Telit Wireless Solutions GE863-QUAD - Manual (Page 34)

GE863-QUAD
GE863-PY
1vv0300715 Rev. 1 - 19/09/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 34 of 79
TIP: environment consideration .
For hands-free/car kit microphone, you must take into account the voice attenuation, due to the
distance between the microphone itself and the talker , when designing the external microphone
amplifier.
Not only, you must consider that the microphone 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 a car cabin usually the distance between the microphone itself and the talker is 40/50cm ; in these
conditions the attenuation can be considered as a thumb rule around 20dB .
For the earphone we shall distinguish two different types: the earphones having the microphone
sustained close to the mouth and the ones 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 earphone having the microphone on the earpiece cable,
while the other kind of earphone shall be threaten as an handset.
TIP: how to compensate the losses in car cabin hands-free condition .
The voice signal , that in the "normal spoken” conditions produces on the microphone membrane an
acoustic pressure of -4,7dBPa at 1kHz , will have a further attenuation of 20dB due the 50cm distance
.
Therefore a microphone having the suggested nominal sensitivity of -45dBV
rms
/Pa,will produce a lower
electrical
equivalent signal :
that means :
Setting the “microphone gain” at +10dB (3 times), the signal in the nominal conditions on the
“Mic_HF” inputs s of GE863-QUAD / PY Telit Module will be :
Hence in these conditions the signal level on the“Mic_HF” input pads of the GE863-QUAD / PY is 10
dB (3 times) lower than the nominal, as suggested.
MicLevel = ( -45) + (-4.7)-20 = -69.7
MicVoltage = 10 ( -49.7 / 20 ) = 0,33* 10 -3
“Mic_HF” Level = 0,33* 10 -3 * 3=1* 10 -3