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LeAudio

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frequency pdms pdms pdms pd ms pd ms
khz ms ms ms ms ms
khz ms ms ms ms ms
khz ms ms ms ms ms
khz ms ms ms ms ms
khz ms ms ms ms ms
khz kbps ms ms ms ms ms
khz kbps ms ms ms ms ms
table typical end toend latencies for bap qos settings
the values in table are for single mono audio streams for stereo applications the latency
increases as we will see the message for implementers is to take care when choosing codec
and qos settings
the quality of service recommendations in tables and of bap include suggestions
for parameters to use in the hci commands to set up unicast or broadcast streams these
are recommendations remember that the controller uses some of these as guidance not as
definitive values covering the
sdu interval which is the same as the frame duration other than for khz
framing requirements all are unframed except for khz which is framed
maximumsdusize which is the same as the
supportedoctetspe rcodecframe
a recommended retransmission number rtn for low latency and high
reliability options
maxtrans portlatency for low latency and high reliability options and
presentati on delay requiring a value of ms to be in the supported range
using the values from these tables may not always produce the expected latencies one of
the reasons for that is the value for presentation delay bap requires all audio sinks to
include a presentation delay of ms within their range of supported values but it should
not be taken as the default value it is what it says in the note at the bottom of the table a
chapter lc latency and qos
value that must be supported by every acceptor acting as an audio sink its a compromise
for interoperability to ensure that everything has time to decode the audio data apply plc
and any additional processing most devices will be able to do better some higher layer
profiles require tighter performance tmap and hap both require acceptors to support a
presentation delay of ms for broad cast but if an initiator sets it to ms that impacts
latency
recalling figure the presentation delay in that example is only around ms lead ing to
an overall latency below m s many hearing aids will be capable of supporting that but it is
highly optimised in unicast where the initiator and acceptor talk to each other they can
agree on a lower value for presentation delay when the initiat or is aware that it is delivering
a low latency use case however a basic broadcast source has no way of knowing the
capabilities of the broadcasts sinks around it so will have to make a decision based on the
use case and a knowledge of the capabilities of broadcast sink s which are on the market and
which its designers expect will access it
a pair of acceptors can act unilaterally if they can communicate with each other by making
a decision to render earlier potentially on the basis of the context t ype for the stream
however that is outside the scope of the bluetooth le audio specifications
returning to the low latency columns in table sam pling frequencies above khz risk
echo effects when they are used for ambient sound applications none of the high
reliability settings are really suitable when reinforcing ambient sound particularly for
broadcast
where an ambient audio stream cannot be heard which is the case with most streaming
music and telephony applications latency becomes far less of a problem unless there is an
accompanying video stream where it could lead to lipsynch problems however in these
cases the video application generally manages the audio stream so can adjust the relative
timing to prevent any issue
rtn the retransmission number benefits from some further explanation the large
values in some of the configurations suggest lots of retransmissions but that is not
necessarily the case rtn is defined in the core vol part e sect as the number
of times that a cis data pdu should be retransmitted from the central to the peripheral or
peripheral to central before it is acknowledged or flushed for broadcast it is simply the
number of times the pdu should be retransmitted vol part e sect as weve
already seen the host is not allowed to specify values for ft pto nse or bn as it
doesnt know what other constraints the controller might have hence rtn is just a
recommendation to help guide the controllers scheduling algorithm
most of the time audio data wont be transmitted rtn times rtn is bette r interpreted as
the maximum number of retransmissions not the average number if an sdu has an ft
greater than and the sdu is transmitted the maximum number of rtn times the
section quality of service qos
following sdu will only have opportunity for transmission with no r etransmissions
possible rtn gives the opportunity to get more transmission slots to accommodate short
bursts of interference but setting it too high can penalise subsequent sdus the average
number of transmission opportunities is always nsebn havin g pointed that out t he
values given in tables and of bap have been well tested and should generally be
followed however as table of bap shows the controller may choose other values
returning to the isochronous channel settings its usefu l to look at what the initiators host
asks for and what it actually gets bap gives an example of how a controller might interpret
the hci parameters it receives based on different resource constraints to understand that
effect we can look at the high reliability setting for the broadcast audio stream
qos configuration thats defined in table of bap and is reproduced in table
below where a maximum transport latency of ms is requested
sdu
interval µs max sdu
octets rtn max transport
latency ms presentation
delay µs
table broadcast audio stream configuration setting for high reliability audio data
table of bap provides recommended link layer parameters for a controller to use
when it receives an lesetci gparameters hci command containing the values of table
these are shown in table which also shows the resulting transport latency and the
percentage of available airtime which is used for each set of parameters
option isointerval
ms bn nse irc pto numb is rtn max transportlatency
monostereo airtime usage
monostereo
or ms
or ms
or ms
table recommended bap ll parameters for the broadcast audio stream qos configuration
the three options in table are possible becau se the maxtransportlatency and rtn