Datasets:
kkboy1
/
LeAudio

Dataset card Data Studio Files
xet
 Community
1
text
stringlengths
1
112
this allows the acce ptor to go to sleep until the next isochronous interval the initiator can
use the time to do other things as many initiators will also be interacting with other bluetooth
devices and possibly sharing their radio and antenna with wi fi that can be useful for the
acceptor turning its receiver off until its ready to do something ag ain can bring a significant
power saving
if the acceptor does not receive the header with the cie bit it will continue to listen for data
in each scheduled subevent but will not receive any packets from the initiator to respond to
controlling audio q uality and robustness
having cover ed the basic timing of transmissions in a cis we can now look at the parameters
which are used to control the quality of the audio and the robustness of the link for many
audio a pplications latency is important for some applications such as when you are listening
to a live stream it is important to minimise the latency particularly if you can hear the ambient
sounds as well on the other hand if youre streaming music thro ugh your phone and cant
hear or see the source latency doesnt matter that much other applications such as gaming
have different priorities and if youre listening to audio while watching a film lipsync becomes
important
the basic audio profile bap can set many of the parameters which affect audio quality and
latency by using the lesetcigparameters hci command well look at how it makes
those choices in chapter but for now we need to understand how the isochronous channel
structure can be configured the key items that an application can request in order to influence
the latency and robustness are
chapter isochronous streams
the maximum transport latency which set s the maximum time that an initiator
can spend transmitting the pdus for a particular cis
the maximum sdu size for both directions of the cis
the sdu interval for both directions
the maximum transport latency affects the overall latency although it is only one element
of it whilst many applications will want to minimi se latency in the real world of wireless you
also need to address the inherent fragility of a wireless link where packets can be lost to
ensure sufficient robustness which translates into rendered voice and music streams without
dropouts clicks and silence we need to use a variety of techniques to help ensure that audio
data gets through in an acceptable timeframe
flush timeout and number of subevent s
the three parameters listed above are inputs to the controller the controller takes the m and
uses them to calculate three parameters that affect the robustness of the bluetooth le audio
link for that cis which are
nse the number of subevent s this specifies the number of subevent s which
will be scheduled in each isochronous interval they are used for the initial
transmission of a cis
pdu and its subsequent retransmissions they may not all be used but it is a fixed
number that are scheduled
ft the flush timeout the flush timeout define s how many consecutive
isochronous intervals can be used to transmit a pdu before it is discarded t he
point at which it is no longer transmitted is called the flush point
bn the burst number which is the number of payloads supplied for
transmission in each cis event
these can be quite difficult to grasp so its useful to look at some simple examples
the number of subevents nse is the most straightforward of the three it is simply the
number of opportunities to transmit an isochronous pdu which are available within each
isochronous interval in the simplest example where only one pdu is supplied for
transmission in each isochronous interval the pdu will be transmitted in the first subevent
and can then be retransmitted a maximum of nse times in the same isochronous
interval if it is a unidirectional cis once the pdus transmission is acknowledged by the
acceptor the controller can set the close isochronous event cie bit in the header of its
next transmission which can have a null pdu payload and any remaining subevent s in
that cis event become free airtime for other radio application s
that is the case where f lush timeout is as the flush point then coincides with the end of
the cis event for the isochronous interval this simple case is illustrated in figure for
the sake of clarity the following examples only involve one acceptor
section connected isochronous streams
figure a unidirectional cis with nse and ft
in this example nse is set to so there are four opportunit ies for each packet to be
transmitted in the fir st cis event none of the four attempts are successful so packet p is
flushed the acceptor will need to try to reconstruct it using some form of packet loss
correction
the second packet p succeeds after the second attempt after which the initiator clos es the
event the third packet p succeeds first time
if the flush timeout is increased then the transmission of a packet can continue over more
isochronous interval s figure illustrates an example where nse remains at but the
flush timeout is increased to
figure an example of ft and nse
this illustrates a problem if you are only using the t wo parameters nse and ft it allows
a packet to dominate the transmission slots until it reaches its flush point in figure the
chapter isochronous streams
first payload p which is having problems getting through occupies all of the subevent s in
the first three isochronous intervals leaving p and p waiting until after the flush point fp
although this situation should not be common it can leave subs equent payloads more
exposed until enough of them get through to bring the system back into equilibrium
burst number
the way to address this problem is to allow more than one payload to be transmitted in a
single isochronous interval so that subevent s in each isochronous interval can be shared
between more than one pdu and not be used exclusively by one of them this is made
possible by taking advantage of burst number bn which is the number of payloads supplied
for transmission i n a cis event in the examples above only one packet has been delivered
in a cis event which is the situation where the cadence of sdu and pdu generation is the
same as the isochronous interval meaning that one encoded ms audio frame becomes
availab le for each ms isochronous interval if we want to make use of bn and were
continuing to sample the incoming audio channel every ms we need to increase the
isochronous interval to a multiple of that so that we have more packets available in each
isochronous interval it mean s that by addressing one problem we are potentially creating
another which is increasing latency
figure the effect of burst number with nse and ft
in figure the isochronous interval has been doubled allowing two packets to be supplied
in each interval the combination of the ms codec frame and ms isochronous interval
means the initiator has two pdus available to transmit within each isochronous interval a
consequence of this is that there are now two flush points in each isochronous interval in
our simple example each flush point occurs after two subevent s for m ore complex