PSB4860 Просмотр технического описания (PDF) - Siemens AG

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PSB4860

Digital Answering Machine with Full Duplex Speakerphone SAM EC

Siemens AG 

PSB 4860

Functional Description

Table 3

SAEPS1

SAEPS2

Echo Cancellation Unit Registers

3

AS

Partial scale (for blocks >= SAEPS2:FB)

3

FB

First block affected by partial scale

The length of the FIR filter can be varied from 127 to 511 taps (15.875ms to 63.875ms).

The taps are grouped into blocks. Each block contains 64 taps.

The performance of the FIR filter can be enhanced by prescaling some or call of the

coefficients of the FIR filter. A coefficient is prescaled by multiplying it by a constant. The

advantage of prescaling is an enhanced precision and consequently an enhanced echo

cancellation. The disadvantage is a reduced echo cancellation performance if the signal

exceeds the maximal coefficient value. More precisely, if a coefficient at tap Ti is scaled

by a factor Ci then the level of the echo (room impulse response) must not exceed Max/

Ci (Max: Maximum PCM value). As an example figure shows a typical room impulse

response.

A

0.5

0.25

t0.25

t

Figure 13 Echo Cancellation Unit - Typical Room Impulse Response

First of all, the echo never exceeds 0.5 of the maximum value. Furthermore the echo

never exceeds 0.25 of the maximum value after time t0.25. Therefore all coefficients can

be scaled by a factor of 2 and all coefficients for taps corresponding to times after t0.25

can be scaled a factor of 4.

The echo cancellation unit provides three parameters for scaling coefficients. The first

parameter (GS) determines a scale for all coefficients. The second parameter (FB)

determines the first block for which an additional scale (PS) takes effect.

This feature can be used for different default settings like large or small rooms.

Semiconductor Group

29

10.97

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