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time the wearer may want to hear a combination of these different inputs aics supports
that flexibility
an important feature of the volume services is that they no tify any changes back to client
devices running the voice control profile this ensures that all potential controller s are kept
up to date with any changes to their state regardless of whether that occurred over a bluetooth
link or from a local volume co ntrol this ensures that they all have a synchronised knowledge
of the volume state so that the user can make changes from any one of them without any
unexpected effects from their having an outdated knowledge of the current state of the volume
level
a complementary pair of specifications micp and mics the microphone control profile
and service are responsible for controlling the microphones that reside within hearing aids
and earbuds nowadays these devices typically contain multiple microphones hearing aids
listen to both ambient sound their primary function as well as audio thats being received
over bluetooth as earbuds get more sophisticated were increasingly seeing similar ambient
sound capabilities being built into them with a growing popularity for some degree of
transparency
micp work s in conjunction with aics and mics to control the overall gain and muting of
multiple microphones they are normally used for control of the captured audio that is
destined for a bluetooth stream but can be used more widely figure illustrates their use
figure the audio input control service aics used with the microphone control service mics
chapter the bluetooth le audio architecture
content control
having specified how streams are set up and managed and how volume and microphone input
is handled we come to content control the content we listen to is generated outside of the
bluetooth specifications it may be streaming music live tv a phone cal l or a video
conference what the content control specifications do is to allow control in terms of starting
stopping answering pausing and selecting the streams these are the types of control which
were embedded into hfp and the audiovideo remote c ontrol profile avrcp which
accompanies adp in bluetooth le audio they are separated out into two sets of
specifications one for telephony in all of its forms and the other for media the key
differentiator is that telephony is about the state of the call or calls which normally reflects
the state of the telephony service whereas media control acts on the state of the stream
when and how its played and how its selected because these are decoupled from the audio
streams they can now be used to help control transitions such as pausing music playback
when you accept a phone call and restoring it when the call is finished for both of these pairs
of specifications the service resides on the primary audio source typically the phone pc
tablet or tv whereas the profile is implemented on the receiving device such as the hearing
aid or earbud as with the rendering and capturing controls multiple devices can act as
client s so telephony and media state can be controlled from a smartwatch a s well as from an
earbud
the media control service mcs resides on the source of audio media and reflects the state
of the audio stream the state machine allows a client using the media control profile mcp
to transition each media source through playing paused and seeking states at its simplest it
allows an earbud to control play and stop however mcs goes far beyond that providing all
of the features which users expect from content players today it also provides higher level
functions where a user can search for tracks modify the playing order set up groups and
adjust the playback speed it defines metadata structur es which can be used to identify the
tracks and uses the existing object transfer service ots to allow a client to perform media
searches on the server or more typically the application behind it all of this means that a
suitably complex device running the media control profile can recreate the controls of a music
player
telephony control is handled in a similar way using the telephon e bearer service tbs which
resides on the device involved in the call typically the phone pc or laptop with the
complementary call control profile ccp controlling the call by writing to the state machine
in the tbs instance tbs and ccp have expanded past the limitations of hands free profile
to accommodate the fact that we now use telephony in many different forms its no longer
just traditional circuit switched and cellular bearers but pc and web based communication
circuit switched is a definition derived from original telephone topology where end toend
connections were made by physically switching using ba nks of relays the term endured until ip
section the bluetooth le audio architecture
and conferencing applications using multiple different types of bearer service tbs exposes
the state of the call using a generic state machine it supports multiple calls call handling and
joining caller id inband and out of band rin gtone selection and exposes call information
such as signal strength
both tbs and mcs acknowledge the fact that there may be multiple sources of media and
multiple different call applications on the server devices to accommodate this both can be
instantiated multiple times once for each instance of an application this allows a client
with the complementary profile to control each application separately alternatively a single
instance of the servic e can be used with the media or call device using its specific
implementation to direct the profile commands to the correct application the single instance
variants of tbs and mcs are known as the generic telephon e bearer service gtbs and
generic media control service gmcs and are included in the tbs and mcs specifications
respectively well look at th ese in more detail in chapter
transition and coordination control
next we come to the transition and coordination control specifications their purpose is
to glue the other specifications together providing a way for the top level profiles to call down
to them without having to concern themselves with the fine detail of setting things up
one of the major enhancements in i sochronous channels is the ability to stream audio to
multiple different devices and render it at exactly the same time the most common
application of this is in streaming stereo music to left and right earbuds speakers or hearing
aids the topology an d syncnronisation of rendering are handled in the core and bap but
ensuring that control operations occur together whether thats changing volume or
transitioning between connections is not thats where the coordinated set identification
profile csip and coordinated set identification service csis come in
where two or more bluetooth le audio devices are expected to be used together they are
called a coordinated set and can be associated with each other by use of the coordinated set
identification service this allows other profiles in particular cap to treat them as a single
entity it i ntroduces the concepts of lock and rank to ensure that when there is a transition
between audio connections whether thats to a new unicast or broadcast stream the members
of the set always react together this prevents a new c onnection only being applied to a subset
of the devices in the set such as a tv connecting to your right earbud while your phone
connects to your left devices designed to be members of coordinated set s are generally
configured as set members during man ufacture