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PSB4860

Digital Answering Machine with Full Duplex Speakerphone SAM EC

Siemens AG 

PSB 4860

Functional Description

Two pairs of coefficients, GDS/PDS when speech is detected, and GDN/PDN in case of

noise, offer a different echo handling for speech and non-speech.

With speech, even if very strong resonances are present, the performance will not be

worsened by the high GDS needed. Only when speech is detected, a high reserve

prevents clipping. A time period ET [ms] after speech end, the parameters of the

comparator are switched to the “noise” values. If both sets of the parameters are equal,

ET has no function.

Table 5 Speech Comparator Parameters

Parameter # of bytes Range

Comment

G

1

– 48 to + 48 dB

Base Gain

GDS

1

0 to 48 dB

Gain Reserve (Speech)

PDS

1

0.025 to 6 dB/ms Peak Decrement (Speech)

GDN

1

0 to 48 dB

Gain Reserve (Noise)

PDN

1

0.025 to 6 dB/ms Peak Decrement (Noise)

ET

1

0 to 992 ms

Time to Switch from speech to noise

parameters

2.1.3.3 Attenuation Control

The attenuation control unit controls the attenuation stages GHX and GHR and performs

state switching. The programmable attenuation ATT is completely switched to GHX

(GHR) in receive state (transmit state). In idle state both GHX and GHR attenuate by

ATT/2.

In addition, attenuation is also influenced by the automatic gain control stages (AGCX,

AGCR).

State switching depends on the signals of one speech comparator and the

corresponding speech detector. While each state is associated with the programmed

attenuation, the time is takes to reach the steady-state attenuation after a state switch

can be programmed (TSW).

If the current state is either transmit or receive and no speech on either side has been

detected for time TW then idle state is entered. To smoothen the transition, the

attenuation is incremented (decremented) by DS until the evenly distribution ATT/2 for

both GHX and GHR is reached.

Table 6 shows the parameters for the attenuation unit. Note that TSW is dependent on

the current attenuation by the formula Tsw = SW × ATT .

Semiconductor Group

37

10.97

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