Telit Wireless Solutions GE863-PY, GE863-QUAD manual TIP environment consideration

GE863-QUAD

GE863-PY

1vv0300715 Rev. 1 - 19/09/06

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 -45dBVrms/Pa,will produce a lower electrical

equivalent signal :

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

that means :

MicVoltage = 10 ( -49.7 / 20 ) = 0,33* 10 -3

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 :

“Mic_HF” Level = 0,33* 10 -3* 3=1* 10 -3

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.

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