USB Device Class Definition for Audio Devices
Release 1.0 March 18, 1998 96
Pitch Control, etc.) If the request specifies an unknown CS to that endpoint, the control pipe must indicate
a stall.
For a description of the parameter block for the endpoint Control Selectors, see Section 5.2.3.2.3,
“Endpoint Controls.”
5.2.3.2.3 Endpoint Controls
5.2.3.2.3.1 Sampling Frequency Control
The Sampling Frequency Control is used to set the initial sampling frequency for an isochronous audio
data endpoint. This allows the endpoints’ clock recovery system to lock onto the incoming clock much
faster. Adaptive endpoints can benefit from this. The Sampling Frequency Control can support all possible
Control attributes (CUR, MIN, MAX, and RES). The settings for the CUR, MIN, and MAX attributes can
range from 0 Hz (0x000000) to 8388607 Hz (0x7FFFFF) in steps of 1 Hz (0x0001). The Sampling
Frequency Control honors the request to the best of its abilities. If the endpoint operates at a fixed
sampling frequency, setting this Control has no effect. If the endpoint supports a discrete number of
sampling frequencies, setting the tSampleFreq value to a non-supported value causes the Control to
round it to the closest available value. This also happens when the sampling frequency is set outside the
range for a continuous sampling frequency endpoint. It will report the rounded setting when queried
during a Get Control request.
Table 5-58: Sampling Frequency Control Parameter Block
Control Selector SAMPLING_FREQ_CONTROL
wLength 3
Offset Field Size Value Description
0 tSampleFreq 3 Number The sampling frequency expressed in Hz.
5.2.3.2.3.2 Pitch Control
The Pitch Control enables or disables the ability of an adaptive endpoint to dynamically track its sampling
frequency. The Control is necessary because the clock recovery circuitry must be informed whether it
should allow for relatively large swings in the sampling frequency. A Pitch Control can have only the
current setting attribute (CUR). The position of a Pitch Control CUR attribute can be either TRUE or
FALSE.
Table 5-59: Pitch Control Parameter Block
Control Selector PITCH_CONTROL
wLength 1
Offset Field Size Value Description
0 bPitchEnable 1 Bool Pitch Control on when TRUE, off when
FALSE.
USB Device Class Definition for Audio Devices
Release 1.0 March 18, 1998 97
5.2.4 Additional Requests
5.2.4.1 Memory Requests
The Host can interact with an addressable Entity (Terminal, Unit or endpoint) within the audio function
in a very generic way. The Entity presents a memory space to the Host whose layout depends on the
implementation. The Memory request provides full access to this memory space.
5.2.4.1.1 Set Memory Request
This request is used to download a parameter block into a particular Entity of the audio function.
Table 5-60: Set Memory Request Values
bmRequest
Type
bRequest wValue wIndex wLength Data
00100001B Entity ID
and
Interface
00100010B
SET_MEM Offset
Endpoint
Length of
parameter
block
Parameter
block
The bRequest field indicates that the MEM attribute of the Entity is addressed.
The wValue field specifies a zero-based offset value that can be used to access only parts of the Entity’s
memory space.
The layout of the parameter block is implementation dependent. A device is required to reevaluate its
memory space at the end of each Set Memory request.
5.2.4.1.2 Get Memory Request
This request is used to upload a parameter block from a particular Entity of the audio function.
Table 5-61: Get Memory Request Values
bmRequest
Type
bRequest wValue wIndex wLength Data
10100001B Entity ID
and
Interface
10100010B
GET_MEM Offset
Endpoint
Length of
parameter
block
Parameter
block
The bRequest field indicates that the MEM attribute of the Entity is addressed.
The wValue field specifies a zero-based offset value that can be used to access only parts of the Entity’s
parameter space.
The layout of the parameter block is implementation dependent.
5.2.4.2 Get Status Request
This request is used to retrieve status information from an Entity within the audio function.
USB Device Class Definition for Audio Devices
Release 1.0 March 18, 1998 98
Table 5-62: Get Status Request Values
bmRequest
Type
bRequest wValue wIndex wLength Data
10100001B Entity ID
and
Interface
10100010B
GET_STAT Zero
Endpoint
Status
message
length
Status
message
The bRequest field contains the GET_STAT constant, defined in Section A.9, “Audio Class-Specific
Request Codes.”
The wValue field is currently unused and must be set to zero.
The wLength field specifies the number of bytes to return. If the status message is longer than the
wLength field, only the initial bytes of the status message are returned. If the status message is shorter
than the wLength field, the function indicates the end of the control transfer by sending short packet
when further data is requested.
The contents of the status message is reserved for future use. For the time being, a null packet should be
returned in the data stage of the control transfer.
USB Device Class Definition for Audio Devices
Release 1.0 March 18, 1998 99
Appendix A. Audio Device Class Codes
A.1 Audio Interface Class Code
Table A-1: Audio Interface Class Code
Audio Interface Class Code Value
AUDIO 0x01
A.2 Audio Interface Subclass Codes
Table A-2: Audio Interface Subclass Codes
Audio Subclass Code Value
SUBCLASS_UNDEFINED 0x00
AUDIOCONTROL 0x01
AUDIOSTREAMING 0x02
MIDISTREAMING 0x03
A.3 Audio Interface Protocol Codes
Table A-3: Audio Interface Protocol Codes
Audio Protocol Code Value
PR_PROTOCOL_UNDEFINED 0x00
A.4 Audio Class-Specific Descriptor Types
Table A-4: Audio Class-specific Descriptor Types
Descriptor Type Value
CS_UNDEFINED 0x20
CS_DEVICE 0x21
CS_CONFIGURATION 0x22
CS_STRING 0x23
CS_INTERFACE 0x24
CS_ENDPOINT 0x25
USB Device Class Definition for Audio Devices
Release 1.0 March 18, 1998 100
A.5 Audio Class-Specific AC Interface Descriptor Subtypes
Table A-5: Audio Class-Specific AC Interface Descriptor Subtypes
Descriptor Subtype Value
AC_DESCRIPTOR_UNDEFINED 0x00
HEADER 0x01
INPUT_TERMINAL 0x02
OUTPUT_TERMINAL 0x03
MIXER_UNIT 0x04
SELECTOR_UNIT 0x05
FEATURE_UNIT 0x06
PROCESSING_UNIT 0x07
EXTENSION_UNIT 0x08
A.6 Audio Class-Specific AS Interface Descriptor Subtypes
Table A-6: Audio Class-Specific AS Interface Descriptor Subtypes
Descriptor Subtype Value
AS_DESCRIPTOR_UNDEFINED 0x00
AS_GENERAL 0x01
FORMAT_TYPE 0x02
FORMAT_SPECIFIC 0x03
A.7 Processing Unit Process Types
Table A-7: Processing Unit Process Types
wProcessType Value
PROCESS_UNDEFINED 0x00
UP/DOWNMIX_PROCESS 0x01
DOLBY_PROLOGIC_PROCESS 0x02
3D_STEREO_EXTENDER_PROCESS 0x03
REVERBERATION_PROCESS 0x04
最終更新：2011年05月02日 03:35
