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LeAudio

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too many compromises and that it would be better to do a clean sheet design on top of the
core low energy specification
the proposal for the core was to implement a new feature called isochronous channels
which could carry audio strea ms in bluetooth le alongside an existing acl channel the
acl channel would be used to con figure set up and con trol the streams as well as carry ing
isochronous means a sequence of events which are repeated at equal time intervals
acl channels are asynch ronous con nection oriented logical transport s which are the used for
control requests and response s in bluetooth le gatt transactions they carry all of the control
information in bluetooth le audio with the sole exception of broadcast control subevents and
form the control plane an acl link must always be present during the life of a cis
chapter the bluetooth le audio architecture
more generic control information such as volume telephony and media control the
isochronous channel s could support unidirectional or bidirectional audio streams and
multiple isochronous channels could be set up with multiple devices this separated out the
audio data and control planes which makes bluetooth le audio far more flexible
it was import ant that the audio connections were robust which meant they needed to support
multiple retransmissions to cope with the fact that some transmissions might suffer from
interference for unicast streams there is an acknack acknowledgement scheme so tha t
retransmission s could stop once the transmitter knew that data had been received for
broadcast where there is no feedback the source would need to unconditionally retransmit
the audio packets during the investigation of robustness it became apparent that the
frequency hopping scheme used to protect le devices against interference could be
improved so that was added as another requirement
broadcast required some new concepts particularly in terms of how devices could find a
broadcast with out the presence of a connection bluetooth le uses advertisements to let
devices announce their presence devices wanting to make a connection scan for these
advertisements then connect to the device they discover to obtain the details of what it
suppo rts how to connect including information on when its transmitting what its hopping
sequence is and what it does with the requirements of bluetooth le audio t hat require s a
lot more information than can be fitted into a normal bluetooth le advertise ment to
overcome this limitation the core added a new feature of extended advertisement s ea and
periodic advertising trains pa which allow this information to be carried in data packets on
general data channels which are not normally used for advert ising to accompany this it
added new procedures for a receiving device to use this information to determine where the
broadcast audio streams were located and synchronise to them
the requirement that an external device can help find a broadcast st ream added a requirement
that it could then inform the receiver of how to connect to that stream essentially an ability
for the receiver to ask for directions from a remote control and be told where to go thats
accomplished by a core feature called pa st periodic advertising synchronisation transfer
which is key to making broadcast acquisition simple past is a really useful feature for hearing
aids as scanning takes a lot of power minimizing scanning in is a useful feature to help
prolong the battery life of a hearing aid
the hearing aid requirements also resulted in a few other features being added to the core
requirements primarily around performance and power saving the first was an ability for
bluetooth uses an adaptive frequency hopping afh scheme to avoid interference constantly
changing the frequency channel a device uses to transmit data based on an analysis of current
interference the sequence of channels to use is called the hopping scheme which needs to be
conveyed from the central device to all of its peripherals
section the use cases
the new codec to be implemented in the host or in the controller the latter makes it easier
for hardware implementations which are generally more power efficient the second was to
put constraints on the maximum time a transmission or reception needed to last which
impacted the design of the pac ket structure within isochronous channels the reason behind
this is that many hearing aids use primary zinc air batteries because of their high power
density however this battery chemistry relies on limiting current spikes and high power
current dra w failing to observe th ese restrictions results in a very significant reduction of
battery life meeting the m shaped the overall design of the isochronous channels
two final additions to the core requirements which came in fairly late in the developme nt
were the introduction of the isochronous adaptation layer isoal and the enhanced
attribute protocol eatt
isoal allows devices to convert s ervice data units sdus from the upper layer to
differently sized p rotocol data units p dus at the link l ayer and vice versa the reason
this is needed is to cope with devices which may be using the recommended timing settings
for the new lc codec which is optimised for ms frames alongside connections to older
bluetooth devices which run at a ms timing interval
eatt is an enhancement to the standard attribute protocol att of bluetooth le to allow
multiple instances of the att protocol to run concurrently
the extended advertising features were adopted in the core release with isochronous
channel s eatt and isoal in the more recent core release paving the way for all of the
other bluetooth le audio specifications to be built on top of them
doing e verything that hfp and adp can do
as the consumer electronics industry began to recognise the potential of the bluetooth le
audio features which addressed many of the problems they had identified over the years they
made a pragmatic request for a third round of requirements to ensure that bluetooth le
audio would be able to do everything that adp and hfp could do they made the point
that nobody would want to use bluetooth le audio instead of bluetooth classic audio if the
user experience was worse
these requirements increased the performance requirements on the new codec and introduced
a far more complex set of requirements for media and telephony control the original hearing
aid requirements included quite limited control functionality for inter actions with phones
assuming that most users would directly control the more complex features on their phone or
tv not least because hearing aids have such a limited user interface many consumer audio
products are larger so dont have that limitation as a result new telephony and media control
requirements were added to allow much more sophisticated control
chapter the bluetooth le audio architecture
evolving beyond hfp and adp
the fourth round of requirements was a reflection that audio and telephony applications have
outpaced hfp and adp many calls today are voip and its common to have multiple
different calls arriving on a single device whether thats a laptop tablet or phone bluetooth
technology needed a better way of handling calls from multiple different bearers similarly
adp hadnt anticipated streaming and the search requirements that come with it as it was
written at a time when users owned local copies of music and rarely did anything more
complex than selecting local files today products needed much more sophist icated media
control they also needed to be able to support voice commands without interrupting a music
stream
the complexity in todays phone and conferencing apps where users handle multiple types of