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served basis so you can have multiple commanders allowing you to use whichever comes to
section remote controls commanders
hand first when you want to change volume or mute your hearing aids because all of the
functions are explicitly specified it also means that multip le devices can implement them
interoperably in chapter well look at some of the ways in which they are likely to change
the way we use bluetooth audio devices
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having covered these new concepts we can now dive into the specifications to see how they
work and how they enable new use cases for bluetooth le audio
chapter isochronous streams
chapter isochronous stream s
if youve worked with bluetooth applications in the past youve probably concentrated on
the profiles and barely looked at the core specification thats possible because the bluetooth
classic audio profiles use well defined transport configurations tied in with core settings so
that there is not much need to understand whats happening underneath the profile or its
associated protocol with bluetooth le audio profiles that changes as you have more
potential to affect how the core works than with any of the bluetooth classic audio profiles
in order to try and make the most flexible system possible which would cope not only with
todays audio requirements but also the ones we havent even thought about yet the
specifications had to allow a much greater degree of flexibility to achieve that a fundamental
architectural decision was made to split the audio plane and the control plane what that
means is that new isochronous physical channels have been defined to carry the audio streams
these sit alongside but are separate to the existing acl links of bluetooth le the
isochronous physical channels in the core let you build up a number of isochronous audio
streams which are capable of transporting all types of audio from very low speech quality up
to incredibly high mus ic quality with one exception which well see later on isochronous
stream s contain no control information they are purely for carrying audio the
accompanying acl channels are used to set up the isochronous streams turn them on turn
them off add volume and media control along with all of the other features that we need
using the standard gatt procedures that are part of bluetooth low energy
in order to provide the flexibility that is needed for diffe rent latencies different audio quality
and different levels of robustness developers need to be able to control the way these
isochronous stream s are configured thats done quite high up in the profile stack of the
generic audio framework it means th at when you start working on bluetooth le audio
applications even if youre just using the top level profiles you still need to know a fair amount
about how the underlying isochronous stream s work thats different from what you would
have experienced i n most previous bluetooth applications to help understand the overall
architecture of bluetooth le audio we need to look at how those isochronous stream s were
developed what they do and how to use them
bluetooth le audio topologies
up until now t he bluetooth specification has largely been concerned with peer topeer
connections a central device makes a connection to a peripheral device and they exchange
the generic attribute profile defines the procedures which are used with the attribute protocol in
bluetooth low energy
from december the bluetooth sig like many other standards organisations implemented a
policy of replacing words which are considered to have negative connotations that means that the
specifications no longer use the traditional engineering terminology of master and slave when
section bluetooth le audio topologies
data its a very constrained topology various companies have developed proprietary
extensions to add flexibility as weve seen with true wireless stereo earbuds but isochronous
stream s were developed to cope with a much wider range of topologies than even these
proprietary solutions could provide as well as connecting a mobile phone to a pair of
headphones or a single speaker bluetooth le audio needed the ability to send separate left
and right signals to a left earbud and a right earbud it also need ed to be able to send the same
information to more than one set of earbuds and scale up the number of devices and streams
there are two types of isochronous stream unicast and broadcast unicast connections
known as connected isochronous stream s cis are the closest to existing bluetooth audio
use cases connected in this sense means that they are transferring audio data between two
devices with an acknowledgement scheme between the two devices to provide flow control
connected isochronous str eams have an acl control channel that is up and running
throughout the lifetime of the cis which is carrying the audio data
a very similar structure of broadcast isochronous stream s bis is used for broadc ast but
theres a major difference with broadcast a device that transmits the isochronous stream s
has no knowledge of how many devices may be out there receiving the audio theres no
connection between devices and no need for an acl link at its sim plest broadcast is purely
promiscuous however it is possible to add control links to broadcast at the core level
there is a clear distinction between connected and broadcast isochronous stream s which is
based on whether data is acknowledged or not however many applications will switch
between unicast and broadcast to fulfil different use cases without the user being aware of
what is happening but for the time being well concentrate on the basics
broadcast allows multiple devices to hear the same thing in the same way as fm radio or
broadcast tv the requirements for bluetooth le audio broadcast capability were initially
driven by the hearing aid application of telecoils where multiple people wearing hearing aids
in a public venue can list en to the same signal telecoils are relatively low audio quality used
mostly for speech with the higher quality possible with bluetooth le audio along with a
significantly lower installation cost the industry envisaged a much wider range of applica tions
and usage traditional telecoil locations like conference centres theatres and places of
worship public information such as flight announcements train departures and bus times
would become accessible to everyone with a headset and earbud not ju st people wearing
hearing aids broadcast is also applicable for more personal applications where a group of
people can listen to the same tv or share music from their mobile phones that last example
shows how bluetooth le audio applications can inv isibly switch the underlying protocols
back and forth if you are streaming music from your phone to your earbuds its probably
describing co mmunications but have replaced them with central and peripheral a full list of these
changes can be found at wwwbluetoothcomlanguagemappingappropriate language mapping
table
chapter isochronous streams
using a connected isochronous stream when your friends come along and you ask them
do you want to listen to this too you r music sharing application will switch your phone
from a private unicast connection to an encrypted broadcast connection so that you can all
hear the same music whether thats on earbuds hearing aids headphones or speakers at the
application layer listening to the music and sharing it with any number of other people should
be seamless the users dont need to know about broadcast or unicast but there will be a lot
going on underneath during that transition
the building blocks for these differen t use cases are essentially the same there are some
differences between the connect ed isochronous stream s that are used for unicast use cases