In Windows Vista and earlier, the operating system managed a list of audio (playback) devices, and one of them was always specified as the ‘default’. This is how audio devices may have looked on such a system:

1. Speakers (Default)
2. USB Headset
3. Realtek rear audio jack

Applications using Windows’ built-in sound functions such as PlaySound() always played to the default device, while games and other more sophisticated software could enumerate the available audio devices and allow the user to choose, while still typically selecting whichever device is marked as ‘default’ if the user doesn’t express a preference.

Windows 7 changed the game a little bit, by introducing a new, virtual device called the “Default Audio Device”. If a user running Windows 7 on the same hardware as above opens up Control Panel and goes into Sound, they will still see the same three devices, so nothing appears to have changed. However, applications that enumerate the available devices now see something different:

1. Default Audio Device (Default)
2. Speakers
3. USB Headset
4. Realtek rear audio jack

In the above scenario, the speakers are still the default *physical* device (and the device that the user sees as Default in control panel), however the new, virtual device is the default device to applications. I fan application selects either device 1 or 2 in the list above, they will both play to the speakers. So, why implement this feature and change the relationship between what the application sees and the user sees?

The primary benefit is that the Default Audio Device automatically remaps to the default physical device whenever it changes, *instantly*. Whereas on previous versions of Windows if you did something like start playing a song in WinAmp with the Speakers as the default audio, then changed the default to the USB Headset, your music would continue to play through the speakers until the start of the next song (or whenever the application closed and reopened it’s audio channel).

Implementing Support in Applications

I strongly, strongly encourage you to ensure you support playback to the Default Audio Device in your application. Having a program or game that *doesn’t* change automatically these days is enough to make me stop using it (looking at Diablo III here!). If you’re using the awesome BASS Audio Library (which I continue to endorse wholeheartedly), here’s how you do it:

BASS_SetConfig(BASS_CONFIG_DEF_DEFAULT,true); // Use Default device
BASS_Init(-1,Frequency,0,0,nil); // Initialize audio

Now, if WinAmp is playing to the (virtual) Default Audio Device, and the physical default changes, the music changes to the new device *instantly*. WinAmp doesn’t have to poll or register a callback event or do anything. For users who switch between devices several times a day, support for this feature is a godsend!

Changing Default Device Programatically

The only problem? Microsoft haven’t provided any way to programmatically change the default audio device. They really really want you to do it manually, through the control panel each time. That’s really painful; the best solution I have found so far is using scripting to simulate opening the control panel and changing the defaults, similar to what’s described here. Not that elegant, but it gets the job done.