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PSB4860

Digital Answering Machine with Full Duplex Speakerphone SAM EC

Siemens AG 

PSB 4860

Functional Description

even if recording or playback is going on. The echo suppression unit is also used to

provide additional attenuation if the echo cancellation unit cannot provide all of the

required attenuation itself.

2.1.2

Echo Cancellation

A simplified block diagram of the echo cancellation unit is shown in figure 12.

microphone

Control

NLMS

-

FIR

Filter

line out

loudspeaker

line in

Figure 12 Echo Cancellation Unit - Block Diagram

The echo cancellation unit consists of an finite impulse response filter (FIR) that models

the expected acoustic echo, an NLMS based adaption unit and a control unit. The

expected echo is subtracted from the actual input signal from the microphone. If the

model is exact and the echo does not exceed the length of the filter then the echo can

be completely cancelled. However, even if this ideal state can be achieved for one given

moment the acoustic echo usually changes over time. Therefore the NLMS unit

continuously adapts the coefficients of the FIR filter. This adaption process is steered by

the control unit. As an example, the adaption is inhibited as long as double talk is

detected by the control unit. Furthermore the control unit informs the echo suppression

unit about the achieved echo return loss.

Table 3 shows the registers associated with the echo cancellation unit.

Table 3 Echo Cancellation Unit Registers

Register # of Bits Name

Comment

SAELEN 9

LEN

Length of FIR filter

SAEATT 15

ATT

Attenuation reduction during double-talk

SAEGS 3

GS

Global scale (all blocks)

Semiconductor Group

28

10.97

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