UC864-G Hardware User Guide
DRAFT
- 29/10/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 38 of 66
TIP: Definition of the nominal sensitivity of the microphone lines.
The nominal sensitivity of the microphone lines indicates the voltage level on the UC864-G 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
st
differential
microphone path
.
In this case the maximum differential input
voltage to “
Mic_MT
” lines is 365mV
rms
(1,03V
pp
) corresponding to –8,76dBV.
Now we can calculate the maximum voltage gain of an external microphone amplifier
G
A
:
(
)
[
]
dBV
G
dB
MicLevel
A
76
,
8
20
−
=
+
+
[
]
76
,
8
20
7
,
49
−
=
+
+
−
A
G
A
G
−
=
+
−
20
9
,
40
dB
G
A
94
,
20
=
You can set G
A
=
+20dB
to use standard resistor values .
MicLevel = ( -45) + (-4.7) = -49.7 dB
Vrms
MicVoltage = 10
( -49.7 / 20 )
= 3.3* 10
-3
V
rms
