From 5063efe064f8d46c52ea3fb03bcaf46dbb4f1353 Mon Sep 17 00:00:00 2001
From: Oliver Jowett <oliver@mutability.co.uk>
Date: Sat, 12 Nov 2016 17:13:36 +0000
Subject: [PATCH] Add #ifdef-ed code to display mode A/C signals graphically.

---
 demod_2400.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)

diff --git a/demod_2400.c b/demod_2400.c
index 32cdb8a..26496f0 100644
--- a/demod_2400.c
+++ b/demod_2400.c
@@ -21,6 +21,10 @@
 
 #include <assert.h>
 
+#ifdef MODEAC_DEBUG
+#include <gd.h>
+#endif
+
 // 2.4MHz sampling rate version
 //
 // When sampling at 2.4MHz we have exactly 6 samples per 5 symbols.
@@ -368,6 +372,85 @@ void demodulate2400(struct mag_buf *mag)
 }
 
 
+#ifdef MODEAC_DEBUG
+
+static int yscale(unsigned signal)
+{
+    return (int) (299 - 299.0 * signal / 65536.0);
+}
+
+static void draw_modeac(uint16_t *m, unsigned modeac, unsigned f1_clock, unsigned noise_threshold, unsigned signal_threshold, unsigned bits, unsigned noisy_bits, unsigned uncertain_bits)
+{
+    // 25 bits at 87*60MHz
+    // use 1 pixel = 30MHz = 1087 pixels
+
+    gdImagePtr im = gdImageCreate(1088, 300);
+    int red = gdImageColorAllocate(im, 255, 0, 0);
+    int brightgreen = gdImageColorAllocate(im, 0, 255, 0);
+    int darkgreen = gdImageColorAllocate(im, 0, 180, 0);
+    int blue = gdImageColorAllocate(im, 0, 0, 255);
+    int grey = gdImageColorAllocate(im, 200, 200, 200);
+    int white = gdImageColorAllocate(im, 255, 255, 255);
+    int black = gdImageColorAllocate(im, 0, 0, 0);
+
+    gdImageFilledRectangle(im, 0, 0, 1087, 299, white);
+
+    // draw samples
+    for (unsigned pixel = 0; pixel < 1088; ++pixel) {
+        int clock_offset = (pixel - 150) * 2;
+        int bit = clock_offset / 87;
+        int sample = (f1_clock + clock_offset) / 25;
+        int bitoffset = clock_offset % 87;
+        int color;
+
+        if (sample < 0)
+            continue;
+
+        if (clock_offset < 0 || bit >= 20) {
+            color = grey;
+        } else if (bitoffset < 27 && (uncertain_bits & (1 << (19-bit)))) {
+            color = red;
+        } else if (bitoffset >= 27 && (noisy_bits & (1 << (19-bit)))) {
+            color = red;
+        } else if (bitoffset >= 27) {
+            color = grey;
+        } else if (bits & (1 << (19-bit))) {
+            color = brightgreen;
+        } else {
+            color = darkgreen;
+        }
+
+        gdImageLine(im, pixel, 299, pixel, yscale(m[sample]), color);
+    }
+
+    // draw bit boundaries
+    for (unsigned bit = 0; bit < 20; ++bit) {
+        unsigned clock = 87 * bit;
+        unsigned pixel0 = clock / 2 + 150;
+        unsigned pixel1 = (clock + 27) / 2 + 150;
+
+        gdImageLine(im, pixel0, 0, pixel0, 299, (bit == 0 || bit == 14) ? black : grey);
+        gdImageLine(im, pixel1, 0, pixel1, 299, (bit == 0 || bit == 14) ? black : grey);
+    }
+
+    // draw thresholds
+    gdImageLine(im, 0, yscale(noise_threshold), 1087, yscale(noise_threshold), blue);
+    gdImageLine(im, 0, yscale(signal_threshold), 1087, yscale(signal_threshold), blue);
+
+    // save it
+
+    static int file_counter;
+    char filename[PATH_MAX];
+    sprintf(filename, "modeac_%04X_%04d.png", modeac, ++file_counter);
+    fprintf(stderr, "writing %s\n", filename);
+
+    FILE *pngout = fopen(filename, "wb");
+    gdImagePng(im, pngout);
+    fclose(pngout);
+    gdImageDestroy(im);
+}
+
+#endif
 
 //////////
 ////////// MODE A/C
@@ -548,6 +631,10 @@ void demodulate2400AC(struct mag_buf *mag)
             ((bits & 0x00040) ? 0x0004 : 0) |  // D4
             ((bits & 0x00004) ? 0x0080 : 0);   // SPI
 
+#ifdef MODEAC_DEBUG
+        draw_modeac(m, modeac, f1_clock, noise_threshold, signal_threshold, bits, noisy_bits, uncertain_bits);
+#endif
+
         // This message looks good, submit it
 
         // compute message receive time as block-start-time + difference in the 12MHz clock