Adaptive Filters and Applications - Digital Signal Processing: Fundamentals and Applications - page 480

d A ðnÞ¼x A ðnÞþx B ðnÞ . If the leakage x B ðnÞ returns back to speaker B, it becomes an annoying echo.
To prevent the echo, the adaptive filter at the speaker A site uses the incoming signal from speaker B as
an input and makes its output approximate to the leaked speaker B signal by adjusting its filter
coefficients; that is,

N 1
i ¼ 0 wðiÞx B ðn iÞ z x B ðnÞ

y A ðnÞ¼ (10.21)

As shown in Figure 10.21B , the estimated echo y A ðnÞ z x B ðnÞ is subtracted from the outgoing signal,
thus producing the signal that contains only speech A; that is, e A ðnÞ z x A ðnÞ . As a result, the echo of
speaker B is removed. We can illustrate similar operations for the adaptive filter used at the speaker B
site. In practice, an FIR adaptive filter with several hundred coefficients or more is commonly used to
effectively cancel the echo. If nonlinearities are concerned in the echo path, a corresponding nonlinear
adaptive canceller can be used to improve the performance of the echo cancellation.
Other forms of adaptive filters and other applications are beyond the scope of this topic. The reader
is referred to the references for further development.

10.5 LABORATORY EXAMPLES USING THE TMS320C6713 DSK
The implementation for system modeling in Section 10.4.3 is shown in Figure 10.22 , where the input is
fed from a function generator. The unknown system is a bandpass filter with a lower cutoff frequency
of 1,400 Hz and upper cutoff frequency of 1,600 Hz. As shown in Figure 10.22 , the left input channel
(Left Line In [LCI]) is used for the input while the left output channel (Left Line Out [LCO]) and the
right output channel (Right Line Out [RCO]) are designated as the system output and error output,

d ()

Unknown system

e ()

Input

x ()

y ()

Output

Adaptive
FIR filter

yn ()

xn ()

Signal
Generator

Left Line In
(LCI)

Left Line Out
(LCO)

TI TMS320C6713
DSP Board

Oscilloscope

Right Line In
(RCI)

Right Line Out
(RCO)

en ()

FIGURE 10.22
Setup for system modeling using the LMS adaptive filter.