Page 15
Audio Gain settings table
Gain Setting (dB)
CH1 G0
CH1 G1
CH2 G2
AGC
0
No Jumper
No Jumper
No Jumper
No Jumper
12
Pre -Linked
No Jumper
No Jumper
No Jumper
15
No Jumper
Solder Link
No Jumper
No Jumper
18
Solder Link
Solder Link
No Jumper
No Jumper
21
No Jumper
No Jumper
Solder Link
No Jumper
24
Solder Link
No Jumper
Solder Link
No Jumper
27
No Jumper
Solder Link
Solder Link
No Jumper
30
Solder Link
Solder Link
Solder Link
No Jumper
AGC (0 to 30)
X
X
X
Solder Link
A gain setting for different microphone levels is given in the Appendix on page 15. The calculations
are relatively simple. With the Input Voltage (RMS) = 10
(dBV/20)
and further conversion to: Volts Peak
to Peak = 2.828 * RMS. To save time you can use the chart by taking, for instance, a Electret
microphone giving about -44dBV and a 1901 gain setting of 24dB should create a signal of -20dBV
(by simple addition), which should give a signal of about 282mV P-P maximum.
Using the AGC action
As previously mentioned, the module has an AGC system that may be used. This requires all the
Gain setting links to be removed and a link to be placed over the AGC pads.
The AGC looks for voice patterns and then sets the appropriate gain setting at that specific time to
get almost maximum audio level output. The DSP then holds the level until it gets the next speech
pattern and sets the gain to get the maximum level this point. As such the AGC is stepped, rather
than a slow ramp up and ramp down with audio level. It does however behave well under many
audio and noise conditions.
RFI
We’ve already mentioned the use of filtering or use of separate supplies to bias /power the
microphones and while the 900 module has in-built filtering to reduce any noise and rf ingress, any
input signals should be treated with further filtering both on the signal and bias lines.
