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are only recommendations to guide the controller when it works out its scheduling
depending on what else it is doing it will normally need to take other constraints into
account the three options in table are recommended link layer setting s for the
following three cases
option which has the lowest airtime is optimise d for coexistence with wi fi and
other bluetooth devices operating at ms schedules using a larger isochronous
interval to carry multiple ms frames provides the largest gaps for wi fi
operation if the broadcast source is a phone or a pc and is u sing wi fi to obtain
the music stream to send over bluetooth le audio thats very important this
option uses the least airtime for the bluetooth le audio transmissions but has the
chapter lc latency and qos
highest latency
option is designed to provide the highest reliability and lowest latency
maximising the number of retransmissions in each frame it uses the greatest
amount of airtime so is only really suitable for broadcast devices which have no
other ghz radio operations
option aims for a balanced approach betwee n reliability and coexistence it
would be a good choice for a broadcaster which is using wi fi to obtain the music
stream but which has no other coexistence issues to contend with
these are only three of many possible combinations which can be chosen b y a chips
scheduler over time others may be added to the recommended list as the industry
acquires more experience with bluetooth le audio
before leaving this subject table shows the overall latencies for stereo broadcast streams
using each of these three options with ms presentation delay mandated by tmap and
hap and the baseline ms of bap the controlle r will inform the host of which
parameters it has chosen but the host has no control over what that choice will be that
will be down to the scheduling algorithm in the chip designers should be aware of this
variation if the latency for your applicat ion is critical ask your silicon manufacturer for
details of what choices they are making in their controller scheduling
overall latency
pdms pdms
option ms ms
option ms ms
option ms ms
table overall latency for a broadcast stereo stream using the bap ll options for the qos configuration
airtime and retransmissions
we touched on airtime in table although the lc codec is more efficient than
previous bluetooth codecs as the bitrate is increased the packets take up an increasing
amount of the available airtime the figures in table for option show that a stereo
stream using these parameters accounts for almost of the available airtime that
doesnt include the airtim e required for advertising other active bluetooth links or any other
ghz radio activity
section quality of service qos
figure airtime usage for different unidirectional qos configurations
figure shows the effect of the higher bitrates in the qos configurations using a ms
frame size as well as the impact of more ret ransmissions specified by the high reliability
settings for the khz sampling configurations the airtime is the same for low latency
and high reliability as a minimum retransmission number of is recommended for each
because of the greater susceptibi lity of the larger packets to interference
broadcast source s have no idea of whether their retransmissions have been received so have
to transmit every packet unicast initiators only need to retransmit packets if they fail to
receive an acknowledgemen t that means that in many cases a unicast initiator will transmit
far fewer packets providing additional airtime for any other resource on the device which
needs it however if the link budget for the connection is poor as it may be for devices like
earbuds and hearing aids particularly when used outside they may need to transmit the
maximum number of retransmissions so the airtime must be allocated for this eventuality
airtime is a limited resource and increasing the audio quality and reliabilit y of a stream eats
into it one of the design requirements for bluetooth le audio was the ability to support
multi ple audio streams allowing a tv or cinema to transmit soundtracks in multiple
languages looking at figure its clear that is not possible with any of the khz
sampling configurations unless multiple radios are employed to transmit each different
stereo language stream in contrast a low latency khz or khz configuration could
easily cope with two different stereo streams if product designers want to take advantage of
bluetooth le audio s ability to transmit multiple streams they need to c onsider the airtime
implications which brings us to audio quality
chapter lc latency and qos
audio quality
since the early days of electronic audio reproduction a small group of users have constantly
pressed for higher quality that led to technical advances in recording and reproduction
along with marketing of terms like hi fi as well as real technical advances it saw the
appearance of pseudo scientific fashions such as oxygen free copper cables and gold plated
connectors to ensure that the analogue signals were not degraded digital technology
didnt lessen the enthusiasm for these despite the fact that a gold plated usb connector is
an anachronism audio devotees k ept clamouring for enhanced quality with the digital age
seeing calls for even higher sampling rates lossless codecs and increased output levels t he
fact that m any people over now have a level of hearing loss which means they are
incapable of hearin g any of these improvements doesnt stop product marketing managers
pushing for ever higher audio quality
in its early days bluetooth audio attracted a fair amount of negative coverage some was
welldeserved as some adp headsets had resource constrained codec implementations
the transducers used for rendering audio were also relatively primitive much has changed
since then with massive development in headphones earbuds and speakers to the point
where few users have concerns over bluetooth technology as an audio solution and it is
routinely used in top end audio equipment costing thousands of dollars
during the lc codec development the bluetooth sig commissioned independent research
on its audio quality compared with other codecs the results confirm that it offers
equivalent or better subjective performance to other codecs across the entire spec trum from
khz to khz sampling examples of lc encoded audio streams are available to listen to
at the bluetooth sig website
the tests were performed by a bank of expert listeners using high quality wired headphones
in audio listening booths the y were asked to rate each sample of sound against the
reference recording grading how close they felt it was to the original the tests used the
mushra protocol with a mixture of sounds from the ebus test recordings
it is always difficult to rel ate test results like these which are done in perfect listening
conditions with the real world experience because they are subjective however i have
tried to describe the general application for the different sampling frequencies in table