diff --git a/include/dsp.hpp b/include/dsp.hpp
index 3806b6e4..dda07723 100644
--- a/include/dsp.hpp
+++ b/include/dsp.hpp
@@ -8,14 +8,17 @@
 namespace rack {
 
 
-/** S must be a power of 2 */
+/** A simple cyclic buffer.
+S must be a power of 2.
+push() is constant time O(1)
+*/
 template <typename T, size_t S>
 struct RingBuffer {
-	T data[S] = {};
+	T data[S];
 	size_t start = 0;
 	size_t end = 0;
 
-	size_t mask(size_t i) {
+	size_t mask(size_t i) const {
 		return i & (S - 1);
 	}
 	void push(T t) {
@@ -25,26 +28,29 @@ struct RingBuffer {
 	T shift() {
 		return data[mask(start++)];
 	}
-	bool empty() {
+	bool empty() const {
 		return start >= end;
 	}
-	bool full() {
+	bool full() const {
 		return end - start >= S;
 	}
-	size_t size() {
+	size_t size() const {
 		return end - start;
 	}
 };
 
 
-/** S must be a power of 2 */
+/** A cyclic buffer which maintains a valid linear array of size S by keeping a copy of the buffer in adjacent memory.
+S must be a power of 2.
+push() is constant time O(2) relative to RingBuffer
+*/
 template <typename T, size_t S>
 struct DoubleRingBuffer {
-	T data[S*2] = {};
+	T data[S*2];
 	size_t start = 0;
 	size_t end = 0;
 
-	size_t mask(size_t i) {
+	size_t mask(size_t i) const {
 		return i & (S - 1);
 	}
 	void push(T t) {
@@ -55,21 +61,51 @@ struct DoubleRingBuffer {
 	T shift() {
 		return data[mask(start++)];
 	}
-	bool empty() {
+	bool empty() const {
 		return start >= end;
 	}
-	bool full() {
+	bool full() const {
 		return end - start >= S;
 	}
-	size_t size() {
+	size_t size() const {
 		return end - start;
 	}
+	/** Returns a pointer to S consecutive elements for appending.
+	If any data is appended, you must call endIncr afterwards.
+	Pointer is invalidated when any other method is called.
+	*/
+	T *endData() {
+		return &data[mask(end)];
+	}
+	void endIncr(size_t n) {
+		size_t mend = mask(end) + n;
+		if (mend > S) {
+			// Copy data backward from the doubled block to the main block
+			memcpy(data, &data[S], sizeof(T) * (mend - S));
+			// Don't bother copying forward
+		}
+		end += n;
+	}
+	/** Returns a pointer to S consecutive elements for consumption
+	If any data is consumed, call startIncr afterwards.
+	*/
+	const T *startData() const {
+		return &data[mask(start)];
+	}
+	void startIncr(size_t n) {
+		start += n;
+	}
 };
 
 
+/** A cyclic buffer which maintains a valid linear array of size S by sliding along a larger block of size N.
+The linear array of S elements are moved back to the start of the block once it outgrows past the end.
+This happens every N - S pushes, so the push() time is O(1 + S / (N - S)).
+For example, a float buffer of size 64 in a block of size 1024 is nearly as efficient as RingBuffer.
+*/
 template <typename T, size_t S, size_t N>
 struct AppleRingBuffer {
-	T data[N] = {};
+	T data[N];
 	size_t start = 0;
 	size_t end = 0;
 
@@ -85,19 +121,34 @@ struct AppleRingBuffer {
 	T shift() {
 		return data[start++];
 	}
-	bool empty() {
+	bool empty() const {
 		return start >= end;
 	}
-	bool full() {
+	bool full() const {
 		return end - start >= S;
 	}
-	size_t size() {
+	size_t size() const {
 		return end - start;
 	}
+	/** Returns a pointer to S consecutive elements for appending, requesting to append n elements.
+	*/
+	T *endData(size_t n) {
+		// TODO
+		return &data[end];
+	}
+	/** Returns a pointer to S consecutive elements for consumption
+	If any data is consumed, call startIncr afterwards.
+	*/
+	const T *startData() const {
+		return &data[start];
+	}
+	void startIncr(size_t n) {
+		// This is valid as long as n < S
+		start += n;
+	}
 };
 
 
-template <size_t S>
 struct SampleRateConverter {
 	SRC_STATE *state;
 	SRC_DATA data;
@@ -116,17 +167,13 @@ struct SampleRateConverter {
 	void setRatio(float r) {
 		data.src_ratio = r;
 	}
-	void push(const float *in, int length) {
-		float out[S];
+	void process(float *in, int inLength, float *out, int outLength) {
 		data.data_in = in;
-		data.input_frames = length;
+		data.input_frames = inLength;
 		data.data_out = out;
-		data.output_frames = S;
+		data.output_frames = outLength;
 		src_process(state, &data);
 	}
-	void push(float in) {
-		push(&in, 1);
-	}
 };
 
 
diff --git a/include/Rack.hpp b/include/rack.hpp
similarity index 87%
rename from include/Rack.hpp
rename to include/rack.hpp
index 44f576bc..cc06cfea 100644
--- a/include/Rack.hpp
+++ b/include/rack.hpp
@@ -4,17 +4,14 @@
 #include <list>
 #include <vector>
 #include <memory>
+#include <set>
+#include <thread>
+#include <mutex>
 #include "widgets.hpp"
 
 
 namespace rack {
 
-extern std::string gApplicationName;
-extern std::string gApplicationVersion;
-
-extern Scene *gScene;
-extern RackWidget *gRackWidget;
-
 ////////////////////
 // Plugin manager
 ////////////////////
@@ -75,10 +72,6 @@ void drawImage(NVGcontext *vg, Vec pos, int imageId);
 // rack.cpp
 ////////////////////
 
-// TODO Find a clean way to make this a variable
-#define SAMPLE_RATE 44100
-
-
 struct Wire;
 
 struct Module {
@@ -106,17 +99,27 @@ struct Wire {
 	float value = 0.0;
 };
 
-void rackInit();
-void rackDestroy();
-void rackStart();
-void rackStop();
-// Does not transfer ownership
-void rackAddModule(Module *module);
-void rackRemoveModule(Module *module);
-// Does not transfer ownership
-void rackConnectWire(Wire *wire);
-void rackDisconnectWire(Wire *wire);
-void rackSetParamSmooth(Module *module, int paramId, float value);
+struct Rack {
+	Rack();
+	~Rack();
+	/** Launches rack thread */
+	void start();
+	void stop();
+	void run();
+	void step();
+	/** Does not transfer pointer ownership */
+	void addModule(Module *module);
+	void removeModule(Module *module);
+	/** Does not transfer pointer ownership */
+	void addWire(Wire *wire);
+	void removeWire(Wire *wire);
+	void setParamSmooth(Module *module, int paramId, float value);
+
+	float sampleRate;
+
+	struct Impl;
+	Impl *impl;
+};
 
 ////////////////////
 // Optional helpers for plugins
@@ -186,6 +189,17 @@ Screw *createScrew(Vec pos) {
 	return screw;
 }
 
+////////////////////
+// Globals
+////////////////////
+
+extern std::string gApplicationName;
+extern std::string gApplicationVersion;
+
+extern Scene *gScene;
+extern RackWidget *gRackWidget;
+
+extern Rack *gRack;
 
 } // namespace rack
 
diff --git a/include/util.hpp b/include/util.hpp
index ee1b4a4a..c19ffccc 100644
--- a/include/util.hpp
+++ b/include/util.hpp
@@ -3,6 +3,7 @@
 #include <stdint.h>
 #include <math.h>
 #include <random>
+#include <string>
 
 
 namespace rack {
@@ -183,5 +184,12 @@ struct Rect {
 	}
 };
 
+////////////////////
+// Helper functions
+////////////////////
+
+/** Converts a printf format string and optional arguments into a std::string */
+std::string stringf(const char *format, ...);
+
 
 } // namespace rack
diff --git a/src/Rack.cpp b/src/Rack.cpp
new file mode 100644
index 00000000..b14df79f
--- /dev/null
+++ b/src/Rack.cpp
@@ -0,0 +1,216 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <math.h>
+#include <chrono>
+#include <condition_variable>
+
+#include "rack.hpp"
+
+
+namespace rack {
+
+/** Threads which obtain a VIPLock will cause wait() to block for other less important threads.
+This does not provide the VIPs with an exclusive lock. That should be left up to another mutex shared between the less important thread.
+*/
+struct VIPMutex {
+	int count = 0;
+	std::condition_variable cv;
+	std::mutex countMutex;
+
+	/** Blocks until there are no remaining VIPLocks */
+	void wait() {
+		std::unique_lock<std::mutex> lock(countMutex);
+		while (count > 0)
+			cv.wait(lock);
+	}
+};
+
+struct VIPLock {
+	VIPMutex &m;
+	VIPLock(VIPMutex &m) : m(m) {
+		std::unique_lock<std::mutex> lock(m.countMutex);
+		m.count++;
+	}
+	~VIPLock() {
+		std::unique_lock<std::mutex> lock(m.countMutex);
+		m.count--;
+		lock.unlock();
+		m.cv.notify_all();
+	}
+};
+
+
+struct Rack::Impl {
+	bool running = false;
+
+	std::mutex mutex;
+	std::thread audioThread;
+	VIPMutex vipMutex;
+
+	std::set<Module*> modules;
+	// Merely used for keeping track of which module inputs point to which module outputs, to prevent pointer mistakes and make the rack API more rigorous
+	std::set<Wire*> wires;
+
+	// Parameter interpolation
+	Module *smoothModule = NULL;
+	int smoothParamId;
+	float smoothValue;
+};
+
+
+Rack::Rack() {
+	impl = new Rack::Impl();
+	sampleRate = 44100.0;
+}
+
+Rack::~Rack() {
+	// Make sure there are no wires or modules in the rack on destruction. This suggests that a module failed to remove itself before the GUI was destroyed.
+	assert(impl->wires.empty());
+	assert(impl->modules.empty());
+	delete impl;
+}
+
+void Rack::start() {
+	impl->running = true;
+	impl->audioThread = std::thread(&Rack::run, this);
+}
+
+void Rack::stop() {
+	impl->running = false;
+	impl->audioThread.join();
+}
+
+void Rack::run() {
+	// Every time the rack waits and locks a mutex, it steps this many frames
+	const int stepSize = 32;
+
+	while (impl->running) {
+		impl->vipMutex.wait();
+
+		auto start = std::chrono::high_resolution_clock::now();
+		{
+			std::lock_guard<std::mutex> lock(impl->mutex);
+			for (int i = 0; i < stepSize; i++) {
+				step();
+			}
+		}
+		auto end = std::chrono::high_resolution_clock::now();
+		auto duration = std::chrono::nanoseconds((long) (0.9 * 1e9 * stepSize / sampleRate)) - (end - start);
+		// Avoid pegging the CPU at 100% when there are no "blocking" modules like AudioInterface, but still step audio at a reasonable rate
+		std::this_thread::sleep_for(duration);
+	}
+}
+
+void Rack::step() {
+	// Param interpolation
+	if (impl->smoothModule) {
+		float value = impl->smoothModule->params[impl->smoothParamId];
+		const float lambda = 60.0; // decay rate is 1 graphics frame
+		const float snap = 0.0001;
+		float delta = impl->smoothValue - value;
+		if (fabsf(delta) < snap) {
+			impl->smoothModule->params[impl->smoothParamId] = impl->smoothValue;
+			impl->smoothModule = NULL;
+		}
+		else {
+			value += delta * lambda / sampleRate;
+			impl->smoothModule->params[impl->smoothParamId] = value;
+		}
+	}
+	// Step all modules
+	std::chrono::time_point<std::chrono::high_resolution_clock> start, end;
+	for (Module *module : impl->modules) {
+		// Start clock for CPU usage
+		start = std::chrono::high_resolution_clock::now();
+		// Step module by one frame
+		module->step();
+		// Stop clock and smooth step time value
+		end = std::chrono::high_resolution_clock::now();
+		std::chrono::duration<float> diff = end - start;
+		float elapsed = diff.count() * sampleRate;
+		const float lambda = 1.0;
+		module->cpuTime += (elapsed - module->cpuTime) * lambda / sampleRate;
+	}
+}
+
+void Rack::addModule(Module *module) {
+	assert(module);
+	VIPLock vipLock(impl->vipMutex);
+	std::lock_guard<std::mutex> lock(impl->mutex);
+	// Check that the module is not already added
+	assert(impl->modules.find(module) == impl->modules.end());
+	impl->modules.insert(module);
+}
+
+void Rack::removeModule(Module *module) {
+	assert(module);
+	VIPLock vipLock(impl->vipMutex);
+	std::lock_guard<std::mutex> lock(impl->mutex);
+	// Remove parameter interpolation which point to this module
+	if (module == impl->smoothModule) {
+		impl->smoothModule = NULL;
+	}
+	// Check that all wires are disconnected
+	for (Wire *wire : impl->wires) {
+		assert(wire->outputModule != module);
+		assert(wire->inputModule != module);
+	}
+	auto it = impl->modules.find(module);
+	if (it != impl->modules.end()) {
+		impl->modules.erase(it);
+	}
+}
+
+void Rack::addWire(Wire *wire) {
+	assert(wire);
+	VIPLock vipLock(impl->vipMutex);
+	std::lock_guard<std::mutex> lock(impl->mutex);
+	// It would probably be good to reset the wire voltage
+	wire->value = 0.0;
+	// Check that the wire is not already added
+	assert(impl->wires.find(wire) == impl->wires.end());
+	assert(wire->outputModule);
+	assert(wire->inputModule);
+	// Check that the inputs/outputs are not already used by another cable
+	for (Wire *wire2 : impl->wires) {
+		assert(wire2 != wire);
+		assert(!(wire2->outputModule == wire->outputModule && wire2->outputId == wire->outputId));
+		assert(!(wire2->inputModule == wire->inputModule && wire2->inputId == wire->inputId));
+	}
+	// Connect the wire to inputModule
+	impl->wires.insert(wire);
+	wire->inputModule->inputs[wire->inputId] = &wire->value;
+	wire->outputModule->outputs[wire->outputId] = &wire->value;
+}
+
+void Rack::removeWire(Wire *wire) {
+	assert(wire);
+	VIPLock vipLock(impl->vipMutex);
+	std::lock_guard<std::mutex> lock(impl->mutex);
+	// Disconnect wire from inputModule
+	wire->inputModule->inputs[wire->inputId] = NULL;
+	wire->outputModule->outputs[wire->outputId] = NULL;
+
+	auto it = impl->wires.find(wire);
+	assert(it != impl->wires.end());
+	impl->wires.erase(it);
+}
+
+void Rack::setParamSmooth(Module *module, int paramId, float value) {
+	VIPLock vipLock(impl->vipMutex);
+	std::lock_guard<std::mutex> lock(impl->mutex);
+	// Check existing parameter interpolation
+	if (impl->smoothModule) {
+		if (!(impl->smoothModule == module && impl->smoothParamId == paramId)) {
+			// Jump param value to smooth value
+			impl->smoothModule->params[impl->smoothParamId] = impl->smoothValue;
+		}
+	}
+	impl->smoothModule = module;
+	impl->smoothParamId = paramId;
+	impl->smoothValue = value;
+}
+
+
+} // namespace rack
diff --git a/src/core/AudioInterface.cpp b/src/core/AudioInterface.cpp
index a02ec7a5..f21f514c 100644
--- a/src/core/AudioInterface.cpp
+++ b/src/core/AudioInterface.cpp
@@ -37,21 +37,25 @@ struct AudioInterface : Module {
 	PaStream *stream = NULL;
 	int numOutputs;
 	int numInputs;
-	int numFrames;
+	int bufferFrame;
 	float outputBuffer[1<<14] = {};
 	float inputBuffer[1<<14] = {};
-	int bufferFrame;
 	// Used because the GUI thread and Rack thread can both interact with this class
 	std::mutex mutex;
 
+	// Set these and call openDevice()
+	int deviceId = -1;
+	float sampleRate = 44100.0;
+	int blockSize = 256;
+
 	AudioInterface();
 	~AudioInterface();
 	void step();
 
 	int getDeviceCount();
 	std::string getDeviceName(int deviceId);
-	// Use -1 as the deviceId to close the current device
-	void openDevice(int deviceId);
+	void openDevice();
+	void closeDevice();
 };
 
 
@@ -59,10 +63,11 @@ AudioInterface::AudioInterface() {
 	params.resize(NUM_PARAMS);
 	inputs.resize(NUM_INPUTS);
 	outputs.resize(NUM_OUTPUTS);
+	closeDevice();
 }
 
 AudioInterface::~AudioInterface() {
-	openDevice(-1);
+	closeDevice();
 }
 
 void AudioInterface::step() {
@@ -85,14 +90,14 @@ void AudioInterface::step() {
 		setf(outputs[i], inputBuffer[numOutputs * bufferFrame + i] * 5.0);
 	}
 
-	if (++bufferFrame >= numFrames) {
+	if (++bufferFrame >= blockSize) {
 		bufferFrame = 0;
 		PaError err;
 
 		// Input
 		// (for some reason, if you write the output stream before you read the input stream, PortAudio can segfault on Windows.)
 		if (numInputs > 0) {
-			err = Pa_ReadStream(stream, inputBuffer, numFrames);
+			err = Pa_ReadStream(stream, inputBuffer, blockSize);
 			if (err) {
 				// Ignore buffer underflows
 				if (err != paInputOverflowed) {
@@ -103,7 +108,7 @@ void AudioInterface::step() {
 
 		// Output
 		if (numOutputs > 0) {
-			err = Pa_WriteStream(stream, outputBuffer, numFrames);
+			err = Pa_WriteStream(stream, outputBuffer, blockSize);
 			if (err) {
 				// Ignore buffer underflows
 				if (err != paOutputUnderflowed) {
@@ -123,29 +128,13 @@ std::string AudioInterface::getDeviceName(int deviceId) {
 	if (!info)
 		return "";
 	const PaHostApiInfo *apiInfo = Pa_GetHostApiInfo(info->hostApi);
-	char name[1024];
-	snprintf(name, sizeof(name), "%s: %s (%d in, %d out)", apiInfo->name, info->name, info->maxInputChannels, info->maxOutputChannels);
-	return name;
+	return stringf("%s: %s (%d in, %d out)", apiInfo->name, info->name, info->maxInputChannels, info->maxOutputChannels);
 }
 
-void AudioInterface::openDevice(int deviceId) {
+void AudioInterface::openDevice() {
+	closeDevice();
 	std::lock_guard<std::mutex> lock(mutex);
 
-	// Close existing device
-	if (stream) {
-		PaError err;
-		err = Pa_CloseStream(stream);
-		if (err) {
-			// This shouldn't happen:
-			printf("Failed to close audio stream: %s\n", Pa_GetErrorText(err));
-		}
-		stream = NULL;
-	}
-	numOutputs = 0;
-	numInputs = 0;
-
-	numFrames = 256;
-	bufferFrame = 0;
 	// Open new device
 	if (deviceId >= 0) {
 		PaError err;
@@ -173,10 +162,10 @@ void AudioInterface::openDevice(int deviceId) {
 		inputParameters.hostApiSpecificStreamInfo = NULL;
 
 		// Don't use stream parameters if 0 input or output channels
-		PaStreamParameters *outputP = numOutputs == 0 ? NULL : &outputParameters;
-		PaStreamParameters *inputP = numInputs == 0 ? NULL : &inputParameters;
-
-		err = Pa_OpenStream(&stream, inputP, outputP, SAMPLE_RATE, numFrames, paNoFlag, NULL, NULL);
+		err = Pa_OpenStream(&stream,
+			numInputs == 0 ? NULL : &outputParameters,
+			numOutputs == 0 ? NULL : &inputParameters,
+			sampleRate, blockSize, paNoFlag, NULL, NULL);
 		if (err) {
 			printf("Failed to open audio stream: %s\n", Pa_GetErrorText(err));
 			return;
@@ -190,12 +179,30 @@ void AudioInterface::openDevice(int deviceId) {
 	}
 }
 
+void AudioInterface::closeDevice() {
+	std::lock_guard<std::mutex> lock(mutex);
+
+	if (stream) {
+		PaError err;
+		err = Pa_CloseStream(stream);
+		if (err) {
+			// This shouldn't happen:
+			printf("Failed to close audio stream: %s\n", Pa_GetErrorText(err));
+		}
+		stream = NULL;
+	}
+	numOutputs = 0;
+	numInputs = 0;
+	bufferFrame = 0;
+}
+
 
 struct AudioItem : MenuItem {
 	AudioInterface *audioInterface;
 	int deviceId;
 	void onAction() {
-		audioInterface->openDevice(deviceId);
+		audioInterface->deviceId = deviceId;
+		audioInterface->openDevice();
 	}
 };
 
@@ -224,6 +231,81 @@ struct AudioChoice : ChoiceButton {
 		overlay->addChild(menu);
 		gScene->setOverlay(overlay);
 	}
+	void step() {
+		std::string name = audioInterface->getDeviceName(audioInterface->deviceId);
+		if (name.empty())
+			text = "(no device)";
+		else
+			text = name;
+	}
+};
+
+
+struct SampleRateItem : MenuItem {
+	AudioInterface *audioInterface;
+	float sampleRate;
+	void onAction() {
+		audioInterface->sampleRate = sampleRate;
+		audioInterface->openDevice();
+	}
+};
+
+struct SampleRateChoice : ChoiceButton {
+	AudioInterface *audioInterface;
+	void onAction() {
+		MenuOverlay *overlay = new MenuOverlay();
+		Menu *menu = new Menu();
+		menu->box.pos = getAbsolutePos().plus(Vec(0, box.size.y));
+
+		const float sampleRates[6] = {44100, 48000, 88200, 96000, 176400, 192000};
+		for (int i = 0; i < 6; i++) {
+			SampleRateItem *item = new SampleRateItem();
+			item->audioInterface = audioInterface;
+			item->sampleRate = sampleRates[i];
+			item->text = stringf("%.0f", sampleRates[i]);
+			menu->pushChild(item);
+		}
+
+		overlay->addChild(menu);
+		gScene->setOverlay(overlay);
+	}
+	void step() {
+		this->text = stringf("%.0f", audioInterface->sampleRate);
+	}
+};
+
+
+struct BlockSizeItem : MenuItem {
+	AudioInterface *audioInterface;
+	int blockSize;
+	void onAction() {
+		audioInterface->blockSize = blockSize;
+		audioInterface->openDevice();
+	}
+};
+
+struct BlockSizeChoice : ChoiceButton {
+	AudioInterface *audioInterface;
+	void onAction() {
+		MenuOverlay *overlay = new MenuOverlay();
+		Menu *menu = new Menu();
+		menu->box.pos = getAbsolutePos().plus(Vec(0, box.size.y));
+
+		const int blockSizes[6] = {128, 256, 512, 1024, 2048, 4096};
+		for (int i = 0; i < 6; i++) {
+			BlockSizeItem *item = new BlockSizeItem();
+			item->audioInterface = audioInterface;
+			item->blockSize = blockSizes[i];
+			item->text = stringf("%d", blockSizes[i]);
+			menu->pushChild(item);
+		}
+
+		overlay->addChild(menu);
+		gScene->setOverlay(overlay);
+	}
+	void step() {
+		this->text = stringf("%d", audioInterface->blockSize);
+	}
 };
 
 
@@ -242,13 +324,48 @@ AudioInterfaceWidget::AudioInterfaceWidget() : ModuleWidget(new AudioInterface()
 	}
 
 	{
-		AudioChoice *audioChoice = new AudioChoice();
-		audioChoice->audioInterface = dynamic_cast<AudioInterface*>(module);
-		audioChoice->text = "Audio device";
-		audioChoice->box.pos = Vec(margin, yPos);
-		audioChoice->box.size.x = box.size.x - 10;
-		addChild(audioChoice);
-		yPos += audioChoice->box.size.y + 2*margin;
+		Label *label = new Label();
+		label->box.pos = Vec(margin, yPos);
+		label->text = "Audio device";
+		addChild(label);
+		yPos += label->box.size.y + margin;
+
+		AudioChoice *choice = new AudioChoice();
+		choice->audioInterface = dynamic_cast<AudioInterface*>(module);
+		choice->box.pos = Vec(margin, yPos);
+		choice->box.size.x = box.size.x - 10;
+		addChild(choice);
+		yPos += choice->box.size.y + 2*margin;
+	}
+
+	{
+		Label *label = new Label();
+		label->box.pos = Vec(margin, yPos);
+		label->text = "Sample rate";
+		addChild(label);
+		yPos += label->box.size.y + margin;
+
+		SampleRateChoice *choice = new SampleRateChoice();
+		choice->audioInterface = dynamic_cast<AudioInterface*>(module);
+		choice->box.pos = Vec(margin, yPos);
+		choice->box.size.x = box.size.x - 10;
+		addChild(choice);
+		yPos += choice->box.size.y + 2*margin;
+	}
+
+	{
+		Label *label = new Label();
+		label->box.pos = Vec(margin, yPos);
+		label->text = "Block size";
+		addChild(label);
+		yPos += label->box.size.y + margin;
+
+		BlockSizeChoice *choice = new BlockSizeChoice();
+		choice->audioInterface = dynamic_cast<AudioInterface*>(module);
+		choice->box.pos = Vec(margin, yPos);
+		choice->box.size.x = box.size.x - 10;
+		addChild(choice);
+		yPos += choice->box.size.y + 2*margin;
 	}
 
 	{
diff --git a/src/core/MidiInterface.cpp b/src/core/MidiInterface.cpp
index 67227939..0c72504d 100644
--- a/src/core/MidiInterface.cpp
+++ b/src/core/MidiInterface.cpp
@@ -92,9 +92,7 @@ std::string MidiInterface::getPortName(int portId) {
 	const PmDeviceInfo *info = Pm_GetDeviceInfo(portId);
 	if (!info)
 		return "";
-	char name[1024];
-	snprintf(name, sizeof(name), "%s: %s (%s)", info->interf, info->name, info->input ? "input" : "output");
-	return name;
+	return stringf("%s: %s (%s)", info->interf, info->name, info->input ? "input" : "output");
 }
 
 void MidiInterface::openPort(int portId) {
diff --git a/src/core/core.hpp b/src/core/core.hpp
index d99ad211..179a15dd 100644
--- a/src/core/core.hpp
+++ b/src/core/core.hpp
@@ -1,7 +1,9 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
-rack::Plugin *coreInit();
+using namespace rack;
+
+Plugin *coreInit();
 
 void audioInit();
 void midiInit();
@@ -10,12 +12,12 @@ void midiInit();
 // module widgets
 ////////////////////
 
-struct AudioInterfaceWidget : rack::ModuleWidget {
+struct AudioInterfaceWidget : ModuleWidget {
 	AudioInterfaceWidget();
 	void draw(NVGcontext *vg);
 };
 
-struct MidiInterfaceWidget : rack::ModuleWidget {
+struct MidiInterfaceWidget : ModuleWidget {
 	MidiInterfaceWidget();
 	void draw(NVGcontext *vg);
 };
diff --git a/src/gui.cpp b/src/gui.cpp
index 4313abb0..fea68a12 100644
--- a/src/gui.cpp
+++ b/src/gui.cpp
@@ -1,5 +1,5 @@
 #include <unistd.h>
-#include "Rack.hpp"
+#include "rack.hpp"
 
 #include <GL/glew.h>
 #include <GLFW/glfw3.h>
diff --git a/src/main.cpp b/src/main.cpp
index 9ada1bac..800fed4d 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -4,7 +4,7 @@
 	#include <libgen.h>
 #endif
 
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
@@ -12,14 +12,16 @@ namespace rack {
 std::string gApplicationName = "Virtuoso Rack";
 std::string gApplicationVersion = "v0.0.0 alpha";
 
+Rack *gRack;
+
 } // namespace rack
 
 
 using namespace rack;
 
 int main() {
-	// Set working directory
 #if defined(APPLE)
+	// macOS workaround for setting the working directory to the location of the .app
 	{
 		CFBundleRef bundle = CFBundleGetMainBundle();
 		CFURLRef bundleURL = CFBundleCopyBundleURL(bundle);
@@ -32,19 +34,18 @@ int main() {
 	}
 #endif
 
-
-	rackInit();
+	gRack = new Rack();
 	guiInit();
 	pluginInit();
 	gRackWidget->loadPatch("autosave.json");
 
-	rackStart();
+	gRack->start();
 	guiRun();
-	rackStop();
+	gRack->stop();
 
 	gRackWidget->savePatch("autosave.json");
 	guiDestroy();
-	rackDestroy();
+	delete gRack;
 	pluginDestroy();
 	return 0;
 }
diff --git a/src/plugin.cpp b/src/plugin.cpp
index 41e46f31..3870c76f 100644
--- a/src/plugin.cpp
+++ b/src/plugin.cpp
@@ -7,7 +7,7 @@
 	#include <glob.h>
 #endif
 
-#include "Rack.hpp"
+#include "rack.hpp"
 #include "core/core.hpp"
 
 
diff --git a/src/rack.cpp b/src/rack.cpp
deleted file mode 100644
index 11d5947b..00000000
--- a/src/rack.cpp
+++ /dev/null
@@ -1,202 +0,0 @@
-#include <stdio.h>
-#include <stdlib.h>
-#include <assert.h>
-#include <math.h>
-#include <set>
-#include <chrono>
-#include <thread>
-#include <mutex>
-#include <condition_variable>
-
-#include "Rack.hpp"
-
-
-namespace rack {
-
-static std::thread thread;
-static std::mutex mutex;
-static bool running;
-
-static std::set<Module*> modules;
-// Merely used for keeping track of which module inputs point to which module outputs, to prevent pointer mistakes and make the rack API rigorous
-static std::set<Wire*> wires;
-
-// Parameter interpolation
-static Module *smoothModule = NULL;
-static int smoothParamId;
-static float smoothValue;
-
-
-// HACK
-static std::mutex requestMutex;
-static std::condition_variable requestCv;
-static bool request = false;
-
-struct RackRequest {
-	RackRequest() {
-		std::unique_lock<std::mutex> lock(requestMutex);
-		request = true;
-	}
-	~RackRequest() {
-		std::unique_lock<std::mutex> lock(requestMutex);
-		request = false;
-		lock.unlock();
-		requestCv.notify_all();
-	}
-};
-// END HACK
-
-
-void rackInit() {
-}
-
-void rackDestroy() {
-	// Make sure there are no wires or modules in the rack on destruction. This suggests that a module failed to remove itself before the GUI was destroyed.
-	assert(wires.empty());
-	assert(modules.empty());
-}
-
-void rackStep() {
-	// Param interpolation
-	if (smoothModule) {
-		float value = smoothModule->params[smoothParamId];
-		const float lambda = 60.0; // decay rate is 1 graphics frame
-		const float snap = 0.0001;
-		float delta = smoothValue - value;
-		if (fabsf(delta) < snap) {
-			smoothModule->params[smoothParamId] = smoothValue;
-			smoothModule = NULL;
-		}
-		else {
-			value += delta * lambda / SAMPLE_RATE;
-			smoothModule->params[smoothParamId] = value;
-		}
-	}
-	// Step all modules
-	std::chrono::time_point<std::chrono::high_resolution_clock> start, end;
-	for (Module *module : modules) {
-		// Start clock for CPU usage
-		start = std::chrono::high_resolution_clock::now();
-		// Step module by one frame
-		module->step();
-		// Stop clock and smooth step time value
-		end = std::chrono::high_resolution_clock::now();
-		std::chrono::duration<float> diff = end - start;
-		float elapsed = diff.count() * SAMPLE_RATE;
-		const float lambda = 1.0;
-		module->cpuTime += (elapsed - module->cpuTime) * lambda / SAMPLE_RATE;
-	}
-}
-
-static void rackRun() {
-	// Every time the rack waits and locks a mutex, it steps this many frames
-	const int stepSize = 32;
-
-	while (running) {
-		auto start = std::chrono::high_resolution_clock::now();
-		// This lock is to make sure the GUI gets higher priority than this thread
-		{
-			std::unique_lock<std::mutex> lock(requestMutex);
-			while (request)
-				requestCv.wait(lock);
-		}
-		{
-			std::lock_guard<std::mutex> lock(mutex);
-			for (int i = 0; i < stepSize; i++) {
-				rackStep();
-			}
-		}
-		auto end = std::chrono::high_resolution_clock::now();
-		auto duration = std::chrono::nanoseconds((long) (0.9 * 1e9 * stepSize / SAMPLE_RATE)) - (end - start);
-		std::this_thread::sleep_for(duration);
-	}
-}
-
-void rackStart() {
-	running = true;
-	thread = std::thread(rackRun);
-}
-
-void rackStop() {
-	running = false;
-	thread.join();
-}
-
-void rackAddModule(Module *module) {
-	assert(module);
-	RackRequest rm;
-	std::lock_guard<std::mutex> lock(mutex);
-	// Check that the module is not already added
-	assert(modules.find(module) == modules.end());
-	modules.insert(module);
-}
-
-void rackRemoveModule(Module *module) {
-	assert(module);
-	RackRequest rm;
-	std::lock_guard<std::mutex> lock(mutex);
-	// Remove parameter interpolation which point to this module
-	if (module == smoothModule) {
-		smoothModule = NULL;
-	}
-	// Check that all wires are disconnected
-	for (Wire *wire : wires) {
-		assert(wire->outputModule != module);
-		assert(wire->inputModule != module);
-	}
-	auto it = modules.find(module);
-	if (it != modules.end()) {
-		modules.erase(it);
-	}
-}
-
-void rackConnectWire(Wire *wire) {
-	assert(wire);
-	RackRequest rm;
-	std::lock_guard<std::mutex> lock(mutex);
-	// It would probably be good to reset the wire voltage
-	wire->value = 0.0;
-	// Check that the wire is not already added
-	assert(wires.find(wire) == wires.end());
-	assert(wire->outputModule);
-	assert(wire->inputModule);
-	// Check that the inputs/outputs are not already used by another cable
-	for (Wire *wire2 : wires) {
-		assert(wire2 != wire);
-		assert(!(wire2->outputModule == wire->outputModule && wire2->outputId == wire->outputId));
-		assert(!(wire2->inputModule == wire->inputModule && wire2->inputId == wire->inputId));
-	}
-	// Connect the wire to inputModule
-	wires.insert(wire);
-	wire->inputModule->inputs[wire->inputId] = &wire->value;
-	wire->outputModule->outputs[wire->outputId] = &wire->value;
-}
-
-void rackDisconnectWire(Wire *wire) {
-	assert(wire);
-	RackRequest rm;
-	std::lock_guard<std::mutex> lock(mutex);
-	// Disconnect wire from inputModule
-	wire->inputModule->inputs[wire->inputId] = NULL;
-	wire->outputModule->outputs[wire->outputId] = NULL;
-
-	auto it = wires.find(wire);
-	assert(it != wires.end());
-	wires.erase(it);
-}
-
-void rackSetParamSmooth(Module *module, int paramId, float value) {
-	// Check existing parameter interpolation
-	if (smoothModule) {
-		if (!(smoothModule == module && smoothParamId == paramId)) {
-			// Jump param value to smooth value
-			smoothModule->params[smoothParamId] = smoothValue;
-		}
-	}
-	smoothModule = module;
-	smoothParamId = paramId;
-	smoothValue = value;
-}
-
-
-} // namespace rack
diff --git a/src/util.cpp b/src/util.cpp
index 38c89f49..becac658 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -1,4 +1,6 @@
 #include "util.hpp"
+#include <stdio.h>
+#include <stdarg.h>
 
 
 namespace rack {
@@ -23,5 +25,20 @@ float randomNormal(){
 	return normalDist(rng);
 }
 
+std::string stringf(const char *format, ...) {
+	va_list ap;
+	va_start(ap, format);
+	size_t size = vsnprintf(NULL, 0, format, ap);
+	if (size < 0)
+		return "";
+	size++;
+	char *buf = new char[size];
+	vsnprintf(buf, size, format, ap);
+	va_end(ap);
+	std::string s = buf;
+	delete[] buf;
+	return s;
+}
+
 
 } // namespace rack
diff --git a/src/widgets/Button.cpp b/src/widgets/Button.cpp
index 4a286453..23a49903 100644
--- a/src/widgets/Button.cpp
+++ b/src/widgets/Button.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/ChoiceButton.cpp b/src/widgets/ChoiceButton.cpp
index c126af8f..3e8fc75a 100644
--- a/src/widgets/ChoiceButton.cpp
+++ b/src/widgets/ChoiceButton.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/InputPort.cpp b/src/widgets/InputPort.cpp
index ec8909a3..04c7c931 100644
--- a/src/widgets/InputPort.cpp
+++ b/src/widgets/InputPort.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Knob.cpp b/src/widgets/Knob.cpp
index 55247807..d85299e5 100644
--- a/src/widgets/Knob.cpp
+++ b/src/widgets/Knob.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Label.cpp b/src/widgets/Label.cpp
index 740708f2..aeaa32a2 100644
--- a/src/widgets/Label.cpp
+++ b/src/widgets/Label.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Light.cpp b/src/widgets/Light.cpp
index 1e9edefe..16e079bd 100644
--- a/src/widgets/Light.cpp
+++ b/src/widgets/Light.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Menu.cpp b/src/widgets/Menu.cpp
index bd950358..38ba4283 100644
--- a/src/widgets/Menu.cpp
+++ b/src/widgets/Menu.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/MenuEntry.cpp b/src/widgets/MenuEntry.cpp
index 15cfd560..84ef8d8e 100644
--- a/src/widgets/MenuEntry.cpp
+++ b/src/widgets/MenuEntry.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/MenuItem.cpp b/src/widgets/MenuItem.cpp
index 23538ac5..4b37bf51 100644
--- a/src/widgets/MenuItem.cpp
+++ b/src/widgets/MenuItem.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/MenuLabel.cpp b/src/widgets/MenuLabel.cpp
index e62691c5..eb6903ed 100644
--- a/src/widgets/MenuLabel.cpp
+++ b/src/widgets/MenuLabel.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/MenuOverlay.cpp b/src/widgets/MenuOverlay.cpp
index fc67c7d8..2d2f2a92 100644
--- a/src/widgets/MenuOverlay.cpp
+++ b/src/widgets/MenuOverlay.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/ModulePanel.cpp b/src/widgets/ModulePanel.cpp
index 413725f4..7a4b0e60 100644
--- a/src/widgets/ModulePanel.cpp
+++ b/src/widgets/ModulePanel.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/ModuleWidget.cpp b/src/widgets/ModuleWidget.cpp
index 91b2af1d..4d7349be 100644
--- a/src/widgets/ModuleWidget.cpp
+++ b/src/widgets/ModuleWidget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
@@ -6,7 +6,7 @@ namespace rack {
 ModuleWidget::ModuleWidget(Module *module) {
 	this->module = module;
 	if (module) {
-		rackAddModule(module);
+		gRack->addModule(module);
 	}
 }
 
@@ -14,7 +14,7 @@ ModuleWidget::~ModuleWidget() {
 	// Make sure WireWidget destructors are called *before* removing `module` from the rack.
 	disconnectPorts();
 	if (module) {
-		rackRemoveModule(module);
+		gRack->removeModule(module);
 		delete module;
 	}
 }
@@ -101,10 +101,9 @@ void ModuleWidget::draw(NVGcontext *vg) {
 
 	// CPU usage text
 	if (module) {
-		char text[32];
-		snprintf(text, sizeof(text), "%.2f%%", module->cpuTime * 10);
+		std::string text = stringf("%.1f%%", module->cpuTime * 100.0);
 		nvgSave(vg);
-		bndSlider(vg, box.pos.x, box.pos.y, box.size.x, BND_WIDGET_HEIGHT, BND_CORNER_ALL, BND_DEFAULT, module->cpuTime, text, NULL);
+		bndSlider(vg, box.pos.x, box.pos.y, box.size.x, BND_WIDGET_HEIGHT, BND_CORNER_ALL, BND_DEFAULT, module->cpuTime, text.c_str(), NULL);
 		nvgRestore(vg);
 	}
 }
diff --git a/src/widgets/OutputPort.cpp b/src/widgets/OutputPort.cpp
index 7cc74cfa..52a69c75 100644
--- a/src/widgets/OutputPort.cpp
+++ b/src/widgets/OutputPort.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/ParamWidget.cpp b/src/widgets/ParamWidget.cpp
index 902ec278..233c8829 100644
--- a/src/widgets/ParamWidget.cpp
+++ b/src/widgets/ParamWidget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
@@ -23,7 +23,7 @@ void ParamWidget::onChange() {
 		return;
 
 	// moduleWidget->module->params[paramId] = value;
-	rackSetParamSmooth(module, paramId, value);
+	gRack->setParamSmooth(module, paramId, value);
 }
 
 
diff --git a/src/widgets/Port.cpp b/src/widgets/Port.cpp
index 122e506b..8d5608e5 100644
--- a/src/widgets/Port.cpp
+++ b/src/widgets/Port.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/QuantityWidget.cpp b/src/widgets/QuantityWidget.cpp
index 483b4ff4..07794005 100644
--- a/src/widgets/QuantityWidget.cpp
+++ b/src/widgets/QuantityWidget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/RackWidget.cpp b/src/widgets/RackWidget.cpp
index de9d6d83..0db0b99a 100644
--- a/src/widgets/RackWidget.cpp
+++ b/src/widgets/RackWidget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 #include <algorithm>
 
 
diff --git a/src/widgets/Scene.cpp b/src/widgets/Scene.cpp
index 8f2dfa43..727d0111 100644
--- a/src/widgets/Scene.cpp
+++ b/src/widgets/Scene.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Screw.cpp b/src/widgets/Screw.cpp
index 0cf9caeb..ac7e154d 100644
--- a/src/widgets/Screw.cpp
+++ b/src/widgets/Screw.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/ScrollBar.cpp b/src/widgets/ScrollBar.cpp
index b013fad1..b66de879 100644
--- a/src/widgets/ScrollBar.cpp
+++ b/src/widgets/ScrollBar.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/ScrollWidget.cpp b/src/widgets/ScrollWidget.cpp
index 78c6fc17..c4a33632 100644
--- a/src/widgets/ScrollWidget.cpp
+++ b/src/widgets/ScrollWidget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Slider.cpp b/src/widgets/Slider.cpp
index 65db5f02..29328b44 100644
--- a/src/widgets/Slider.cpp
+++ b/src/widgets/Slider.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/SpriteWidget.cpp b/src/widgets/SpriteWidget.cpp
index dc1d6f52..d1f97eb0 100644
--- a/src/widgets/SpriteWidget.cpp
+++ b/src/widgets/SpriteWidget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Toolbar.cpp b/src/widgets/Toolbar.cpp
index 27c1920f..9f8cdd80 100644
--- a/src/widgets/Toolbar.cpp
+++ b/src/widgets/Toolbar.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Tooltip.cpp b/src/widgets/Tooltip.cpp
index baea5703..3bf8de6b 100644
--- a/src/widgets/Tooltip.cpp
+++ b/src/widgets/Tooltip.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
diff --git a/src/widgets/Widget.cpp b/src/widgets/Widget.cpp
index 1f8d9569..6c661d8d 100644
--- a/src/widgets/Widget.cpp
+++ b/src/widgets/Widget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 #include <algorithm>
 
 
diff --git a/src/widgets/WireWidget.cpp b/src/widgets/WireWidget.cpp
index 63e1860a..95106eb9 100644
--- a/src/widgets/WireWidget.cpp
+++ b/src/widgets/WireWidget.cpp
@@ -1,4 +1,4 @@
-#include "Rack.hpp"
+#include "rack.hpp"
 
 
 namespace rack {
@@ -70,7 +70,7 @@ WireWidget::~WireWidget() {
 
 void WireWidget::updateWire() {
 	if (wire) {
-		rackDisconnectWire(wire);
+		gRack->removeWire(wire);
 		delete wire;
 		wire = NULL;
 	}
@@ -80,7 +80,7 @@ void WireWidget::updateWire() {
 		wire->outputId = outputPort->outputId;
 		wire->inputModule = inputPort->module;
 		wire->inputId = inputPort->inputId;
-		rackConnectWire(wire);
+		gRack->addWire(wire);
 	}
 }