Fix export alignment (#7916)

Ardour's playback is aligned to master-out:
 "When the playback clock reads 01:00:00:00, the sample corresponding
  to 01:00:00:00 is audible at the speaker(s)"

When exporting, and grabbing data from output ports, the signal
is offset by the master-bus physical playback latency. This was
compensated for, but lead to initial silence in the exported file.

New approach is to start capturing export data during pre-roll,
at the time when playback is written to the output buffers.

To also shaves off a common offset to make this work with
realtime export. Effectively this emulates processing with
disconnected master-output port, while still keeping any
latency of effects on the master-bus itself.

Last but not least: jack updates latencies when freewheeling,
(setting HW latency to zero). The callback arrives asynchronously
some time after enabling freewheeling, but after Export
Ports have been configured. Those callbacks are ignored.

1 files changed, 35 insertions, 13 deletions

diff --git a/libs/ardour/export_graph_builder.cc b/libs/ardour/export_graph_builder.cc
index f4d6d21f2d..eb74d44fd4 100644
--- a/libs/ardour/export_graph_builder.cc
+++ b/libs/ardour/export_graph_builder.cc
@@ -77,20 +77,33 @@ ExportGraphBuilder::~ExportGraphBuilder ()
 {
 }
 
-int
+samplecnt_t
 ExportGraphBuilder::process (samplecnt_t samples, bool last_cycle)
 {
 	assert(samples <= process_buffer_samples);
 
+	sampleoffset_t off = 0;
 	for (ChannelMap::iterator it = channels.begin(); it != channels.end(); ++it) {
 		Sample const * process_buffer = 0;
 		it->first->read (process_buffer, samples);
-		ConstProcessContext<Sample> context(process_buffer, samples, 1);
+
+		if (session.remaining_latency_preroll () >= _master_align + samples) {
+			/* Skip processing during pre-roll, only read/write export ringbuffers */
+			return 0;
+		}
+
+		off = 0;
+		if (session.remaining_latency_preroll () > _master_align) {
+			off = session.remaining_latency_preroll () - _master_align;
+			assert (off < samples);
+		}
+
+		ConstProcessContext<Sample> context(&process_buffer[off], samples - off, 1);
 		if (last_cycle) { context().set_flag (ProcessContext<Sample>::EndOfInput); }
 		it->second->process (context);
 	}
 
-	return 0;
+	return samples - off;
 }
 
 bool
@@ -126,6 +139,7 @@ ExportGraphBuilder::reset ()
 	intermediates.clear ();
 	analysis_map.clear();
 	_realtime = false;
+	_master_align = 0;
 }
 
 void
@@ -148,17 +162,25 @@ ExportGraphBuilder::set_current_timespan (boost::shared_ptr<ExportTimespan> span
 void
 ExportGraphBuilder::add_config (FileSpec const & config, bool rt)
 {
-	ExportChannelConfiguration::ChannelList const & channels =
-		config.channel_config->get_channels();
-	for(ExportChannelConfiguration::ChannelList::const_iterator it = channels.begin();
-	    it != channels.end(); ++it) {
-		(*it)->set_max_buffer_size(process_buffer_samples);
+	/* calculate common latency, shave off master-bus hardware playback latency (if any) */
+	_master_align = session.master_out() ? session.master_out()->output()->connected_latency (true) : 0;
+
+	ExportChannelConfiguration::ChannelList const & channels = config.channel_config->get_channels();
+
+	for(ExportChannelConfiguration::ChannelList::const_iterator it = channels.begin(); it != channels.end(); ++it) {
+		_master_align = std::min (_master_align, (*it)->common_port_playback_latency ());
+	}
+
+	/* now set-up port-data sniffing and delay-ringbuffers */
+	for(ExportChannelConfiguration::ChannelList::const_iterator it = channels.begin(); it != channels.end(); ++it) {
+		(*it)->prepare_export (process_buffer_samples, _master_align);
 	}
 
 	_realtime = rt;
 
-	// If the sample rate is "session rate", change it to the real value.
-	// However, we need to copy it to not change the config which is saved...
+	/* If the sample rate is "session rate", change it to the real value.
+	 * However, we need to copy it to not change the config which is saved...
+	 */
 	FileSpec new_config (config);
 	new_config.format.reset(new ExportFormatSpecification(*new_config.format, false));
 	if(new_config.format->sample_rate() == ExportFormatBase::SR_Session) {
@@ -166,14 +188,14 @@ ExportGraphBuilder::add_config (FileSpec const & config, bool rt)
 		new_config.format->set_sample_rate(ExportFormatBase::nearest_sample_rate(session_rate));
 	}
 
-
 	if (!new_config.channel_config->get_split ()) {
 		add_split_config (new_config);
 		return;
 	}
 
-	// Split channel configurations are split into several channel configurations,
-	// each corresponding to a file, at this stage
+	/* Split channel configurations are split into several channel configurations,
+	 * each corresponding to a file, at this stage
+	 */
 	typedef std::list<boost::shared_ptr<ExportChannelConfiguration> > ConfigList;
 	ConfigList file_configs;
 	new_config.channel_config->configurations_for_files (file_configs);