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User Manual (1.0.11)
EasyVR 3
11
Microphone
The microphone provided with the EasyVR 3 module is an omnidirectional electret condenser microphone
(Horn EM9745P-382):
Sensitivity -38dB (0dB=1V/Pa @1KHz)
Load Impedance 2.2K
Operating Voltage 3V
Almost flat frequency response in the range 100Hz
– 20kHz
The microphone circuit is optimized for use at ARMS_LENGTH (default, about 60cm) or FAR_MIC distance
settings.
If you use a microphone with different specifications the recognition accuracy may be adversely affected.
Differences in rated load impedance and sensitivity can be compensated to a certain extent by changing the
microphone gain. This can be done in several ways:
Replacing the internal gain resistor R4 (1.2k
Ω)
Adding an external resistor Rx going in parallel with R4 (it can only reduce gain, useful for HEADSET
distance settings)
Removing the internal resistor R4 and using only the external resistor Rx
MIC
VM
RET
3V
AGND
R4
1.2k
Ω
Module
Internals
External
Microphone
+
-
Rx
Optional
Microphone circuit
Modifying gain resistance
You can calculate the overall microphone gain resistance using the formula below:
Rs
is the optimal microphone gain resistance
I
is the impedance rating of the microphone
G
is the desired overall system gain, defined as follows:
1. If the module is configured for HEADSET microphone distance (typically a few centimeters from the
user’s mouth), then the overall system gain should be -49 dB (0dB=1v/Pa@1KHz);
2. If the module is configured for ARMS_LENGTH microphone distance (typically 60-90 cm from the
user's mouth
– this is the default setting of EasyVR), then the overall system gain should be -44 dB;
3. If the module is configured for FAR_MIC microphone distance (up to about 3 meters from the user's
mouth), then the overall system gain should be -43 dB.
S
is the sensitivity rating of the microphone you want to use, and it is specified in
–dB in the microphone’s
specification
3
.
3
Converting uBars to Pascal: microphone manufacturers specify the sensitivity referencing to uBars or
Pascal. If the microphone sensitivity is referenced to uBars, simply add 20 dB to the rating. For example, -58
dB/uBars + 20dB = -38 dBV/Pa.