new acoustic echo pptvbhjn
TRANSCRIPT
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ACOUSTIC ECHO
CANCELLATION FOR DOUBLETALK
BYS IMRANA 07H81A0417
FATIMA TAYYBA
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INTRODUCTION
The purpose of this project is to cancel
Acoustic echo.Before implementing in real
time on DSP kit.
we have simulated in Matlab and found theresponse for different speech signals.
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BACKGROUND
SPEECH SIGNAL.
ECHO SIGNAL
Echo is a phenomenon in which a delayed and
distorted version of an original sound or electrical
signal is reflected back to the source.
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Adaptive Filter
The general adaptive filter ,filters the input signal
so that it resembles (in some sense) the desired
signal input. An adaptive FIR or IIR filter designs
itself based on the characteristics of the input
signal to the filter.
It iteratively alter its parameters.
It minimizes the error between some desiredsignal and some reference signal.As shown in
figure.
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Block Diagram Of AdaptiveFilter
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Diagram Explanation
we are considering that the desired signal(d) is corr-
upted and our aim is to eliminate the undesired part to
get signal.For this purpose we are using adaptive
filter.It uses algorithm which provides a set of taps.The tap
weights could then be implemented as the correspond-
ing filter, inversed and applied to the received signal
thus producing the original signal in its original
form.
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Adaptive Filtering System
Configurations
There are four major types of adaptive filtering
configurations.
Adaptive system identification. Adaptive echo/noise cancellation.
Inverse system identification.
Linear predictive coding.
Note:
All of the above systems are similar in the implementation ofthe algorithm, but different in system Configuration.
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ACOUSTIC ECHO
Acoustic echo occurs when an audio signal is
reverberated in a real environment, resulting in
the original intended signal plus attenuated, time
delayed images of this signal.
When a signal is received by the system, it is
output through the loudspeaker into an acoustic
environment. This signal is reverberated within
the environment and returned to the system via
the microphone input.
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Cont..
These reverberated signals contain time
delayed images of the original signal, which are
then returned to the original sender. The
occurrence of acoustic echo in speechtransmission causes signal interference and
reduced quality of communication.
The method used to cancel the echo signal is
known as adaptive filtering.
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SOURCES OF ACOUSTIC
ECHO
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ACOUSTIC ECHO
CANCELLER
The echo canceller mimics the transfer function of theecho path (or room acoustic) to synthesize a replica ofthe echo, and then subtracts that replica from the
combined echo and near-end speech (or disturbance)signal to obtain the near-end signal alone.
However, the transfer function is unknown in practice,and so it must be identified. The solution to thisproblem is to use an adaptive filter
An adaptive filter algorithmically alters its parametersin order to minimize a function of the differencebetween the desired output d(n) and its actual outputy(n). This function is known as the cost function of theadaptive algorithm.
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ACOUSTIC ECHO
CANCELLER
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CONT
Here the filter H(n) represents the impulse response of
the acoustic environment, W(n) represents the adaptive
filter used to cancel the echo signal. The adaptive filteraims to equate its output y(n) to the desired output d(n)
(the signal reverberated within the acoustic
environment). At each iteration the error signal,
e(n)=d(n)-y(n), is fed back into the filter, where thefilter characteristics are altered accordingly.
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Conventional block diagram
microphone ejnj
uj
dj pn
n
xjloudspeaker
Adaptive
filter
Step size
control
+
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`block diagram
Main
ADF
StepSize
Control
+
+ +
Sub adf
microphone
Loud speaker
nj
pn
n
dj
UbjUcj
xj
ej
j
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Contd..
In addition to the main adaptive filter in our project
we are using the sub adaptive filter to cancel the
echo.
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LMS ALGORITHM
The LMS algorithm is a linear adaptive filter algorithm,which in general consists of two basic processes.
1. A filter process, which involves
a.Computing the output of a linear filter inresponse to an input signal.
b.Generating an estimation error bycomparing this output with a desiredresponse.
2. An adaptive process, which involves theautomatic adjustment of the parameters of thefilter in accordance with the estimation error.
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IMPLEMENTATION OF LMS
ALGORITHM
Each iteration of the LMS algorithm requires 3 distinct
steps in this order:
1. The output of the FIR filter, y(n) is calculated
using equation
2. The value of the error estimation is calculated
using equation
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CONT..
3. The tap weights of the FIR vector are updated in
preparation for the next iteration, by
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REAL TIME
IMPLEMENTATION
Real Time Processing:
Real-time processing means that the processingmust keep pace with some external event .
Real Time Implementation involves:
A/D converter is used to take the input Thisinput signal is processed by the processor and
the output is send through D/A.To read and write data multi channel buffer
serial port is used
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Thank you