midi_clock_slave: fix wrong calculation of loop error

since transport_frame refers to cycle start, the should be position has
to account for position of the midi clock event.

3 files changed, 23 insertions, 11 deletions

diff --git a/libs/ardour/ardour/slave.h b/libs/ardour/ardour/slave.h
index d736a0948a..364929eaab 100644
--- a/libs/ardour/ardour/slave.h
+++ b/libs/ardour/ardour/slave.h
@@ -183,13 +183,14 @@ class Slave {
 class ISlaveSessionProxy {
   public:
 	virtual ~ISlaveSessionProxy() {}
-	virtual TempoMap&  tempo_map()                 const   { return *((TempoMap *) 0); }
-	virtual framecnt_t frame_rate()                const   { return 0; }
-	virtual pframes_t  frames_per_cycle()          const   { return 0; }
-	virtual framepos_t audible_frame ()            const   { return 0; }
-	virtual framepos_t transport_frame ()          const   { return 0; }
-	virtual pframes_t  frames_since_cycle_start () const   { return 0; }
-	virtual framepos_t frame_time ()               const   { return 0; }
+	virtual TempoMap&  tempo_map()                  const   { return *((TempoMap *) 0); }
+	virtual framecnt_t frame_rate()                 const   { return 0; }
+	virtual pframes_t  frames_per_cycle()           const   { return 0; }
+	virtual framepos_t audible_frame ()             const   { return 0; }
+	virtual framepos_t transport_frame ()           const   { return 0; }
+	virtual pframes_t  frames_since_cycle_start ()  const   { return 0; }
+	virtual pframes_t  sample_time_at_cycle_start() const   { return 0; }
+	virtual framepos_t frame_time ()                const   { return 0; }
 
 	virtual void request_locate (framepos_t /*frame*/, bool with_roll = false) {
 		(void) with_roll;
@@ -211,6 +212,7 @@ class SlaveSessionProxy : public ISlaveSessionProxy {
 	framepos_t audible_frame ()              const;
 	framepos_t transport_frame ()            const;
 	pframes_t  frames_since_cycle_start ()   const;
+	pframes_t  sample_time_at_cycle_start()  const;
 	framepos_t frame_time ()                 const;
 
 	void request_locate (framepos_t frame, bool with_roll = false);
diff --git a/libs/ardour/midi_clock_slave.cc b/libs/ardour/midi_clock_slave.cc
index 2e5e4b67ff..25f92a482c 100644
--- a/libs/ardour/midi_clock_slave.cc
+++ b/libs/ardour/midi_clock_slave.cc
@@ -123,6 +123,8 @@ MIDIClock_Slave::update_midi_clock (Parser& /*parser*/, framepos_t timestamp)
 		return;
 	}
 
+	pframes_t cycle_offset = timestamp - session->sample_time_at_cycle_start();
+
 	calculate_one_ppqn_in_frames_at(should_be_position);
 
 	framepos_t elapsed_since_start = timestamp - first_timestamp;
@@ -131,7 +133,6 @@ MIDIClock_Slave::update_midi_clock (Parser& /*parser*/, framepos_t timestamp)
 	if (_starting || last_timestamp == 0) {
 		midi_clock_count = 0;
 
-
 		first_timestamp = timestamp;
 		elapsed_since_start = should_be_position;
 
@@ -156,7 +157,7 @@ MIDIClock_Slave::update_midi_clock (Parser& /*parser*/, framepos_t timestamp)
 		// we use session->transport_frame() instead of t1 here
 		// because t1 is used to calculate the transport speed,
 		// so the loop will compensate for accumulating rounding errors
-		error = (double(should_be_position) - double(session->transport_frame()));
+		error = (double(should_be_position) - (double(session->transport_frame()) + double(cycle_offset)));
 		e = error / double(session->frame_rate());
 		current_delta = error;
 
@@ -167,7 +168,7 @@ MIDIClock_Slave::update_midi_clock (Parser& /*parser*/, framepos_t timestamp)
 	}
 
 	DEBUG_TRACE (DEBUG::MidiClock, string_compose ("clock #%1 @ %2 should-be %3 transport %4 error %5 appspeed %6 "
-						       "read-delta %7 should-be delta %8 t1-t0 %9 t0 %10 t1 %11 framerate %12\n",
+						       "read-delta %7 should-be delta %8 t1-t0 %9 t0 %10 t1 %11 framerate %12 engine %13\n",
 						       midi_clock_count,                                          // #
 						       elapsed_since_start,                                       // @
 						       should_be_position,                                        // should-be
@@ -179,7 +180,9 @@ MIDIClock_Slave::update_midi_clock (Parser& /*parser*/, framepos_t timestamp)
 						       (t1 - t0) * session->frame_rate(),                         // t1-t0
 						       t0 * session->frame_rate(),                                // t0
 						       t1 * session->frame_rate(),                                // t1
-						       session->frame_rate()                                      // framerate
+						       session->frame_rate(),                                      // framerate
+						       session->frame_time()
+
 	));
 
 	last_timestamp = timestamp;
diff --git a/libs/ardour/slave.cc b/libs/ardour/slave.cc
index 2aed535133..c0678ce252 100644
--- a/libs/ardour/slave.cc
+++ b/libs/ardour/slave.cc
@@ -59,6 +59,13 @@ SlaveSessionProxy::frames_since_cycle_start() const
 	return session.engine().samples_since_cycle_start();
 }
 
+pframes_t
+SlaveSessionProxy::sample_time_at_cycle_start() const
+{
+	return session.engine().sample_time_at_cycle_start();
+}
+
+
 framepos_t
 SlaveSessionProxy::frame_time() const
 {