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, 5 insertions, 3 deletions

diff --git a/libs/ardour/ardour/export_channel.h b/libs/ardour/ardour/export_channel.h
index 2d002ee1e4..ca23eadc67 100644
--- a/libs/ardour/ardour/export_channel.h
+++ b/libs/ardour/ardour/export_channel.h
@@ -50,7 +50,8 @@ class LIBARDOUR_API ExportChannel : public boost::less_than_comparable<ExportCha
 
 	virtual ~ExportChannel () {}
 
-	virtual void set_max_buffer_size(samplecnt_t) { }
+	virtual samplecnt_t common_port_playback_latency () const { return 0; }
+	virtual void prepare_export (samplecnt_t max_samples, sampleoffset_t common_latency) { }
 
 	virtual void read (Sample const *& data, samplecnt_t samples) const = 0;
 	virtual bool empty () const = 0;
@@ -74,7 +75,8 @@ class LIBARDOUR_API PortExportChannel : public ExportChannel
 	PortExportChannel ();
 	~PortExportChannel ();
 
-	void set_max_buffer_size(samplecnt_t samples);
+	samplecnt_t common_port_playback_latency () const;
+	void prepare_export (samplecnt_t max_samples, sampleoffset_t common_latency);
 
 	void read (Sample const *& data, samplecnt_t samples) const;
 	bool empty () const { return ports.empty(); }
@@ -171,7 +173,7 @@ class LIBARDOUR_API RouteExportChannel : public ExportChannel
         static void create_from_route(std::list<ExportChannelPtr> & result, boost::shared_ptr<Route> route);
 
   public: // ExportChannel interface
-	void set_max_buffer_size(samplecnt_t samples);
+	void prepare_export (samplecnt_t max_samples, sampleoffset_t common_latency);
 
 	void read (Sample const *& data, samplecnt_t samples) const;
 	bool empty () const { return false; }