libs/ardour/session_ltc.cc

   1 /*
   2   Copyright (C) 2012 Paul Davis
   3   Written by Robin Gareus <robin@gareus.org>
   4
   5   This program is free software; you can redistribute it and/or modify
   6   it under the terms of the GNU General Public License as published by
   7   the Free Software Foundation; either version 2 of the License, or
   8   (at your option) any later version.
   9
  10   This program is distributed in the hope that it will be useful,
  11   but WITHOUT ANY WARRANTY; without even the implied warranty of
  12   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13   GNU General Public License for more details.
  14
  15   You should have received a copy of the GNU General Public License
  16   along with this program; if not, write to the Free Software
  17   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  18
  19 */
  20
  21 #include "timecode/time.h"
  22
  23 #include "ardour/audioengine.h"
  24 #include "ardour/audio_port.h"
  25 #include "ardour/debug.h"
  26 #include "ardour/io.h"
  27 #include "ardour/session.h"
  28 #include "ardour/slave.h"
  29
  30 #include "i18n.h"
  31
  32 using namespace std;
  33 using namespace ARDOUR;
  34 using namespace PBD;
  35 using namespace Timecode;
  36
  37 /* really verbose timing debug */
  38 //#define LTC_GEN_FRAMEDBUG
  39 //#define LTC_GEN_TXDBUG
  40
  41 #ifndef MAX
  42 #define MAX(a,b) ( (a) > (b) ? (a) : (b) )
  43 #endif
  44 #ifndef MIN
  45 #define MIN(a,b) ( (a) < (b) ? (a) : (b) )
  46 #endif
  47
  48 /* LTC signal should have a rise time of 25 us +/- 5 us.
  49  * yet with most sound-cards a square-wave of 1-2 sample
  50  * introduces ringing and small oscillations.
  51  * https://en.wikipedia.org/wiki/Gibbs_phenomenon
  52  * A low-pass filter in libltc can reduce this at
  53  * the cost of being slightly out of spec WRT to rise-time.
  54  *
  55  * This filter is adaptive so that fast vari-speed signals
  56  * will not be affected by it.
  57  */
  58 #define LTC_RISE_TIME(speed) MIN (100, MAX(40, (4000000 / ((speed==0)?1:speed) / engine().sample_rate())))
  59
  60 #define TV_STANDARD(tcf) \
  61         (timecode_to_frames_per_second(tcf)==25.0 ? LTC_TV_625_50 : \
  62          timecode_has_drop_frames(tcf)? LTC_TV_525_60 : LTC_TV_FILM_24)
  63
  64 void
  65 Session::ltc_tx_initialize()
  66 {
  67         ltc_enc_tcformat = config.get_timecode_format();
  68
  69         ltc_tx_parse_offset();
  70         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX init sr: %1 fps: %2\n", nominal_frame_rate(), timecode_to_frames_per_second(ltc_enc_tcformat)));
  71         ltc_encoder = ltc_encoder_create(nominal_frame_rate(),
  72                         timecode_to_frames_per_second(ltc_enc_tcformat),
  73                         TV_STANDARD(ltc_enc_tcformat), 0);
  74
  75         ltc_encoder_set_bufsize(ltc_encoder, nominal_frame_rate(), 23.0);
  76         ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(1.0));
  77
  78         /* buffersize for 1 LTC frame: (1 + sample-rate / fps) bytes
  79          * usually returned by ltc_encoder_get_buffersize(encoder)
  80          *
  81          * since the fps can change and A3's  min fps: 24000/1001 */
  82         ltc_enc_buf = (ltcsnd_sample_t*) calloc((nominal_frame_rate() / 23), sizeof(ltcsnd_sample_t));
  83         ltc_speed = 0;
  84         ltc_prev_cycle = -1;
  85         ltc_tx_reset();
  86         ltc_tx_resync_latency();
  87         Xrun.connect_same_thread (*this, boost::bind (&Session::ltc_tx_reset, this));
  88         engine().GraphReordered.connect_same_thread (*this, boost::bind (&Session::ltc_tx_resync_latency, this));
  89         restarting = false;
  90 }
  91
  92 void
  93 Session::ltc_tx_cleanup()
  94 {
  95         DEBUG_TRACE (DEBUG::LTC, "LTC TX cleanup\n");
  96         free(ltc_enc_buf);
  97         ltc_enc_buf = NULL;
  98         ltc_encoder_free(ltc_encoder);
  99         ltc_encoder = NULL;
 100 }
 101
 102 void
 103 Session::ltc_tx_resync_latency()
 104 {
 105         DEBUG_TRACE (DEBUG::LTC, "LTC TX resync latency\n");
 106         if (!deletion_in_progress()) {
 107                 boost::shared_ptr<Port> ltcport = ltc_output_port();
 108                 if (ltcport) {
 109                         ltcport->get_connected_latency_range(ltc_out_latency, true);
 110                 }
 111         }
 112 }
 113
 114 void
 115 Session::ltc_tx_reset()
 116 {
 117         DEBUG_TRACE (DEBUG::LTC, "LTC TX reset\n");
 118         ltc_enc_pos = -9999; // force re-start
 119         ltc_buf_len = 0;
 120         ltc_buf_off = 0;
 121         ltc_enc_byte = 0;
 122         ltc_enc_cnt = 0;
 123
 124         ltc_encoder_reset(ltc_encoder);
 125 }
 126
 127 void
 128 Session::ltc_tx_parse_offset() {
 129         Timecode::Time offset_tc;
 130         Timecode::parse_timecode_format(config.get_timecode_generator_offset(), offset_tc);
 131         offset_tc.rate = timecode_frames_per_second();
 132         offset_tc.drop = timecode_drop_frames();
 133         timecode_to_sample(offset_tc, ltc_timecode_offset, false, false);
 134         ltc_timecode_negative_offset = !offset_tc.negative;
 135         ltc_prev_cycle = -1;
 136 }
 137
 138 void
 139 Session::ltc_tx_recalculate_position()
 140 {
 141         SMPTETimecode enctc;
 142         Timecode::Time a3tc;
 143         ltc_encoder_get_timecode(ltc_encoder, &enctc);
 144
 145         a3tc.hours   = enctc.hours;
 146         a3tc.minutes = enctc.mins;
 147         a3tc.seconds = enctc.secs;
 148         a3tc.frames  = enctc.frame;
 149         a3tc.rate = timecode_to_frames_per_second(ltc_enc_tcformat);
 150         a3tc.drop = timecode_has_drop_frames(ltc_enc_tcformat);
 151
 152         Timecode::timecode_to_sample (a3tc, ltc_enc_pos, true, false,
 153                 (double)frame_rate(),
 154                 config.get_subframes_per_frame(),
 155                 ltc_timecode_negative_offset, ltc_timecode_offset
 156                 );
 157         restarting = false;
 158 }
 159
 160 void
 161 Session::ltc_tx_send_time_code_for_cycle (framepos_t start_frame, framepos_t end_frame,
 162                                           double target_speed, double current_speed,
 163                                           pframes_t nframes)
 164 {
 165         assert (nframes > 0);
 166
 167         Sample *out;
 168         pframes_t txf = 0;
 169         boost::shared_ptr<Port> ltcport = ltc_output_port();
 170
 171         Buffer& buf (ltcport->get_buffer (nframes));
 172
 173         if (!ltc_encoder || !ltc_enc_buf) {
 174                 return;
 175         }
 176
 177         SyncSource sync_src = Config->get_sync_source();
 178         if (engine().freewheeling() || !Config->get_send_ltc()
 179             /* TODO
 180              * decide which time-sources we can generated LTC from.
 181              * Internal, JACK or sample-synced slaves should be fine.
 182              * talk to oofus.
 183              *
 184              || (config.get_external_sync() && sync_src == LTC)
 185              || (config.get_external_sync() && sync_src == MTC)
 186             */
 187              ||(config.get_external_sync() && sync_src == MIDIClock)
 188                 ) {
 189                 return;
 190         }
 191
 192         out = dynamic_cast<AudioBuffer*>(&buf)->data ();
 193
 194         /* range from libltc (38..218) || - 128.0  -> (-90..90) */
 195         const float ltcvol = Config->get_ltc_output_volume()/(90.0); // pow(10, db/20.0)/(90.0);
 196
 197         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX %1 to %2 / %3 | lat: %4\n", start_frame, end_frame, nframes, ltc_out_latency.max));
 198
 199         /* all systems go. Now here's the plan:
 200          *
 201          *  1) check if fps has changed
 202          *  2) check direction of encoding, calc speed, re-sample existing buffer
 203          *  3) calculate frame and byte to send aligned to jack-period size
 204          *  4) check if it's the frame/byte that is already in the queue
 205          *  5) if (4) mismatch, re-calculate offset of LTC frame relative to period size
 206          *  6) actual LTC audio output
 207          *  6a) send remaining part of already queued frame; break on nframes
 208          *  6b) encode new LTC-frame byte
 209          *  6c) goto 6a
 210          *  7) done
 211          */
 212
 213         // (1) check fps
 214         TimecodeFormat cur_timecode = config.get_timecode_format();
 215         if (cur_timecode != ltc_enc_tcformat) {
 216                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX1: TC format mismatch - reinit sr: %1 fps: %2\n", nominal_frame_rate(), timecode_to_frames_per_second(cur_timecode)));
 217                 if (ltc_encoder_reinit(ltc_encoder, nominal_frame_rate(),
 218                                         timecode_to_frames_per_second(cur_timecode),
 219                                         TV_STANDARD(cur_timecode), 0
 220                                         )) {
 221                         PBD::error << _("LTC encoder: invalid framerate - LTC encoding is disabled for the remainder of this session.") << endmsg;
 222                         ltc_tx_cleanup();
 223                         return;
 224                 }
 225                 ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(ltc_speed));
 226                 ltc_enc_tcformat = cur_timecode;
 227                 ltc_tx_parse_offset();
 228                 ltc_tx_reset();
 229         }
 230
 231         /* LTC is max. 30 fps */
 232         if (timecode_to_frames_per_second(cur_timecode) > 30) {
 233                 return;
 234         }
 235
 236         // (2) speed & direction
 237
 238         /* speed 0 aka transport stopped is interpreted as rolling forward.
 239          * keep repeating current frame
 240          */
 241 #define SIGNUM(a) ( (a) < 0 ? -1 : 1)
 242         bool speed_changed = false;
 243
 244         /* port latency compensation:
 245          * The _generated timecode_ is offset by the port-latency,
 246          * therefore the offset depends on the direction of transport.
 247          */
 248         framepos_t cycle_start_frame;
 249
 250         if (current_speed < 0) {
 251                 cycle_start_frame = (start_frame - ltc_out_latency.max);
 252         } else if (current_speed > 0) {
 253                 cycle_start_frame = (start_frame + ltc_out_latency.max);
 254         } else {
 255                 /* There is no need to compensate for latency when not rolling
 256                  * rather send the accurate NOW timecode
 257                  * (LTC encoder compenates latency by sending earlier timecode)
 258                  */
 259                 cycle_start_frame = start_frame;
 260         }
 261
 262         /* LTC TV standard offset */
 263         if (current_speed != 0) {
 264                 /* ditto - send "NOW" if not rolling */
 265                 cycle_start_frame -= ltc_frame_alignment(frames_per_timecode_frame(), TV_STANDARD(cur_timecode));
 266         }
 267
 268         /* cycle-start may become negative due to latency compensation */
 269         if (cycle_start_frame < 0) { cycle_start_frame = 0; }
 270
 271         double new_ltc_speed = (double)(labs(end_frame - start_frame) * SIGNUM(current_speed)) / (double)nframes;
 272         if (nominal_frame_rate() != frame_rate()) {
 273                 new_ltc_speed *= (double)nominal_frame_rate() / (double)frame_rate();
 274         }
 275
 276         if (SIGNUM(new_ltc_speed) != SIGNUM (ltc_speed)) {
 277                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport changed direction\n");
 278                 ltc_tx_reset();
 279         }
 280
 281         if (ltc_speed != new_ltc_speed
 282                         /* but only once if, current_speed changes to 0. In that case
 283                          * new_ltc_speed is > 0 because (end_frame - start_frame) == jack-period for no-roll
 284                          * but ltc_speed will still be 0
 285                          */
 286                         && (current_speed != 0 || ltc_speed != current_speed)
 287                         ) {
 288                 /* check ./libs/ardour/interpolation.cc  CubicInterpolation::interpolate
 289                  * if target_speed != current_speed we should interpolate, too.
 290                  *
 291                  * However, currency in A3 target_speed == current_speed for each process cycle
 292                  * (except for the sign and if target_speed > 8.0).
 293                  * Besides, above speed calculation uses the difference (end_frame - start_frame).
 294                  * end_frame is calculated from 'frames_moved' which includes the interpolation.
 295                  * so we're good.
 296                  */
 297                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: speed change old: %1 cur: %2 tgt: %3 ctd: %4\n", ltc_speed, current_speed, target_speed, fabs(current_speed) - target_speed, new_ltc_speed));
 298                 speed_changed = true;
 299                 ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(new_ltc_speed));
 300         }
 301
 302         if (end_frame == start_frame || fabs(current_speed) < 0.1 ) {
 303                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport is not rolling or absolute-speed < 0.1\n");
 304                 /* keep repeating current frame
 305                  *
 306                  * an LTC generator must be able to continue generating LTC when Ardours transport is in stop
 307                  * some machines do odd things if LTC goes away:
 308                  * e.g. a tape based machine (video or audio), some think they have gone into park if LTC goes away,
 309                  * so unspool the tape from the playhead. That might be inconvenient.
 310                  * If LTC keeps arriving they remain in a stop position with the tape on the playhead.
 311                  */
 312                 new_ltc_speed = 0;
 313                 if (!Config->get_ltc_send_continuously()) {
 314                         ltc_speed = new_ltc_speed;
 315                         return;
 316                 }
 317                 if (start_frame != ltc_prev_cycle) {
 318                         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: no-roll seek from %1 to %2 (%3)\n", ltc_prev_cycle, start_frame, cycle_start_frame));
 319                         ltc_tx_reset();
 320                 }
 321         }
 322
 323         if (fabs(new_ltc_speed) > 10.0) {
 324                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: speed is out of bounds.\n");
 325                 ltc_tx_reset();
 326                 return;
 327         }
 328
 329         if (ltc_speed == 0 && new_ltc_speed != 0) {
 330                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport started rolling - reset\n");
 331                 ltc_tx_reset();
 332         }
 333
 334         /* the timecode duration corresponding to the samples that are still
 335          * in the buffer. Here, the speed of previous cycle is used to calculate
 336          * the alignment at the beginning of this cycle later.
 337          */
 338         double poff = (ltc_buf_len - ltc_buf_off) * ltc_speed;
 339
 340         if (speed_changed && new_ltc_speed != 0) {
 341                 /* we need to re-sample the existing buffer.
 342                  * "make space for the en-coder to catch up to the new speed"
 343                  *
 344                  * since the LTC signal is a rectangular waveform we can simply squeeze it
 345                  * by removing samples or duplicating samples /here and there/.
 346                  *
 347                  * There may be a more elegant way to do this, in fact one could
 348                  * simply re-render the buffer using ltc_encoder_encode_byte()
 349                  * but that'd require some timecode offset buffer magic,
 350                  * which is left for later..
 351                  */
 352
 353                 double oldbuflen = (double)(ltc_buf_len - ltc_buf_off);
 354                 double newbuflen = (double)(ltc_buf_len - ltc_buf_off) * fabs(ltc_speed / new_ltc_speed);
 355
 356                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: bufOld %1 bufNew %2 | diff %3\n",
 357                                         (ltc_buf_len - ltc_buf_off), newbuflen, newbuflen - oldbuflen
 358                                         ));
 359
 360                 double bufrspdiff = rint(newbuflen - oldbuflen);
 361
 362                 if (abs(bufrspdiff) > newbuflen || abs(bufrspdiff) > oldbuflen) {
 363                         DEBUG_TRACE (DEBUG::LTC, "LTC TX2: resampling buffer would destroy information.\n");
 364                         ltc_tx_reset();
 365                         poff = 0;
 366                 } else if (bufrspdiff != 0 && newbuflen > oldbuflen) {
 367                         int incnt = 0;
 368                         double samples_to_insert = ceil(newbuflen - oldbuflen);
 369                         double avg_distance = newbuflen / samples_to_insert;
 370                         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: resample buffer insert: %1\n", samples_to_insert));
 371
 372                         for (int rp = ltc_buf_off; rp < ltc_buf_len - 1; ++rp) {
 373                                 const int ro = rp - ltc_buf_off;
 374                                 if (ro < (incnt*avg_distance)) continue;
 375                                 const ltcsnd_sample_t v1 = ltc_enc_buf[rp];
 376                                 const ltcsnd_sample_t v2 = ltc_enc_buf[rp+1];
 377                                 if (v1 != v2 && ro < ((incnt+1)*avg_distance)) continue;
 378                                 memmove(&ltc_enc_buf[rp+1], &ltc_enc_buf[rp], ltc_buf_len-rp);
 379                                 incnt++;
 380                                 ltc_buf_len++;
 381                         }
 382                 } else if (bufrspdiff != 0 && newbuflen < oldbuflen) {
 383                         double samples_to_remove = ceil(oldbuflen - newbuflen);
 384                         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: resample buffer - remove: %1\n", samples_to_remove));
 385                         if (oldbuflen <= samples_to_remove) {
 386                                 ltc_buf_off = ltc_buf_len= 0;
 387                         } else {
 388                                 double avg_distance = newbuflen / samples_to_remove;
 389                                 int rmcnt = 0;
 390                                 for (int rp = ltc_buf_off; rp < ltc_buf_len - 1; ++rp) {
 391                                         const int ro = rp - ltc_buf_off;
 392                                         if (ro < (rmcnt*avg_distance)) continue;
 393                                         const ltcsnd_sample_t v1 = ltc_enc_buf[rp];
 394                                         const ltcsnd_sample_t v2 = ltc_enc_buf[rp+1];
 395                                         if (v1 != v2 && ro < ((rmcnt+1)*avg_distance)) continue;
 396                                         memmove(&ltc_enc_buf[rp], &ltc_enc_buf[rp+1], ltc_buf_len-rp-1);
 397                                         ltc_buf_len--;
 398                                         rmcnt++;
 399                                 }
 400                         }
 401                 }
 402         }
 403
 404         ltc_prev_cycle = start_frame;
 405         ltc_speed = new_ltc_speed;
 406         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: transport speed %1.\n", ltc_speed));
 407
 408         // (3) bit/sample alignment
 409         Timecode::Time tc_start;
 410         framepos_t tc_sample_start;
 411
 412         /* calc timecode frame from current position - round down to nearest timecode */
 413         Timecode::sample_to_timecode(cycle_start_frame, tc_start, true, false,
 414                         timecode_frames_per_second(),
 415                         timecode_drop_frames(),
 416                         (double)frame_rate(),
 417                         config.get_subframes_per_frame(),
 418                         ltc_timecode_negative_offset, ltc_timecode_offset
 419                         );
 420
 421         /* convert timecode back to sample-position */
 422         Timecode::timecode_to_sample (tc_start, tc_sample_start, true, false,
 423                 (double)frame_rate(),
 424                 config.get_subframes_per_frame(),
 425                 ltc_timecode_negative_offset, ltc_timecode_offset
 426                 );
 427
 428         /* difference between current frame and TC frame in samples */
 429         frameoffset_t soff = cycle_start_frame - tc_sample_start;
 430         if (current_speed == 0) {
 431                 soff = 0;
 432         }
 433         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX3: A3cycle: %1 = A3tc: %2 +off: %3\n",
 434                                 cycle_start_frame, tc_sample_start, soff));
 435
 436
 437         // (4) check if alignment matches
 438         const double fptcf = frames_per_timecode_frame();
 439
 440         /* maximum difference of bit alignment in audio-samples.
 441          *
 442          * if transport and LTC generator differs more than this, the LTC
 443          * generator will be re-initialized
 444          *
 445          * due to rounding error and variations in LTC-bit duration depending
 446          * on the speed, it can be off by +- ltc_speed audio-samples.
 447          * When the playback speed changes, it can actually reach +- 2 * ltc_speed
 448          * in the cycle _after_ the speed changed. The average delta however is 0.
 449          */
 450         double maxdiff;
 451
 452         if (config.get_external_sync() && slave()) {
 453                 maxdiff = slave()->resolution();
 454         } else {
 455                 maxdiff = ceil(fabs(ltc_speed))*2.0;
 456                 if (nominal_frame_rate() != frame_rate()) {
 457                         maxdiff *= 3.0;
 458                 }
 459                 if (ltc_enc_tcformat == Timecode::timecode_23976 || ltc_enc_tcformat == Timecode::timecode_24976) {
 460                         maxdiff *= 15.0;
 461                 }
 462         }
 463
 464         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: enc: %1 + %2 - %3 || buf-bytes: %4 enc-byte: %5\n",
 465                                 ltc_enc_pos, ltc_enc_cnt, poff, (ltc_buf_len - ltc_buf_off), poff, ltc_enc_byte));
 466
 467         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: enc-pos: %1  | d: %2\n",
 468                                 ltc_enc_pos + ltc_enc_cnt - poff,
 469                                 rint(ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame
 470                                 ));
 471
 472         if (ltc_enc_pos < 0
 473                         || (ltc_speed != 0 && fabs(ceil(ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame) > maxdiff)
 474                         ) {
 475
 476                 // (5) re-align
 477                 ltc_tx_reset();
 478
 479                 /* set frame to encode */
 480                 SMPTETimecode tc;
 481                 tc.hours = tc_start.hours;
 482                 tc.mins = tc_start.minutes;
 483                 tc.secs = tc_start.seconds;
 484                 tc.frame = tc_start.frames;
 485                 ltc_encoder_set_timecode(ltc_encoder, &tc);
 486
 487                 /* workaround for libltc recognizing 29.97 and 30000/1001 as drop-frame TC.
 488                  * In A3 30000/1001 or 30 fps can be drop-frame.
 489                  */
 490                 LTCFrame ltcframe;
 491                 ltc_encoder_get_frame(ltc_encoder, &ltcframe);
 492                 ltcframe.dfbit = timecode_has_drop_frames(cur_timecode)?1:0;
 493                 ltc_encoder_set_frame(ltc_encoder, &ltcframe);
 494
 495
 496                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: now: %1 trs: %2 toff %3\n", cycle_start_frame, tc_sample_start, soff));
 497
 498                 int32_t cyc_off;
 499                 if (soff < 0 || soff >= fptcf) {
 500                         /* session framerate change between (2) and now */
 501                         ltc_tx_reset();
 502                         return;
 503                 }
 504
 505                 if (ltc_speed < 0 ) {
 506                         /* calculate the byte that starts at or after the current position */
 507                         ltc_enc_byte = floor((10.0 * soff) / (fptcf));
 508                         ltc_enc_cnt = ltc_enc_byte * fptcf / 10.0;
 509
 510                         /* calculate difference between the current position and the byte to send */
 511                         cyc_off = soff- ceil(ltc_enc_cnt);
 512
 513                 } else {
 514                         /* calculate the byte that starts at or after the current position */
 515                         ltc_enc_byte = ceil((10.0 * soff) / fptcf);
 516                         ltc_enc_cnt = ltc_enc_byte * fptcf / 10.0;
 517
 518                         /* calculate difference between the current position and the byte to send */
 519                         cyc_off = ceil(ltc_enc_cnt) - soff;
 520
 521                         if (ltc_enc_byte == 10) {
 522                                 ltc_enc_byte = 0;
 523                                 ltc_encoder_inc_timecode(ltc_encoder);
 524                         }
 525                 }
 526
 527                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX5 restart encoder: soff %1 byte %2 cycoff %3\n",
 528                                         soff, ltc_enc_byte, cyc_off));
 529
 530                 if ( (ltc_speed < 0 && ltc_enc_byte !=9 ) || (ltc_speed >= 0 && ltc_enc_byte !=0 ) ) {
 531                         restarting = true;
 532                 }
 533
 534                 if (cyc_off >= 0 && cyc_off <= (int32_t) nframes) {
 535                         /* offset in this cycle */
 536                         txf= rint(cyc_off / fabs(ltc_speed));
 537                         memset(out, 0, cyc_off * sizeof(Sample));
 538                 } else {
 539                         /* resync next cycle */
 540                         memset(out, 0, nframes * sizeof(Sample));
 541                         return;
 542                 }
 543
 544                 ltc_enc_pos = tc_sample_start;
 545
 546                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX5 restart @ %1 + %2 - %3 |  byte %4\n",
 547                                         ltc_enc_pos, ltc_enc_cnt, cyc_off, ltc_enc_byte));
 548         }
 549         else if (ltc_speed != 0 && (fptcf / ltc_speed / 80) > 3 ) {
 550                 /* reduce (low freq) jitter.
 551                  * The granularity of the LTC encoder speed is 1 byte =
 552                  * (frames-per-timecode-frame / 10) audio-samples.
 553                  * Thus, tiny speed changes [as produced by some slaves]
 554                  * may not have any effect in the cycle when they occur,
 555                  * but they will add up over time.
 556                  *
 557                  * This is a linear approx to compensate for this jitter
 558                  * and prempt re-sync when the drift builds up.
 559                  *
 560                  * However, for very fast speeds - when 1 LTC bit is
 561                  * <= 3 audio-sample - adjusting speed may lead to
 562                  * invalid frames.
 563                  *
 564                  * To do better than this, resampling (or a rewrite of the
 565                  * encoder) is required.
 566                  */
 567                 ltc_speed -= ((ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame) / engine().sample_rate();
 568         }
 569
 570
 571         // (6) encode and output
 572         while (1) {
 573 #ifdef LTC_GEN_TXDBUG
 574                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.1 @%1  [ %2 / %3 ]\n", txf, ltc_buf_off, ltc_buf_len));
 575 #endif
 576                 // (6a) send remaining buffer
 577                 while ((ltc_buf_off < ltc_buf_len) && (txf < nframes)) {
 578                         const float v1 = ltc_enc_buf[ltc_buf_off++] - 128.0;
 579                         const Sample val = (Sample) (v1*ltcvol);
 580                         out[txf++] = val;
 581                 }
 582 #ifdef LTC_GEN_TXDBUG
 583                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.2 @%1  [ %2 / %3 ]\n", txf, ltc_buf_off, ltc_buf_len));
 584 #endif
 585
 586                 if (txf >= nframes) {
 587                         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX7 enc: %1 [ %2 / %3 ] byte: %4 spd %5 fpp %6 || nf: %7\n",
 588                                                 ltc_enc_pos, ltc_buf_off, ltc_buf_len, ltc_enc_byte, ltc_speed, nframes, txf));
 589                         break;
 590                 }
 591
 592                 ltc_buf_len = 0;
 593                 ltc_buf_off = 0;
 594
 595                 // (6b) encode LTC, bump timecode
 596
 597                 if (ltc_speed < 0) {
 598                         ltc_enc_byte = (ltc_enc_byte + 9)%10;
 599                         if (ltc_enc_byte == 9) {
 600                                 ltc_encoder_dec_timecode(ltc_encoder);
 601                                 ltc_tx_recalculate_position();
 602                                 ltc_enc_cnt = fptcf;
 603                         }
 604                 }
 605
 606                 int enc_frames;
 607
 608                 if (restarting) {
 609                         /* write zero bytes -- don't touch encoder until we're at a frame-boundary
 610                          * otherwise the biphase polarity may be inverted.
 611                          */
 612                         enc_frames = fptcf / 10.0;
 613                         memset(&ltc_enc_buf[ltc_buf_len], 127, enc_frames * sizeof(ltcsnd_sample_t));
 614                 } else {
 615                         if (ltc_encoder_encode_byte(ltc_encoder, ltc_enc_byte, (ltc_speed==0)?1.0:(1.0/ltc_speed))) {
 616                                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.3 encoder error byte %1\n", ltc_enc_byte));
 617                                 ltc_encoder_buffer_flush(ltc_encoder);
 618                                 ltc_tx_reset();
 619                                 return;
 620                         }
 621                         enc_frames = ltc_encoder_get_buffer(ltc_encoder, &(ltc_enc_buf[ltc_buf_len]));
 622                 }
 623
 624 #ifdef LTC_GEN_FRAMEDBUG
 625                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.3 encoded %1 bytes for LTC-byte %2 at spd %3\n", enc_frames, ltc_enc_byte, ltc_speed));
 626 #endif
 627                 if (enc_frames <=0) {
 628                         DEBUG_TRACE (DEBUG::LTC, "LTC TX6.3 encoder empty buffer.\n");
 629                         ltc_encoder_buffer_flush(ltc_encoder);
 630                         ltc_tx_reset();
 631                         return;
 632                 }
 633
 634                 ltc_buf_len += enc_frames;
 635                 if (ltc_speed < 0)
 636                         ltc_enc_cnt -= fptcf/10.0;
 637                 else
 638                         ltc_enc_cnt += fptcf/10.0;
 639
 640                 if (ltc_speed >= 0) {
 641                         ltc_enc_byte = (ltc_enc_byte + 1)%10;
 642                         if (ltc_enc_byte == 0 && ltc_speed != 0) {
 643                                 ltc_encoder_inc_timecode(ltc_encoder);
 644 #if 0 /* force fixed parity -- scope debug */
 645                                 LTCFrame f;
 646                                 ltc_encoder_get_frame(ltc_encoder, &f);
 647                                 f.biphase_mark_phase_correction=0;
 648                                 ltc_encoder_set_frame(ltc_encoder, &f);
 649 #endif
 650                                 ltc_tx_recalculate_position();
 651                                 ltc_enc_cnt = 0;
 652                         } else if (ltc_enc_byte == 0) {
 653                                 ltc_enc_cnt = 0;
 654                                 restarting=false;
 655                         }
 656                 }
 657 #ifdef LTC_GEN_FRAMEDBUG
 658                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.4 enc-pos: %1 + %2 [ %4 / %5 ] spd %6\n", ltc_enc_pos, ltc_enc_cnt, ltc_buf_off, ltc_buf_len, ltc_speed));
 659 #endif
 660         }
 661
 662         dynamic_cast<AudioBuffer*>(&buf)->set_written (true);
 663         return;
 664 }