Some parametrization.

[c64tapwav] / decode.c

diff --git a/decode.c b/decode.c
index e20c8c9d39743f3b0c011fe1d25bc754f81f9152..7bbb250f973ce0f307bd3062e45573b7a11a90d9 100644 (file)
--- a/decode.c
+++ b/decode.c
@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <string.h>
 #include <math.h>
 #include <unistd.h>
 #include <assert.h>
@@ -7,7 +8,22 @@
 
 #define LANCZOS_RADIUS 30
 #define BUFSIZE 4096
-#define HYSTERESIS_LIMIT 1000
+#define HYSTERESIS_LIMIT 3000
+#define SAMPLE_RATE 44100
+#define C64_FREQUENCY 985248
+#define TAP_RESOLUTION 8
+
+#define SYNC_PULSE_START 1000
+#define SYNC_PULSE_END 15000
+#define SYNC_PULSE_LENGTH 380.0
+#define SYNC_TEST_TOLERANCE 1.10
+
+struct tap_header {
+       char identifier[12];
+       char version;
+       char reserved[3];
+       unsigned int data_len;
+};
 
 double sinc(double x)
 {
@@ -75,6 +91,11 @@ double find_zerocrossing(const std::vector<short> &pcm, int x)
        return 0.5f * (upper + lower);
 }
 
+struct pulse {
+       double time;  // in seconds from start
+       double len;   // in seconds
+};
+       
 int main(int argc, char **argv)
 {
        std::vector<short> pcm;
@@ -98,18 +119,23 @@ int main(int argc, char **argv)
                printf("%d\n", in[i]);
        }
 #endif
+
+       std::vector<pulse> pulses;  // in seconds
+
+       // Find the flanks.
        int last_bit = -1;
        double last_upflank = -1;
        int last_max_level = 0;
-       for (int i = 0; i < pcm.size(); ++i) {
+       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!
-                       double t = find_zerocrossing(pcm, i - 1) * (123156.0/44100.0);
+                       double t = find_zerocrossing(pcm, i - 1) * (1.0 / SAMPLE_RATE);
                        if (last_upflank > 0) {
-//                             fprintf(stderr, "length: %f (0x%x)\n", t - last_upflank, lrintf(t - last_upflank));
-                               int len = lrintf(t - last_upflank);
-                               printf("0x%x\n", len);
+                               pulse p;
+                               p.time = t;
+                               p.len = t - last_upflank;
+                               pulses.push_back(p);
                        }
                        last_upflank = t;
                        last_max_level = 0;
@@ -117,4 +143,66 @@ int main(int argc, char **argv)
                last_max_level = std::max(last_max_level, abs(pcm[i]));
                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 -> 380.0 (change %+.2f%%)\n",
+                       mean_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",
+                                       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);
+       }
+
+       std::vector<char> tap_data;
+       for (unsigned i = 0; i < pulses.size(); ++i) {
+               double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
+               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",
+                                       pulses[i].time, cycles);
+               }
+               if (len <= 255) {
+                       tap_data.push_back(len);
+               } else {
+                       int overflow_len = lrintf(cycles);
+                       tap_data.push_back(0);
+                       tap_data.push_back(overflow_len & 0xff);
+                       tap_data.push_back((overflow_len >> 8) & 0xff);
+                       tap_data.push_back(overflow_len >> 16);
+               }
+       }
+
+       tap_header hdr;
+       memcpy(hdr.identifier, "C64-TAPE-RAW", 12);
+       hdr.version = 1;
+       hdr.reserved[0] = hdr.reserved[1] = hdr.reserved[2] = 0;
+       hdr.data_len = tap_data.size();
+
+       fwrite(&hdr, sizeof(hdr), 1, stdout);
+       fwrite(tap_data.data(), tap_data.size(), 1, stdout);
 }