Datasets:
kkboy1
/
LeAudio

Dataset card Data Studio Files
xet
 Community
1
text
stringlengths
1
112
x bis event effectively spreading the audio data transmissions over five events and adding
a total of ms to the latency compared with what it would be with pto in this figure
were only showing the arrows for group and group in that first bis event x otherwise
the figure becomes very cluttered and difficult to read
figure pretransmissions with nse bn irc and pto
finally in figure weve set nse to burst number and irc to and the pre
transmission offset is also with these settings each bis event includes the two packets
for that current event transmitted twice one after the other as we saw in figure with
the final two subevent s used for packets two events in the future because bn is the
isochronous interval will be ms as the pto is p and p come from two isochronous
intervals in the future so the latency is ms greater than the simple example of figure
these bis parameters allow for some very flexible transmission schemes in order to cope with
different requirements in terms of latency and robustness in the next chapter well see how
they can be used for different broadcast situations
in conclusion b oth flush timeout and pre transmission offset increase latency for a cis
flush timeout helps to share battery savings between both initiator and acceptor because
the use of acknowledgements means that events can be closed with a bis wher e there are
no acknowledgements a broadcast transmitter has to transmit at every subevent pto
effectively gives all of the power savings to the acceptor by providing a diversity of
transmission that gives it the best chance of acquiring a packet
section broadcast isochronous streams
the c ontrol subevent
the control subevent core vol b does not need to be included within every big
event typically control subevent s are used for items such as channel map updates so only
occur occasionally when that information needs to be provided to all of the devices that a re
receiving the broadcast audio streams the advantage of using control subevent s is that
devices no longer need to scan to find out the basic big information such as the hopping
channels that are being used th is minimise s the amount of work that a rec eiver has to do to
ensure that it stays synchronised with a big and the bises within it without them a
broadcast sink would need to continue to receive the periodic advertising train and
periodically examine the biginfo to discover any changes
a contro l subevent is transmitted in six consecutive big events it may then be transmitted
in any subsequent big event but only one control event can be transmitted at a time every
bis header for a bis subevent in a big event which includes a control subevent must have
the control subevent transmission flag cstf set to to signal the presence of a control
subevent in that big event its control subevent sequence number cssn will tell a
receiver if it is the same as one which they have already received control events use the same
frequency hopping scheme as every other subevent taking the index of the first bis event in
the same big event
big synchronisation
as with connected isochronous stream s individual receiving devices typically a pair of
earbud s or hearing aids dont necessarily know about each others existence to keep their
audio in synchronisation they need to use information thats included within the big to
understand when they need to render it to enable them to do this a big synchroni sation
point is defined which coincides with the end of the transmission of the audio data because
we have no acknowledgments to take into account in broadcast that big synchronisation
point is located exactly at the end of the last bis transmission of the last bis in a big
normally that will be coincident with the end of the big event however for the case when
there is a control subevent present the big synchronisation point remains at the end of the
last bis subevent transmission whilst the big e vent extends to the end of the control
subevent as shown in figure
chapter isochronous streams
figure big synchronisation
at the big synchronisation point every device that is listening to any of the broadcast
isochronous stream s within that big will know that every other device has received its data
hence the big synchr onisation point is a fixed point in time at which they can apply the
presentation delay this is defined higher up the stack and dictates the point at which audio
needs to be rendered the important point here is that audio is n ot rendered at the big
synchronisation point its rendered a t the end of the presentation delay which commences
at the big synchronisation point as broadcasting consists only of transmissions from a
broadcast s ource there is no concept of presentatio n delay applied to captured data coming
back from an acceptor
figure the application of presentation delay in a big
the derivation of the big synchronisation point is also a little different for broadcast unlike
unicast every bis has the same basic timing parameters thats because they need to be
defined in the biginfo and there is not enough space to allow different setting s for individual
bises this means that the big synchro nisation point is a fixed interval from the big anchor
section broadcast isochronous streams
point both of which are known by every acceptor as theyre in the biginfo in contrast
each cis uses a cissyncdelay based on its own anchor point as it doesnt know the
relative timings of any other cises and cant assume they are the same as its own
hci commands for bises
as with connected isochronous streams a host application can define the sdu interval the
maximum sdu size and the maximum transport latency which is the maximum time
allowed for the transmi ssion of a bis data pdu as with a cig it is up to the scheduler in
the controller to use these to guide it in defining the actual link layer parameters ie nse
bn irc and pto
setting up a big is much sim pler than a cis largely because there is no communication with
any of the receiving devices so only two hci commands are required lecreate big
configures and creates a big with a n umbis number of bises within it and the
leterminatebig command end s and removes it there are no options to add or remove
bises everything is done in a single operation both commands apply only to an initiator
which is transmitting one or more bigs
there are two similar commands for an acceptor which want s to recei ve one or more bises
from within a big a process which is called synchronising with a bis they are the
lebigcreate sync command and lebig terminatesync command but before we
get to those we need to look at how an acceptor can find a broadcaster and connect to it
finding broadcast audio stream s
when we looked at connected isochronous stream s we didnt talk about how the devices
initiated the connection the reason is that devices using connected isochronous stream s
connect in exactly the same way as every other bluetooth le device using the normal
advertising and scanning procedures they pair bond set up acl links discover each others
features and then get on with setting up the streams if they are a coordinated set cap
procedures ensure that all members of that set have the same procedures applied to them
with broadcast audio stre ams none of that pairing and negotiation process happens because
there is no connection instead receiving devices need to find a way to discover what is being
transmitted when it is being transmitted and work out how to synchronise to it