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generated locally in the ac ceptor so only needs a control signal not the presence of an audio stream |
chapter new concepts in bluetooth le audio |
user experience is likely to result from providing an out of band ring tone which is generated |
by the earbuds using ccp and tbs the user can hear this mixed into their existing audio |
stream and decide whether to accept or reject the call if they reject the call they can continue |
listening to their or iginal stream in most cases ringtone is best used to manage whether |
devices are allowed to interrupt the current audio application with incoming phone calls we |
will cover how out of band ringtones are handled in chapter |
the unspecified context type is a catch all category every acceptor has to support the |
unspecified context type but do esnt need to make it available when an acceptor does |
set the unspecified context type as available it is saying that it will accept any context type |
other than ones it has specifically s aid it will not support as implementers get used to context |
type s some will use this to allow the central device typically a phone to be in charge of the |
audio use case in much the same way it is for adp and hfp an acceptor that just sets |
unspecified as available is effectively allowing the phone to take full control of what it is |
sent we ll discuss the concepts of supported and available in more detail in chapter |
all of the context type s are independent of e ach other the use of any one of them does |
not imply or require that another one needs to be supported unless that requirement is |
imposed by a top level profile as an example support for the ringtone context type does |
not generally imply or require support for the conversational context type as ringtone |
could be used by itself for an extension bell to alert someone with hearing loss of an incoming |
phone call on a land line phone |
context type s are used by both initiators and acceptors to provide information about the |
use case intended for a stream allowing them to make a decision about whether to accept a |
stream to accomplish this they are used in a range of characteristics and ltv metadata |
structures youll find them used in this way in the following places |
supportedaudiocontexts characteristic pacs |
availableaudiocontexts characteristic pacs also used in a server |
announcement bap |
preferred audio contexts ltv structure metadata in a pac record see also bap |
streaming audio contexts ltv structure metadata used by an initiator to label an |
audio stream bap |
an audio stream can be associated with more than one context type although the intention |
is that the context type value represents the current use case the streaming audio contexts |
metadata has procedu res to allow a device to update the bitfield values as the use case changes |
ltvs are triplet structures consisting of a statement of the length of the structure its type and |
the parameter values in that order |
section availability |
this is typically used where an established stream is used for multiple use cases an example |
would be where an audio source mixes in audio from different applications such as a satnav |
message that could interrupt music in this case it is efficient to continue to use the same |
stream assuming its qos parameters are suitable and just update the current context type |
value |
availability |
bluetooth le audio supports far more poss ibilities and combinations than bluetooth classic |
audio to enable devices to make informed choices about what theyre doing they not only |
need the ability to tell each other about the use case theyre engaging in which is the reason |
for the context typ es but also which ones theyre interested in participating in in the future |
this is where availability comes in |
as weve seen above context type s are used by acceptors to signal whether they can take |
part in a use case thats a ccomplished in two ways an acceptor uses the |
availableaudiocontexts characteristic defined in pacs to state which of its supported |
audio context type s can currently be used to establish an audio stream audio sources |
whether initiators or acceptors also use the streaming audio contexts ltv structure in the |
metadata of their codec configurations to inform an audio sink of the use cases that are |
associated with an audio stream |
bluetooth le audio devices wanting to establish unicast audio streams can also use context |
type s to signal their availability before they even start an audio stream by including the |
streaming audiocontext s ltv structure in th eir advertising pdus in a similar way |
initiators acting as broadcaster s include this in the metadata section of their periodic |
advertisement s so that broadcast sink s and broadcast assistant s can filter out the use cases |
they want from those they are n ot interested in receiving |
for broadcaster s you should note that if the streamingaudiocontexts field is not present |
in a codec ids metadata which well get to in chapter it will be interpreted as meaning |
that the only supportedaudiocontexts value is unspecified so every broadcast sink can |
synchronise to it unless they have specifically set unspecified as non available |
audio location |
with every previous bluetooth audio specification there was a sin gle bl uetooth le audio |
source and a sin gle bl uetooth le audio sink the audio stream was sent as mono or stereo |
and it was left to the audio sink to interpret how it was rendered bluetooth le audio is |
intended to address applications with multiple speakers or earbuds it ha s the ability to |
optimise airtime for each audio sink by only sending it the audio stream it needs in general |
for a pair of earbuds the left earbud only receives the left audio stream and the right earbud |
only receives the right audio stream that m eans that each audio sink can minimi se the time |
that its receiver is on thereby reducing its power consumption |
chapter new concept s in bluetooth le audio |
to accomplish th is devices need to know what spatial information they are meant to receive |
for example a left or a right stream from a ster eo input they do this by specifying an audio |
location these are defined in the bluetooth generic audio assigned numbers and follow |
the categorization of cta gs table codes for speaker placement the values are |
expresse d as bits in a four octet wide bitfield the most common audio locations are shown |
in table |
audio location value bitmap |
front left x bit |
front right x bit |
front centre x bit |
low frequency effects front woofer x bit |
back left x bit |
back right x bit |
prohibited x |
table common audio location values |
every acceptor which can receive an audio stream must set at least one audio location |
leaving the entire bitfield blank is not allowed an audio location is normally set at |
manufacture but in some cases it may be changeable by the user for instance a speaker |
could have an application or a physical switch to set it to front left or front right |
note that mono is not a location as mono is a property of the stream not the physical |
rendering device a single channel speaker would normally set both the front left and front |
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