16.2.5 Comfort Noise Generator
The CNG:
■
Creates a spectrally and temporally consistent noise floor for the far-end listener
■
Adaptively inserts noise modelled from the noise present at the microphone into gaps introduced when
attenuation is applied by the non-linear processing of the AEC
16.2.6 Equalisation
The equalisation filters:
■
Have independent equalisation modules provided in the send and receive signal paths:
■
Each module comprises of 5 bands of equalisation using cascaded 2
nd
order IIR filters
■
Are fully configurable using a graphical tuning tool
■
Provide static compensation for the frequency response of transducers in the system
16.2.7 Automatic Gain Control
The AGC block attempts to:
■
Normalise the amplitude of the incoming audio signal to a desired range to increase perceived loudness
■
Reduce distortion due to clipping
■
Reduce amplitude variance observed from different users, phones, and networks
Maintaining a consistent long-term loudness for the speech ensures it is more easily heard by the listener and it also
provides the subsequent processing block a larger amplitude signal to process. The behaviour of the AGC differs
from a dynamic range audio compressor. The convergence time for the AGC is much slower to reduce the non-
linear distortion.
16.2.8 Packet Loss Concealment
Bit errors and packet loss can occur in the Bluetooth transmission due to a variety of reasons, e.g. Wi-Fi interference
or RF signal degradation due to distance or physical objects. As a result of these errors, the user hears glitches
referred to as
pops
and
clicks
in the audio stream. The PLC block improves the receive path audio quality in the
presence of bit and packet errors within the Bluetooth link by using a variety of techniques such as pitch-based
waveform substitution.
The PLC significantly improves dealing with bit errors, using the BFI output from the firmware. The DSP calculates
an average BER and selectively applies the PLC to the incoming data. This optimises audio quality for a variety of
bit errors and packet loss conditions. The PLC is enabled in all modes.
16.2.9 Adaptive Equalisation
The adaptive equalisation block improves the intelligibility of the receive path voice signal in the presence of near‑end
noise by altering the spectral shape of the receive path signal while maintaining the overall power level. The adaptive
equaliser can also compensate for variations in voice transmission channels.
16.2.10 Auxiliary Stream Mix
The auxiliary stream mixer enables the system to seamlessly mix audio signals such as tones, beeps and voice
prompts with the incoming SCO stream. This avoids any interruption to the SCO stream and as a result prevents
any speech from being lost.
16.2.11 Clipper
The clipper block intentionally limits the amplitude of the receive signal prior to the reference input of the AEC to
more accurately model the behaviour of the post reference input blocks such as the DAC, power amplifier, and the
loudspeaker. This processing block can significantly improve the echo performance in cost-optimised loudspeakers.
Advance Information
This material is subject to CSR's non-disclosure agreement
© Cambridge Silicon Radio Limited 2011
Page 98 of 110
CS-209182-DSP1
CSR8640 BGA
Data Sheet
