Tutorial: Looping audio using the AudioSampleBuffer class (advanced)

This tutorial shows how to play and loop audio stored in an AudioSampleBuffer object using thread-safe techniques. A technique for loading the audio data on a background thread is also introduced.

Level: Advanced

Platforms: Windows, macOS, Linux

Classes: ReferenceCountedObject, ReferenceCountedArray, Thread, AudioBuffer

Getting started

This tutorial leads on from Tutorial: Looping audio using the AudioSampleBuffer class. If you haven't done so already, you should read that tutorial first.

Download the demo project for this tutorial here: PIP | ZIP. Unzip the project and open the first header file in the Projucer.

If you need help with this step, see Tutorial: Projucer Part 1: Getting started with the Projucer.

The demo project

The demo project implements similar behaviour to the demo project from Tutorial: Looping audio using the AudioSampleBuffer class. It allows the user to open an audio file that is loaded into a buffer and played in a loop. One major difference in this tutorial is that the audio system is kept running, rather than shutting it down each time we browse for a file. This is achieved by using some helpful classes for communicating between threads in a thread-safe manner.

Thread-safe techniques

You should recall in Tutorial: Looping audio using the AudioSampleBuffer class how we solved the potential problem of the audio thread and the message thread accessing potentially incomplete or corrupted data. Just before we browsed for a file we shut down the audio system. Then, once a file was selected, we opened the file and restarted the audio system. This is clearly an impractical and cumbersome method in a real application!

Reference-counted objects

The ReferenceCountedObject class is a useful tool for passing messages and data between threads. Here, we store our AudioSampleBuffer object and the playback position in a ReferenceCountedObject class. To help with debugging, and to help illustrate how the class works, we also include name member (although this isn't strictly necessary for the class to function):

class ReferenceCountedBuffer : public juce::ReferenceCountedObject
{
public:
typedef juce::ReferenceCountedObjectPtr<ReferenceCountedBuffer> Ptr;
ReferenceCountedBuffer (const juce::String& nameToUse,
int numChannels,
int numSamples)
: name (nameToUse),
buffer (numChannels, numSamples)
{
DBG (juce::String ("Buffer named '") + name + "' constructed. numChannels = " + juce::String (numChannels) + ", numSamples = " + juce::String (numSamples));
}
~ReferenceCountedBuffer()
{
DBG (juce::String ("Buffer named '") + name + "' destroyed");
}
juce::AudioSampleBuffer* getAudioSampleBuffer()
{
return &buffer;
}
int position = 0;
private:
juce::String name;
};

The typedef at the top of the class is an important part in implementing a ReferenceCountedObject subclass. Rather than storing our ReferenceCountedBuffer object in a raw pointer, we store it in a ReferenceCountedBuffer::Ptr type. It is this that manages the reference count of the object (incrementing and decrementing as necessary) and its lifetime (deleting the object when the reference count reaches zero). We can also store an array of ReferenceCountedBuffer objects using the ReferenceCountedArray class.

In our MainContentComponent class we store both an array and a single instance:

juce::SpinLock mutex;
juce::ReferenceCountedArray<ReferenceCountedBuffer> buffers;
ReferenceCountedBuffer::Ptr currentBuffer;

The buffers member keeps hold of our buffers in the array until we are absolutely sure they are no longer needed by the audio thread. The currentBuffer member holds the currently selected buffer.

Implementing the background thread

Our MainContentComponent class inherits from the Thread class:

class MainContentComponent : public juce::AudioAppComponent,
private juce::Thread
{
public:

This is used to implement our background thread. Our overridden Thread::run() function is as follows:

void run() override
{
while (! threadShouldExit())
{
checkForBuffersToFree();
wait (500);
}
}

Here, we check whether there are any buffers to be freed, then our thread waits for 500ms or to be woken up (using the Thread::notify() function). Essentially, this means that the check will occur at least every 500ms. The checkForBuffersToFree() function searches through our buffers array to see if any buffers can be freed:

void checkForBuffersToFree()
{
for (auto i = buffers.size(); --i >= 0;) // [1]
{
ReferenceCountedBuffer::Ptr buffer (buffers.getUnchecked (i)); // [2]
if (buffer->getReferenceCount() == 2) // [3]
buffers.remove (i);
}
}
  • [1]: It is useful to remember to iterate over the array in reverse in these situations. It is easier to avoid corrupting the array index access if we remove items as we iterate over the array.
  • [2]: This retains a copy of a buffer at the specified index.
  • [3]: If the reference count at this point is equal to 2 then we know that the audio thread can't be using the buffer and we can remove it from the array. One of these two references will be in the buffers and the other will be in the local buffer variable. The removed buffer will delete itself as the buffer variable goes out of scope (as this will be the last remaining reference).

Of course, we need to start the thread as our application starts, which we do in our MainContentComponent constructor:

startThread();
}

Opening the file

Our openButtonClicked() function is similar to the openButtonClicked() function from Tutorial: Looping audio using the AudioSampleBuffer class with some minor differences:

void openButtonClicked()
{
chooser = std::make_unique<juce::FileChooser> ("Select a Wave file shorter than 2 seconds to play...",
juce::File{},
"*.wav");
auto chooserFlags = juce::FileBrowserComponent::openMode
| juce::FileBrowserComponent::canSelectFiles;
chooser->launchAsync (chooserFlags, [this] (const juce::FileChooser& fc)
{
auto file = fc.getResult();
if (file == juce::File{})
return;
std::unique_ptr<juce::AudioFormatReader> reader (formatManager.createReaderFor (file));
if (reader != nullptr)
{
auto duration = (float) reader->lengthInSamples / reader->sampleRate;
if (duration < 2)
{
ReferenceCountedBuffer::Ptr newBuffer = new ReferenceCountedBuffer (file.getFileName(),
(int) reader->numChannels,
(int) reader->lengthInSamples);
reader->read (newBuffer->getAudioSampleBuffer(), 0, (int) reader->lengthInSamples, 0, true, true);
{
const juce::SpinLock::ScopedLockType lock (mutex);
currentBuffer = newBuffer;
}
buffers.add (newBuffer);
}
else
{
// handle the error that the file is 2 seconds or longer..
}
}
});
}

Here are the differences:

  • Allocate a new instance of our ReferenceCountedBuffer class.
  • Read the audio data into the AudioSampleBuffer object that it contains.
  • Make it the current buffer.
  • Add it to our array of buffers.

To clear the current buffer we can just set its value to nullptr:

void clearButtonClicked()
{
const juce::SpinLock::ScopedLockType lock (mutex);
currentBuffer = nullptr;
}

Playing the buffer

Our getNextAudioBlock() function is similar to the getNextAudioBlock() function from Tutorial: Looping audio using the AudioSampleBuffer class except we need to access our current ReferenceCountedBuffer object and the AudioSampleBuffer object it contains.

void getNextAudioBlock (const juce::AudioSourceChannelInfo& bufferToFill) override
{
auto retainedCurrentBuffer = [&]() -> ReferenceCountedBuffer::Ptr // [4]
{
const juce::SpinLock::ScopedTryLockType lock (mutex);
if (lock.isLocked())
return currentBuffer;
return nullptr;
}();
if (retainedCurrentBuffer == nullptr) // [5]
{
bufferToFill.clearActiveBufferRegion();
return;
}
auto* currentAudioSampleBuffer = retainedCurrentBuffer->getAudioSampleBuffer(); // [6]
auto position = retainedCurrentBuffer->position; // [7]
auto numInputChannels = currentAudioSampleBuffer->getNumChannels();
auto numOutputChannels = bufferToFill.buffer->getNumChannels();
auto outputSamplesRemaining = bufferToFill.numSamples;
auto outputSamplesOffset = 0;
while (outputSamplesRemaining > 0)
{
auto bufferSamplesRemaining = currentAudioSampleBuffer->getNumSamples() - position;
auto samplesThisTime = juce::jmin (outputSamplesRemaining, bufferSamplesRemaining);
for (auto channel = 0; channel < numOutputChannels; ++channel)
{
bufferToFill.buffer->copyFrom (channel,
bufferToFill.startSample + outputSamplesOffset,
*currentAudioSampleBuffer,
channel % numInputChannels,
position,
samplesThisTime);
}
outputSamplesRemaining -= samplesThisTime;
outputSamplesOffset += samplesThisTime;
position += samplesThisTime;
if (position == currentAudioSampleBuffer->getNumSamples())
position = 0;
}
retainedCurrentBuffer->position = position; // [8]
}

The important changes are:

  • [4]: We retain a copy of the currentBuffer member. After this point in the function it doesn't matter if the currentBuffer member is changed on another thread since we have taken a local copy. Note that we use a try lock here, so that the audio thread doesn't get stuck waiting to access the currentBuffer in the case that another thread is currently modifying it.
  • [5]: We output silence if the currentBuffer member was null when we took a copy.
  • [6]: We access the AudioSampleBuffer object within the ReferenceCountedBuffer object.
  • [7]: We get the current playback position for the buffer.
  • [8]: After modifying the current playback position, we store it back in the ReferenceCountedBuffer object.

This algorithm ensures that ReferenceCountedBuffer objects aren't deleted on the the audio thread. It is not a good idea to allocate or free memory on the audio thread. The ReferenceCountedBuffer objects will only be deleted on our background thread.

Reading the audio on the background thread

Our application still reads the audio data on the message thread. This is not ideal since this blocks the message thread and large files could take some time to load. In fact, we can also use our background thread to perform this task.

Passing the file path to the background thread

First, add the following member to the MainContentComponent class:

juce::CriticalSection pathMutex;
juce::String chosenPath;

Now change the openButtonClicked() function to swap the full path of the file into this member. Swapping strings is not technically thread-safe, so we also need to take a lock to ensure that no other threads try to modify chosenPath while this thread is using it.

void openButtonClicked()
{
chooser = std::make_unique<juce::FileChooser> ("Select a Wave file shorter than 2 seconds to play...",
juce::File{},
"*.wav");
auto chooserFlags = juce::FileBrowserComponent::openMode
| juce::FileBrowserComponent::canSelectFiles;
chooser->launchAsync (chooserFlags, [this] (const juce::FileChooser& fc)
{
auto file = fc.getResult();
if (file == juce::File{})
return;
auto path = file.getFullPathName();
{
const juce::ScopedLock lock (pathMutex);
chosenPath.swapWith (path);
}
notify();
});
}

Here we also wake up the background thread since we are going to call a function on the background thread to open the file.

Accessing the path from the background thread

Our run() function should be updated as follows:

void run() override
{
while (! threadShouldExit())
{
checkForPathToOpen();
checkForBuffersToFree();
wait (500);
}
}

The checkForPathToOpen() function checks the chosenPath member by swapping it into a local variable. Again, swapping is not thread-safe, so we must take a lock before accessing chosenPath.

void checkForPathToOpen()
{
juce::String pathToOpen;
{
const juce::ScopedLock lock (pathMutex);
pathToOpen.swapWith (chosenPath);
}
if (pathToOpen.isNotEmpty())
{
juce::File file (pathToOpen);
std::unique_ptr<juce::AudioFormatReader> reader (formatManager.createReaderFor (file));
if (reader.get() != nullptr)
{
auto duration = (float) reader->lengthInSamples / reader->sampleRate;
if (duration < 2)
{
ReferenceCountedBuffer::Ptr newBuffer = new ReferenceCountedBuffer (file.getFileName(),
(int) reader->numChannels,
(int) reader->lengthInSamples);
reader->read (newBuffer->getAudioSampleBuffer(), 0, (int) reader->lengthInSamples, 0, true, true);
{
const juce::SpinLock::ScopedLockType lock (mutex);
currentBuffer = newBuffer;
}
buffers.add (newBuffer);
}
else
{
// handle the error that the file is 2 seconds or longer..
}
}
}
}

If the pathToOpen variable is an empty string then we know there isn't a new file to open. The remainder of the code in this function should be familiar to you.

Run the application again and it should still function correctly.

Note
The final code for this section can be found in the LoopingAudioSampleBufferAdvancedTutorial_02.h file of the demo project.

Summary

This tutorial has introduced some useful techniques for passing data between threads, especially in an audio application. After reading this tutorial you should be able to:

  • Implement a subclass of the ReferenceCountedObject class.
  • Maintain the lifetime of a ReferenceCountedObject in a multi-threaded application.
  • Implement a background thread to perform tasks such as deleting objects that are no longer needed and performing file reading operations.

See also

gl::buffers
const GLuint * buffers
Definition: juce_gl.h:1460
gl::name
GLuint const GLchar * name
Definition: juce_gl.h:1567
int
typedef int(UNITY_INTERFACE_API *createCallback)(UnityAudioEffectState *state)
jmin
constexpr Type jmin(Type a, Type b)
Returns the smaller of two values.
Definition: juce_MathsFunctions.h:109
AudioSampleBuffer
AudioBuffer< float > AudioSampleBuffer
A multi-channel buffer of 32-bit floating point audio samples.
Definition: juce_AudioSampleBuffer.h:1280
ScopedLock
CriticalSection::ScopedLockType ScopedLock
Automatically locks and unlocks a CriticalSection object.
Definition: juce_CriticalSection.h:189
JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR
#define JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR(className)
This is a shorthand way of writing both a JUCE_DECLARE_NON_COPYABLE and JUCE_LEAK_DETECTOR macro for ...
Definition: juce_PlatformDefs.h:247
gl::buffer
GLenum GLfloat * buffer
Definition: juce_gl.h:871
DBG
#define DBG(textToWrite)
Writes a string to the standard error stream.
Definition: juce_PlatformDefs.h:148
gl::path
GLsizei const GLchar *const * path
Definition: juce_gl.h:4970