session_ltc.cc

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/*
  Copyright (C) 2012 Paul Davis
  Written by Robin Gareus <robin@gareus.org>

  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

*/

#include "timecode/time.h"

#include "ardour/audioengine.h"
#include "ardour/audio_port.h"
#include "ardour/debug.h"
#include "ardour/io.h"
#include "ardour/session.h"
#include "ardour/slave.h"

#include "i18n.h"

using namespace std;
using namespace ARDOUR;
using namespace PBD;
using namespace Timecode;

/* really verbose timing debug */
//#define LTC_GEN_FRAMEDBUG
//#define LTC_GEN_TXDBUG

#ifndef MAX
#define MAX(a,b) ( (a) > (b) ? (a) : (b) )
#endif
#ifndef MIN
#define MIN(a,b) ( (a) < (b) ? (a) : (b) )
#endif

/* LTC signal should have a rise time of 25 us +/- 5 us.
 * yet with most sound-cards a square-wave of 1-2 sample
 * introduces ringing and small oscillations.
 * https://en.wikipedia.org/wiki/Gibbs_phenomenon
 * A low-pass filter in libltc can reduce this at
 * the cost of being slightly out of spec WRT to rise-time.
 *
 * This filter is adaptive so that fast vari-speed signals
 * will not be affected by it.
 */
#define LTC_RISE_TIME(speed) MIN (100, MAX(40, (4000000 / ((speed==0)?1:speed) / engine().sample_rate())))

#define TV_STANDARD(tcf) \
        (timecode_to_frames_per_second(tcf)==25.0 ? LTC_TV_625_50 : \
         timecode_has_drop_frames(tcf)? LTC_TV_525_60 : LTC_TV_FILM_24)

void
Session::ltc_tx_initialize()
{
        ltc_enc_tcformat = config.get_timecode_format();

        ltc_tx_parse_offset();
        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)));
        ltc_encoder = ltc_encoder_create(nominal_frame_rate(),
                        timecode_to_frames_per_second(ltc_enc_tcformat),
                        TV_STANDARD(ltc_enc_tcformat), 0);

        ltc_encoder_set_bufsize(ltc_encoder, nominal_frame_rate(), 23.0);
        ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(1.0));

        /* buffersize for 1 LTC frame: (1 + sample-rate / fps) bytes
         * usually returned by ltc_encoder_get_buffersize(encoder)
         *
         * since the fps can change and A3's  min fps: 24000/1001 */
        ltc_enc_buf = (ltcsnd_sample_t*) calloc((nominal_frame_rate() / 23), sizeof(ltcsnd_sample_t));
        ltc_speed = 0;
        ltc_prev_cycle = -1;
        ltc_tx_reset();
        ltc_tx_resync_latency();
        Xrun.connect_same_thread (*this, boost::bind (&Session::ltc_tx_reset, this));
        engine().GraphReordered.connect_same_thread (*this, boost::bind (&Session::ltc_tx_resync_latency, this));
        restarting = false;
}

void
Session::ltc_tx_cleanup()
{
        DEBUG_TRACE (DEBUG::LTC, "LTC TX cleanup\n");
        free(ltc_enc_buf);
        ltc_enc_buf = NULL;
        ltc_encoder_free(ltc_encoder);
        ltc_encoder = NULL;
}

void
Session::ltc_tx_resync_latency()
{
        DEBUG_TRACE (DEBUG::LTC, "LTC TX resync latency\n");
        if (!deletion_in_progress()) {
                boost::shared_ptr<Port> ltcport = ltc_output_port();
                if (ltcport) {
                        ltcport->get_connected_latency_range(ltc_out_latency, true);
                }
        }
}

void
Session::ltc_tx_reset()
{
        DEBUG_TRACE (DEBUG::LTC, "LTC TX reset\n");
        ltc_enc_pos = -9999; // force re-start
        ltc_buf_len = 0;
        ltc_buf_off = 0;
        ltc_enc_byte = 0;
        ltc_enc_cnt = 0;

        ltc_encoder_reset(ltc_encoder);
}

void
Session::ltc_tx_parse_offset() {
        Timecode::Time offset_tc;
        Timecode::parse_timecode_format(config.get_timecode_generator_offset(), offset_tc);
        offset_tc.rate = timecode_frames_per_second();
        offset_tc.drop = timecode_drop_frames();
        timecode_to_sample(offset_tc, ltc_timecode_offset, false, false);
        ltc_timecode_negative_offset = !offset_tc.negative;
        ltc_prev_cycle = -1;
}

void
Session::ltc_tx_recalculate_position()
{
        SMPTETimecode enctc;
        Timecode::Time a3tc;
        ltc_encoder_get_timecode(ltc_encoder, &enctc);

        a3tc.hours   = enctc.hours;
        a3tc.minutes = enctc.mins;
        a3tc.seconds = enctc.secs;
        a3tc.frames  = enctc.frame;
        a3tc.rate = timecode_to_frames_per_second(ltc_enc_tcformat);
        a3tc.drop = timecode_has_drop_frames(ltc_enc_tcformat);

        Timecode::timecode_to_sample (a3tc, ltc_enc_pos, true, false,
                (double)frame_rate(),
                config.get_subframes_per_frame(),
                ltc_timecode_negative_offset, ltc_timecode_offset
                );
        restarting = false;
}

void
Session::ltc_tx_send_time_code_for_cycle (framepos_t start_frame, framepos_t end_frame,
                                          double target_speed, double current_speed,
                                          pframes_t nframes)
{
        assert (nframes > 0);

        Sample *out;
        pframes_t txf = 0;
        boost::shared_ptr<Port> ltcport = ltc_output_port();

        Buffer& buf (ltcport->get_buffer (nframes));

        if (!ltc_encoder || !ltc_enc_buf) {
                return;
        }

        SyncSource sync_src = Config->get_sync_source();
        if (engine().freewheeling() || !Config->get_send_ltc()
            /* TODO
             * decide which time-sources we can generated LTC from.
             * Internal, JACK or sample-synced slaves should be fine.
             * talk to oofus.
             *
             || (config.get_external_sync() && sync_src == LTC)
             || (config.get_external_sync() && sync_src == MTC)
            */
             ||(config.get_external_sync() && sync_src == MIDIClock)
                ) {
                return;
        }

        out = dynamic_cast<AudioBuffer*>(&buf)->data ();

        /* range from libltc (38..218) || - 128.0  -> (-90..90) */
        const float ltcvol = Config->get_ltc_output_volume()/(90.0); // pow(10, db/20.0)/(90.0);

        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX %1 to %2 / %3 | lat: %4\n", start_frame, end_frame, nframes, ltc_out_latency.max));

        /* all systems go. Now here's the plan:
         *
         *  1) check if fps has changed
         *  2) check direction of encoding, calc speed, re-sample existing buffer
         *  3) calculate frame and byte to send aligned to jack-period size
         *  4) check if it's the frame/byte that is already in the queue
         *  5) if (4) mismatch, re-calculate offset of LTC frame relative to period size
         *  6) actual LTC audio output
         *  6a) send remaining part of already queued frame; break on nframes
         *  6b) encode new LTC-frame byte
         *  6c) goto 6a
         *  7) done
         */

        // (1) check fps
        TimecodeFormat cur_timecode = config.get_timecode_format();
        if (cur_timecode != ltc_enc_tcformat) {
                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)));
                if (ltc_encoder_reinit(ltc_encoder, nominal_frame_rate(),
                                        timecode_to_frames_per_second(cur_timecode),
                                        TV_STANDARD(cur_timecode), 0
                                        )) {
                        PBD::error << _("LTC encoder: invalid framerate - LTC encoding is disabled for the remainder of this session.") << endmsg;
                        ltc_tx_cleanup();
                        return;
                }
                ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(ltc_speed));
                ltc_enc_tcformat = cur_timecode;
                ltc_tx_parse_offset();
                ltc_tx_reset();
        }

        /* LTC is max. 30 fps */
        if (timecode_to_frames_per_second(cur_timecode) > 30) {
                return;
        }

        // (2) speed & direction

        /* speed 0 aka transport stopped is interpreted as rolling forward.
         * keep repeating current frame
         */
#define SIGNUM(a) ( (a) < 0 ? -1 : 1)
        bool speed_changed = false;

        /* port latency compensation:
         * The _generated timecode_ is offset by the port-latency,
         * therefore the offset depends on the direction of transport.
         *
         * latency is compensated by adding it to the timecode to
         * be generated. e.g. if the signal will reach the output in
         * N samples time from now, generate the timecode for (now + N).
         *
         * sample-sync is achieved by further calculating the difference
         * between the timecode and the session-transport and offsetting the
         * buffer.
         *
         * The timecode is generated directly in the Session process callback
         * using _transport_frame. It requires that the session has set the
         * port's playback latency to worst_playback_latency() prior to
         * calling ltc_tx_send_time_code_for_cycle().
         */
        framepos_t cycle_start_frame;

        if (current_speed < 0) {
                cycle_start_frame = (start_frame - ltc_out_latency.max + worst_playback_latency());
        } else if (current_speed > 0) {
                cycle_start_frame = (start_frame + ltc_out_latency.max - worst_playback_latency());
        } else {
                /* There is no need to compensate for latency when not rolling
                 * rather send the accurate NOW timecode
                 * (LTC encoder compenates latency by sending earlier timecode)
                 */
                cycle_start_frame = start_frame;
        }

        /* LTC TV standard offset */
        if (current_speed != 0) {
                /* ditto - send "NOW" if not rolling */
                cycle_start_frame -= ltc_frame_alignment(frames_per_timecode_frame(), TV_STANDARD(cur_timecode));
        }

        /* cycle-start may become negative due to latency compensation */
        if (cycle_start_frame < 0) { cycle_start_frame = 0; }

        double new_ltc_speed = (double)(labs(end_frame - start_frame) * SIGNUM(current_speed)) / (double)nframes;
        if (nominal_frame_rate() != frame_rate()) {
                new_ltc_speed *= (double)nominal_frame_rate() / (double)frame_rate();
        }

        if (SIGNUM(new_ltc_speed) != SIGNUM (ltc_speed)) {
                DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport changed direction\n");
                ltc_tx_reset();
        }

        if (ltc_speed != new_ltc_speed
                        /* but only once if, current_speed changes to 0. In that case
                         * new_ltc_speed is > 0 because (end_frame - start_frame) == jack-period for no-roll
                         * but ltc_speed will still be 0
                         */
                        && (current_speed != 0 || ltc_speed != current_speed)
                        ) {
                /* check ./libs/ardour/interpolation.cc  CubicInterpolation::interpolate
                 * if target_speed != current_speed we should interpolate, too.
                 *
                 * However, currency in A3 target_speed == current_speed for each process cycle
                 * (except for the sign and if target_speed > 8.0).
                 * Besides, above speed calculation uses the difference (end_frame - start_frame).
                 * end_frame is calculated from 'frames_moved' which includes the interpolation.
                 * so we're good.
                 */
                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));
                speed_changed = true;
                ltc_encoder_set_filter(ltc_encoder, LTC_RISE_TIME(new_ltc_speed));
        }

        if (end_frame == start_frame || fabs(current_speed) < 0.1 ) {
                DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport is not rolling or absolute-speed < 0.1\n");
                /* keep repeating current frame
                 *
                 * an LTC generator must be able to continue generating LTC when Ardours transport is in stop
                 * some machines do odd things if LTC goes away:
                 * e.g. a tape based machine (video or audio), some think they have gone into park if LTC goes away,
                 * so unspool the tape from the playhead. That might be inconvenient.
                 * If LTC keeps arriving they remain in a stop position with the tape on the playhead.
                 */
                new_ltc_speed = 0;
                if (!Config->get_ltc_send_continuously()) {
                        ltc_speed = new_ltc_speed;
                        return;
                }
                if (start_frame != ltc_prev_cycle) {
                        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));
                        ltc_tx_reset();
                }
        }

        if (fabs(new_ltc_speed) > 10.0) {
                DEBUG_TRACE (DEBUG::LTC, "LTC TX2: speed is out of bounds.\n");
                ltc_tx_reset();
                return;
        }

        if (ltc_speed == 0 && new_ltc_speed != 0) {
                DEBUG_TRACE (DEBUG::LTC, "LTC TX2: transport started rolling - reset\n");
                ltc_tx_reset();
        }

        /* the timecode duration corresponding to the samples that are still
         * in the buffer. Here, the speed of previous cycle is used to calculate
         * the alignment at the beginning of this cycle later.
         */
        double poff = (ltc_buf_len - ltc_buf_off) * ltc_speed;

        if (speed_changed && new_ltc_speed != 0) {
                /* we need to re-sample the existing buffer.
                 * "make space for the en-coder to catch up to the new speed"
                 *
                 * since the LTC signal is a rectangular waveform we can simply squeeze it
                 * by removing samples or duplicating samples /here and there/.
                 *
                 * There may be a more elegant way to do this, in fact one could
                 * simply re-render the buffer using ltc_encoder_encode_byte()
                 * but that'd require some timecode offset buffer magic,
                 * which is left for later..
                 */

                double oldbuflen = (double)(ltc_buf_len - ltc_buf_off);
                double newbuflen = (double)(ltc_buf_len - ltc_buf_off) * fabs(ltc_speed / new_ltc_speed);

                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: bufOld %1 bufNew %2 | diff %3\n",
                                        (ltc_buf_len - ltc_buf_off), newbuflen, newbuflen - oldbuflen
                                        ));

                double bufrspdiff = rint(newbuflen - oldbuflen);

                if (abs(bufrspdiff) > newbuflen || abs(bufrspdiff) > oldbuflen) {
                        DEBUG_TRACE (DEBUG::LTC, "LTC TX2: resampling buffer would destroy information.\n");
                        ltc_tx_reset();
                        poff = 0;
                } else if (bufrspdiff != 0 && newbuflen > oldbuflen) {
                        int incnt = 0;
                        double samples_to_insert = ceil(newbuflen - oldbuflen);
                        double avg_distance = newbuflen / samples_to_insert;
                        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: resample buffer insert: %1\n", samples_to_insert));

                        for (int rp = ltc_buf_off; rp < ltc_buf_len - 1; ++rp) {
                                const int ro = rp - ltc_buf_off;
                                if (ro < (incnt*avg_distance)) continue;
                                const ltcsnd_sample_t v1 = ltc_enc_buf[rp];
                                const ltcsnd_sample_t v2 = ltc_enc_buf[rp+1];
                                if (v1 != v2 && ro < ((incnt+1)*avg_distance)) continue;
                                memmove(&ltc_enc_buf[rp+1], &ltc_enc_buf[rp], ltc_buf_len-rp);
                                incnt++;
                                ltc_buf_len++;
                        }
                } else if (bufrspdiff != 0 && newbuflen < oldbuflen) {
                        double samples_to_remove = ceil(oldbuflen - newbuflen);
                        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: resample buffer - remove: %1\n", samples_to_remove));
                        if (oldbuflen <= samples_to_remove) {
                                ltc_buf_off = ltc_buf_len= 0;
                        } else {
                                double avg_distance = newbuflen / samples_to_remove;
                                int rmcnt = 0;
                                for (int rp = ltc_buf_off; rp < ltc_buf_len - 1; ++rp) {
                                        const int ro = rp - ltc_buf_off;
                                        if (ro < (rmcnt*avg_distance)) continue;
                                        const ltcsnd_sample_t v1 = ltc_enc_buf[rp];
                                        const ltcsnd_sample_t v2 = ltc_enc_buf[rp+1];
                                        if (v1 != v2 && ro < ((rmcnt+1)*avg_distance)) continue;
                                        memmove(&ltc_enc_buf[rp], &ltc_enc_buf[rp+1], ltc_buf_len-rp-1);
                                        ltc_buf_len--;
                                        rmcnt++;
                                }
                        }
                }
        }

        ltc_prev_cycle = start_frame;
        ltc_speed = new_ltc_speed;
        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX2: transport speed %1.\n", ltc_speed));

        // (3) bit/sample alignment
        Timecode::Time tc_start;
        framepos_t tc_sample_start;

        /* calc timecode frame from current position - round down to nearest timecode */
        Timecode::sample_to_timecode(cycle_start_frame, tc_start, true, false,
                        timecode_frames_per_second(),
                        timecode_drop_frames(),
                        (double)frame_rate(),
                        config.get_subframes_per_frame(),
                        ltc_timecode_negative_offset, ltc_timecode_offset
                        );

        /* convert timecode back to sample-position */
        Timecode::timecode_to_sample (tc_start, tc_sample_start, true, false,
                (double)frame_rate(),
                config.get_subframes_per_frame(),
                ltc_timecode_negative_offset, ltc_timecode_offset
                );

        /* difference between current frame and TC frame in samples */
        frameoffset_t soff = cycle_start_frame - tc_sample_start;
        if (current_speed == 0) {
                soff = 0;
        }
        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX3: A3cycle: %1 = A3tc: %2 +off: %3\n",
                                cycle_start_frame, tc_sample_start, soff));


        // (4) check if alignment matches
        const double fptcf = frames_per_timecode_frame();

        /* maximum difference of bit alignment in audio-samples.
         *
         * if transport and LTC generator differs more than this, the LTC
         * generator will be re-initialized
         *
         * due to rounding error and variations in LTC-bit duration depending
         * on the speed, it can be off by +- ltc_speed audio-samples.
         * When the playback speed changes, it can actually reach +- 2 * ltc_speed
         * in the cycle _after_ the speed changed. The average delta however is 0.
         */
        double maxdiff;

        if (config.get_external_sync() && slave()) {
                maxdiff = slave()->resolution();
        } else {
                maxdiff = ceil(fabs(ltc_speed))*2.0;
                if (nominal_frame_rate() != frame_rate()) {
                        maxdiff *= 3.0;
                }
                if (ltc_enc_tcformat == Timecode::timecode_23976 || ltc_enc_tcformat == Timecode::timecode_24976) {
                        maxdiff *= 15.0;
                }
        }

        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: enc: %1 + %2 - %3 || buf-bytes: %4 enc-byte: %5\n",
                                ltc_enc_pos, ltc_enc_cnt, poff, (ltc_buf_len - ltc_buf_off), poff, ltc_enc_byte));

        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: enc-pos: %1  | d: %2\n",
                                ltc_enc_pos + ltc_enc_cnt - poff,
                                rint(ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame
                                ));

        if (ltc_enc_pos < 0
                        || (ltc_speed != 0 && fabs(ceil(ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame) > maxdiff)
                        ) {

                // (5) re-align
                ltc_tx_reset();

                /* set frame to encode */
                SMPTETimecode tc;
                tc.hours = tc_start.hours;
                tc.mins = tc_start.minutes;
                tc.secs = tc_start.seconds;
                tc.frame = tc_start.frames;
                ltc_encoder_set_timecode(ltc_encoder, &tc);

                /* workaround for libltc recognizing 29.97 and 30000/1001 as drop-frame TC.
                 * In A3 30000/1001 or 30 fps can be drop-frame.
                 */
                LTCFrame ltcframe;
                ltc_encoder_get_frame(ltc_encoder, &ltcframe);
                ltcframe.dfbit = timecode_has_drop_frames(cur_timecode)?1:0;
                ltc_encoder_set_frame(ltc_encoder, &ltcframe);


                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX4: now: %1 trs: %2 toff %3\n", cycle_start_frame, tc_sample_start, soff));

                int32_t cyc_off;
                if (soff < 0 || soff >= fptcf) {
                        /* session framerate change between (2) and now */
                        ltc_tx_reset();
                        return;
                }

                if (ltc_speed < 0 ) {
                        /* calculate the byte that starts at or after the current position */
                        ltc_enc_byte = floor((10.0 * soff) / (fptcf));
                        ltc_enc_cnt = ltc_enc_byte * fptcf / 10.0;

                        /* calculate difference between the current position and the byte to send */
                        cyc_off = soff- ceil(ltc_enc_cnt);

                } else {
                        /* calculate the byte that starts at or after the current position */
                        ltc_enc_byte = ceil((10.0 * soff) / fptcf);
                        ltc_enc_cnt = ltc_enc_byte * fptcf / 10.0;

                        /* calculate difference between the current position and the byte to send */
                        cyc_off = ceil(ltc_enc_cnt) - soff;

                        if (ltc_enc_byte == 10) {
                                ltc_enc_byte = 0;
                                ltc_encoder_inc_timecode(ltc_encoder);
                        }
                }

                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX5 restart encoder: soff %1 byte %2 cycoff %3\n",
                                        soff, ltc_enc_byte, cyc_off));

                if ( (ltc_speed < 0 && ltc_enc_byte !=9 ) || (ltc_speed >= 0 && ltc_enc_byte !=0 ) ) {
                        restarting = true;
                }

                if (cyc_off >= 0 && cyc_off <= (int32_t) nframes) {
                        /* offset in this cycle */
                        txf= rint(cyc_off / fabs(ltc_speed));
                        memset(out, 0, cyc_off * sizeof(Sample));
                } else {
                        /* resync next cycle */
                        memset(out, 0, nframes * sizeof(Sample));
                        return;
                }

                ltc_enc_pos = tc_sample_start;

                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX5 restart @ %1 + %2 - %3 |  byte %4\n",
                                        ltc_enc_pos, ltc_enc_cnt, cyc_off, ltc_enc_byte));
        }
        else if (ltc_speed != 0 && (fptcf / ltc_speed / 80) > 3 ) {
                /* reduce (low freq) jitter.
                 * The granularity of the LTC encoder speed is 1 byte =
                 * (frames-per-timecode-frame / 10) audio-samples.
                 * Thus, tiny speed changes [as produced by some slaves]
                 * may not have any effect in the cycle when they occur,
                 * but they will add up over time.
                 *
                 * This is a linear approx to compensate for this jitter
                 * and prempt re-sync when the drift builds up.
                 *
                 * However, for very fast speeds - when 1 LTC bit is
                 * <= 3 audio-sample - adjusting speed may lead to
                 * invalid frames.
                 *
                 * To do better than this, resampling (or a rewrite of the
                 * encoder) is required.
                 */
                ltc_speed -= ((ltc_enc_pos + ltc_enc_cnt - poff) - cycle_start_frame) / engine().sample_rate();
        }


        // (6) encode and output
        while (1) {
#ifdef LTC_GEN_TXDBUG
                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.1 @%1  [ %2 / %3 ]\n", txf, ltc_buf_off, ltc_buf_len));
#endif
                // (6a) send remaining buffer
                while ((ltc_buf_off < ltc_buf_len) && (txf < nframes)) {
                        const float v1 = ltc_enc_buf[ltc_buf_off++] - 128.0;
                        const Sample val = (Sample) (v1*ltcvol);
                        out[txf++] = val;
                }
#ifdef LTC_GEN_TXDBUG
                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.2 @%1  [ %2 / %3 ]\n", txf, ltc_buf_off, ltc_buf_len));
#endif

                if (txf >= nframes) {
                        DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX7 enc: %1 [ %2 / %3 ] byte: %4 spd %5 fpp %6 || nf: %7\n",
                                                ltc_enc_pos, ltc_buf_off, ltc_buf_len, ltc_enc_byte, ltc_speed, nframes, txf));
                        break;
                }

                ltc_buf_len = 0;
                ltc_buf_off = 0;

                // (6b) encode LTC, bump timecode

                if (ltc_speed < 0) {
                        ltc_enc_byte = (ltc_enc_byte + 9)%10;
                        if (ltc_enc_byte == 9) {
                                ltc_encoder_dec_timecode(ltc_encoder);
                                ltc_tx_recalculate_position();
                                ltc_enc_cnt = fptcf;
                        }
                }

                int enc_frames;

                if (restarting) {
                        /* write zero bytes -- don't touch encoder until we're at a frame-boundary
                         * otherwise the biphase polarity may be inverted.
                         */
                        enc_frames = fptcf / 10.0;
                        memset(&ltc_enc_buf[ltc_buf_len], 127, enc_frames * sizeof(ltcsnd_sample_t));
                } else {
                        if (ltc_encoder_encode_byte(ltc_encoder, ltc_enc_byte, (ltc_speed==0)?1.0:(1.0/ltc_speed))) {
                                DEBUG_TRACE (DEBUG::LTC, string_compose("LTC TX6.3 encoder error byte %1\n", ltc_enc_byte));
                                ltc_encoder_buffer_flush(ltc_encoder);
                                ltc_tx_reset();
                                return;
                        }
                        enc_frames = ltc_encoder_get_buffer(ltc_encoder, &(ltc_enc_buf[ltc_buf_len]));
                }

#ifdef LTC_GEN_FRAMEDBUG
                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));
#endif
                if (enc_frames <=0) {
                        DEBUG_TRACE (DEBUG::LTC, "LTC TX6.3 encoder empty buffer.\n");
                        ltc_encoder_buffer_flush(ltc_encoder);
                        ltc_tx_reset();
                        return;
                }

                ltc_buf_len += enc_frames;
                if (ltc_speed < 0)
                        ltc_enc_cnt -= fptcf/10.0;
                else
                        ltc_enc_cnt += fptcf/10.0;

                if (ltc_speed >= 0) {
                        ltc_enc_byte = (ltc_enc_byte + 1)%10;
                        if (ltc_enc_byte == 0 && ltc_speed != 0) {
                                ltc_encoder_inc_timecode(ltc_encoder);
#if 0 /* force fixed parity -- scope debug */
                                LTCFrame f;
                                ltc_encoder_get_frame(ltc_encoder, &f);
                                f.biphase_mark_phase_correction=0;
                                ltc_encoder_set_frame(ltc_encoder, &f);
#endif
                                ltc_tx_recalculate_position();
                                ltc_enc_cnt = 0;
                        } else if (ltc_enc_byte == 0) {
                                ltc_enc_cnt = 0;
                                restarting=false;
                        }
                }
#ifdef LTC_GEN_FRAMEDBUG
                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));
#endif
        }

        dynamic_cast<AudioBuffer*>(&buf)->set_written (true);
        return;
}