Telit GM862-QUAD/-QUAD-PY
Hardware User Guide
1vv0300692, Rev. ISSUE#0, - 04/10/2005
Reproduction forbidden without DAI Telecom written authorization – All Right reserved – Right of modification reserved
page 24 of 69
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 GM862-QUAD pins 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. pins of the GM862-QUAD will be:
Voltage @ GM862-QUAD_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.
Another thing to consider, especially for cabin car use, is the fact that the external mic. will pick up
also ambient noise; to overcome this problem it is preferable to set the gain of the microphone 10dB
lower with respect to the calculated value for a nominal sensitivity. The corresponding reduction in
signal level will be compensated by an increased voice volume of the talker which will speak louder
because of the ambient noise.
