With some attempt at vsync prediction, seeks to smooth audio/video output.

There's plenty more work to do here, but hopefully it takes the issue immediately off the table.

ClockReceiver/VSyncPredictor.hpp

@@ -0,0 +1,63 @@
+//
+//  VSyncPredictor.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/06/2020.
+//  Copyright © 2020 Thomas Harte. All rights reserved.
+//
+
+#ifndef VSyncPredictor_hpp
+#define VSyncPredictor_hpp
+
+#include "TimeTypes.hpp"
+
+namespace Time {
+
+/*!
+	For platforms that provide no avenue into vsync tracking other than block-until-sync,
+	this class tracks: (i) how long frame draw takes; and (ii) the apparent frame period; in order
+	to suggest when you should next start drawing.
+*/
+class VSyncPredictor {
+	public:
+		void begin_redraw() {
+			redraw_begin_time_ = nanos_now();
+		}
+
+		void end_redraw() {
+			const auto redraw_length = nanos_now() - redraw_begin_time_;
+			redraw_period_ =
+				((redraw_period_ * 9) +
+				((redraw_length) * 1)) / 10;
+		}
+
+		void announce_vsync() {
+			const auto vsync_time = nanos_now();
+			if(last_vsync_) {
+				// Use an IIR to try to converge on frame times.
+				vsync_period_ =
+					((vsync_period_ * 9) +
+					((vsync_time - last_vsync_) * 1)) / 10;
+			}
+			last_vsync_ = vsync_time;
+		}
+
+		Nanos suggested_draw_time() {
+			// TODO: this is a very simple version of how this calculation
+			// should be made. It's tracking the average amount of time these
+			// things take, therefore will often be wrong. Deviations need to
+			// be accounted for.
+			return last_vsync_ + vsync_period_ - redraw_period_;
+		}
+
+	private:
+		Nanos redraw_begin_time_ = 0;
+		Nanos last_vsync_ = 0;
+
+		Nanos vsync_period_ = 1'000'000'000 / 60;	// Seems like a good first guess.
+		Nanos redraw_period_ = 0;
+};
+
+}
+
+#endif /* VSyncPredictor_hpp */

OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj

@@ -1317,6 +1317,7 @@
 		4B98A05D1FFAD3F600ADF63B /* CSROMFetcher.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = CSROMFetcher.mm; sourceTree = "<group>"; };
 		4B98A0601FFADCDE00ADF63B /* MSXStaticAnalyserTests.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = MSXStaticAnalyserTests.mm; sourceTree = "<group>"; };
 		4B98A1CD1FFADEC400ADF63B /* MSX ROMs */ = {isa = PBXFileReference; lastKnownFileType = folder; path = "MSX ROMs"; sourceTree = "<group>"; };
+		4B996B2D2496DAC2001660EF /* VSyncPredictor.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = VSyncPredictor.hpp; sourceTree = "<group>"; };
 		4B9BE3FE203A0C0600FFAE60 /* MultiSpeaker.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = MultiSpeaker.cpp; sourceTree = "<group>"; };
 		4B9BE3FF203A0C0600FFAE60 /* MultiSpeaker.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = MultiSpeaker.hpp; sourceTree = "<group>"; };
 		4B9D0C4A22C7D70900DE1AD3 /* 68000BCDTests.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = 68000BCDTests.mm; sourceTree = "<group>"; };
@@ -3855,8 +3856,9 @@
 				4B8A7E85212F988200F2BBC6 /* DeferredQueue.hpp */,
 				4BB06B211F316A3F00600C7A /* ForceInline.hpp */,
 				4B80214322EE7C3E00068002 /* JustInTime.hpp */,
-				4B449C942063389900A095C8 /* TimeTypes.hpp */,
 				4B644ED023F0FB55006C0CC5 /* ScanSynchroniser.hpp */,
+				4B449C942063389900A095C8 /* TimeTypes.hpp */,
+				4B996B2D2496DAC2001660EF /* VSyncPredictor.hpp */,
 			);
 			name = ClockReceiver;
 			path = ../../ClockReceiver;

OSBindings/Qt/ClockSignal.pro

@@ -1,4 +1,4 @@
-QT       += core gui multimedia
+QT       += core gui multimedia quickwidgets
 
 greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
 

OSBindings/Qt/mainwindow.cpp

@@ -47,7 +47,6 @@ void MainWindow::createActions() {
 	openAct->setStatusTip(tr("Open an existing file"));
 	connect(openAct, &QAction::triggered, this, &MainWindow::open);
 	fileMenu->addAction(openAct);
-
 }
 
 void MainWindow::open() {
@@ -143,7 +142,7 @@ void MainWindow::launchMachine() {
 			// Install audio output if required.
 			const auto audio_producer = machine->audio_producer();
 			if(audio_producer) {
-				static constexpr size_t samplesPerBuffer = 256;
+				static constexpr size_t samplesPerBuffer = 2048;	// TODO: select this dynamically; it's very high.
 				const auto speaker = audio_producer->get_speaker();
 				if(speaker) {
 					const QAudioDeviceInfo &defaultDeviceInfo = QAudioDeviceInfo::defaultOutputDevice();

OSBindings/Qt/scantargetwidget.cpp

@@ -13,15 +13,17 @@ void ScanTargetWidget::initializeGL() {
 	glClearColor(0.5, 0.5, 1.0, 1.0);
 
 	// Follow each swapped frame with an additional update.
-	connect(this, SIGNAL(frameSwapped()), this, SLOT(update()));
+	connect(this, &QOpenGLWidget::frameSwapped, this, &ScanTargetWidget::vsync);
 //	qDebug() << "share context: " << bool(context()->shareGroup());
 }
 
 void ScanTargetWidget::paintGL() {
 	glClear(GL_COLOR_BUFFER_BIT);
 	if(scanTarget) {
+		vsync_predictor_.begin_redraw();
 		scanTarget->update(width(), height());
 		scanTarget->draw(width(), height());
+		vsync_predictor_.end_redraw();
 
 //		static int64_t start = 0;
 //		static int frames = 0;
@@ -34,15 +36,26 @@ void ScanTargetWidget::paintGL() {
 	}
 }
 
+void ScanTargetWidget::vsync() {
+	vsync_predictor_.announce_vsync();
+
+	const auto time_now = Time::nanos_now();
+	const auto delay_time = ((vsync_predictor_.suggested_draw_time() - time_now) / 1'000'000) - 5;	// TODO: the extra 5 is a random guess.
+	if(delay_time > 0) {
+		QTimer::singleShot(delay_time, this, SLOT(repaint()));
+	} else {
+		repaint();
+	}
+}
+
 void ScanTargetWidget::resizeGL(int w, int h) {
-    glViewport(0,0,w,h);
+	glViewport(0,0,w,h);
 }
 
 Outputs::Display::OpenGL::ScanTarget *ScanTargetWidget::getScanTarget() {
 	makeCurrent();
 	if(!scanTarget) {
 		scanTarget = std::make_unique<Outputs::Display::OpenGL::ScanTarget>(defaultFramebufferObject());
-		QTimer::singleShot(500, this, SLOT(update()));	// TODO: obviously this is nonsense.
 	}
 	return scanTarget.get();
 }

OSBindings/Qt/scantargetwidget.h

@@ -4,6 +4,7 @@
 #include <QOpenGLWidget>
 
 #include "../../Outputs/OpenGL/ScanTarget.hpp"
+#include "../../ClockReceiver/VSyncPredictor.hpp"
 
 class ScanTargetWidget : public QOpenGLWidget
 {
@@ -20,8 +21,12 @@ class ScanTargetWidget : public QOpenGLWidget
 
 	private:
 		// This should be created only once there's an OpenGL context. So it
-		// can't be done at creation time.4
+		// can't be done at creation time.
 		std::unique_ptr<Outputs::Display::OpenGL::ScanTarget> scanTarget;
+		Time::VSyncPredictor vsync_predictor_;
+
+	private slots:
+		void vsync();
 };
 
 #endif // SCANTARGETWIDGET_H