Telit GE863-QUAD / GE863-PY
Hardware User guide
1vv0300715, Rev. ISSUE#0, - 21/02/06
Reproduction forbidden without Telit Communication written authorization – All Right reserved – Right of modification reserved
page 28 of 71
•
echo
canceller
type:
car
kit
hands
free
TIP: Due to the difference in the echo canceller type, the internal audio path is suited for handset
applications while the external audio path is suited for car kit hands free. The headset applications
should be made by using the external audio path but DISABLING the echo canceller by software AT
command. If the echo canceller is left active with the headset, then some echo might be introduced
by the echo cancel algorithm.
The nominal sensitivity of the line indicates the voltage level on the GE863-QUAD/PY pads that
should be present during "normal spoken" conditions: for a handset a voice signal coming from
the mouth of the talker at 7 cm of distance from the microphone, producing an acoustic pressure
of -4,7 dB
Pa
(@ 1 kHz) on the microphone membrane.
For example:
With the internal mic. having the suggested nominal sensitivity -45dB
Vrms/Pa
at the "normal spoken" conditions: -4.7dB
Pa
on the microphone membrane.
At that acoustic pressure the voltage output from the microphone is:
Voltage Output (dB) = ( -45) + (-4.7) = -49.7 dB
Vrms
corresponding to:
Voltage Output = 10
( -49.7 / 20 )
= 3.3* 10
-3
V
rms
by having the microphone gain set to +24 dB (corresponding to 15.8 times) the signal in the
nominal conditions on the input mic. pads of the GE863-QUAD/PY will be:
Voltage @ GE863-QUAD/PY_mic = 3.3 * 10
-3
* 15.8 = 51 mV
rms
During the spoken conditions the signal varies according to the volume of the voice of the talker,
usually a rough thumb rule for the dynamic range may be considered:
* +20dB (10 times) at maximum voice level (talker screaming)
* -50 dB (1/300 times) at minimum voice level (talker whispering).
For the handsfree/car kit microphone the voice attenuation due to the distance between the
microphone and the talker must be taken into account when designing the microphone amplifier.
For a car cabin usually the distance between the talker and the mic. is about 50cm; in these
conditions the attenuation can be considered as a thumb rule around 20dB.
