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().frame_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_tx_reset();
  85         ltc_tx_resync_latency();
  86         Xrun.connect_same_thread (*this, boost::bind (&Session::ltc_tx_reset, this));
  87         engine().GraphReordered.connect_same_thread (*this, boost::bind (&Session::ltc_tx_resync_latency, this));
  88         restarting = false;
  89 }
  90
  91 void
  92 Session::ltc_tx_cleanup()
  93 {
  94         DEBUG_TRACE (DEBUG::LTC, "LTC TX cleanup\n");
  95         free(ltc_enc_buf);
  96         ltc_enc_buf = NULL;
  97         ltc_encoder_free(ltc_encoder);
  98         ltc_encoder = NULL;
  99 }
 100
 101 void
 102 Session::ltc_tx_resync_latency()
 103 {
 104         DEBUG_TRACE (DEBUG::LTC, "LTC TX resync latency\n");
 105         if (!deletion_in_progress()) {
 106                 boost::shared_ptr<Port> ltcport = ltc_output_port();
 107                 if (ltcport) {
 108                         ltcport->get_connected_latency_range(ltc_out_latency, true);
 109                 }
 110         }
 111 }
 112
 113 void
 114 Session::ltc_tx_reset()
 115 {
 116         DEBUG_TRACE (DEBUG::LTC, "LTC TX reset\n");
 117         ltc_enc_pos = -9999; // force re-start
 118         ltc_buf_len = 0;
 119         ltc_buf_off = 0;
 120         ltc_enc_byte = 0;
 121         ltc_enc_cnt = 0;
 122
 123         ltc_encoder_reset(ltc_encoder);
 124 }
 125
 126 void
 127 Session::ltc_tx_parse_offset() {
 128         Timecode::Time offset_tc;
 129         Timecode::parse_timecode_format(config.get_timecode_generator_offset(), offset_tc);
 130         offset_tc.rate = timecode_frames_per_second();
 131         offset_tc.drop = timecode_drop_frames();
 132         timecode_to_sample(offset_tc, ltc_timecode_offset, false, false);
 133         ltc_timecode_negative_offset = !offset_tc.negative;
 134 }
 135
 136 void
 137 Session::ltc_tx_recalculate_position()
 138 {
 139         SMPTETimecode enctc;
 140         Timecode::Time a3tc;
 141         ltc_encoder_get_timecode(ltc_encoder, &enctc);
 142
 143         a3tc.hours   = enctc.hours;
 144         a3tc.minutes = enctc.mins;
 145         a3tc.seconds = enctc.secs;
 146         a3tc.frames  = enctc.frame;
 147         a3tc.rate = timecode_to_frames_per_second(ltc_enc_tcformat);
 148         a3tc.drop = timecode_has_drop_frames(ltc_enc_tcformat);
 149
 150         Timecode::timecode_to_sample (a3tc, ltc_enc_pos, true, false,
 151                 (double)frame_rate(),
 152                 config.get_subframes_per_frame(),
 153                 ltc_timecode_negative_offset, ltc_timecode_offset
 154                 );
 155         restarting = false;
 156 }
 157
 158 void
 159 Session::ltc_tx_send_time_code_for_cycle (framepos_t start_frame, framepos_t end_frame,
 160                                           double target_speed, double current_speed,
 161                                           pframes_t nframes)
 162 {
 163         assert (nframes > 0);
 164
 165         Sample *out;
 166         pframes_t txf = 0;
 167         boost::shared_ptr<Port> ltcport = ltc_output_port();
 168
 169         Buffer& buf (ltcport->get_buffer (nframes));
 170
 171         if (!ltc_encoder || !ltc_enc_buf) {
 172                 return;
 173         }
 174
 175         SyncSource sync_src = Config->get_sync_source();
 176         if (engine().freewheeling() || !Config->get_send_ltc()
 177             /* TODO
 178              * decide which time-sources we can generated LTC from.
 179              * Internal, JACK or sample-synced slaves should be fine.
 180              * talk to oofus.
 181              *
 182              || (config.get_external_sync() && sync_src == LTC)
 183              || (config.get_external_sync() && sync_src == MTC)
 184             */
 185              ||(config.get_external_sync() && sync_src == MIDIClock)
 186                 ) {
 187                 return;
 188         }
 189
 190         out = dynamic_cast<AudioBuffer*>(&buf)->data ();
 191
 192         /* range from libltc (38..218) || - 128.0  -> (-90..90) */
 193         const float ltcvol = Config->get_ltc_output_volume()/(90.0); // pow(10, db/20.0)/(90.0);
 194
 195         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX %1 to %2 / %3 | lat: %4\n", start_frame, end_frame, nframes, ltc_out_latency.max));
 196
 197         /* all systems go. Now here's the plan:
 198          *
 199          *  1) check if fps has changed
 200          *  2) check direction of encoding, calc speed, re-sample existing buffer
 201          *  3) calculate frame and byte to send aligned to jack-period size
 202          *  4) check if it's the frame/byte that is already in the queue
 203          *  5) if (4) mismatch, re-calculate offset of LTC frame relative to period size
 204          *  6) actual LTC audio output
 205          *  6a) send remaining part of already queued frame; break on nframes
 206          *  6b) encode new LTC-frame byte
 207          *  6c) goto 6a
 208          *  7) done
 209          */
 210
 211         // (1) check fps
 212         TimecodeFormat cur_timecode = config.get_timecode_format();
 213         if (cur_timecode != ltc_enc_tcformat) {
 214                 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)));
 215                 if (ltc_encoder_reinit(ltc_encoder, nominal_frame_rate(),
 216                                         timecode_to_frames_per_second(cur_timecode),
 217                                         TV_STANDARD(cur_timecode), 0
 218                                         )) {
 219                         PBD::error << _("LTC encoder: invalid framerate - LTC encoding is disabled for the remainder of this session.") << endmsg;
 220                         ltc_tx_cleanup();
 221                         return;
 222                 }
 223                 ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(ltc_speed));
 224                 ltc_enc_tcformat = cur_timecode;
 225                 ltc_tx_parse_offset();
 226                 ltc_tx_reset();
 227         }
 228
 229         /* LTC is max. 30 fps */
 230         if (timecode_to_frames_per_second(cur_timecode) > 30) {
 231                 return;
 232         }
 233
 234         // (2) speed & direction
 235
 236         /* speed 0 aka transport stopped is interpreted as rolling forward.
 237          * keep repeating current frame
 238          */
 239 #define SIGNUM(a) ( (a) < 0 ? -1 : 1)
 240         bool speed_changed = false;
 241
 242         /* port latency compensation:
 243          * The _generated timecode_ is offset by the port-latency,
 244          * therefore the offset depends on the direction of transport.
 245          */
 246         framepos_t cycle_start_frame = (current_speed < 0) ? (start_frame - ltc_out_latency.max) : (start_frame + ltc_out_latency.max);
 247
 248         /* LTC TV standard offset */
 249         cycle_start_frame -= ltc_frame_alignment(frames_per_timecode_frame(), TV_STANDARD(cur_timecode));
 250
 251         /* cycle-start may become negative due to latency compensation */
 252         if (cycle_start_frame < 0) { cycle_start_frame = 0; }
 253
 254         double new_ltc_speed = (double)(labs(end_frame - start_frame) * SIGNUM(current_speed)) / (double)nframes;
 255         if (nominal_frame_rate() != frame_rate()) {
 256                 new_ltc_speed *= (double)nominal_frame_rate() / (double)frame_rate();
 257         }
 258
 259         if (SIGNUM(new_ltc_speed) != SIGNUM (ltc_speed)) {
 260                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport changed direction\n");
 261                 ltc_tx_reset();
 262         }
 263
 264         if (ltc_speed != new_ltc_speed) {
 265                 /* check ./libs/ardour/interpolation.cc  CubicInterpolation::interpolate
 266                  * if target_speed != current_speed we should interpolate, too.
 267                  *
 268                  * However, currency in A3 target_speed == current_speed for each process cycle
 269                  * (except for the sign and if target_speed > 8.0).
 270                  * Besides, above speed calculation uses the difference (end_frame - start_frame).
 271                  * end_frame is calculated from 'frames_moved' which includes the interpolation.
 272                  * so we're good.
 273                  */
 274                 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));
 275                 speed_changed = true;
 276                 ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(new_ltc_speed));
 277         }
 278
 279         if (end_frame == start_frame || fabs(current_speed) < 0.1 ) {
 280                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport is not rolling or absolute-speed < 0.1\n");
 281                 /* keep repeating current frame
 282                  *
 283                  * an LTC generator must be able to continue generating LTC when Ardours transport is in stop
 284                  * some machines do odd things if LTC goes away:
 285                  * e.g. a tape based machine (video or audio), some think they have gone into park if LTC goes away,
 286                  * so unspool the tape from the playhead. That might be inconvenient.
 287                  * If LTC keeps arriving they remain in a stop position with the tape on the playhead.
 288                  */
 289                 new_ltc_speed = 0;
 290                 if (!Config->get_ltc_send_continuously()) {
 291                         ltc_speed = new_ltc_speed;
 292                         return;
 293                 }
 294         }
 295
 296         if (fabs(new_ltc_speed) > 10.0) {
 297                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: speed is out of bounds.\n");
 298                 ltc_tx_reset();
 299                 return;
 300         }
 301
 302         if (ltc_speed == 0 && new_ltc_speed != 0) {
 303                 DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport started rolling - reset\n");
 304                 ltc_tx_reset();
 305         }
 306
 307         /* the timecode duration corresponding to the samples that are still
 308          * in the buffer. Here, the speed of previous cycle is used to calculate
 309          * the alignment at the beginning of this cycle later.
 310          */
 311         double poff = (ltc_buf_len - ltc_buf_off) * ltc_speed;
 312
 313         if (speed_changed && new_ltc_speed != 0) {
 314                 /* we need to re-sample the existing buffer.
 315                  * "make space for the en-coder to catch up to the new speed"
 316                  *
 317                  * since the LTC signal is a rectangular waveform we can simply squeeze it
 318                  * by removing samples or duplicating samples /here and there/.
 319                  *
 320                  * There may be a more elegant way to do this, in fact one could
 321                  * simply re-render the buffer using ltc_encoder_encode_byte()
 322                  * but that'd require some timecode offset buffer magic,
 323                  * which is left for later..
 324                  */
 325
 326                 double oldbuflen = (double)(ltc_buf_len - ltc_buf_off);
 327                 double newbuflen = (double)(ltc_buf_len - ltc_buf_off) * fabs(ltc_speed / new_ltc_speed);
 328
 329                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: bufOld %1 bufNew %2 | diff %3\n",
 330                                         (ltc_buf_len - ltc_buf_off), newbuflen, newbuflen - oldbuflen
 331                                         ));
 332
 333                 double bufrspdiff = rint(newbuflen - oldbuflen);
 334
 335                 if (abs(bufrspdiff) > newbuflen || abs(bufrspdiff) > oldbuflen) {
 336                         DEBUG_TRACE (DEBUG::LTC, "LTC TX2: resampling buffer would destroy information.\n");
 337                         ltc_tx_reset();
 338                         poff = 0;
 339                 } else if (bufrspdiff != 0 && newbuflen > oldbuflen) {
 340                         int incnt = 0;
 341                         double samples_to_insert = ceil(newbuflen - oldbuflen);
 342                         double avg_distance = newbuflen / samples_to_insert;
 343                         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: resample buffer insert: %1\n", samples_to_insert));
 344
 345                         for (int rp = ltc_buf_off; rp < ltc_buf_len - 1; ++rp) {
 346                                 const int ro = rp - ltc_buf_off;
 347                                 if (ro < (incnt*avg_distance)) continue;
 348                                 const ltcsnd_sample_t v1 = ltc_enc_buf[rp];
 349                                 const ltcsnd_sample_t v2 = ltc_enc_buf[rp+1];
 350                                 if (v1 != v2 && ro < ((incnt+1)*avg_distance)) continue;
 351                                 memmove(&ltc_enc_buf[rp+1], &ltc_enc_buf[rp], ltc_buf_len-rp);
 352                                 incnt++;
 353                                 ltc_buf_len++;
 354                         }
 355                 } else if (bufrspdiff != 0 && newbuflen < oldbuflen) {
 356                         double samples_to_remove = ceil(oldbuflen - newbuflen);
 357                         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: resample buffer - remove: %1\n", samples_to_remove));
 358                         if (oldbuflen <= samples_to_remove) {
 359                                 ltc_buf_off = ltc_buf_len= 0;
 360                         } else {
 361                                 double avg_distance = newbuflen / samples_to_remove;
 362                                 int rmcnt = 0;
 363                                 for (int rp = ltc_buf_off; rp < ltc_buf_len - 1; ++rp) {
 364                                         const int ro = rp - ltc_buf_off;
 365                                         if (ro < (rmcnt*avg_distance)) continue;
 366                                         const ltcsnd_sample_t v1 = ltc_enc_buf[rp];
 367                                         const ltcsnd_sample_t v2 = ltc_enc_buf[rp+1];
 368                                         if (v1 != v2 && ro < ((rmcnt+1)*avg_distance)) continue;
 369                                         memmove(&ltc_enc_buf[rp], &ltc_enc_buf[rp+1], ltc_buf_len-rp-1);
 370                                         ltc_buf_len--;
 371                                         rmcnt++;
 372                                 }
 373                         }
 374                 }
 375         }
 376
 377         ltc_speed = new_ltc_speed;
 378         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: transport speed %1.\n", ltc_speed));
 379
 380         // (3) bit/sample alignment
 381         Timecode::Time tc_start;
 382         framepos_t tc_sample_start;
 383
 384         /* calc timecode frame from current position - round down to nearest timecode */
 385         Timecode::sample_to_timecode(cycle_start_frame, tc_start, true, false,
 386                         timecode_frames_per_second(),
 387                         timecode_drop_frames(),
 388                         (double)frame_rate(),
 389                         config.get_subframes_per_frame(),
 390                         ltc_timecode_negative_offset, ltc_timecode_offset
 391                         );
 392
 393         /* convert timecode back to sample-position */
 394         Timecode::timecode_to_sample (tc_start, tc_sample_start, true, false,
 395                 (double)frame_rate(),
 396                 config.get_subframes_per_frame(),
 397                 ltc_timecode_negative_offset, ltc_timecode_offset
 398                 );
 399
 400         /* difference between current frame and TC frame in samples */
 401         frameoffset_t soff = cycle_start_frame - tc_sample_start;
 402         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX3: A3cycle: %1 = A3tc: %2 +off: %3\n",
 403                                 cycle_start_frame, tc_sample_start, soff));
 404
 405
 406         // (4) check if alignment matches
 407         const double fptcf = frames_per_timecode_frame();
 408
 409         /* maximum difference of bit alignment in audio-samples.
 410          *
 411          * if transport and LTC generator differs more than this, the LTC
 412          * generator will be re-initialized
 413          *
 414          * due to rounding error and variations in LTC-bit duration depending
 415          * on the speed, it can be off by +- ltc_speed audio-samples.
 416          * When the playback speed changes, it can actually reach +- 2 * ltc_speed
 417          * in the cycle _after_ the speed changed. The average delta however is 0.
 418          */
 419         double maxdiff;
 420
 421         if (config.get_external_sync() && slave()) {
 422                 maxdiff = slave()->resolution();
 423         } else {
 424                 maxdiff = ceil(fabs(ltc_speed))*2.0;
 425                 if (nominal_frame_rate() != frame_rate()) {
 426                         maxdiff *= 3.0;
 427                 }
 428                 if (ltc_enc_tcformat == Timecode::timecode_23976 || ltc_enc_tcformat == Timecode::timecode_24976) {
 429                         maxdiff *= 15.0;
 430                 }
 431         }
 432
 433         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: enc: %1 + %2 - %3 || buf-bytes: %4 enc-byte: %5\n",
 434                                 ltc_enc_pos, ltc_enc_cnt, poff, (ltc_buf_len - ltc_buf_off), poff, ltc_enc_byte));
 435
 436         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: enc-pos: %1  | d: %2\n",
 437                                 ltc_enc_pos + ltc_enc_cnt - poff,
 438                                 rint(ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame
 439                                 ));
 440
 441         if (ltc_enc_pos < 0
 442                         || (ltc_speed != 0 && fabs(ceil(ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame) > maxdiff)
 443                         ) {
 444
 445                 // (5) re-align
 446                 ltc_tx_reset();
 447
 448                 /* set frame to encode */
 449                 SMPTETimecode tc;
 450                 tc.hours = tc_start.hours;
 451                 tc.mins = tc_start.minutes;
 452                 tc.secs = tc_start.seconds;
 453                 tc.frame = tc_start.frames;
 454                 ltc_encoder_set_timecode(ltc_encoder, &tc);
 455
 456                 /* workaround for libltc recognizing 29.97 and 30000/1001 as drop-frame TC.
 457                  * In A3 30000/1001 or 30 fps can be drop-frame.
 458                  */
 459                 LTCFrame ltcframe;
 460                 ltc_encoder_get_frame(ltc_encoder, &ltcframe);
 461                 ltcframe.dfbit = timecode_has_drop_frames(cur_timecode)?1:0;
 462                 ltc_encoder_set_frame(ltc_encoder, &ltcframe);
 463
 464
 465                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: now: %1 trs: %2 toff %3\n", cycle_start_frame, tc_sample_start, soff));
 466
 467                 uint32_t cyc_off;
 468                 if (soff < 0 || soff >= fptcf) {
 469                         /* session framerate change between (2) and now */
 470                         ltc_tx_reset();
 471                         return;
 472                 }
 473
 474                 if (ltc_speed < 0 ) {
 475                         /* calculate the byte that starts at or after the current position */
 476                         ltc_enc_byte = floor((10.0 * soff) / (fptcf));
 477                         ltc_enc_cnt = ltc_enc_byte * fptcf / 10.0;
 478
 479                         /* calculate difference between the current position and the byte to send */
 480                         cyc_off = soff- ceil(ltc_enc_cnt);
 481
 482                 } else {
 483                         /* calculate the byte that starts at or after the current position */
 484                         ltc_enc_byte = ceil((10.0 * soff) / fptcf);
 485                         ltc_enc_cnt = ltc_enc_byte * fptcf / 10.0;
 486
 487                         /* calculate difference between the current position and the byte to send */
 488                         cyc_off = ceil(ltc_enc_cnt) - soff;
 489
 490                         if (ltc_enc_byte == 10) {
 491                                 ltc_enc_byte = 0;
 492                                 ltc_encoder_inc_timecode(ltc_encoder);
 493                         }
 494                 }
 495
 496                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX5 restart encoder: soff %1 byte %2 cycoff %3\n",
 497                                         soff, ltc_enc_byte, cyc_off));
 498
 499                 if ( (ltc_speed < 0 && ltc_enc_byte !=9 ) || (ltc_speed >= 0 && ltc_enc_byte !=0 ) ) {
 500                         restarting = true;
 501                 }
 502
 503                 if (cyc_off > 0 && cyc_off <= nframes) {
 504                         /* offset in this cycle */
 505                         txf= rint(cyc_off / fabs(ltc_speed));
 506                         memset(out, 0, cyc_off * sizeof(Sample));
 507                 } else {
 508                         /* resync next cycle */
 509                         memset(out, 0, nframes * sizeof(Sample));
 510                         return;
 511                 }
 512
 513                 ltc_enc_pos = tc_sample_start;
 514
 515                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX5 restart @ %1 + %2 - %3 |  byte %4\n",
 516                                         ltc_enc_pos, ltc_enc_cnt, cyc_off, ltc_enc_byte));
 517         }
 518         else if (ltc_speed != 0 && (fptcf / ltc_speed / 80) > 3 ) {
 519                 /* reduce (low freq) jitter.
 520                  * The granularity of the LTC encoder speed is 1 byte =
 521                  * (frames-per-timecode-frame / 10) audio-samples.
 522                  * Thus, tiny speed changes [as produced by some slaves]
 523                  * may not have any effect in the cycle when they occur,
 524                  * but they will add up over time.
 525                  *
 526                  * This is a linear approx to compensate for this jitter
 527                  * and prempt re-sync when the drift builds up.
 528                  *
 529                  * However, for very fast speeds - when 1 LTC bit is
 530                  * <= 3 audio-sample - adjusting speed may lead to
 531                  * invalid frames.
 532                  *
 533                  * To do better than this, resampling (or a rewrite of the
 534                  * encoder) is required.
 535                  */
 536                 ltc_speed -= ((ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame) / engine().frame_rate();
 537         }
 538
 539
 540         // (6) encode and output
 541         while (1) {
 542 #ifdef LTC_GEN_TXDBUG
 543                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.1 @%1  [ %2 / %3 ]\n", txf, ltc_buf_off, ltc_buf_len));
 544 #endif
 545                 // (6a) send remaining buffer
 546                 while ((ltc_buf_off < ltc_buf_len) && (txf < nframes)) {
 547                         const float v1 = ltc_enc_buf[ltc_buf_off++] - 128.0;
 548                         const Sample val = (Sample) (v1*ltcvol);
 549                         out[txf++] = val;
 550                 }
 551 #ifdef LTC_GEN_TXDBUG
 552                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.2 @%1  [ %2 / %3 ]\n", txf, ltc_buf_off, ltc_buf_len));
 553 #endif
 554
 555                 if (txf >= nframes) {
 556                         DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX7 enc: %1 [ %2 / %3 ] byte: %4 spd %5 fpp %6 || nf: %7\n",
 557                                                 ltc_enc_pos, ltc_buf_off, ltc_buf_len, ltc_enc_byte, ltc_speed, nframes, txf));
 558                         break;
 559                 }
 560
 561                 ltc_buf_len = 0;
 562                 ltc_buf_off = 0;
 563
 564                 // (6b) encode LTC, bump timecode
 565
 566                 if (ltc_speed < 0) {
 567                         ltc_enc_byte = (ltc_enc_byte + 9)%10;
 568                         if (ltc_enc_byte == 9) {
 569                                 ltc_encoder_dec_timecode(ltc_encoder);
 570                                 ltc_tx_recalculate_position();
 571                                 ltc_enc_cnt = fptcf;
 572                         }
 573                 }
 574
 575                 int enc_frames;
 576
 577                 if (restarting) {
 578                         /* write zero bytes -- don't touch encoder until we're at a frame-boundary
 579                          * otherwise the biphase polarity may be inverted.
 580                          */
 581                         enc_frames = fptcf / 10.0;
 582                         memset(&ltc_enc_buf[ltc_buf_len], 127, enc_frames * sizeof(ltcsnd_sample_t));
 583                 } else {
 584                         if (ltc_encoder_encode_byte(ltc_encoder, ltc_enc_byte, (ltc_speed==0)?1.0:(1.0/ltc_speed))) {
 585                                 DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.3 encoder error byte %1\n", ltc_enc_byte));
 586                                 ltc_encoder_buffer_flush(ltc_encoder);
 587                                 ltc_tx_reset();
 588                                 return;
 589                         }
 590                         enc_frames = ltc_encoder_get_buffer(ltc_encoder, &(ltc_enc_buf[ltc_buf_len]));
 591                 }
 592
 593 #ifdef LTC_GEN_FRAMEDBUG
 594                 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));
 595 #endif
 596                 if (enc_frames <=0) {
 597                         DEBUG_TRACE (DEBUG::LTC, "LTC TX6.3 encoder empty buffer.\n");
 598                         ltc_encoder_buffer_flush(ltc_encoder);
 599                         ltc_tx_reset();
 600                         return;
 601                 }
 602
 603                 ltc_buf_len += enc_frames;
 604                 if (ltc_speed < 0)
 605                         ltc_enc_cnt -= fptcf/10.0;
 606                 else
 607                         ltc_enc_cnt += fptcf/10.0;
 608
 609                 if (ltc_speed >= 0) {
 610                         ltc_enc_byte = (ltc_enc_byte + 1)%10;
 611                         if (ltc_enc_byte == 0 && ltc_speed != 0) {
 612                                 ltc_encoder_inc_timecode(ltc_encoder);
 613 #if 0 /* force fixed parity -- scope debug */
 614                                 LTCFrame f;
 615                                 ltc_encoder_get_frame(ltc_encoder, &f);
 616                                 f.biphase_mark_phase_correction=0;
 617                                 ltc_encoder_set_frame(ltc_encoder, &f);
 618 #endif
 619                                 ltc_tx_recalculate_position();
 620                                 ltc_enc_cnt = 0;
 621                         } else if (ltc_enc_byte == 0) {
 622                                 ltc_enc_cnt = 0;
 623                                 restarting=false;
 624                         }
 625                 }
 626 #ifdef LTC_GEN_FRAMEDBUG
 627                 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));
 628 #endif
 629         }
 630
 631         dynamic_cast<AudioBuffer*>(&buf)->set_written (true);
 632         return;
 633 }