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MP-3 player

Profiles: A2DP + AVRCP
Protocols: AVDTP + AVCTP

Piconet
Profile: A2DP
Profile: AVRCP Protocol: AVDTP
Protocol: AVCTP

Remote Headset
control

Figure 7.26 Simultaneous audio streaming and control connections to different devices.

each dealing with a certain frequency range of the input signal. Subsequently, a scaling
factor is calculated for each sub‐band, which gives an indication of the loudness of the
signal in the sub‐band. The scaling factors are then compared with each other to facili-
tate encoding of more important sub‐bands with a higher number of bits. The recom-
mendation for the number of bits to be used for this purpose ranges from 19 for
mid‐quality mono audio channels up to 55 for high‐quality joint stereo channels. The
results of the different sub‐bands are then compressed with a variable compression fac-
tor. Using the lowest compression factor to achieve the highest audio quality finally
results in a bit stream of about 345 kbit/s.
To transfer user commands from the audio sink device (e.g. headset) like volume control,
next/previous track, pause, and so on back to the audio source device (e.g. MP‐3 player),
the Audio/Video Remote Control Profile (AVRCP) [21] is used. The profile uses the
AVCTP [22] as shown in Figure 7.26 to send the commands from controller devices and to
receive responses from target devices. To achieve interoperability between controller and
target devices, the remote control profile specifies the following target device categories:
Category 1: Player/recorder
●
Category 2: Monitor/amplifier
●
Category 3: Tuner
●
Category 4: Menu.
●
Depending on the device category, the standard then defines a number of control
commands (operation IDs) and indicates for each device category whether the support
is mandatory or optional. Here are some examples of standardized control commands:
‘select’, ‘up’, ‘right’, ‘root menu’, ‘setup menu’, ‘channel up’, ‘channel down’, ‘volume up’,
‘volume down’, ‘play’, ‘stop’, ‘pause’, ‘eject’, ‘forward’ and ‘backward’. Vendor‐specific
control commands can be added to the list of commands, which, however, reduces
interoperability between devices and should therefore only be used with care.
It must be noted that there is no interaction between the audio streaming session that
uses the A2DP profile and a control session that uses the remote control profile. Thus,