Changing the volume of audio devices

Q: How do I change the volume of an audio device?

A: To change the volume of audio hardware you must first obtain an AudioDevice and determine if the device has volume control that is settable. You can get the default output device or the system output device by using the method AudioHardwareGetProperty and the constants kAudioHardwarePropertyDefaultOutputDevice and kAudioHardwarePropertyDefaultSystemOutputDevice .

Note: Every device may not support master volume control or individual channel control.

Listing 1: Getting the default output device

    UInt32 size;
    AudioDeviceID outputDevice;
    AudioHardwareGetProperty(kAudioHardwarePropertyDefaultOuputDevice,
                                              &size,
                                              &outputDevice);

After obtaining the device, you must check if it has volume control and if this property is writeable.

Listing 2: Getting the device capabilities

bool CAAudioHardwareDevice::HasProperty(
    UInt32 inChannel,
    CAAudioHardwareDeviceSectionID inSection,
    AudioHardwarePropertyID inPropertyID) const
{
    OSStatus theError = AudioDeviceGetPropertyInfo(mAudioDeviceID,
            inChannel, inSection, inPropertyID, NULL, NULL);
    return theError == 0;
}

bool CAAudioHardwareDevice::PropertyIsSettable(
    UInt32 inChannel,
    CAAudioHardwareDeviceSectionID inSection,
    AudioHardwarePropertyID inPropertyID) const
{
    Boolean isWritable = false;
    OSStatus theError = AudioDeviceGetPropertyInfo(mAudioDeviceID, inChannel,
        inSection, inPropertyID, NULL, &isWritable);
    ThrowIfError(theError, CAException(theError),
        "CAAudioHardwareDevice::PropertyIsSettable: got "
        "an error getting info about a property");
    return isWritable != 0;
}

bool CAAudioHardwareDevice::HasVolumeControl(
    UInt32 inChannel,
    CAAudioHardwareDeviceSectionID inSection) const
{
    return HasProperty(inChannel, inSection, kAudioDevicePropertyVolumeScalar);
}

bool CAAudioHardwareDevice::VolumeControlIsSettable(
    UInt32 inChannel,
    CAAudioHardwareDeviceSectionID inSection) const
{
    return PropertyIsSettable(inChannel, inSection, kAudioDevicePropertyVolumeScalar);
}

Then you can set the volume of the device by using the constant kAudioDevicePropertyVolumeScalar within AudioDeviceSetProperty. You must specify a Float32 value between 0 and 1 that will scale the volume of the device.

Listing 3: Setting the volume on master channel


AudioDeviceSetProperty(theDevice,
                         NULL, //time stamp not needed
                         0, //channel 0 is master channel
                         false,  //for an output device
                         kAudioDevicePropertyVolumeScalar,
                         sizeof(Float32),
                         &theValue );

References

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Document Revision History

DateNotes
2006-05-02First Version

Posted: 2006-05-02

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