User's Manual

Telit GE863-QUAD / GE863-PY

Hardware User guide

1vv0300715, Rev. ISSUE#0, - 21/02/06

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.

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

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

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. pads of the GE863-QUAD/PY will be:

Voltage @ GE863-QUAD/PY__extmic = 3.3 * 10

-4

* 3 = 1 mV

rms

Hence in these conditions the signal level on the input pads of the external mic. of the GE863-

QUAD/PY 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 EVK2 all the microphone input jacks have the hot wire connected to the central pole.