3.1.2.5. Audio Lines Connection
The modem supports one microphone input and one speaker output.
3.1.2.5.1. Microphone
The microphone inputs are connected in differential mode to reject common mode
noise and TDMA noise. The microphone inputs have already included biasing for an
electret microphone (0.5mA and 2V) and are ESD protected. An electret microphone
may be directly connected to these inputs allowing an easy connection to a headset.
The microphone gain can be adjusted by AT+VGT and the transmit digital gain can
be adjusted by AT+WDGT. Refer to documents the AT Commands Interface Guide
at http://www.usr.com/support/3500 for more information about these AT commands.
The following table shows the pin assignments of the microphone input.
Table 9. Microphone Pin Description for USR3500
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