A few of my favorite Win7 Sound features – Stream Switching

Way back when I was in college, I learned Lisp using a derivative of Lisp called MACLISP (named for MIT’s project MAC, not for anything that came from a fruity company in Cupertino).  One of the coolest features that MACLISP offered was the (DWIM) command – basically if you had a typo when entering an s-list into MACLISP, you could type (DWIM) and the MACLISP interpreter would fix your code for you (and yeah, it usually got it somewhat wrong :)).

Stream Switching is a DWIM feature in the audio stack.  If an application is rendering to a default audio device and the device is removed, the audio stream will automatically switch to the new device.  The same happens if the default device changes for other reasons (if the user changes the default device for example) or if the sample rate on the device changes (this can happen with certain types of audio hardware allow external controls to change the sample rate for the device).

We were able to figure out how to implement the stream switching logic in a fashion that causes it to work without requiring changes from 3rd party applications, which is really cool because it allows us to enable new scenarios without breaking appcompat – as long as the application is rendering to a default endpoint, we’ll stream switch the application without the app being notified.

If an application is rendering to a specific endpoint, we’re not going to stream switch when the endpoint is removed – we don’t know the reasons for the application choosing that particular endpoint so we don’t attempt to second guess the applications intent (maybe the user has asked that their music player only play through the headphones and not through the speakers because playing through the speakers would disturb the baby).

We also don’t support stream switching if the application is using WASAPI (the low level audio rendering APIs) to render audio.  That’s for a number of reasons, but mostly it’s because the presumption is that any application that is using WASAPI is using a low level rendering API and thus doesn’t want this kind of behavior.


The stream switching logic is really cool in action, especially if you’ve got a machine which supports dynamic jack detection – when you’re watching a DVD in windows media player and you plug in a pair of headphones, poof – the audio gets redirected to the headphones just like you’d expect it to.