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 25 of 69
For the headset we shall distinguish two different types: the headsets having the microphone
sustained close to the mouth and the headsets having the microphone on the earpiece cable.
The same considerations for the additional voice attenuation due to the distance from the
microphone and the noise pick up can be made for the headset having the microphone on the
earpiece cable, while the other kind of headset shall be threaten as an handset.
For example:
With the external mic. having the suggested nominal sensitivity -45dB
Vrms/Pa
at the "normal spoken" conditions: -4.7dB
Pa
at 7 cm from the mouth of the talker and with a
further attenuation of 20dB due to the distance from the microphone (about 50 cm).
At that acoustic pressure the voltage output from the microphone is:
Voltage Output (dB) = ( -45) + (-4.7) - 20 = -69.7 dB
Vrms
corresponding to:
Voltage Output = 10
( -69.7 / 20 )
= 3.3* 10
-4
V
rms
by having the microphone gain set to +10 dB (corresponding to 3 times) the signal in the nominal
conditions on the input external mic. pins of the GM862-QUAD will be:
Voltage @ GM862-QUAD__extmic = 3.3 * 10
-4
* 3 = 1 mV
rms
Hence in these conditions the signal level on the input pins of the external mic. of the GM862-
QUAD is 10 dB (3 times) lower than the nominal, as suggested.
The microphones usually need a biasing network that provides the necessary DC current to the mic.,
this will be explained further on.
In the EVK all the microphone input jacks have the hot wire connected to the central pole.
