Speakerphone/car kit hands free design
LZT 123 7895 R1A
7
of this selection may introduce too many higher order acoustic frequency
components for the customer application. In this case both circuits may
be replaced with low noise analogue equivalents. It is recommended that
these still be chosen with a 2W drive and an audio-frequency gain around
12dB.
3.3 GR47 Audio Parameters
3.3.1 Configuration
Figure 2 and Table 4, in the appendix, present the system block level
diagram for the GR47 audio routing and the parameters used within those
blocks. The AT command used to set the audio parameters is
AT*E2EAMS
The parameters used in the Sony Ericsson example circuit are:
n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
AT*E2EAMS
1 1 0 3 0 1 8 4 3 3 1 1 1 0 0 0 0 1 1 0
Table 3 - Working Parameters for Simple Car-kit Application
During testing by Sony Ericsson, the performance was optimised in the
chosen surroundings by modifying the audio levels of the microphone path
and speaker path. These modifications were made by changing the
values written to parameters n=1 and n=2. Parameter 1 (TxPGA) was
adjusted between levels -10dB and 0dB. Parameter 2 (RxPGA) was
adjusted between levels -5dB and +5dB. When modified within this range
and set up as a simple car-kit it was possible to achieve a comfortable
audio environment where both parties could speak and be heard at a
comfortable level with minimal or no disruption caused by acoustic echo.
Note :
The parameters in Table 3 give a TxPGA setting of -2.5dB and an
RxPGA setting of -2.5dB.
3.4 Operation
The audio performance achieved following these guidelines should be
acceptable for most simple applications. During testing as a simple car-
kit, audio volume within the car was acceptable even at speed. Speech
from the microphone inside the car was loud and clear although some
background road noise is also heard.
During the early part of each call, the far-end party may hear audible signs
of acoustic training which are tolerable and are not distinguishable as
