User's Manual

GC864 Hardware User Guide

1vv0300733 Rev. 0 - 12/06/06

TIP: definition of the nominal sensitivity of the microphone lines .

The nominal sensitivity of the microphone lines indicates the voltage level on the GC864 pins present

during "normal spoken" conditions.

For a handset , the "normal spoken” conditions take place when the talker mouth is 7cm far from the

microphone ; under these conditions the voice will produce an acoustic pressure of -4,7dBPa @1kHz

on the microphone membrane .

TIP: electrical equivalent signal and operating voice levels .

At "normal spoken" conditions, a microphone having the suggested nominal sensitivity of -

45dBV

rms

/Pa , will produce

the electrical equivalent signal :

that means :

During a call , this level varies according to the volume of the talker voice; usually the following rough

thumb rule for the dynamic range may be used :

1) the talker is screaming . This is the strongest voice level condition : the signal increases by

+20dB ;

2) the talker is whispering. This is the lowest voice level condition: the voice level decreases by

–50dB .

These changes must be considered for designing the external microphone amplifier .

TIP: example of external microphone amplifier calculation .

Let’s suppose to use the 1

differential microphone path .In this case the maximum differential input

voltage to “Mic_MT” lines is 365mV

rms

(1,03V

) corresponding to –8,76dBV.

Now we can calculate the maximum voltage gain of an external microphone amplifier G

()

[]

dBVGdBMicLevel

76,820 −

[]

76,8207,49 −=++−

G−=+− 209,40

dBG

94,20= you can set G

= +20dB to use standard resistor values .

MicLevel = ( -45) + (-4.7) = -49.7 dB

Vrms

MicVoltage = 10

(

49.7 / 20 )

= 3.3* 10

rms