1. Using the USB Audio Class Driver – Best Practices and Common Pitfalls
By Daniel “DJ” Sisolak (USBAudio.sys owner/developer)

2. Common Pitfalls in Creating USB Audio Class Compliant Devices
Descriptor errors or missing descriptors (Vista much more stringent than XP)
Units that do not support required controls
Bad/Unreasonable Control Ranges
Interface/Endpoint descriptor values unusable
Endian problems

3. Feature Unit: Most Common Error
Number of Controls MUST match channel count PLUS one control for Master.
Example (from actual device):
INPUT TERMINAL:
BYTE Length: x0c
BYTE DescriptorType: 0x24
BYTE DescriptorSubtype: 0x02
BYTE bUnitID: x01
SHORT wTerminalType: 0x0603
BYTE bAssocTerminal: 0x00
BYTE bNrChannels: x02
SHORT wChannelConfig: 0x0003
BYTE iChannelNames: 0x00
BYTE iTerminal: x00
FEATURE UNIT:
BYTE Length:
BYTE DescriptorType: 0x24
BYTE DescriptorSubtype: 0x06
BYTE bUnitID: x02
BYTE bSourceID: x01
BYTE bControlSize: x01
BYTE bmaControls:
0x03
0x00
BYTE iFeature: x00
0x09
0x0a
Missing Control
0x00
Corollary: All required Feature Unit controls (GETMIN, GETMAX, GETRES, GETCUR and SETCUR) MUST be supported

4. Channel Configuration Does Not Match Channel Count
Usually found in (but not confined to) Input Terminal
Channel Configuration CAN be 0 (no set configuration)
If the ChannelConfig field is used it MUST match the number of channels
Example (again from actual device):
INPUT TERMINAL:
BYTE Length: x0c
BYTE DescriptorType: 0x24
BYTE DescriptorSubtype: 0x02
BYTE bUnitID: x01
SHORT wTerminalType: 0x0201
BYTE bAssocTerminal: 0x00
BYTE bNrChannels: x02
SHORT wChannelConfig: 0x0001
BYTE iChannelNames: 0x00
BYTE iTerminal: x00
Missing Channel
Location

5. Selector Unit CANNOT take inputs with different Channel Counts or Configurations
Channel Count/Configuration at output becomes nebulous
Use Mixer Unit if necessary
INPUT TERMINAL:
BYTE Length: x0c
BYTE DescriptorType: 0x24
BYTE DescriptorSubtype: 0x02
BYTE bUnitID: x01
SHORT wTerminalType: 0x0201
BYTE bAssocTerminal: 0x00
BYTE bNrChannels: x08
SHORT wChannelConfig: 0x063f
BYTE iChannelNames: 0x00
BYTE iTerminal: x00
INPUT TERMINAL:
BYTE Length: x0c
BYTE DescriptorType: 0x24
BYTE DescriptorSubtype: 0x02
BYTE bUnitID: x04
SHORT wTerminalType: 0x0201
BYTE bAssocTerminal: 0x00
BYTE bNrChannels: x02
SHORT wChannelConfig: 0x0003
BYTE iChannelNames: 0x00
BYTE iTerminal: x00
Incompatible
SELECTOR UNIT:
BYTE Length: x08
BYTE DescriptorType: 0x24
BYTE DescriptorSubtype: 0x05
BYTE bUnitID: x07
BYTE bNrInPins: x01
BYTE baSourceID[0]: 0x01
BYTE baSourceID[1]: 0x04
BYTE iSelector x00

6. Formula for byte count from Specification:
Mixer Unit MUST have Control Bits in its Bitmap for Each Input Channel to Mix into Each Output Channel
Formula for byte count from Specification:
IF ((n x m) MOD 8) <> 0 THEN
N = ((n x m) DIV 8) + 1
ELSE
N = ((n x m) DIV 8)
Where:
N is the number of necessary bytes to represent the mix.
n is the total number of input channels from all input sources
m is the total number of output channels from the mixer
All Controls represented with “programmable” Bit (1) must support a query for Current, Minimum, Maximum and Step Resolution values.
All Controls represented with “not programmable” Bit (0) must support a query for Current value.

7. Selector Units Must Support Queries for Current Value
Even if there is only a single input to the Unit
Corollary: Selector Unit should NOT be necessary if it only has a single input source.
Should also support a query for Maximum value. (Minimum should always be 1)

8. MIDI Only Devices Are Required by the Specification to Include a Control Interface
Some confusion created by a bug in the MIDI portion of the USB Audio driver in Windows XP. (Send mail to for further discussion about this compatibility issue)
From the USB MIDI Specification (unfortunately somewhat hidden in Appendix B):
“…the AudioControl interface is mandatory and therefore both the standard AC interface descriptor and the class specific AC interface descriptor must be present…”

9. Units Should Have “Reasonable” Controls and Ranges
Volume controls should have a range of more than a few dB and would usually max out at 0 dB. (-96db to 0db common)
Selector Maximum value should not exceed number of inputs
Microphone probably should not have a Bass control

10. Interfaces and Endpoints Should Have “Reasonable” Values
Lock Delay should not be a reasonable value (ie not 1)
Corollary: Lock delay implies stream start delay
If Endpoint indicates a sample rate control, the interface must support more than one sample rate.
Every Streaming interface must have one and only one associated Terminal Unit.
There MUST be a Zero Bandwidth Alternate Setting for every streaming interface (except MIDI)
Entering a non-zero Delay value is helpful (Class Specific Interface Descriptor) to determine the delay through the system as a whole and to give accurate audio position information. (most often this is set to 0).

11. Best Practices When Developing a Class Compliant USB Audio Device
MaxPacketSize match to Sample Rates/Multiple Alternate Settings
“Communications” oriented devices
Microphone Array Support
Asynchronous Render Endpoint (does work in XP, better in Vista)
Follow guidelines laid out on Web Page (will be updated soon).
Tools are being developed to help validate descriptors/devices.

12. MaxPacketSize Match to Sample Rates/Multiple Alternate Settings
If you have a device that supports multiple sample rates consider using Alternate Settings if the rates are disparate. Consider breaking up contiguous ranges as well. This saves bus bandwidth.
Example: MaxPacketSize set to accommodate largest rate (864 bytes)
CS FORMAT TYPE DESCRIPTOR:
BYTE Length: x11
BYTE DescriptorType: 0x24
BYTE DescriptorSubType: 0x02
BYTE bFormatType: x01
BYTE bNumberOfChannels: 0x06
BYTE bSlotSize: x03
BYTE bBitsPerSample: 0x18
BYTE bSampleFreqType: 0x03
3BYTE SampleRate[0] x001f40 (8000)
3BYTE SampleRate[1] x00ac44 (44100)
3BYTE SampleRate[2] x00bb80 (48000)
Alternate Setting?

13. “Communications” Oriented Devices
Recommend 16kHz Sample Rate support
New code (Vista SP1 and XP QFE) will prevent devices from becoming preferred audio devices when plugged in to the PC if they have the correct terminal type:
0x306 - Communication speaker
Section 2.4 – Bi-Directional Terminal Types
Section 2.5 – Telephony Terminal Types
( Sections from USB Device Class Definition for Terminal Types V 1.0)

14. Microphone Array Support
Implemented in the device via a GetMemory request to the Input Terminal that represents the ADC for the array.
Specifications are posted for the descriptor that should be retrieved from the terminal:

15. Asynchronous Render Endpoint (does work in XP, better in Vista)
Don’t specify a refresh rate value less than 4 (16ms) in the endpoint descriptor for the feedback endpoint
Example:
ENDPOINT DESCRIPTOR:
BYTE Length: x09
BYTE DescriptorType: 0x05
BYTE EndpointAddress: 0x81
BYTE bmAttributes: 0x01
SHORT MaxPacketSize: 0x0003
BYTE Interval: x01
BYTE Refresh: x05
BYTE SynchAddress: 0x00
Good Value (32ms)

16. Follow Guidelines laid out in UAA Document
This document should be updated soon (mostly with the info found in this presentation)

17. Tools are Being Developed to Help Create/Validate Descriptors/Devices
Descriptor Validation/Topology Generator
Descriptor Generator (built from graphic input tool)
Test Driver (?) driver that will enumerate the errors instead of failing to load at all.
Tools are USB Audio 1.0 based (for now)

18. Future of USB Audio
USB Audio 2.0 specification has been completed but at this time we are still trying to determine how it would fit into our development plan.
If you have plans to implement USB Audio 2.0 please let us know.
Currently it is possible to stream audio using the USB Audio 1.0 specification/Class driver on a USB 2.0. (Requires specific behavior and descriptor values)

19. QUESTIONS???