Add some heuristics to deal with shorter initial sync periods.
authorSteinar H. Gunderson <sgunderson@bigfoot.com>
Thu, 5 Mar 2015 20:01:53 +0000 (21:01 +0100)
committerSteinar H. Gunderson <sgunderson@bigfoot.com>
Thu, 5 Mar 2015 20:01:53 +0000 (21:01 +0100)
decode.cpp

index c5aa966..8ed2f1b 100644 (file)
@@ -15,7 +15,7 @@
 #define C64_FREQUENCY 985248
 
 #define SYNC_PULSE_START 1000
-#define SYNC_PULSE_END 15000
+#define SYNC_PULSE_END 20000
 #define SYNC_PULSE_LENGTH 378.0
 #define SYNC_TEST_TOLERANCE 1.10
 
@@ -50,6 +50,59 @@ struct pulse {
        double time;  // in seconds from start
        double len;   // in seconds
 };
+
+// Calibrate on the first ~25k pulses (skip a few, just to be sure).
+double calibrate(const std::vector<pulse> &pulses) {
+       if (pulses.size() < SYNC_PULSE_END) {
+               fprintf(stderr, "Too few pulses, not calibrating!\n");
+               return 1.0;
+       }
+
+       int sync_pulse_end = -1;
+       double sync_pulse_stddev = -1.0;
+
+       // Compute the standard deviation (to check for uneven speeds).
+       // If it suddenly skyrockets, we assume that sync ended earlier
+       // than we thought (it should be 25000 cycles), and that we should
+       // calibrate on fewer cycles.
+       for (int try_end : { 2000, 4000, 5000, 7500, 10000, 15000, SYNC_PULSE_END }) {
+               double sum2 = 0.0;
+               for (int i = SYNC_PULSE_START; i < try_end; ++i) {
+                       double cycles = pulses[i].len * C64_FREQUENCY;
+                       sum2 += (cycles - SYNC_PULSE_LENGTH) * (cycles - SYNC_PULSE_LENGTH);
+               }
+               double stddev = sqrt(sum2 / (try_end - SYNC_PULSE_START - 1));
+               if (sync_pulse_end != -1 && stddev > 5.0 && stddev / sync_pulse_stddev > 1.3) {
+                       fprintf(stderr, "Stopping at %d sync pulses because standard deviation would be too big (%.2f cycles); shorter-than-usual trailer?\n",
+                               sync_pulse_end, stddev);
+                       break;
+               }
+               sync_pulse_end = try_end;
+               sync_pulse_stddev = stddev;
+       }
+       fprintf(stderr, "Sync pulse length standard deviation: %.2f cycles\n",
+               sync_pulse_stddev);
+
+       double sum = 0.0;
+       for (int i = SYNC_PULSE_START; i < sync_pulse_end; ++i) {
+               sum += pulses[i].len;
+       }
+       double mean_length = C64_FREQUENCY * sum / (sync_pulse_end - SYNC_PULSE_START);
+       double calibration_factor = SYNC_PULSE_LENGTH / mean_length;
+       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 downflank at %.6f was detected at %.0f cycles; misdetect?\n",
+                               pulses[i].time, cycles);
+               }
+       }
+
+       return calibration_factor;
+}
        
 int main(int argc, char **argv)
 {
@@ -115,39 +168,7 @@ int main(int argc, char **argv)
                last_bit = bit;
        }
 
-       // Calibrate on the first ~25k pulses (skip a few, just to be sure).
-       double calibration_factor = 1.0f;
-       if (pulses.size() < SYNC_PULSE_END) {
-               fprintf(stderr, "Too few pulses, not calibrating!\n");
-       } else {
-               double sum = 0.0;
-               for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
-                       sum += pulses[i].len;
-               }
-               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 -> %.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 downflank at %.6f was detected at %.0f cycles; misdetect?\n",
-                                       pulses[i].time, cycles);
-                       }
-               }
-
-               // Compute the standard deviation (to check for uneven speeds).
-               double sum2 = 0.0;
-               for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
-                       double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
-                       sum2 += (cycles - SYNC_PULSE_LENGTH) * (cycles - SYNC_PULSE_LENGTH);
-               }
-               double stddev = sqrt(sum2 / (SYNC_PULSE_END - SYNC_PULSE_START - 1));
-               fprintf(stderr, "Sync pulse length standard deviation: %.2f cycles\n",
-                       stddev);
-       }
+       double calibration_factor = calibrate(pulses);
 
        FILE *fp = fopen("cycles.plot", "w");
        std::vector<char> tap_data;