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LeAudio

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frame size ms or ms
the actual size for khz sampling is generated
automatically when or ms is set
bytes per frame payload s per
channel to
number of audio channels typically or but limited only by the profile or
implementatio n
table lc configuration parameters
chapter lc latency and qos
the bits per sample at the encoder and decoder are local settings and may be different in
most cases they are set to
for unicast streams an acceptor can expose which combinations of these values it supports
along with a preference for which configuration is used with a specific use case an initiator
makes a selection from the supported values exposed by each acceptor to configure each
audio stream
to limit these to a sensible number of combinations bap defines sixteen configurations for
the lc codec to help drive interoperability these are reproduced below in table and
cover sampling frequencies of khz to khz these can be found in bap tables
unicast server unicast client and broadcast source and broadcast
sink
codec
configuration
setting supported
sampling
frequency supported
frame
duration supported
octets per
codec frame bitrate
kbps
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
ms
mandated by bap for acceptors and initiators acting as unicast audio sink s or audio
source s and broadcast sources
mandated by bap for acceptors acting as unicast audio sink s or broadcast sinks
table bap defined codec configuration settings
for broadcast streams where the broadcast source has no knowledge of the receiving
devices it has to make a unilateral decision on the lc parameters it will use bap curre ntly
mandates that every broadcast receiver must be capable of decoding ms lc frames
encoded at khz with a byte sdu and khz with a byte sdu so a broadcast
source using these values knows that its audio streams can be decoded by every bluetooth
section the lc codec
le audio device
some top level audio profiles mandate support for receiving higher quality encoded lc
audio streams tmap mandates support for a range of codec configurat ions that employ
khz sampling ms and ms frames and a variety of bitrates however a broadcast
source with no connection to all of the receiving devices cannot know whether or not such a
stream can be decoded well look at the implications for b roadcast design later in the
chapter
packet loss concealment plc
an annoying feature of wireless transmission is that packets get lost for bluetooth which
shares the ghz spectrum with wi fi baby monitor s and a host of other wireless
products thats generally as a result of interference the bluetooth specification is one of
the most robust radio standards employing adaptive frequency hopping to try and avoid any
interference but there are occasions w hen data will be lost if the audio source is a phone or
a pc which is also using wi fi or has other bluetooth peripherals there will also be
occasions when they need priority which results in a bluetooth le audio transmission
being missed well look at ways to mitigate these later on but the reality is that occasionally
a packet will be irretrievably lost
unfortunately losing a packet in an audio stream is very noticeable and something that
annoys users to try and conceal it a number of techniques have evolved inserting silence
is generally annoying unless it happens to be preceded by a silent or ve ry quiet moment
repeating the previous frame may work but becomes noticeable if there are consecutive
missed frames
to provide a better listening experience the industry has developed a range of packet loss
concealment algorithms which attempt to co nceal the missing audio by predicting what it
was most likely to be these work very well with voice and generally quite well with music
although if a segment with or close to an attack transient is lost that is difficult to conceal
the lc specifica tion includes a packet loss concealment algorithm which has been
developed to match the lc codec it is applied whenever the b ad frame indication flag
signals a lost or corrupted frame or when the decoder detects an internal bit error it is
recommende d that it or an alternative plc algorithm is always used
chapter lc latency and qos
lc latency
we looked at the basics of latency at the start of the chapter but its important to
understand it in more detail
figure the component parts of latency
figure illustrates the main components of latency showing how it is built up across the
initiator and acceptor any frame based codec starts by imposing a delay due to the
sampling of the audio for a ms frame length the standard frame length in le audio
that accounts for the first ms of latency once the incoming audio frame has been
sampled the encoding can start which for lc takes about ms before it has a fully
encoded sdu to pass forward for transmission
the diagram shows transmission starting immediately if the receiver gets a valid pack et on