[wdmaudiodev] Re: Why would you want USB Audio 2.0 in Windows?

  • From: Geert Knapen <geert@xxxxxxxxxxxxxxxxxxx>
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Thu, 22 Oct 2009 09:16:39 -0700


My name is Geert Knapen and I am the Chair of the USB Audio Device Class Working Group.

We specifically started the Audio 2.0 effort back when USB 2.0 came out to support High Speed. Audio is probably one of the few device classes where the timing of the underlying transport has an influence on the class spec. Indeed, since one of the design goals of the class was to spread out the samples as evenly as possible over frames, we had to revise the class spec to deal with microframes now. Besides this fundamental change, there were a significant amount of changes made to improve the overall specification.

One that stands out - at least in my mind - was the addition of a well defined notification mechanism that was completely lacking from Audio 1.0. In Audio 2.0, the messages going over the interrupt pipe are well defined, which makes it possible to accurately report back to the Host if something inside the audio function changed due to external events. This also makes it possible to design Controls that can be locally modified (changing the volume, for example) and the resulting change can be reported back to the Host via a notification. This creates a tight binding between the UI (the volume button on the device) and the Audio Control (the Volume Control). In Audio 1.0, the binding needs to be created by the Host. The UI element (the volume button) is a HID control and the Host needs to interpret the HID report and generate the correct corresponding audio request (the change in the Volume Control). When there are several Volume Controls incorporated in the device, the Host has a hard time figuring out to which of the Controls to associate with the HID control.

This is just one example (and I can get more) where Audio 2.0 clearly has an advantage over Audio 1.0.

However, having said this, it is probably very late in the game to create an Audio 2.0 class spec. As you may know, an effort is underway to create an AV specification that would eventually supersede both the Audio 2.0 and the Video 1.1 class spec. All of the functionality of the Audio 2.0 and Video 1.1 spec can be found in this new AV spec plus some additional functionality, especially in the video realm (multi-window video composition, support for metadata such as subtitles, and much more).

So maybe if there is going to be an effort to create a class spec in the audio domain, it would be worthwhile to look into this new AV spec and see if that is something that may be of interest to Windows platform users.

I would be more than happy to provide more information if you are interested. Thank you.

Kindest Regards,
Geert Knapen
USB Audio Device Class Chair
USB-IF Director of the Board
President & Consultant to ST-Ericsson

JW House

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