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call along with audio streaming means that they make more frequent transitions between
devices and applications the inherent difference in architecture between hfp and adp has
always made that difficult resulting in a set of best practice rules which make up the multi
profile specification for hfp a nd adp the new bluetooth le audio architecture was
going to have to go beyond that and incorporate multi profile support by design with robust
and interoperable transitions between devices and applications as well as between unicast and
broadcast
as more people in the consumer space started to understand how telecoil and the broadcast
features of hearing aids worked they began to realise that broadcast might have some very
interesting mass consumer applications at the top of their list was the real isation that i t could
be used for sharing music that could be friends sharing music from their phones silent
discos or silent background music in coffee shops and public spaces public broadcast
installations such as those designed to provide travel information for hearing aid wearers
would now be accessible to everyone with a bluetooth headset the concept of audio sharing
which well examine in more detail in chapter was born
potential new u se cases started to proliferate if we could synchronise stereo channels for
two earbuds why not for surround sound companies were keen to make sure that it
supported smart watches and wristbands which could act as remote controls or even be
audio so urces with embedded mp players the low latencies were exciting for the gaming
community microwave ovens could tell you when your dinners cooked you can tell that
idea came from an engineer the number of use cases continued to grow as companies sa w
how it could benefit their customers their product strategies and affect the future use of
voice and music
voice over internet protocol which is used for voice in most pc and mobile ott over the top
telephony applications
section the bluetooth le audio architecture
the number of features has meant it has taken a long time to complete the specification
what is gratifying is that most of the new use cases wh ich have been raised in the last few
years have not needed us to go back and reopen the specifications weve found that they
were already supported by the features that had been defined that suggests the bluetooth
le audio standards have been well desi gned and are able to support the audio applications
we have today as well as new audio applications which are yet to come
the bluetooth le audio architecture
the bluetooth le audio architecture has been built up in layers as has every other bluetooth
specification before it this is illustrated in figure which shows the main new specification
blocks relating to bluetooth le audio with key existing ones greyed ou t or dotted
at the bottom we have the core which contains the radio and link layer collectively known
as the controller it is responsible for sending bluetooth packets over the air on top of that
is the host which has the task of telling the core what to do for any specific application the
separation between the controller and host is historic reflecting the days when a bl uetooth
radio would be sold in a usb stick or a pcmcia card with the host implemented as a
software application on a pc today both host and controller are often integrated into a
single chip
in the host there i s a new structure called the generic au dio framework or gaf this is an
audio middleware which contains all of the functions which are considered to be generic ie
features which are likely to be used by more than one audio application the core and the
gaf are the heart of bluetooth le a udio they provide great flexibility finally at the top
of the stack we have what are termed top level profiles which add application specific
information to the gaf specifications
figure the bluetooth le audio architecture
chapter the bluetooth le audio archit ecture
it is perfectly possible to build interopera ble bl uetooth le audio applications using just the
gaf specifications the individual specification s within it have been defined to ensure a base
level of interoperability which would enable any two bluetooth le audio devices to transfer
audio between them the top level profile specifications largely add features specific to a
particular type of audi o application mandating features which gaf only defines as optional
and adding application specific functionality the intention is that the top level profiles are
relatively simple building on features from within the gaf
at first glance the bluetoo th le audio architecture looks complex as weve ended up with
different specifications inside the generic audio framework as well as an expanded core
and the new lc codec but theres a logic to this each specification is trying to encapsulate
the specific elements of the way you set up and control different aspects of an audio stream
in the rest of this chapter ill briefly explain each one and how they fit together then in the
rest of the book well look at how each individual specification works and how they interact
profiles and services
all of the specifications within the gaf are classified as profiles or services using the standard
bluetooth le gatt model depicted in figure
figure the bluetooth le profile and service model
in bluetooth le profiles and services can be considered as clients and servers the service
is implemented where the state resides whereas profile specifications describe how the state
behaves and includes procedures to manage it service specifications define one or more
characteristics which can represent individual features or the states of a state machine they
can also be control points which cause transitions between the states of a state machine
profiles act on the se characteristics reading or writing them and being notified when ever the
values change multiple devices each acting a s clients can operate on a server
state refers to the value of a parameter or the current position within a state machine
section the bluetooth le audio architecture
traditionally in classic bluetooth profiles which didnt have a corresponding service there
was a simple one toone relationship with only one client and one server with everything
described in a profile specifica tion in bluetooth le audio the many toone topology is
much more common particularly in features like volume control and the selection of a
broadcast source where a user may have multiple devices implementing the profile
specification and acting as a client in most cases these act on a first come first served basis
the number of different control profiles that can be used in bluetooth le audio drove the
eatt enhancement to the core profiles and services communicate using the attribute
protocol att but att assume s that only one command is occurring at a time if more
than one is happening the second command can be delayed because att is a blocking
protocol to get around this the extended attribute pro tocol eatt was added in core
release allow ing multiple att instances to operate at the same time
figure provides an overview of the bluetooth le audio architecture putting a name or
more precisely a set of letters to all of the specifications which make up the gaf along
with the f our in the current top level profiles the dotted boxes indicate sets of profiles and
services which work together i n most cases there is a one toone relationship of a profile and
a service although in the case of the basic audio profile bap and the voice control profile