Trigger on down-flanks instead of up-flanks.

[c64tapwav] / decode.cpp

diff --git a/decode.cpp b/decode.cpp
index 7bbb250f973ce0f307bd3062e45573b7a11a90d9..1a5ee74856c5c635e37ea434213307114964be6c 100644 (file)
--- a/decode.cpp
+++ b/decode.cpp
@@ -6,7 +6,8 @@
 #include <vector>
 #include <algorithm>
 
-#define LANCZOS_RADIUS 30
+#include "interpolate.h"
+
 #define BUFSIZE 4096
 #define HYSTERESIS_LIMIT 3000
 #define SAMPLE_RATE 44100
@@ -15,7 +16,7 @@
 
 #define SYNC_PULSE_START 1000
 #define SYNC_PULSE_END 15000
-#define SYNC_PULSE_LENGTH 380.0
+#define SYNC_PULSE_LENGTH 378.0
 #define SYNC_TEST_TOLERANCE 1.10
 
 struct tap_header {
@@ -25,45 +26,6 @@ struct tap_header {
        unsigned int data_len;
 };
 
-double sinc(double x)
-{
-       if (fabs(x) < 1e-6) {
-               return 1.0f - fabs(x);
-       } else {
-               return sin(x) / x;
-       }
-}
-
-#if 1
-double weight(double x)
-{
-       if (fabs(x) > LANCZOS_RADIUS) {
-               return 0.0f;
-       }
-       return sinc(M_PI * x) * sinc(M_PI * x / LANCZOS_RADIUS);
-}
-#else
-double weight(double x)
-{
-       if (fabs(x) > 1.0f) {
-               return 0.0f;
-       }
-       return 1.0f - fabs(x);
-}
-#endif
-
-double interpolate(const std::vector<short> &pcm, double i)
-{
-       int lower = std::max<int>(ceil(i - LANCZOS_RADIUS), 0);
-       int upper = std::min<int>(floor(i + LANCZOS_RADIUS), pcm.size() - 1);
-       double sum = 0.0f;
-
-       for (int x = lower; x <= upper; ++x) {
-               sum += pcm[x] * weight(i - x);
-       }
-       return sum;
-}
-       
 // between [x,x+1]
 double find_zerocrossing(const std::vector<short> &pcm, int x)
 {
@@ -74,14 +36,14 @@ double find_zerocrossing(const std::vector<short> &pcm, int x)
                return x + 1;
        }
 
-       assert(pcm[x + 1] > 0);
-       assert(pcm[x] < 0);
+       assert(pcm[x + 1] < 0);
+       assert(pcm[x] > 0);
 
-       double lower = x;
-       double upper = x + 1;
+       double upper = x;
+       double lower = x + 1;
        while (upper - lower > 1e-6) {
                double mid = 0.5f * (upper + lower);
-               if (interpolate(pcm, mid) > 0) {
+               if (lanczos_interpolate(pcm, mid) > 0) {
                        upper = mid;
                } else {
                        lower = mid;
@@ -124,23 +86,42 @@ int main(int argc, char **argv)
 
        // Find the flanks.
        int last_bit = -1;
-       double last_upflank = -1;
-       int last_max_level = 0;
+       double last_downflank = -1;
        for (unsigned i = 0; i < pcm.size(); ++i) {
                int bit = (pcm[i] > 0) ? 1 : 0;
-               if (bit == 1 && last_bit == 0 && last_max_level > HYSTERESIS_LIMIT) {
-                       // up-flank!
+               if (bit == 0 && last_bit == 1) {
+                       // Check if we ever go up above HYSTERESIS_LIMIT before we dip down again.
+                       bool true_pulse = false;
+                       unsigned j;
+                       int min_level_after = 32767;
+                       for (j = i; j < pcm.size(); ++j) {
+                               min_level_after = std::min<int>(min_level_after, pcm[j]);
+                               if (pcm[j] > 0) break;
+                               if (pcm[j] < -HYSTERESIS_LIMIT) {
+                                       true_pulse = true;
+                                       break;
+                               }
+                       }
+
+                       if (!true_pulse) {
+#if 0
+                               fprintf(stderr, "Ignored down-flank at %.6f seconds due to hysteresis (%d < %d).\n",
+                                       double(i) / SAMPLE_RATE, -min_level_after, HYSTERESIS_LIMIT);
+#endif
+                               i = j;
+                               continue;
+                       } 
+
+                       // down-flank!
                        double t = find_zerocrossing(pcm, i - 1) * (1.0 / SAMPLE_RATE);
-                       if (last_upflank > 0) {
+                       if (last_downflank > 0) {
                                pulse p;
                                p.time = t;
-                               p.len = t - last_upflank;
+                               p.len = t - last_downflank;
                                pulses.push_back(p);
                        }
-                       last_upflank = t;
-                       last_max_level = 0;
+                       last_downflank = t;
                }
-               last_max_level = std::max(last_max_level, abs(pcm[i]));
                last_bit = bit;
        }
 
@@ -155,14 +136,14 @@ int main(int argc, char **argv)
                }
                double mean_length = C64_FREQUENCY * sum / (SYNC_PULSE_END - SYNC_PULSE_START);
                calibration_factor = SYNC_PULSE_LENGTH / mean_length;
-               fprintf(stderr, "Calibrated sync pulse length: %.2f -> 380.0 (change %+.2f%%)\n",
-                       mean_length, 100.0 * (calibration_factor - 1.0));
+               fprintf(stderr, "Calibrated sync pulse length: %.2f -> %.2f (change %+.2f%%)\n",
+                       mean_length, SYNC_PULSE_LENGTH, 100.0 * (calibration_factor - 1.0));
 
                // Check for pulses outside +/- 10% (sign of misdetection).
                for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
                        double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
                        if (cycles < SYNC_PULSE_LENGTH / SYNC_TEST_TOLERANCE || cycles > SYNC_PULSE_LENGTH * SYNC_TEST_TOLERANCE) {
-                               fprintf(stderr, "Sync cycle with upflank at %.6f was detected at %.0f cycles; misdetect?\n",
+                               fprintf(stderr, "Sync cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
                                        pulses[i].time, cycles);
                        }
                }
@@ -178,12 +159,14 @@ int main(int argc, char **argv)
                        stddev);
        }
 
+       FILE *fp = fopen("cycles.plot", "w");
        std::vector<char> tap_data;
        for (unsigned i = 0; i < pulses.size(); ++i) {
                double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
+               fprintf(fp, "%f %f\n", pulses[i].time, cycles);
                int len = lrintf(cycles / TAP_RESOLUTION);
                if (i > SYNC_PULSE_END && (cycles < 100 || cycles > 800)) {
-                       fprintf(stderr, "Cycle with upflank at %.6f was detected at %.0f cycles; misdetect?\n",
+                       fprintf(stderr, "Cycle with downflank at %.6f was detected at %.0f cycles; misdetect?\n",
                                        pulses[i].time, cycles);
                }
                if (len <= 255) {
@@ -196,6 +179,7 @@ int main(int argc, char **argv)
                        tap_data.push_back(overflow_len >> 16);
                }
        }
+       fclose(fp);
 
        tap_header hdr;
        memcpy(hdr.identifier, "C64-TAPE-RAW", 12);