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dsPIC ? DSC Acoustic Echo Cancellation Library User’s Guide
4.3
AEC ALGORITHM OVERVIEW
This section describes the AEC algorithm. A conceptual block diagram illustrating the
operation of the AEC algorithm is shown in Figure 4-3 .
FIGURE 4-3:
AEC ALGORITHM FUNCTIONAL BLOCKS
S IN
Acoustic path
from speaker to
microphone
R OUT
Nonlinear
Processor
(NLP)
Normalized
Least Mean
Square
(NLMS)
Adaptive Filter
The AEC algorithm can be divided into these functions:
Howling Control
Double Talk
Detector
(DTD)
S OUT
R IN
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?
?
?
Normalized Least Mean Square (NLMS) Adaptive Filter
Double Talk Detector (DTD)
Nonlinear Processor (NLP)
Howling Control
A typical AEC system involves these signals:
?
?
?
?
Far-end speech receive input (R IN )
Near-end speech send output (S OUT )
Far-end speech output (R OUT ), usually sent to a local speaker
Near-end speech input (S IN ), usually received from a local microphone
The systems in which the near-end speaker and microphone do not have sufficient
acoustic separation, the S IN signal not only contains the microphone input (presumably
spoken by a talker at the near-end), but also an undesirable echo generated by the
acoustic path from the speaker to the microphone. This signal is then transmitted to the
far-end through the communication channel (wired or wireless), with the result that the
listener at the other end hears a perceptible echo of his/her own speech. Traditionally,
this problem was avoided by allowing only one person to talk at any given time (that is,
by not allowing “double talk”).
An AEC algorithm consists of an adaptive filter and various associated control
functions, which not only eliminate the acoustic echo but also enables double talk (that
is, full-duplex operation). This algorithm operates at the same communicating node at
which the echo was generated. The control functions used in the AEC algorithm, in
conjunction with the NLMS adaptive filter, are Double Talk Detector, Nonlinear
Processor and Howling Control.
DS70134F-page 34
? 2004-2011 Microchip Technology Inc.
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