Merge pull request #14 from joshua-jerred/sstv_fix

SSTV fix

docs/sstv.md

@@ -0,0 +1,19 @@
+# SSTV
+
+## Robot36 quick notes
+
+Audio sample rate of 48000, so 1728000 total samples. This means 7200 samples,
+150ms, per scan line.
+
+|                   |                                          |
+| ----------------- | ---------------------------------------- |
+| *VIS Code*        | 8d (decimal)                             |
+| Color Mode        | Y, R-Y, B-Y                              |
+| Scan Sequence     | Y, R-Y (even lines) - Y, B-Y (odd lines) |
+| Number of Lines   | 240                                      |
+| Transmission Time | 36 seconds                               |
+| Image Size        | 320x240                                  |
+
+## Useful links
+
+https://vu2nsb.com/cw-digital-radio/slow-scan-tv-sstv/

include/SignalEasel/sstv.hpp

@@ -69,6 +69,7 @@ class Modulator : public signal_easel::Modulator {
   enum class ColorFormat { _4_2_0_, _4_2_2_, NONE };
   enum class ColorType { Y, Cb, Cr };
 
+  void addPreamble();
   void encodeAdjustedImage(SstvImage &image);
   void readImageLine(SstvImage &image, uint32_t line_number);
   void encodeScanLine(uint32_t line_number);
@@ -85,6 +86,8 @@ class Modulator : public signal_easel::Modulator {
   uint32_t line_width_ = 0;
   uint32_t num_of_lines_ = 0;
 
+  uint8_t vis_code_ = 0;
+
   double y_scan_line_time_ = 0;
   double c_scan_line_time_ = 0;
 

src/sstv/sstv_modulator.cpp

@@ -27,6 +27,9 @@ Modulator::Modulator(sstv::Settings settings) : settings_(std::move(settings)) {
     color_format_ = ColorFormat::_4_2_0_;
     line_width_ = 320;
     num_of_lines_ = 240;
+
+    vis_code_ = 0b10001000;
+
     y_scan_line_time_ = 88.0;
     c_scan_line_time_ = 44.0;
 
@@ -64,13 +67,41 @@ void Modulator::encodeImage(std::string input_image_path) {
     }
 
     scan_line_buffer_.reserve(line_width_);
-
+    addPreamble();
     encodeAdjustedImage(image);
+
   } catch (const std::exception &e) {
     throw Exception(Exception::Id::SSTV_IMAGE_TOOLS_ERROR, e.what());
   }
 }
 
+void Modulator::addPreamble() {
+  auto visCodeBitFrequncy = [](uint8_t vis_code, uint8_t bit_number) {
+    return vis_code & (1 << bit_number) ? 1100 : 1300;
+  };
+
+  pulse(100, 1900);
+  pulse(100, 1500);
+  pulse(100, 1900);
+  pulse(100, 1500);
+  pulse(100, 2300);
+  pulse(100, 1500);
+  pulse(100, 2300);
+  pulse(100, 1500);
+
+  pulse(300, 1900); // Leader Tone
+  pulse(10, 1500);  // Break
+  pulse(300, 1900); // Leader Tone
+
+  pulse(30, 1200); // Start Bit
+
+  for (int i = 0; i < 8; i++) {
+    pulse(30, visCodeBitFrequncy(vis_code_, i));
+  }
+
+  pulse(21, 1200); // Stop Bit
+}
+
 void Modulator::encodeAdjustedImage(SstvImage &image) {
   for (uint32_t i = 0; i < num_of_lines_; i++) {
     // for (int i = 0; i < 1; i++) {
@@ -167,7 +198,7 @@ void Modulator::writeBuffer(const double total_time, ColorType ycbcr_colors) {
   int previous_pixel_index = -1;
   int frequency = 0;
   double color = 0;
-  for (int i = 0; i < num_of_samples; i++) {
+  for (int i = 1; i <= num_of_samples; i++) {
     int new_pixel_index =
         (int)((double)i / (double)num_of_samples * (double)line_width_);
     if (new_pixel_index != previous_pixel_index) {