LG PX8700 Service Manual page 34

PX8700
2.3 Circuit Description
2.3.1 Keypad/LCD and Receptacle Part
Once the keypad is pressed, the key signals are sent out to MSM6550 for processing. In addition, when the key
is pressed, the keypad/LCD lights up through the use of 18 LEDs. The terminal status and operation are
displayed on the screen for the user with the characters and icons on the LCD.
Moreover, it exchanges audio signals and data with external sources through the receptacle, and then receives
power from the battery or external batteries.
2.3.2 Audio Processing Part
MIC signals are amplified through OP AMP, inputted into the audio codec(included in MSM6550) and
converted into digital signals. Oppositely, digital audio signals are converted into analog signals after going
through the audio codec. These signals are amplified at the audio amplifier and transmitted to the ear-piece. The
signals from MSM6550 activate the ringer by using signals generated in the timer in MSM6550.
2.3.3 MSM Part
MSM6550 is the core element of CDMA system terminal that includes ARM926EJ-S microprocessor core. It
supports both CDMA and Digital FM, operating in both the cellular and PCS spectrums. The subsystems within
the MSM6550 include a CDMA processor, a DFM processor, a multi-standard Vocoder, an integrated CODEC
with earpiece and microphone amplifiers, general-purpose ADC for subsytem monitoring, an ARM926EJ-S
microprocessor, and an RS-232 serial interfaces supporting forward and reverse link MDR data communications
of 230.4 Kbps simultaneously. And it also contains complete digital modulation and demodulation systems for
both CDMA and AMPS cellular standards, as specified in IS-95-A/B/C.
In MSM, coded symbols are interleaved in order to cope with multi-path fading. Each data channel is scrambled
by the long code PN sequence of the user in order to ensure the confidentiality of calls. Moreover, binary
quadrature codes are used based on walsh functions in order to discern each channel. Data created thus are
4-phase modulated by one pair of Pilot PN code and they are used to create I and Q data.
When received, I and Q data are demodulated into symbols by the demodulator, and then de-interleaved in
reverse to the case of transmission. Then, the errors of data received from viterbi decoder are detected and
corrected. They are voice-decoded at the vocoder in order to output digital voice data.
- 34 -
LG Electronics Inc.