A wrapper class for the DirectSound API

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Introduction

This article focuses on giving an example of using the DirectX API in PC game software development by using a DirectSound wrapper class.

The DirectSound wrapper class

The wrapper class has the functionality to handle various aspects of sound playback.

The structure of the wrapper class

The DirectSound object uses double buffers to manipulate the sound data processing and playback. The DirectSound object creates a single primary buffer to deal with output devices to play the sound data. The DirectSound object creates multiple second buffers to load, store various sound data, and configure sound data performance. The object loads and mixes sound data from the secondary buffers and sends the processed sound data to the primary buffer to play it through the output device.

The wrapper class contains a pointer to the DirectSound object, a primary buffer object (a pointer of type DirectSoundBuffer), and the secondary buffer array (DirectSoundBuffer object array)

DirectSound object in the wrapper class

DirectSound is a COM interface, so the DirectSound object is the COM component object (a pointer to DirectSound). The DirectSound object is the core of sound playback and all of buffers, both primary buffer and secondary buffers are created by it, so it must be instantiated before doing any sound play back.

Because the DirectSound object is a COM object, it can be instantiated by either the generic way of creation COM components, or by calling DirectSoundCreate. In my wrapper class, I find the generic method of creating a COM object is more reliable. Remember: The DirectX interfaces are a subset of the COM interfaces, so before you use them, call CoInitialize/ CoInitializeEx, and call CoUninitialize at the end of your application.

// To start...
BOOL CSoundManager::CoInitializeDSound(void)
{
    HRESULT  hr;
    GUID     guID;

    // Clear the GUID
    ::memset(&guID, 0, sizeof(GUID));
 
    // Initialize COM service, create STA 
    hr = ::CoInitialize(NULL);
    ................................. 
    .................................
    // Create the IDirectSound object with the realiable way (create
    // standard COM object)
    hr = CoCreateInstance(CLSID_DirectSound, NULL, CLSCTX_INPROC_SERVER,
              IID_IDirectSound, (void**)&m_pIDS);
    ................................. 
    ................................
    // Initialize the IDirectSound object
    hr = IDirectSound_Initialize(m_pIDS, &guID);
    ................................. 
    .................................
    return TRUE;
}

// To finish up...
void CSoundManager::CoUnInitializeDSound(void)
{
    if(m_pIDS)
    {
        m_pIDS->Release();
        m_pIDS = NULL;
        ::CoUninitialize();
    }	
}

Primary Buffer

The primary buffer is a DirectSoundBuffer object, and its main task is to receive the processed sound data from the DirectSound object and send it to the output device to play. The primary’s format settings determine the sound play effect of the output device. The primary’s format is a WAVEFORMATEX structure and must be set before play.

Second Buffer

The second buffer loads sound data and sets the sound performance, then sends the sound to the  DirectSound object. The second buffer is a DirectSoundBuffer object and is created by the DirectSound object.

In my library, I use a wrapper class of DirectSoundBuffer for the second buffer. This wrapper class has the DirectSoundBuffer object, and the methods to set sound performances, such as adjustment of volume, frequency and channel.

A special feature of the DirectSoundBuffer wrapper class is the special sound effect of Fade-In (volume up step by step) and Fade-Out (volume down step by step).

To avoid the concurrency of incoming function calls, a mutex object is used in the DirectSoundBuffer wrapper class to synchronize the function calls.

Timer Event

The DirectSound wrapper class needs a timer to monitor and manipulate the sound status and performance in a special time interval. The DirectSound wrapper class is a generic class, so it is hard to use a windows timer in the class. As an alternative, a waitable timer is used in the class. A thread is created to check the timer event when the class object is instantiated.

The test program

The test program is a Dialog box application written in MFC. It tests the various wrapper class features, such as volume, frequency, channel, looping, mixing and fade-in and fade-out, etc.