LG MG200 Service Manual page 28

3.5.3 Audio Uplink processing
The VUL path includes two input stages. The first stage is a microphone amplifier, compatible with
electret microphones containing a FET buffer with open drain output. The microphone amplifier has
a gain of typically 25.6 dB (±1 dB) and provides an external voltage of 2.0 V or 2.5 V to bias the
microphone (MICBIAS). The auxiliary audio input can be used as an alternative source for higher
level speech signals. This stage performs single-ended-to differential conversion and provides a
programmable gain of 4.6 dB or 28.2 dB. The third stage is a headset microphone amplifier,
compatible with electret microphones. The headset microphone amplifier has a gain of typically 18
dB and provides an external voltage of 2.0 V or 2.5 V to bias the headset microphone
(HSMICBIAS). When one of the input stages (MICI, AUXI, HSMICP) is in use, the two other input
stages are disabled and powered down. The resulting fully differential signal is fed to the analog-
to-digital converter (ADC). The ADC conversion slope depends on the value of the internal voltage
reference. Analog-to-digital conversion is performed by a third-order
sampling rate of 1 MHz. Output of the ADC is fed to a speech digital filter, which performs the
decimation down to 8 kHz and band-limits the signal with both low-pass and high-pass transfer
functions. Programmable gain can be set digitally from -12 dB to +12 dB in 1-dB steps and is
programmed with bits 4-0 (VULPG(4:0)) of the voiceband uplink register. The speech samples are
then transmitted to the DSP via the VSP at a rate of 8 kHz. There are 15 meaningful output bits.
Programmable functions of the VUL path, power-up, input selection, and gain are controlled by the
BSP or the USP via the serial interfaces.
The VUL path can be powered down by bit 0 (VULON) of the power down register.
MICBIAS
Bias
Generator
HAMICBIAS
Microphone
MICIP
Amplifier
MICIN
25.6 dB
Auxiliary
Amplifier
AUXI
4.6 dB
28.2 dB
Auxiliary
HSMICP
Amplifier
18 dB
SINC
Modulator
Filter
2.48 dB
fs 1 = 1 MHz fs 2 = 40 KHz
Figure 13. Uplink Path
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3. H/W CIRCUIT DESCRIPTION
modulator with a
Side Tone to Voice Downlink
Up Link IR PGA
Bandpass 12 dB to
Filter 3.52 dB -12 dB
fs 3 = 8 KHz
To Voice
Serial
Interface