PSB4860 Просмотр технического описания (PDF) - Siemens AG

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производитель

PSB4860

Digital Answering Machine with Full Duplex Speakerphone SAM EC

Siemens AG 

PSB 4860

Functional Description

2.1.3.2 Speech Comparators (SC)

The echo suppression unit has two identical speech comparators (SCAS, SCLS). Each

comparator can be programmed individually to accommodate the different system

characteristics of the acoustic interface and the line interface. As SCAS and SCLS are

identical, the following description holds for both SCAS and SCLS.

The SC has two input signals SX and SR, which map to microphone/loudspeaker for

SCAS and line in/line out for SCLS.

In principle, the SC works according to the following equation:

if SX > SR + V then switch state

Therefore, SCAS controls the switching to transmit state and SCLS controls the

switching to receive state. Switching is done only if SX exceeds SR by at least the

expected acoustic level enhancement V which is divided into two parts: G and GD. A

block diagram of the SC is shown in figure 17.

Log. Amp.

SX

Peak Decrement

PDS

PDN

Log. Amp.

SR

Base Gain

G

Gain Reserve Peak Decrement

GDS

GDN

PDS

PDN

Figure 17 Speech Comparator - Block Diagram

At both inputs, logarithmic amplifiers compress the signal range. Hence after the

required signal processing for controlling the acoustic echo, pure logarithmic levels on

both paths are compared.

The main task of the comparator is to control the echo. The internal coupling due to the

direct sound and mechanical resonances are covered by G. The external coupling,

mainly caused by the acoustic feedback, is controlled by GD/PD.

Semiconductor Group

35

10.97

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