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available setting. It will report this rounded setting when queried during a Get Processing Unit Control
request.
Table 5-47: Chorus Modulation Depth Control Parameter Block
Control Selector CHORUS_DEPTH_CONTROL
wLength 2
Offset Field Size Value Description
0 wChorusDepth 2 Number The setting for the attribute of the Chorus
Modulation Depth Control.
5.2.2.5.3.11 Dynamic Range Compressor Compression Ratio Control
The Compression Ratio Control is used to influence the slope of the active part of the static input-tooutput
transfer characteristic of the Dynamic Range Compressor Processing Unit. The Compression Ratio
Control can support all possible Control attributes (CUR, MIN, MAX, and RES). The valid range for the
CUR, MIN, MAX, and RES attributes is from zero (0x0000) to 255.9961 dB/dB (0xFFFF) in steps of
1/256 dB/dB or 0.00390625 dB/dB (0x0001). The Compression Ratio Control honors the request to the
best of its abilities. It may round the wRatio attribute value to its closest available setting. It will report
this rounded setting when queried during a Get Processing Unit Control request.
Table 5-48: Dynamic Range Compressor Ratio Control Parameter Block
Control Selector COMPRESSION_RATIO_CONTROL
wLength 2
Offset Field Size Value Description
0 wRatio 2 Number The setting for the attribute of the
Compression Ratio Control.
5.2.2.5.3.12 Dynamic Range Compressor MaxAmpl Control
The MaxAmpl Control is used to set the upper boundary of the active input range of the compressor. The
MaxAmpl Control can support all possible Control attributes (CUR, MIN, MAX, and RES). The settings
for the CUR, MIN, and MAX attributes can range from -128.0000 dB (0x8000) to 127.9961 dB (0x7FFF)
in steps of 1/256 dB or 0.00390625 dB (0x0001). The settings for the RES attribute can only take positive
values and range from 1/256 dB (0x0001) to +127.9961 dB (0x7FFF). The MaxAmpl Control honors the
request to the best of its abilities. It may round the wMaxAmpl attribute value to its closest available
setting. It will report this rounded setting when queried during a Get Processing Unit Control request.
Table 5-49: Dynamic Range Compressor MaxAmpl Control Parameter Block
Control Selector MAXAMPL_CONTROL
wLength 2
USB Device Class Definition for Audio Devices
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Offset Field Size Value Description
0 wMaxAmpl 2 Number The setting for the attribute of the
MaxAmpl Control.
5.2.2.5.3.13 Dynamic Range Compressor Threshold Control
The Threshold Control is used to set the lower boundary of the active input range of the compressor. The
Threshold Control can support all possible Control attributes (CUR, MIN, MAX, and RES). The settings
for the CUR, MIN, and MAX attributes can range from -128.0000 dB (0x8000) to 127.9961 dB (0x7FFF)
in steps of 1/256 dB or 0.00390625 dB (0x0001). The settings for the RES attribute can only take positive
values and range from 1/256 dB (0x0001) to +127.9961 dB (0x7FFF). The Threshold Control honors the
request to the best of its abilities. It may round the wThreshol attribute value to its closest available
setting. It will report this rounded setting when queried during a Get Processing Unit Control request.
Table 5-50: Dynamic Range Compressor Threshold Control Parameter Block
Control Selector THRESHOLD_CONTROL
wLength 2
Offset Field Size Value Description
0 wThreshold 2 Number The setting for the attribute of the
Threshold Control.
5.2.2.5.3.14 Dynamic Range Compressor Attack Time Control
The Attack Time Control is used to determine the response of the compressor to a step increase in the
input signal level. The Attack Time Control can support all possible Control attributes (CUR, MIN, MAX,
and RES). The settings for the CUR, MIN, MAX, and RES attributes can range from zero (0x0000) to
255.9961 ms (0xFFFF) in steps of 1/256 ms or 0.00390625 ms (0x0001). The Attack Time Control honors
the request to the best of its abilities. It may round the wAttackTime attribute value to its closest available
setting. It will report this rounded setting when queried during a Get Processing Unit Control request.
Table 5-51: Dynamic Range Compressor Attack Time Control Parameter Block
Control Selector ATTACK_TIME_CONTROL
wLength 2
Offset Field Size Value Description
0 wAttackTime 2 Number The setting for the attribute of the Attack
Time Control.
5.2.2.5.3.15 Dynamic Range Compressor Release Time Control
The Release Time Control is used to determine the recovery response of the compressor to a step decrease
in the input signal level. The Release Time Control can support all possible Control attributes (CUR,
MIN, MAX, and RES). The settings for the CUR, MIN, MAX, and RES attributes can range from zero
(0x0000) to 255.9961 ms (0xFFFF) in steps of 1/256 ms or 0.00390625 ms (0x0001). The Release Time
Control honors the request to the best of its abilities. It may round the wReleaseTime attribute value to its
USB Device Class Definition for Audio Devices
Release 1.0 March 18, 1998 93
closest available setting. It will report this rounded setting when queried during a Get Processing Unit
Control request.
Table 5-52: Dynamic Range Compressor Release Time Control Parameter Block
Control Selector RELEASE_TIME_CONTROL
wLength 2
Offset Field Size Value Description
0 wAttackTime 2 Number The setting for the attribute of the
Release Time Control.
5.2.2.6 Extension Unit Control Requests
Because this specification has no knowledge about the inner workings of an Extension Unit, it is
impossible to define requests that are able to manipulate specific Extension Unit Controls. However, this
specification defines a number of Controls an Extension Unit must or can support.
5.2.2.6.1 Set Extension Unit Control Request
This request is used to set an audio Control inside a particular Extension Unit of the audio function.
Table 5-53: Set Extension Unit Control Request Values
bmRequest
Type
bRequest wValue wIndex wLength Data
00100001B SET_CUR
SET_MIN
SET_MAX
SET_RES
CS Extension Unit
ID
and
Interface
Length of
parameter
block
Parameter
block
The bRequest field indicates which attribute the request is manipulating. The MIN, MAX, and RES
attributes are usually not supported for the Set request.
The wValue field specifies the Control Selector (CS) in the high byte and zero in the low byte. The
Control Selector indicates which type of Control this request is manipulating. (Enable Processing, etc.) If
the request specifies an unknown or unsupported CS to that Unit, the control pipe must indicate a stall.
For a description of the parameter block for the Extension Unit Control Selectors, see Section 5.2.2.6.3,
“Extension Unit Controls.”
5.2.2.6.2 Get Extension Unit Control Request
This request returns the attribute setting of a specific audio Control inside an Extension Unit of the audio
function.
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Table 5-54: Get Extension Unit Control Request Values
bmRequest
Type
bRequest wValue wIndex wLength Data
10100001B GET_CUR
GET_MIN
GET_MAX
GET_RES
CS Extension Unit
ID
and
Interface
Length of
parameter
block
Parameter
block
The bRequest field indicates which attribute the request is reading
The wValue field specifies the Control Selector (CS) in the high byte and zero in the low byte. The
Control Selector indicates which type of Control this request is addressing. (Enable Processing, etc.) If the
request specifies an unknown or unsupported CS to that Unit, the control pipe must indicate a stall.
For a description of the parameter block for the Extension Unit Controls, see Section 5.2.2.6.3, “Extension
Unit Controls.”
5.2.2.6.3 Extension Unit Controls
For the time being, this specification only defines a single generic Extension Unit Control, the Enable
Processing Control. Issuing non-supported Control Selectors to an Extension Unit leads to a control pipe
stall.
5.2.2.6.3.1 Enable Processing Control
The Enable Processing Control is used to either enable the functionality of the Extension Unit or bypass
the Extension Unit entirely. In the latter case, default behavior is assumed. Enable Processing Control can
have only the current setting attribute (CUR). The position of an Enable Processing switch can be either
TRUE or FALSE. The current setting of the Control can be queried using a Get Extension Unit Control
request.
Table 5-55: Enable Processing Control Parameter Block
Control Selector XU_ENABLE_CONTROL
wLength 1
Offset Field Size Value Description
0 bOn 1 Bool The setting for the Enable Processing
Control. Enabled when TRUE, disabled
when FALSE.
5.2.3 AudioStreaming Requests
AudioStreaming requests can be directed either to the AudioStreaming interface or to the associated
isochronous data endpoint, depending on the location of the Control to be manipulated.
5.2.3.1 Interface Control Requests
For now, this specification only deals with format-specific AudioStreaming interface requests. Depending
on the Audio Data Format used by the interface (reflected in the wFormatTag field of the class-specific
AS interface descriptor), the set of requests to control the interface may vary. As an example, consider an
interface capable of accepting AC-3 encoded data streams. Such an interface obviously incorporates an
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Release 1.0 March 18, 1998 95
AC-3 decoder to convert the incoming data stream into the logical channels that eventually enter the
audio function through the associated Input Terminal. The same yields for an interface capable of
accepting MPEG-2 encoded data streams. However, the parameter set needed to control the AC-3 decoder
differs substantially from the parameter set needed to control the MPEG-2 decoder. Therefore, the formatspecific
Interface Control requests are described in the USB Audio Data Formats, together with the
definition of the format-specific descriptors.
5.2.3.2 Endpoint Control Requests
The following sections describe the requests an audio function can support for its AudioStreaming
endpoints. The same layout of the parameter blocks is used for both the Set and Get requests.
5.2.3.2.1 Set Endpoint Control Request
This request is used to set an attribute of an endpoint Control inside a particular endpoint of the audio
function.
Table 5-56: Set Endpoint Control Request Values
bmRequest
Type
bRequest wValue wIndex wLength Data
00100010B SET_CUR
SET_MIN
SET_MAX
SET_RES
CS endpoint Length of
parameter
block
Parameter
block
The bRequest field indicates which attribute the request is manipulating. The MIN, MAX, and RES
attributes are usually not supported for the Set request.
The wValue field specifies the Control Selector (CS) in the high byte and the low byte must be set to zero.
The Control Selector indicates which type of Control this request is manipulating (Sampling Frequency,
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.”
5.2.3.2.2 Get Endpoint Control Request
This request returns the attribute setting of a specific endpoint Control inside an endpoint of the audio
function.
Table 5-57: Get Endpoint Control Request Values
bmRequest
Type
bRequest wValue wIndex wLength Data
10100010B GET_CUR
GET_MIN
GET_MAX
GET_RES
CS endpoint Length of
parameter
block
Parameter
block
The bRequest field indicates which attribute the request is reading.
The wValue field specifies the Control Selector (CS) in the high byte and the low byte must be set to zero.
The Control Selector indicates which type of Control this request is manipulating (Sampling Frequency,
最終更新：2011年05月02日 03:34
