodulator(getSample()); wavFM.write(getBytesFromDouble(0), 0, 2); } // putting a tick to the output wav signing the moment when the function returned wav0.write(100); wav0.write(100); wav0.write(100); wav0.write(100); wav0.write(100); wav0.write(100); return val; } public void startVISReceiving() { int bitResult; System.out.print("Header received, starting receiving VIS code: "); oneBitSampleCount = SAMPLERATE*0.030; // waiting half bit time (15ms) for (int i = 0; i < oneBitSampleCount/2; i++) { demodulator(getSample()); wavFM.write(getBytesFromDouble(0), 0, 2); } int VIS = 0; int parity = 0; boolean parityError = false; for (int i = 0; i < 9; i++) { bitResult = getVisBit(); int bit = bitResult == 0 ? 1 : 0; switch (i) { case 7: if (parity != bit) parityError = true; break; case 8: break; // stop bit default: if (bitResult == 0) VIS |= bit << (i); parity ^= bit; System.out.print(bit); } } System.out.println(); System.out.print("VIS: " + VIS); if (parityError) System.out.print(" parity ERROR"); else System.out.print(" parity OK"); if (VIS == 44) // Martin M1 VIS code System.out.println(" VIS MARTIN OK"); else System.out.println(" VIS UNKNOWN ERROR"); // waiting to reach the end of the stop bit for (int i = 0; i < oneBitSampleCount/4+5; i++) { demodulator(getSample()); wavFM.write(getBytesFromDouble(0), 0, 2); } } public int[] waitForEdge(int[] result) { result[0] = result[1]; while (!Thread.interrupted()) { result[1] = demodulator(getSample()); wavFM.write(getBytesFromDouble(0), 0, 2); if (result[0] != result[1] || Thread.interrupted()) break; result[0] = result[1]; }; return result; } public void waitForStart() { int[] result = new int[2]; int edge41Count = 0; result[0] = result[1] = 0; while (!Thread.interrupted()) { result = waitForEdge(result); // this part starts VIS receiving if we receive 41 -> 14 -> 41 transitions (bits 4141) in sequence switch (edge41Count) { case 0: if (result[0] == 4 && result[1] == 1) { edge41Count = 1; System.out.println("Got edge: 41"); continue; } break; case 1: if (result[0] == 1 && result[1] == 4) { // got 41 -> 14 edge41Count = 2; System.out.println("Got edge: 14"); continue; } break; case 2: if (result[0] == 4 && result[1] == 1) { // got 41 -> 14 -> 41 startVISReceiving(); receiveImage(); } break; } edge41Count = 0; } } // 800Hz, beta 0.5, impulse length 50 private double[] FIRLPCoeffs = { +0.0001082461, +0.0034041195, +0.0063570207, +0.0078081648, +0.0060550614, -0.0002142384, -0.0104500335, -0.0211855480, -0.0264527776, -0.0201269304, +0.0004419626, +0.0312014771, +0.0606261038, +0.0727491887, +0.0537028370, -0.0004362161, -0.0779387981, -0.1511168919, -0.1829049634, -0.1390189257, -0.0017097774, +0.2201896764, +0.4894395006, +0.7485289338, +0.9357596142, +1.0040320616, +0.9357596142, +0.7485289338, +0.4894395006, +0.2201896764, -0.0017097774, -0.1390189257, -0.1829049634, -0.1511168919, -0.0779387981, -0.0004362161, +0.0537028370, +0.0727491887, +0.0606261038, +0.0312014771, +0.0004419626, -0.0201269304, -0.0264527776, -0.0211855480, -0.0104500335, -0.0002142384, +0.0060550614, +0.0078081648, +0.0063570207, +0.0034041195, +0.0001082461, }; private double[] xvFIRLP1 = new double[FIRLPCoeffs.length]; public double FIRLowPass1(double sampleIn) { for (int i = 0; i < xvFIRLP1.length-1; i++) xvFIRLP1[i] = xvFIRLP1[i+1]; xvFIRLP1[xvFIRLP1.length-1] = sampleIn / 5.013665674e+00; double sum = 0; for (int i = 0; i <= xvFIRLP1.length-1; i++) sum += (FIRLPCoeffs[i] * xvFIRLP1[i]); return sum; } private double[] xvFIRLP2 = new double[FIRLPCoeffs.length]; public double FIRLowPass2(double sampleIn) { for (int i = 0; i < xvFIRLP2.length-1; i++) xvFIRLP2[i] = xvFIRLP2[i+1]; xvFIRLP2[xvFIRLP2.length-1] = sampleIn / 5.013665674e+00; double sum = 0; for (int i = 0; i <= xvFIRLP2.length-1; i++) sum += (FIRLPCoeffs[i] * xvFIRLP2[i]); return sum; } // moving average private double[] xvMA1 = new double[9]; private double yvMA1prev = 0; public double noiseReductionFilter1(double sampleIn) { for (int i = 0; i < xvMA1.length-1; i++) xvMA1[i] = xvMA1[i+1]; xvMA1[xvMA1.length-1] = sampleIn; yvMA1prev = yvMA1prev+xvMA1[xvMA1.length-1]-xvMA1[0]; return yvMA1prev; } private double[] xvMA2 = new double[xvMA1.length]; private double yvMA2prev = 0; public double noiseReductionFilter2(double sampleIn) { for (int i = 0; i < xvMA2.length-1; i++) xvMA2[i] = xvMA2[i+1]; xvMA2[xvMA2.length-1] = sampleIn; yvMA2prev = yvMA2prev+xvMA2[xvMA2.length-1]-xvMA2[0]; return yvMA2prev; } //private double fmDemodMinVal = 0, fmDemodMaxVal = 0; private double oscPhase = 0; private double realPartPrev = 0, imaginaryPartPrev = 0; public int fmDemodulateLuminance(double sample) { oscPhase += (2 * Math.PI * 2000) / SAMPLERATE; double realPart = Math.cos(oscPhase) * sample; double imaginaryPart = Math.sin(oscPhase) * sample; if (oscPhase >= 2 * Math.PI) oscPhase -= 2 * Math.PI; realPart = FIRLowPass1(realPart); imaginaryPart = FIRLowPass2(imaginaryPart); realPart = noiseReductionFilter1(realPart); imaginaryPart = noiseReductionFilter2(imaginaryPart); sample = (imaginaryPart*realPartPrev-realPart*imaginaryPartPrev)/(realPart*realPart+imaginaryPart*imaginaryPart); realPartPrev = realPart; imaginaryPartPrev = imaginaryPart; sample += 0.2335; // bring the value above 0 /*if (fmDemodMinVal > sample) { // these min/max values were used to calibrate DC demodulator fmDemodMinVal = sample; System.out.println("fmDemodMinVal: " + sample); } if (fmDemodMaxVal < sample) { fmDemodMaxVal = sample; System.out.println("fmDemodMaxVal: " + sample); }*/ wavFM.write(getBytesFromDouble(sample), 0, 2); int luminance = (int)Math.round((sample/0.617)*255); luminance = 255-luminance; //System.out.println("lum: " + luminance); if (luminance > 255) luminance = 255; if (luminance < 0) luminance = 0; return luminance; } public void receiveImage() { double pixelLengthInS = 0.0004576; double syncLengthInS = 0.004862; double porchLengthInS = 0.000572; double separatorLengthInS = 0.000572; double channelLengthInS = pixelLengthInS*320; double channelGStartInS = syncLengthInS + porchLengthInS; double channelBStartInS = channelGStartInS + channelLengthInS + separatorLengthInS; double channelRStartInS = channelBStartInS + channelLengthInS + separatorLengthInS; double lineLengthInS = syncLengthInS + porchLengthInS + channelLengthInS + separatorLengthInS + channelLengthInS + separatorLengthInS + channelLengthInS + separatorLengthInS; double imageLengthInSamples = (lineLengthInS*256)*SAMPLERATE; double t, linet; double nextSyncTime = 0; // clearing displayed image for (int y = 0; y < 256; y++) { for (int x = 0; x < 320; x++) displayedImage[x][y] = new Color(0,0,0,0); } wav2.write(100); wav2.write(100); wav2.write(100); wav2.write(100); wav2.write(100); wav2.write(100); double sample; for (int s = 0; s < imageLengthInSamples && !Thread.interrupted(); s++) { t = s/(double)SAMPLERATE; linet = t % lineLengthInS; sample = getSample(); demodulator(sample); int lum = fmDemodulateLuminance(sample); if (t >= nextSyncTime) { nextSyncTime += lineLengthInS; wav1.write(getBytesFromDouble(1), 0, 2); repaint(); } if (linet >= channelGStartInS && linet < channelGStartInS + channelLengthInS || linet >= channelBStartInS && linet < channelBStartInS + channelLengthInS || linet >= channelRStartInS && linet < channelRStartInS + channelLengthInS) { int y = (int)Math.floor(t/lineLengthInS); if (linet >= channelGStartInS && linet < channelGStartInS + channelLengthInS) { int x = (int)Math.floor(((linet-channelGStartInS)/channelLengthInS)*320); displayedImage[x][y] = new Color(0, lum, 0); } if (linet >= channelBStartInS && linet < channelBStartInS + channelLengthInS) { int x = (int)Math.floor(((linet-channelBStartInS)/channelLengthInS)*320); displayedImage[x][y] = new Color(0, displayedImage[x][y].getGreen(), lum); } if (linet >= channelRStartInS && linet < channelRStartInS + channelLengthInS) { int x = (int)Math.floor(((linet-channelRStartInS)/channelLengthInS)*320); displayedImage[x][y] = new Color(lum, displayedImage[x][y].getGreen(), displayedImage[x][y].getBlue()); //displayedImage[x][y] = new Color(lum, lum, lum); } } } repaint(); System.out.println("Finished rx."); } |
(Lower quality FFT method:
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I'm Nonoo. This is my blog about music, sounds, filmmaking, amateur radio, computers, programming, electronics and other things I'm obsessed with.
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