PSB4860 Ver la hoja de datos (PDF) - Siemens AG

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PSB4860

Digital Answering Machine with Full Duplex Speakerphone SAM EC

Siemens AG 

PSB 4860

Functional Description

Operation of the AGC depends on a threshold level defined by the parameter COM

(value relative to the maximum PCM-value). The regulation speed is controlled by

SPEEDH for signal amplitudes above the threshold and SPEEDL for amplitudes below.

Usually SPEEDH will be chosen to be at least 10 times faster than SPEEDL. The bold

line in Figure 19 depicts the steady-state output level of the AGC as a function of the

input level.

AGC input level

-20 dB -10 dB

max. PCM

Example:

COM = -30 dB

AG_GAIN = 15 dB

AG_ATT = 20 dB

AG_GAIN

-10 dB

AG_ATT

-20 dB

COM

AGC

output

level

Figure 19 Echo Suppression Unit - Automatic Gain Control

For reasons of physiological acceptance the AGC gain is automatically reduced in case

of continuous background noise (e.g. by ventilators). The reduction is programmed via

the NOlS parameter. When the noise level exceeds the threshold determined by NOIS,

the amplification will be reduced by the same amount the noise level is above the

threshold. The current gain/attenuation of the AGC can be read at any time (AG_CUR).

An additional low pass with time constant LP is provided to avoid an immediate response

of the AGC to very short signal bursts.

If SDX detects noise, AGCX is not working. In this case the last gain setting is used.

Regulation starts with this value as soon as SDX detects speech.

Likewise, if SDR detects noise, AGCR is not working. In this case the last gain setting is

used. Regulation starts with this value as soon as SDR detects speech. When the AGC

has been disabled the initial gain used immediately after enabling the AGC can be

programmed. Table 8 shows the parameters of the AGC.

Semiconductor Group

39

10.97

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