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an opcode of x this sets an absolute volume which may be a default value determined
by an application or the client device it may also be the value remembered from the last time
that application was used
if the value is it indicates that the volume control server still has the volumesetting from
its last session which should continue to be used for this session the volume client should
retrieve this value either through a notification or by reading it and use it as the initial value
for this session this can improve the user experience by starting with the same vo lume
settings which had previously been used
assuming a to scale with a resolution of unless youre a spinal tap fan in which case its
twenty three
section volume offset control service
volume offset control service
if all you have is a single mono speaker then the volume control service is all you need as
soon as you are dealing with more than one audio stream whether thats going to a single
acceptor or multiple acceptors you need to include an instance of the volume offset control
service for every rendered audio location all of the characteristics are accessed using
procedures in the volume control profile
the volume offset contr ol service includes four characteristics shown in table all
four of them are mandatory
characteristic mandatory properties optional properties
volume offset state read notify none
volume offset control point write none
audio location read notify write without response
audio output description read notify write without response
table volume offset control characteristics
the volume offset state and volume offset control work in the normal manner the
volume offset state includes two fields volumeoffset and changecounter
field size
volumeoffset octets
changecounter octet
table the volume offset characteristic
the volumeoffset is two octets long as it allows values from to the value of the
volumeoffset is added to the value of volumestate to provide a total volume value for
rendering
the changecounter is the same concept we saw in the volume control service and is used
to ensure that any client issuing a volume offset command is aware of the current status of
the vol ume offset note that although it is the same concept and has the same name it is a
separate instance every instance of vocs as well as the single instance of vcs maintain
their own value of changecounter which is updated when there is any change t o their
volumeoffset or volumestate field value
to effect a change a client writes to the volume offset control point characteristic with the
following parameters
chapter volume audio input and microphone control
parameter size octets value
opcode x set volume offset
changecounter to
volumeoffset to
table the set volume offset parameters opcode x
audio location characteristics
the volume offset control service contains two characteristics relating to the audio location
to which the offset is applied
the first of these is the audio location characteristic this is a octet bitmap which defines
which audio location the offset should be applied to u sing the format of audio locations
from the generic audio assigned numbers normally this will contain a single location as a
separate vocs instance is applied to each audio location this is normally a read only
characteristic set at manufacture but it can also be made writable
the audio output description characteristic allows a text string to be assigned to each audio
location to provide more information for an application eg bedroom left speaker it may
be assigned at manufacture or made ac cessible for users to assign friendly names
audio input control service
the final part of the rendering trio of services is the audio input control service this
acknowledges that ma ny products such as hearing aids earbuds and headphones contain
more than one source of audio streams and that more than one of them may be used at the
same time the most common example is the concurrent use of an external microphone along
with a bluetooth stream for hearing aids this has always been the way they work more
recently with the appearance of transparency modes it has become an increasingly popular
feature in earbuds and headphones so that wearers can be made aware of sounds from the
world around them
an instance of the audio input control service is n ormally included for each separate audio
path which is intended to be rendered on that device if there is a sin gle bl uetooth le audio
path it offers no advantages over vcs so does not need to be included audio inputs which
are not directly fed to the output such as microphones which provide inputs for noise
cancellation would not have their own instance as they would be used for processing another
audio stream which might have its own aics instance
the audio input control service contains six ch aracteristics which are listed in table
all of them are mandatory to support
section audio input control service
name mandatory
properties optional properties
audio input state read notify none
audio input control point write none
audio input type read none
audio input status read notify none
audio input description read notify write without response
gain setting properties read none
table audio input control service characteristics
before jumping into these we need to understand how aics defines gain with volume
vcs and vocs simply define a linear scale from to much like a knob on a hi fi unit
which goes from to and leav es it up to the manufacturer to convert that to what is usually
a decibel based internal mapping with aics the assumption is made that gain will be decibel
based so the figures for gain are in decibel based units however to provide a bit more
flexibility the actual step in gain values can be defined in multiples of db what those
multiples are is a manufacturer specific choice which is exposed in the gain setting properties
characteristic along with minimum and maximum permitted values as s hown in table
name size
octets description