git.sesse.net Git - c64tapwav/blob - decode.cpp

   1 #include <stdio.h>
   2 #include <string.h>
   3 #include <math.h>
   4 #include <unistd.h>
   5 #include <assert.h>
   6 #include <vector>
   7 #include <algorithm>
   8
   9 #include "interpolate.h"
  10
  11 #define BUFSIZE 4096
  12 #define HYSTERESIS_LIMIT 3000
  13 #define SAMPLE_RATE 44100
  14 #define C64_FREQUENCY 985248
  15 #define TAP_RESOLUTION 8
  16
  17 #define SYNC_PULSE_START 1000
  18 #define SYNC_PULSE_END 15000
  19 #define SYNC_PULSE_LENGTH 378.0
  20 #define SYNC_TEST_TOLERANCE 1.10
  21
  22 struct tap_header {
  23         char identifier[12];
  24         char version;
  25         char reserved[3];
  26         unsigned int data_len;
  27 };
  28
  29 // between [x,x+1]
  30 double find_zerocrossing(const std::vector<short> &pcm, int x)
  31 {
  32         if (pcm[x] == 0) {
  33                 return x;
  34         }
  35         if (pcm[x + 1] == 0) {
  36                 return x + 1;
  37         }
  38
  39         assert(pcm[x + 1] > 0);
  40         assert(pcm[x] < 0);
  41
  42         double lower = x;
  43         double upper = x + 1;
  44         while (upper - lower > 1e-6) {
  45                 double mid = 0.5f * (upper + lower);
  46                 if (lanczos_interpolate(pcm, mid) > 0) {
  47                         upper = mid;
  48                 } else {
  49                         lower = mid;
  50                 }
  51         }
  52
  53         return 0.5f * (upper + lower);
  54 }
  55
  56 struct pulse {
  57         double time;  // in seconds from start
  58         double len;   // in seconds
  59 };
  60
  61 int main(int argc, char **argv)
  62 {
  63         std::vector<short> pcm;
  64
  65         while (!feof(stdin)) {
  66                 short buf[BUFSIZE];
  67                 ssize_t ret = fread(buf, 2, BUFSIZE, stdin);
  68                 if (ret >= 0) {
  69                         pcm.insert(pcm.end(), buf, buf + ret);
  70                 }
  71         }
  72
  73 #if 0
  74         for (int i = 0; i < LEN; ++i) {
  75                 in[i] += rand() % 10000;
  76         }
  77 #endif
  78
  79 #if 0
  80         for (int i = 0; i < LEN; ++i) {
  81                 printf("%d\n", in[i]);
  82         }
  83 #endif
  84
  85         std::vector<pulse> pulses;  // in seconds
  86
  87         // Find the flanks.
  88         int last_bit = -1;
  89         double last_upflank = -1;
  90         for (unsigned i = 0; i < pcm.size(); ++i) {
  91                 int bit = (pcm[i] > 0) ? 1 : 0;
  92                 if (bit == 1 && last_bit == 0) {
  93                         // Check if we ever go up above HYSTERESIS_LIMIT before we dip down again.
  94                         bool true_pulse = false;
  95                         unsigned j;
  96                         int max_level_after = -32768;
  97                         for (j = i; j < pcm.size(); ++j) {
  98                                 max_level_after = std::max<int>(max_level_after, pcm[j]);
  99                                 if (pcm[j] < 0) break;
 100                                 if (pcm[j] > HYSTERESIS_LIMIT) {
 101                                         true_pulse = true;
 102                                         break;
 103                                 }
 104                         }
 105
 106                         if (!true_pulse) {
 107 #if 0
 108                                 fprintf(stderr, "Ignored up-flank at %.6f seconds due to hysteresis (%d < %d).\n",
 109                                         double(i) / SAMPLE_RATE, max_level_after, HYSTERESIS_LIMIT);
 110 #endif
 111                                 i = j;
 112                                 continue;
 113                         }
 114
 115                         // up-flank!
 116                         double t = find_zerocrossing(pcm, i - 1) * (1.0 / SAMPLE_RATE);
 117                         if (last_upflank > 0) {
 118                                 pulse p;
 119                                 p.time = t;
 120                                 p.len = t - last_upflank;
 121                                 pulses.push_back(p);
 122                         }
 123                         last_upflank = t;
 124                 }
 125                 last_bit = bit;
 126         }
 127
 128         // Calibrate on the first ~25k pulses (skip a few, just to be sure).
 129         double calibration_factor = 1.0f;
 130         if (pulses.size() < SYNC_PULSE_END) {
 131                 fprintf(stderr, "Too few pulses, not calibrating!\n");
 132         } else {
 133                 double sum = 0.0;
 134                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 135                         sum += pulses[i].len;
 136                 }
 137                 double mean_length = C64_FREQUENCY * sum / (SYNC_PULSE_END - SYNC_PULSE_START);
 138                 calibration_factor = SYNC_PULSE_LENGTH / mean_length;
 139                 fprintf(stderr, "Calibrated sync pulse length: %.2f -> %.2f (change %+.2f%%)\n",
 140                         mean_length, SYNC_PULSE_LENGTH, 100.0 * (calibration_factor - 1.0));
 141
 142                 // Check for pulses outside +/- 10% (sign of misdetection).
 143                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 144                         double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 145                         if (cycles < SYNC_PULSE_LENGTH / SYNC_TEST_TOLERANCE || cycles > SYNC_PULSE_LENGTH * SYNC_TEST_TOLERANCE) {
 146                                 fprintf(stderr, "Sync cycle with upflank at %.6f was detected at %.0f cycles; misdetect?\n",
 147                                         pulses[i].time, cycles);
 148                         }
 149                 }
 150
 151                 // Compute the standard deviation (to check for uneven speeds).
 152                 double sum2 = 0.0;
 153                 for (int i = SYNC_PULSE_START; i < SYNC_PULSE_END; ++i) {
 154                         double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 155                         sum2 += (cycles - SYNC_PULSE_LENGTH) * (cycles - SYNC_PULSE_LENGTH);
 156                 }
 157                 double stddev = sqrt(sum2 / (SYNC_PULSE_END - SYNC_PULSE_START - 1));
 158                 fprintf(stderr, "Sync pulse length standard deviation: %.2f cycles\n",
 159                         stddev);
 160         }
 161
 162         FILE *fp = fopen("cycles.plot", "w");
 163         std::vector<char> tap_data;
 164         for (unsigned i = 0; i < pulses.size(); ++i) {
 165                 double cycles = pulses[i].len * calibration_factor * C64_FREQUENCY;
 166                 fprintf(fp, "%f %f\n", pulses[i].time, cycles);
 167                 int len = lrintf(cycles / TAP_RESOLUTION);
 168                 if (i > SYNC_PULSE_END && (cycles < 100 || cycles > 800)) {
 169                         fprintf(stderr, "Cycle with upflank at %.6f was detected at %.0f cycles; misdetect?\n",
 170                                         pulses[i].time, cycles);
 171                 }
 172                 if (len <= 255) {
 173                         tap_data.push_back(len);
 174                 } else {
 175                         int overflow_len = lrintf(cycles);
 176                         tap_data.push_back(0);
 177                         tap_data.push_back(overflow_len & 0xff);
 178                         tap_data.push_back((overflow_len >> 8) & 0xff);
 179                         tap_data.push_back(overflow_len >> 16);
 180                 }
 181         }
 182         fclose(fp);
 183
 184         tap_header hdr;
 185         memcpy(hdr.identifier, "C64-TAPE-RAW", 12);
 186         hdr.version = 1;
 187         hdr.reserved[0] = hdr.reserved[1] = hdr.reserved[2] = 0;
 188         hdr.data_len = tap_data.size();
 189
 190         fwrite(&hdr, sizeof(hdr), 1, stdout);
 191         fwrite(tap_data.data(), tap_data.size(), 1, stdout);
 192 }