User Manual

GM862-QUAD / PY Hardware User Guide

1vv0300748 Rev. 5 - 20/09/07

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76,8207,49 −=++−

G−=+− 209,40

dBG

94,20= you can set G

= +20dB to use standard resistor values .

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 GM862-QUAD / PY Telit Module will be :

MicLevel = ( -45) + (-4.7)-20 = -69.7

MicVoltage = 10

(

49.7 / 20 )

= 0,33* 10

“Mic_HF” Level = 0,33* 10

* 3=1* 10