UC864-G Hardware User Guide
DRAFT
- 29/10/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 39 of 66
TIP: Environment consideration
For
handsfree/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 a handset.
TIP: How to compensate the losses in the 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 UC864-G Telit Module will be:
Hence in these conditions the signal level on the“
Mic_HF
” input pads of UC864-E 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