Move vari-speed into backend (resample ports)

Previously Ardour used a /local/ per track vari-speed mechanism.
Now that the disk-reader is a latency-compensated processor, the speed
of each disk-reader would need to be maintained locally, offset by each
disk-reader's output latency. Furthermore each disk-reader may
produce a different number of samples, depending on its global alignment.

This commit introduces port-data resampling directly at the engine-level:
Up/down-sample all input ports at the beginning, and down/up-sample output
port-data using the inverse ratio at the end of the session's process
cycle.
The session itself is unaware of the speed-change, and only needs to
handle transport speeds {-1, 0, +1}.

This also allows for aligned cue-monitoring and vari-speed recording,
and also pitch-shifts synthesized MIDI along.

1 files changed, 22 insertions, 1 deletions

diff --git a/libs/ardour/audioengine.cc b/libs/ardour/audioengine.cc
index 03c13e38fd..da512cc51d 100644
--- a/libs/ardour/audioengine.cc
+++ b/libs/ardour/audioengine.cc
@@ -190,6 +190,7 @@ int
 AudioEngine::process_callback (pframes_t nframes)
 {
 	Glib::Threads::Mutex::Lock tm (_process_lock, Glib::Threads::TRY_LOCK);
+	Port::set_speed_ratio (1.0);
 
 	PT_TIMING_REF;
 	PT_TIMING_CHECK (1);
@@ -357,6 +358,14 @@ AudioEngine::process_callback (pframes_t nframes)
 		return 0;
 	}
 
+	if (!_freewheeling || Freewheel.empty()) {
+		// run a list of slaves here
+		// - multiple slaves (ow_many_dsp_threads() in paralell)
+		// - session can pick one (ask for position & speed)
+		// - GUI can display all
+		Port::set_speed_ratio (_session->engine_speed ());
+	}
+
 	/* tell all relevant objects that we're starting a new cycle */
 
 	InternalSend::CycleStart (nframes);
@@ -373,7 +382,19 @@ AudioEngine::process_callback (pframes_t nframes)
 	if (_freewheeling && !Freewheel.empty()) {
 		Freewheel (nframes);
 	} else {
-		_session->process (nframes);
+		if (Port::cycle_nframes () <= nframes) {
+			_session->process (Port::cycle_nframes ());
+		} else {
+			pframes_t remain = Port::cycle_nframes ();
+			while (remain > 0) {
+				pframes_t nf = std::min (remain, nframes);
+				_session->process (nf);
+				remain -= nf;
+				if (remain > 0) {
+					split_cycle (nf);
+				}
+			}
+		}
 	}
 
 	if (_freewheeling) {