Acoustic echo Cancellers
Acoustic echo is inevitable whenever a loudspeaker is placed near a microphone in a full-duplex
communication application. This is the case in speaker-phones, audio and video conferencing, desktop
communication, and many other communication scenarios. Especially hands-free mobile communication kits
for cars are becoming increasingly important due to safety regulations introduced in more and more
countries. In all those and similar communication scenarios, the voice from the loudspeaker (Far End
Speech, FES) is inevitably picked by the microphone (NES) and transmitted back to the remote speaker
as shown in Fig. 1. This makes the remote speaker hear her own voice distorted and delayed by the
communication channel, which is known as echo. The longer the channel delay, the more annoying the echo
becomes until it makes natural conversation impossible and decreases the perceived quality of the
communication service. It is therefore absolutely necessary to avoid transmitting back the echo
picked by the microphone.
Modern full-duplex communication systems make use of an acoustic echo canceller (AEC) to prevent
the echo from being transmitted back to the channel. The AEC is employed in each terminal, and has
completely different requirements than the Network Echo Canceller employed by the telephone network
provider to eliminate the electric echo. The AEC basically estimates the echo and subtracts the
estimated echo from the microphone signal as shown in Fig.1. The resulting signal (RESidual) is
transmitted to the far end speaker through the communication channel.
Several acoustic echo canceller implementations are available in object or source code for
different platforms. The NLMS-AEC is suitable for applications with short reverberation
time, and therefore need short adaptive filters (up to 512 coefficients). The BFDAF-AEC
is suitable for applications with long reverberation time and can afford long processing
delay. The PBFDAF is similar to the BFDAF but solves the delay problem, therefore suitable
for applications requiring long filters (512 to 4000) and require low processing delay.
CANEC, our latest development, is a unisize combined noise and echo canceller algorithm
designed to provide excellent performance in any acoustic environment at very low resources.
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