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Windows Platform Design Notes
Design Information for the Microsoft® Windows® Family of Operating Systems
USB Audio Devices and Windows
Abstract
The Universal Audio Architecture (UAA) describes a class driver architecture for
personal computer (PC) audio solutions supported in the next version of the
Microsoft® Windows® operating system, codenamed “Windows Longhorn.” This
paper provides information about how the USB Audio specifications are
implemented by Usbaudio.sys, the Microsoft UAA class driver for USB audio
devices.
Draft Version 0.3 - April 1, 2003
Contents
Introduction ............................................................................................................................................. 3
Universal Audio Architecture ............................................................................................................. 3
Compatibility of USB Audio Devices with Windows ............................................................................ 3
Streaming Data....................................................................................................................................... 4
Isochronous Endpoint Types .................................................................................................................. 4
Topology ................................................................................................................................................ 4
Interface Descriptors ......................................................................................................................... 4
Multiple Types as Alternate Interfaces ............................................................................................. 11
Control Interface and Unit Descriptors ............................................................................................. 12
String Descriptors ............................................................................................................................ 13
Property Sets ........................................................................................................................................ 13
Standard Audio Properties .............................................................................................................. 13
Feature Unit Properties ................................................................................................................... 14
Processing Unit Properties .............................................................................................................. 16
Device-Specific Properties .............................................................................................................. 19
AC-3 (Type II) Properties ................................................................................................................. 19
Filter-Level Properties ..................................................................................................................... 19
Pin Properties .................................................................................................................................. 20
Pin Data Intersection ............................................................................................................................ 20
USB Audio 2.0 Enhancements ............................................................................................................. 20
Call to Action and Resources ................................................................................................................ 21
USB Audio Devices and Windows - 2
© 2003 Microsoft Corporation. All rights reserved. Draft Version 0.3 - April 1, 2003
This is a preliminary document and may be changed substantially prior to final commercial release of the
software described herein.
The information contained in this document represents the current view of Microsoft Corporation on the
issues discussed as of the date of publication. Because Microsoft must respond to changing market
conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot
guarantee the accuracy of any information presented after the date of publication.
This White Paper is for informational purposes only. MICROSOFT MAKES NO WARRANTIES,
EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS DOCUMENT.
Complying with all applicable copyright laws is the responsibility of the user. Without limiting the rights
under copyright, no part of this document may be reproduced, stored in or introduced into a retrieval
system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or
otherwise), or for any purpose, without the express written permission of Microsoft Corporation.
Microsoft may have patents, patent applications, trademarks, copyrights, or other intellectual property
rights covering subject matter in this document. Except as expressly provided in any written license
agreement from Microsoft, the furnishing of this document does not give you any license to these patents,
trademarks, copyrights, or other intellectual property.
Unless otherwise noted, the example companies, organizations, products, domain names, e-mail
addresses, logos, people, places and events depicted herein are fictitious, and no association with any
real company, organization, product, domain name, email address, logo, person, place or event is
intended or should be inferred.
© 2003 Microsoft Corporation. All rights reserved.
Microsoft, Windows, and Window NT are either registered trademarks or trademarks of Microsoft
Corporation in the United States and/or other countries.
The names of actual companies and products mentioned herein may be the trademarks of their
respective owners.
USB Audio Devices and Windows - 3
© 2003 Microsoft Corporation. All rights reserved. Draft Version 0.3 - April 1, 2003
Introduction
The USB Audio class system driver, Usbaudio.sys, is an AVStream minidriver that
provides driver support on Microsoft® Windows® operating systems for USB Audio
devices.
This paper describes guidelines for developing USB Audio devices to interoperate
with the USB Audio class system driver in Microsoft Windows XP and later versions
of the operating system. This paper also provides information about enhancements
to Usbaudio.sys planned for Windows Longhorn. Audio device manufacturers can
use the information in this paper to ensure compliance with the Microsoft Universal
Audio Architecture (UAA) initiative and to design devices for compatibility with the
Microsoft Windows family of operating systems.
For general information about Windows Driver Model (WDM) audio architecture and
Usbaudio.sys, see the Windows DDK. A preview and documentation for
Usbaudio.sys will be provided in a beta version of the Windows Longhorn DDK.
Universal Audio Architecture
Microsoft is enhancing audio support in Microsoft Windows through the UAA
initiative. The UAA initiative supports important audio technologies through class
drivers that are provided and maintained by Microsoft. For Windows Longhorn,
Microsoft is planning to supply UAA class drivers for USB and IEEE 1394 audio
devices and for internal bus audio solutions including PCI and solutions that comply
with the Intel next-generation audio specification, codenamed “Azalia.”
USB Audio devices that are compatible with Usbaudio.sys on Windows Longhorn
will automatically be UAA-compliant, without additional work on the part of the
manufacturer.
UAA compliance is a proposed future Designed for Windows Logo Program
requirement for hardware. For information about UAA, see “Resources and Call to
Action” at the end of this paper.
Compatibility of USB Audio Devices with Windows
Usbaudio.sys supports a subset of the hardware features described in the USB
Audio specifications. To ensure compatibility with Usbaudio.sys, USB Audio devices
must design their devices as described in this paper and comply with the following
USB Audio specifications:
Universal Serial Bus Device Class Definition for Audio Devices,
Revision 1.0
Universal Serial Bus Device Class Definition for Audio Data Formats,
Revision 1.0
Universal Serial Bus Device Class Definition for Terminal Types,
Revision 1.0
Universal Serial Bus Device Class Definition for MIDI Devices,
Revision 1.0
USB Audio Devices and Windows - 4
© 2003 Microsoft Corporation. All rights reserved. Draft Version 0.3 - April 1, 2003
Streaming Data
The Universal Serial Bus Device Class Definition for Audio Data Formats defines
four categories of data types for USB Audio devices: Type I, Type II, Type III, and
MIDI.
Type I consists of uncompressed pulse-code-modulation (PCM)-based formats.
These formats are all fully supported by Usbaudio.sys with the exception of
signed 8-bit PCM, for which there is no corresponding Windows format.
Type II consists of compressed formats. The USB audio data format
specification defines two compressed formats: AC-3 and MPEG (1 and 2).
Usbaudio.sys implements only the AC-3 format and restricts the Type II AC-3
data path to non-encrypted data. The driver must be able to read the data
buffers sent to it to determine the locations of the frame breaks and the frame
formats used for the data.
Type III formats are based on the IEC 60958 and IEC 61937 formats for
packaging data into what is effectively a PCM-like stream. For this reason,
Usbaudio.sys fully implements Type III but only exposes the AC-3 and MP3
data formats. Other Type III formats are not supported by Usbaudio.sys.
MIDI format communication is performed through a bulk pipe, in contrast with
other formats that take advantage of the isochronous capabilities of the USB
bus. The MIDI specification was not fully supported in Windows XP and earlier
versions of the operating system. In particular, Usbaudio.sys did not support
MIDI elements, which often led to broken topologies and sometimes caused the
system to crash. Full MIDI support as defined in the Universal Serial Bus
Device Class Definition for MIDI Devices is planned for Usbaudio.sys in
Windows Longhorn.
Isochronous Endpoint Types
The USB specification defines three types of isochronous endpoints: Adaptive,
Synchronous, and Asynchronous.
Starting with Windows 98, Usbaudio.sys supported the adaptive and synchronous
endpoints, but it did not implement the asynchronous endpoint correctly. Full
support for asynchronous endpoints in Usbaudio.sys is planned for Windows
Longhorn.
For device compatibility with earlier versions of Windows, vendors may choose to
continue using adaptive endpoints. Keep in mind that the use of a lock delay for
adaptive endpoints adds latency to the start of a stream.
Topology
A USB Audio device describes its capabilities to the system through a series of
device descriptors. These descriptors are defined in the USB Audio specifications.
Device descriptors describe the internal topology, control capabilities, and data
formats for the device.
Interface Descriptors
USB devices are described as a series of interfaces. The USB bus driver, Usbd.sys,
groups associated audio interfaces and creates a single PDO for each group. Of
these interfaces, the streaming interfaces and their alternate interfaces define the
AVStream pins for the driver. Each streaming interface from the device results in a
USB Audio Devices and Windows - 5
© 2003 Microsoft Corporation. All rights reserved. Draft Version 0.3 - April 1, 2003
single pin. Each alternate interface for a streaming interface results in a separate
data range for that pin.
Zero-Bandwidth Interface
At least one of the alternate interfaces for each interface must be a zero-bandwidth
interface. The USB bus driver uses this to free bus bandwidth when the pin is not in
use. The USB bus driver will fail enumeration for any device that does not
implement a zero-bandwidth alternate setting for each interface.
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