LTC slave rewrite #2

git-svn-id: svn://localhost/ardour2/branches/3.0@13313 d708f5d6-7413-0410-9779-e7cbd77b26cf

4 files changed, 153 insertions, 257 deletions

diff --git a/libs/ardour/ardour/slave.h b/libs/ardour/ardour/slave.h
index 8589473d3b..07e3fd0e1d 100644
--- a/libs/ardour/ardour/slave.h
+++ b/libs/ardour/ardour/slave.h
@@ -342,63 +342,47 @@ public:
 	framecnt_t resolution () const;
 	bool requires_seekahead () const { return false; }
 	framecnt_t seekahead_distance() const;
-	bool give_slave_full_control_over_transport_speed() const;
+	bool give_slave_full_control_over_transport_speed() const { return true; }
 
-        Timecode::TimecodeFormat apparent_timecode_format() const;
-        std::string approximate_current_position() const;
+	Timecode::TimecodeFormat apparent_timecode_format() const;
+	std::string approximate_current_position() const;
 	std::string approximate_current_delta() const;
 
   private:
-	int parse_ltc(const jack_nframes_t, const jack_default_audio_sample_t * const, const framecnt_t);
-	bool process_ltc(framepos_t const, framepos_t const, framecnt_t);
-	void init_ltc_dll(framepos_t const, double const);
-	void init_engine_dll (framepos_t, framepos_t);
+	void parse_ltc(const jack_nframes_t, const jack_default_audio_sample_t * const, const framecnt_t);
+	void process_ltc(framepos_t const);
+	void init_engine_dll (framepos_t, int32_t);
 	bool detect_ltc_fps(int, bool);
-	bool detect_ltc_discontinuity(LTCFrameExt *);
 	void reset();
 
-	Session&    session;
-	bool        did_reset_tc_format;
+	Session&       session;
+	bool           did_reset_tc_format;
 	Timecode::TimecodeFormat saved_tc_format;
 
-	LTCDecoder *decoder;
+	LTCDecoder *   decoder;
+	double         frames_per_ltc_frame;
 	Timecode::Time timecode;
-	double      frames_per_ltc_frame;
-	bool        ltc_discontinuity;
-
-	framecnt_t  monotonic_cnt;
-	framecnt_t  last_timestamp;
-	framecnt_t  last_ltc_frame;
-	framepos_t  ltc_transport_pos;
-	double      ltc_speed;
-	frameoffset_t current_delta;
-
-	int         ltc_detect_fps_cnt;
-	int         ltc_detect_fps_max;
-	Timecode::TimecodeFormat ltc_timecode;
-	Timecode::TimecodeFormat a3e_timecode;
-	bool           printed_timecode_warning;
 
-	LTCFrame prev_ltc_frame;
-	uint64_t frames_in_sequence;
-	int      delayedlocked;
+	framecnt_t     monotonic_cnt;
+	framecnt_t     last_timestamp;
+	framecnt_t     last_ltc_frame;
+	double         ltc_speed;
+	frameoffset_t  current_delta;
+	int            delayedlocked;
 
+	int            ltc_detect_fps_cnt;
+	int            ltc_detect_fps_max;
+	bool           printed_timecode_warning;
+	Timecode::TimecodeFormat ltc_timecode;
+	Timecode::TimecodeFormat a3e_timecode;
 
 	/* DLL - chase LTC */
+	int    transport_direction;
+	int    engine_dll_initstate;
 	double t0; ///< time at the beginning of the MTC quater frame
 	double t1; ///< calculated end of the MTC quater frame
 	double e2; ///< second order loop error
-	double b, c, omega; ///< DLL filter coefficients
-
-	/* DLL - sync engine */
-	int    transport_direction;
-	int    engine_dll_initstate;
-	double te0; ///< time at the beginning of the engine process
-	double te1; ///< calculated sync time
-	double ee2; ///< second order loop error
-	double be, ce, oe; ///< DLL filter coefficients
-
-
+	double b, c; ///< DLL filter coefficients
 };
 #endif
 
diff --git a/libs/ardour/audioengine.cc b/libs/ardour/audioengine.cc
index ba7666f3e4..338bc2fd0e 100644
--- a/libs/ardour/audioengine.cc
+++ b/libs/ardour/audioengine.cc
@@ -91,7 +91,7 @@ AudioEngine::AudioEngine (string client_name, string session_uuid)
 	Port::set_engine (this);
 
 #ifdef HAVE_LTC
-	_ltc_input = register_port (DataType::AUDIO, _("LTC in"), Port::IsInput);
+	_ltc_input = register_port (DataType::AUDIO, _("LTC in"), true);
 
 	/* As of October 2012, the LTC source port is the only thing that needs
 	 * to care about Config parameters, so don't bother to listen if we're
diff --git a/libs/ardour/ltc_slave.cc b/libs/ardour/ltc_slave.cc
index d542038658..39a97677a6 100644
--- a/libs/ardour/ltc_slave.cc
+++ b/libs/ardour/ltc_slave.cc
@@ -44,25 +44,21 @@ using namespace Timecode;
 LTC_Slave::LTC_Slave (Session& s)
 	: session (s)
 {
-	frames_per_ltc_frame = session.frames_per_timecode_frame(); // XXX at most 30fps ?
+	frames_per_ltc_frame = session.frames_per_timecode_frame();
 	timecode.rate = session.timecode_frames_per_second();
 	timecode.drop  = session.timecode_drop_frames();
 
-	ltc_transport_pos = 0;
 	did_reset_tc_format = false;
 	delayedlocked = 10;
-	engine_dll_initstate = 0;
 	monotonic_cnt = 0;
 
-	memset(&prev_ltc_frame, 0, sizeof(LTCFrame));
-
 	ltc_timecode = timecode_60; // track changes of LTC timecode
 	a3e_timecode = timecode_60; // track canges of Ardour's timecode
 	printed_timecode_warning = false;
+	ltc_detect_fps_cnt = ltc_detect_fps_max = 0;
 
 	decoder = ltc_decoder_create((int) frames_per_ltc_frame, 128 /*queue size*/);
 	reset();
-	//session.engine().Xrun.connect_same_thread (*this, boost::bind (&LTC_Slave::reset, this));
 }
 
 LTC_Slave::~LTC_Slave()
@@ -74,13 +70,6 @@ LTC_Slave::~LTC_Slave()
 	ltc_decoder_free(decoder);
 }
 
-bool
-LTC_Slave::give_slave_full_control_over_transport_speed() const
-{
-	return true; // DLL align to engine transport
-	// return false; // for Session-level computed varispeed
-}
-
 ARDOUR::framecnt_t
 LTC_Slave::resolution () const
 {
@@ -109,28 +98,31 @@ void
 LTC_Slave::reset()
 {
 	DEBUG_TRACE (DEBUG::LTC, "LTC reset()\n");
-	frames_in_sequence = 0;
-	ltc_detect_fps_cnt = ltc_detect_fps_max = 0;
 	last_timestamp = 0;
 	current_delta = 0;
 	transport_direction = 0;
 	ltc_speed = 0;
-	ltc_decoder_queue_flush(decoder);
+	engine_dll_initstate = 0;
 }
 
-int
+void
 LTC_Slave::parse_ltc(const jack_nframes_t nframes, const jack_default_audio_sample_t * const in, const framecnt_t posinfo)
 {
 	jack_nframes_t i;
 	unsigned char sound[8192];
-	if (nframes > 8192) return 1;
+	if (nframes > 8192) {
+		/* TODO warn once or wrap, loop conversion below
+		 * does A3 support > 8192 spp anyway?
+		 */
+		return;
+	}
 
 	for (i = 0; i < nframes; i++) {
 		const int snd=(int)rint((127.0*in[i])+128.0);
 		sound[i] = (unsigned char) (snd&0xff);
 	}
 	ltc_decoder_write(decoder, sound, nframes, posinfo);
-	return 0;
+	return;
 }
 
 bool
@@ -142,6 +134,7 @@ LTC_Slave::detect_ltc_fps(int frameno, bool df)
 		ltc_detect_fps_max = frameno;
 	}
 	ltc_detect_fps_cnt++;
+
 	if (ltc_detect_fps_cnt > 60)
 	{
 		if (ltc_detect_fps_cnt > ltc_detect_fps_max
@@ -187,8 +180,8 @@ LTC_Slave::detect_ltc_fps(int frameno, bool df)
 						Timecode::timecode_format_name(cur_timecode),
 						Timecode::timecode_format_name(tc_format))
 					<< endmsg;
+				session.config.set_timecode_format (tc_format);
 			}
-			session.config.set_timecode_format (tc_format);
 		} else {
 			/* only warn about TC mismatch */
 			if (ltc_timecode != tc_format) printed_timecode_warning = false;
@@ -208,53 +201,21 @@ LTC_Slave::detect_ltc_fps(int frameno, bool df)
 	return false;
 }
 
-bool
-LTC_Slave::detect_ltc_discontinuity(LTCFrameExt *frame) {
-	bool discontinuity_detected = false;
-	/* detect discontinuities */
-	if (frame->reverse) {
-		ltc_frame_decrement(&prev_ltc_frame, ceil(timecode.rate), 0);
-	} else {
-		ltc_frame_increment(&prev_ltc_frame, ceil(timecode.rate), 0);
-	}
-
-	if (memcmp(&prev_ltc_frame, &frame->ltc, sizeof(LTCFrame))) {
-		discontinuity_detected = true;
-	}
-	memcpy(&prev_ltc_frame, &frame->ltc, sizeof(LTCFrame));
-
-	/* notfify about discontinuities */
-	if (frames_in_sequence > 0 && discontinuity_detected) {
-		DEBUG_TRACE (DEBUG::LTC, "# LTC DISCONTINUITY\n");
-		frames_in_sequence=0;
-		return true;
-	}
-	frames_in_sequence++;
-
-	return false;
-}
-
-bool
-LTC_Slave::process_ltc(framepos_t const now, framepos_t const sess_pos, framecnt_t const nframes)
+void
+LTC_Slave::process_ltc(framepos_t const now)
 {
-	bool have_frame = false;
-	DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC Process eng-tme: %1 eng-pos: %2\n", now, sess_pos));
-
 	LTCFrameExt frame;
-	while (ltc_decoder_read(decoder,&frame)) {
-		bool reinitialize_ltc_dll = false;
+	while (ltc_decoder_read(decoder, &frame)) {
 		SMPTETimecode stime;
 
 		ltc_frame_to_time(&stime, &frame.ltc, 0);
 		timecode.negative  = false;
 		timecode.subframes  = 0;
+
 		/* set timecode.rate and timecode.drop: */
 		if (detect_ltc_fps(stime.frame, (frame.ltc.dfbit)? true : false)) {
 			reset();
-			break;
-		}
-		if (detect_ltc_discontinuity(&frame)) {
-			ltc_discontinuity = true;
+			last_timestamp = 0;
 		}
 
 #if 0 // Devel/Debug
@@ -268,20 +229,8 @@ LTC_Slave::process_ltc(framepos_t const now, framepos_t const sess_pos, framecnt
 			frame.off_end,
 			frame.reverse ? " R" : "  "
 			);
-
-		if (frames_in_sequence < 1) {
-			fprintf(stdout, " ####### FIRST LTC FRAME in SEQ #######\n");
-		}
-
-		if (ltc_discontinuity) {
-			fprintf(stdout, " ####### LTC DISCONTINUITY #######\n");
-		}
 #endif
 
-		if (frames_in_sequence < 1) {
-			continue;
-		}
-
 		/* when a full LTC frame is decoded, the timecode the LTC frame
 		 * is referring has just passed.
 		 * So we send the _next_ timecode which
@@ -291,11 +240,13 @@ LTC_Slave::process_ltc(framepos_t const now, framepos_t const sess_pos, framecnt
 		if (!frame.reverse) {
 			ltc_frame_increment(&frame.ltc, fps_i , 0);
 			ltc_frame_to_time(&stime, &frame.ltc, 0);
+			transport_direction = 1;
 		} else {
 			ltc_frame_decrement(&frame.ltc, fps_i , 0);
 			int off = frame.off_end - frame.off_start;
 			frame.off_start += off;
 			frame.off_end += off;
+			transport_direction = -1;
 		}
 
 		timecode.hours   = stime.hours;
@@ -311,210 +262,172 @@ LTC_Slave::process_ltc(framepos_t const now, framepos_t const sess_pos, framecnt
 			session.config.get_timecode_offset_negative(), session.config.get_timecode_offset()
 			);
 
-		/* (frame.off_end + 1) = start of next LTC frame */
-		double poff = (frame.off_end + 1 - now);
-		ltc_transport_pos = ltc_frame - poff;
-
-#if 0 // vari-speed LTC, no DLL
-			frames_per_ltc_frame = 1 + frame.off_end - frame.off_start;
-#else
-			frames_per_ltc_frame = (double(session.frame_rate()) / timecode.rate);
-#endif
-
-		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC frame: %1 poff: %2 pos: %3\n", ltc_frame, poff, ltc_transport_pos));
-
-		if (ltc_discontinuity) {
-			ltc_discontinuity=false;
-			if (ltc_speed==0
-					|| !locked()
-					|| (ltc_transport_pos - sess_pos) > FLYWHEEL_TIMEOUT)
-			{
-				engine_dll_initstate = 0;
-				reset();
-			}
-			reinitialize_ltc_dll = true;
-		}
-
-		if (last_timestamp == 0
-			|| ((now - last_timestamp) > FLYWHEEL_TIMEOUT)
-			|| (abs(ltc_transport_pos - sess_pos) >  frames_per_ltc_frame)
-			|| (frame.reverse && transport_direction != -1)
-			|| (!frame.reverse && transport_direction != 1)
-			) {
-			reinitialize_ltc_dll = true;
-			engine_dll_initstate = 0;
-			reset();
-		}
-
-		if (reinitialize_ltc_dll) {
-			init_ltc_dll(ltc_transport_pos, frames_per_ltc_frame);
-
-			if (ltc_speed==0 || !locked()) {
-				if (frame.reverse) {
-					transport_direction = -1;
-					ltc_transport_pos -= nframes * rint((2 * frames_per_ltc_frame + poff)/nframes);
-				} else {
-					transport_direction = 1;
-					ltc_transport_pos += nframes * rint((2 * frames_per_ltc_frame + poff)/nframes);
-				}
-			}
-
+		framepos_t cur_timestamp = frame.off_end + 1;
+		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC F: %1 LF: %2  N: %3 L: %4\n", ltc_frame, last_ltc_frame, cur_timestamp, last_timestamp));
+		if (frame.off_end + 1 <= last_timestamp || last_timestamp == 0) {
+			DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC speed: UNCHANGED: %1\n", ltc_speed));
 		} else {
-			// update DLL
-			double e = (double(ltc_frame) - poff - double(sess_pos));
-			t0 = t1;
-			t1 += b * e + e2;
-			e2 += c * e;
-
-			ltc_speed = (t1 - t0) / frames_per_ltc_frame;
-			//current_delta = (ltc_transport_pos - sess_pos);
-			DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC DLL t0:%1 t1:%2 err:%3 spd:%4 ddt:%5\n", t0, t1, e, ltc_speed, e2 - frames_per_ltc_frame));
+			ltc_speed = double(ltc_frame - last_ltc_frame) / double(cur_timestamp - last_timestamp);
+			DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC speed: %1\n", ltc_speed));
+		}
 
+		if (fabs(ltc_speed) > 10.0) {
+			ltc_speed = 0;
 		}
 
-		/* the are equivalent
-		 * TODO: choose the value with larger diff ->
-		 * minimize roundig errors when extrapolating
-		 */
-#if 1
-		last_timestamp = now;
-		last_ltc_frame = ltc_transport_pos;
-#else
 		last_timestamp = frame.off_end + 1;
 		last_ltc_frame = ltc_frame;
-#endif
-
-		have_frame = true;
 	} /* end foreach decoded LTC frame */
-
-	return have_frame;
 }
 
 void
-LTC_Slave::init_ltc_dll(framepos_t const tme, double const dt)
+LTC_Slave::init_engine_dll (framepos_t pos, int32_t inc)
 {
-	omega = 2.0 * M_PI * (2.0 * dt) / double(session.frame_rate());
+	double omega = 2.0 * M_PI * double(inc) / double(session.frame_rate());
 	b = 1.4142135623730950488 * omega;
 	c = omega * omega;
 
-	e2 = dt;
-	t0 = double(tme);
+	e2 = double(ltc_speed * inc);
+	t0 = double(pos);
 	t1 = t0 + e2;
-	DEBUG_TRACE (DEBUG::LTC, string_compose ("[re-]init LTC DLL %1 %2 %3\n", t0, t1, e2));
-}
-
-void
-LTC_Slave::init_engine_dll (framepos_t pos, framepos_t inc)
-{
-	/* the bandwidth of the DLL is a trade-off,
-	 * because the max-speed of the transport in ardour is
-	 * limited to +-8.0, a larger bandwidth would cause oscillations
-	 *
-	 * But this is only really a problem if the user performs manual
-	 * seeks while transport is running and slaved to LTC.
-	 */
-	oe = 2.0 * M_PI * double(inc) / double(session.frame_rate());
-	be = 1.4142135623730950488 * oe;
-	ce = oe * oe;
-
-	ee2 = double(transport_direction * inc);
-	te0 = double(pos);
-	te1 = te0 + ee2;
-	DEBUG_TRACE (DEBUG::LTC, string_compose ("[re-]init Engine DLL %1 %2 %3\n", te0, te1, ee2));
+	DEBUG_TRACE (DEBUG::LTC, string_compose ("[re-]init Engine DLL %1 %2 %3\n", t0, t1, e2));
 }
 
 /* main entry point from session_process.cc
  * called from jack_process callback context
- * so it is OK to use jack_port_get_buffer() etc
+ * so it is OK to use jack_port_get_buffer()
  */
 bool
 LTC_Slave::speed_and_position (double& speed, framepos_t& pos)
 {
-	//framepos_t now = session.engine().frame_time_at_cycle_start();
-	//framepos_t now = session.engine().processed_frames();
-	framepos_t now = monotonic_cnt;
+	bool engine_init_called = false;
+	framepos_t now = session.engine().frame_time_at_cycle_start();
 	framepos_t sess_pos = session.transport_frame(); // corresponds to now
 	framecnt_t nframes = session.engine().frames_per_cycle();
+
 	jack_default_audio_sample_t *in;
 	jack_latency_range_t ltc_latency;
 
-	monotonic_cnt += nframes;
-
 	boost::shared_ptr<Port> ltcport = session.engine().ltc_input_port();
 	ltcport->get_connected_latency_range(ltc_latency, false);
 	in = (jack_default_audio_sample_t*) jack_port_get_buffer (ltcport->jack_port(), nframes);
 
-	DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC_Slave::speed_and_position - TID:%1 | latency: %2\n", ::pthread_self(), ltc_latency.max));
+	frameoffset_t skip = now - (monotonic_cnt + nframes);
+	monotonic_cnt = now;
+	DEBUG_TRACE (DEBUG::LTC, string_compose ("speed_and_position - TID:%1 | latency: %2 | skip %3\n", ::pthread_self(), ltc_latency.max, skip));
 
 	if (last_timestamp == 0) {
 		engine_dll_initstate = 0;
 		delayedlocked++;
 	}
-	else if (engine_dll_initstate != transport_direction) {
+	else if (engine_dll_initstate != transport_direction && ltc_speed != 0) {
 		engine_dll_initstate = transport_direction;
-		init_engine_dll(last_ltc_frame, session.engine().frames_per_cycle());
+		init_engine_dll(last_ltc_frame + rint(ltc_speed * double(2 * nframes + now - last_timestamp)),
+				session.engine().frames_per_cycle());
+		engine_init_called = true;
 	}
 
 	if (in) {
-		parse_ltc(nframes, in, now  + ltc_latency.max );
-		if (!process_ltc(now, sess_pos, nframes) && ltc_speed != 0) {
+		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC Process eng-tme: %1 eng-pos: %2\n", now, sess_pos));
+		/* when the jack-graph changes and if ardour performs
+		 * locates, the audioengine is stopped (skipping frames) while
+		 * jack [time] moves along.
+		 */
+		if (skip > 0) {
+			DEBUG_TRACE (DEBUG::LTC, string_compose("engine skipped %1 frames. Feeding silence to LTC parser.\n", skip));
+			if (skip >= 8192) skip = 8192;
+			unsigned char sound[8192];
+			memset(sound, 0, sizeof(char) * skip);
+			ltc_decoder_write(decoder, sound, nframes, now);
+		} else if (skip != 0) {
+			/* this should never happen. it may if monotonic_cnt, now overflow on 64bit */
+			DEBUG_TRACE (DEBUG::LTC, string_compose("engine skipped %1 frames\n", skip));
+			reset();
 		}
-	}
 
-	/* interpolate position according to speed and time since last LTC-frame*/
-	double speed_flt = ltc_speed;
-	framecnt_t elapsed;
-	if (speed_flt == 0.0f) {
-		elapsed = 0;
-	} else {
-		/* scale elapsed time by the current LTC speed */
-		if (last_timestamp && (now > last_timestamp)) {
-			elapsed = (now - last_timestamp) * speed_flt;
-		} else {
-			elapsed = 0;
-		}
-		/* update engine DLL and calculate speed */
-		const double e = double (last_ltc_frame + elapsed - sess_pos);
-		te0 = te1;
-		te1 += be * e + ee2;
-		ee2 += ce * e;
-		speed_flt = (te1 - te0) / double(session.engine().frames_per_cycle());
-		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC engine DLL t0:%1 t1:%2 err:%3 spd:%4 ddt:%5\n", te0, te1, e, speed_flt, ee2 - session.engine().frames_per_cycle() ));
+		parse_ltc(nframes, in, now + ltc_latency.max );
+		process_ltc(now);
 	}
 
-	pos = last_ltc_frame + elapsed;
-	speed = speed_flt;
-	current_delta = (pos - sess_pos);
-
-	DEBUG_TRACE (DEBUG::LTC, string_compose ("LTCsync spd: %1 pos: %2 | last-pos: %3 elapsed: %4 delta: %5\n",
-						 speed, pos, last_ltc_frame, elapsed, current_delta));
-
-	if (last_timestamp != 0 /* && frames_in_sequence > 8*/) {
+	if (last_timestamp == 0) {
+		DEBUG_TRACE (DEBUG::LTC, "last timestamp == 0\n");
+		speed = 0;
+		pos = session.transport_frame();
+		return true;
+	} else if (ltc_speed != 0) {
 		delayedlocked = 0;
 	}
 
-	if (last_timestamp == 0 || ((now - last_timestamp) > FLYWHEEL_TIMEOUT)) {
-		DEBUG_TRACE (DEBUG::LTC, "LTC no-signal - reset\n");
+	if (abs(now - last_timestamp) > FLYWHEEL_TIMEOUT) {
+		DEBUG_TRACE (DEBUG::LTC, "flywheel timeout\n");
 		reset();
-		engine_dll_initstate = 0;
 		speed = 0;
 		pos = session.transport_frame();
 		return true;
 	}
 
-	if (last_timestamp != 0  && ltc_speed != 0 && ((ltc_transport_pos < 0) || (labs(current_delta) > 10 * session.frame_rate()))) {
-		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC large drift. %1\n", current_delta));
-		// XXX only re-init engine DLL ?
+	/* it take 2 cycles from naught to rolling.
+	 * during these to initial cycles the speed == 0
+	 * 
+	 * the first cycle:
+	 * DEBUG::Slave: slave stopped, move to NNN
+	 * DEBUG::Transport: Request forced locate to NNN
+	 * DEBUG::Slave: slave state 0 @ NNN speed 0 cur delta VERY-LARGE-DELTA avg delta 1800
+	 * DEBUG::Slave: silent motion
+	 * DEBUG::Transport: realtime stop @ NNN
+	 * DEBUG::Transport: Butler transport work, todo = PostTransportStop,PostTransportLocate,PostTransportClearSubstate
+	 *
+	 * [engine skips frames to locate, jack time keeps rolling on]
+	 *
+	 * the second cycle:
+	 *
+	 * DEBUG::LTC: [re-]init Engine DLL
+	 * DEBUG::Slave: slave stopped, move to NNN+
+	 * ...
+	 *
+	 * we need to seek two cycles ahead: 2 * nframes
+	 */
+	if (engine_dll_initstate == 0) {
+		DEBUG_TRACE (DEBUG::LTC, "engine DLL not initialized. ltc_speed\n");
+		speed = 0;
+		pos = last_ltc_frame + rint(ltc_speed * double(2 * nframes + now - last_timestamp));
+		return true;
+	}
+
+	/* interpolate position according to speed and time since last LTC-frame*/
+	double speed_flt = ltc_speed;
+	double elapsed = (now - last_timestamp) * speed_flt;
+
+	if (!engine_init_called) {
+		const double e = elapsed + double (last_ltc_frame - sess_pos);
+		t0 = t1;
+		t1 += b * e + e2;
+		e2 += c * e;
+		speed_flt = (t1 - t0) / double(session.engine().frames_per_cycle());
+		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC engine DLL t0:%1 t1:%2 err:%3 spd:%4 ddt:%5\n", t0, t1, e, speed_flt, e2 - session.engine().frames_per_cycle() ));
+	} else {
+		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC adjusting elapsed (no DLL) from %1 by %2\n", elapsed, (2 * nframes * ltc_speed)));
+		speed_flt = 0;
+		elapsed += 2.0 * nframes * ltc_speed; /* see note above */
+	}
+
+	pos = last_ltc_frame + rint(elapsed);
+	speed = speed_flt;
+	current_delta = (pos - sess_pos);
+
+	if (((pos < 0) || (labs(current_delta) > 2 * session.frame_rate()))) {
+		DEBUG_TRACE (DEBUG::LTC, string_compose ("LTC large drift: %1\n", current_delta));
 		reset();
-		engine_dll_initstate = 0;
 		speed = 0;
 		pos = session.transport_frame();
 		return true;
 	}
 
-#if 0
-	/* provide a 1% deadzone to lock the speed */
-	if (fabs(speed - 1.0) <= 0.01)
+	DEBUG_TRACE (DEBUG::LTC, string_compose ("LTCsync spd: %1 pos: %2 | last-pos: %3 elapsed: %4 delta: %5\n",
+						 speed, pos, last_ltc_frame, elapsed, current_delta));
+
+#if 1 /* provide a .1% deadzone to lock the speed */
+	if (fabs(speed - 1.0) <= 0.001)
 	        speed = 1.0;
 #endif
 
@@ -554,13 +467,11 @@ std::string
 LTC_Slave::approximate_current_delta() const
 {
 	char delta[24];
-	if (last_timestamp == 0 || frames_in_sequence < 2) {
+	if (last_timestamp == 0 || engine_dll_initstate == 0) {
 		snprintf(delta, sizeof(delta), "\u2012\u2012\u2012\u2012");
 	} else if ((monotonic_cnt - last_timestamp) > 2 * frames_per_ltc_frame) {
 		snprintf(delta, sizeof(delta), "flywheel");
 	} else {
-		// TODO if current_delta > 1 frame -> display timecode.
-		// delta >0 if A3's transport is _behind_ LTC
 		snprintf(delta, sizeof(delta), "%s%4" PRIi64 " sm",
 				PLUSMINUS(-current_delta), abs(current_delta));
 	}
diff --git a/libs/ardour/session_process.cc b/libs/ardour/session_process.cc
index 4af6b10b89..1d11de79d3 100644
--- a/libs/ardour/session_process.cc
+++ b/libs/ardour/session_process.cc
@@ -666,6 +666,7 @@ Session::track_slave_state (float slave_speed, framepos_t slave_transport_frame,
 				}
 
 				if (slave_transport_frame != _transport_frame) {
+					DEBUG_TRACE (DEBUG::Slave, string_compose ("require locate to run. eng: %1 -> sl: %2\n", _transport_frame, slave_transport_frame));
 					locate (slave_transport_frame, false, false);
 				}
 				_slave_state = Running;