psb21391 Infineon Technologies Corporation, psb21391 Datasheet - Page 139

• Manufacturer Part Number: psb21391
• Description: Siemens Codec With Upn Transceiver
• Manufacturer: Infineon Technologies Corporation

At the SCAE-input, 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.
Principally, 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 GAE. The external
coupling, mainly caused by the acoustic feedback, is controlled by GDAE/PDAE.
The Gain of the Acoustic Echo (GAE) corresponds to the terminal couplings of the
complete telephone: GAE is the measured or calculated level enhancement between
both receive and transmit inputs of the SCAE (see figure 67). It equals the sum of the
amplification of ALS plus the gain due to the loudspeaker/microphone coupling plus the
TX-amplification of AMIC1 and GX1. To succeed in a sure differentiation between
original speech and echo, it must be guaranteed that the TX-signal does not run into
saturation due to the loudspeaker/microphone coupling. Therefore, it is recommended
to reduce the TX-gain by 10 dB in front of the SCAE at least in the loudest loudspeaker
volume step. To fulfill the sending loudness rating, this gain is realized by the LGAX/
AGCX which follows the SCAE. Of course, the GAE has to be reduced by the same
amount.
To control the acoustic feedback two parameters are necessary: GDAE-features the
actual reserve on the measured GAE. Together with the Peak Decrement (PDAE) it
simulates the echo behaviour at the acoustic side: After RX-speech has ended there is
a short time during which hard couplings through the mechanics and resonances and the
direct echo are present. Till the end of that time (
be at least equal to GAE to prevent clipping caused by these internal couplings. Then,
only the acoustic feedback is present. This coupling, however, is reduced by air
attenuation. For this in general the longer the delay, the smaller the echo being valid.
This echo behaviour is featured by the decrement PDAE.
Data Sheet
129
t
) the level enhancement VAE must
PSB 21391
PSB 21393
2001-03-07
Codec