audio: master-clock playback timing, output clock compensation, buffer config, interpolated speed

- playback timer uses the audio output device as its master clock: the
  PortAudio callback counts consumed frames (including underrun
  zero-fill) so video cannot drift away from what is heard; wall clock
  remains as fallback when no clocked output is running
- output clock compensates for device output latency; new Preferences >
  Audio buffer size setting (0 = auto)
- SampleBuffer::speed() now uses linear interpolation instead of
  nearest-neighbor sampling
- regression tests: audio-clock driven timer (fwd/rev/speed), wall
  clock fallback, interpolation correctness

app/widget/viewer/viewer.cpp

@@ -1384,6 +1384,10 @@ void ViewerWidget::finish_play_preprocess()
 
 	int64_t playback_start_time = get_timestamp();
 
+	// Restart the audio output clock for this playback run; the playback
+	// timer uses it as its master clock
+	AudioManager::instance()->reset_output_clock();
+
 	// Start audio waveform playback
 	if (!prequeued_audio_.isEmpty()) {
 		QString error;

app/widget/viewer/viewerdisplay.cpp

@@ -35,6 +35,7 @@
 #include <QScreen>
 #include <QTextEdit>
 
+#include "audio/audiomanager.h"
 #include "common/define.h"
 #include "common/html.h"
 #include "common/qtutils.h"
@@ -1632,7 +1633,8 @@ void ViewerDisplayWidget::play(const int64_t &start_timestamp,
 	playback_timebase_ = timebase;
 	playback_speed_ = playback_speed;
 
-	timer_.start(start_timestamp, playback_speed, timebase.to_double());
+	timer_.start(start_timestamp, playback_speed, timebase.to_double(),
+				 AudioManager::instance());
 
 	if (start_updating) {
 		connect(this, &ViewerDisplayWidget::frame_swapped, this,

app/widget/viewer/viewerplaybacktimer.cpp

@@ -28,16 +28,36 @@ namespace olive
 
 void ViewerPlaybackTimer::start(const int64_t &start_timestamp,
 								const int &playback_speed,
-								const double &timebase)
+								const double &timebase,
+								const PlaybackAudioClock *audio_clock)
 {
 	timer_.start();
 	start_timestamp_ = start_timestamp;
 	playback_speed_ = playback_speed;
 	timebase_ = timebase * 1000;
+	audio_clock_ = audio_clock;
 }
 
 int64_t ViewerPlaybackTimer::get_timestamp_now() const
 {
+	// The audio output clock is the master clock when available: sound card
+	// consumption is what the viewer must stay in sync with, and unlike the
+	// wall clock it cannot drift away from what is actually heard
+	if (audio_clock_) {
+		const double audio_seconds = audio_clock_->seconds();
+		if (audio_seconds >= 0.0) {
+			// At speeds other than 1x the audio tempo is scaled, so one
+			// output second corresponds to |speed| timeline seconds. The
+			// result is already in timeline frames, so it is applied
+			// signed rather than multiplied by the speed again.
+			const double timeline_ms =
+				audio_seconds * 1000.0 * qAbs(playback_speed_);
+			const int64_t frames_since_start = qFloor(timeline_ms / timebase_);
+			return start_timestamp_ +
+				frames_since_start * (playback_speed_ < 0 ? -1 : 1);
+		}
+	}
+
 	int64_t real_time = timer_.elapsed();
 
 	int64_t frames_since_start =

app/widget/viewer/viewerplaybacktimer.h

@@ -26,6 +26,7 @@
 #include <QElapsedTimer>
 
 #include "common/define.h"
+#include "common/playbackaudioclock.h"
 
 namespace olive
 {
@@ -33,7 +34,8 @@ namespace olive
 class ViewerPlaybackTimer {
 public:
 	void start(const int64_t &start_timestamp, const int &playback_speed,
-			   const double &timebase);
+			   const double &timebase,
+			   const PlaybackAudioClock *audio_clock = nullptr);
 
 	int64_t get_timestamp_now() const;
 
@@ -44,6 +46,8 @@ private:
 	int playback_speed_;
 
 	double timebase_;
+
+	const PlaybackAudioClock *audio_clock_ = nullptr;
 };
 
 }