LG U8110 Service Manual page 56

3. TECHNICAL BRIEF
3.4.4.2 Voice Call Uplink Mode (Receiver, Speaker, Headset)
This section provides a detailed description of the Voice Call TX functions.
Figure. Voice call Uplink Scheme
The Uplink supports two microphones and two auxiliary inputs to the speech encoder blocks. Both
microphone inputs are compatible with an electric microphone.
The VINCENNE internal voltage source (CCO) provides the necessary drive current for the electric
microphone. The voltage source is via I2C programmable to supply 2.2V or 2.4V. But the voltage
source of our Model is to supply 2.4V.
The auxiliary audio inputs can be used as an alternative source of speech, a source from an external
microphone or as an analog loop connection. Figure shows that the audio inputs are fed to the
transmit PGAs, which enables to adjust the total gain in the product for different sensitivities of the
microphones and spread in the transmit paths. The ADCs are followed by the transmit band pass
filters, which accept the maximum output swing that the microphone preamplifiers can deliver without
clipping, and maintain a good signal-to-noise ratio. The high pass filter in the TX-paths can be disabled
via I2C; still removing the DC offset from the signal. For one of the two transmit paths, a transmit gain
control amplifier precedes the final encoding of the PCM output.
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