Introduction - Nokia 8208 Service Manual

RM-384
System Module and User Interface

Introduction

Phone description
The logic part consists of MSM6800 ARM926TDMI microprocessor-combined CDMA ASIC, flash ROM , SRAM. MSM6
800
is CDMA ASIC chipset implemented for CDMA terminal's system control and baseband digital signal processing. TC
XO clock and external resonator are indispensable for the operation of MSM6800
A.These elements generate the ARM core CPU clock built in MSM6800
A.Major parts used in the logic part are as follows:
1. MSM6800A
ARM Processor Core + CDMA Signal Processing + EVRC ASIC
1.1. CDMACORE
RF interface is connected to the external RF circuit. It has data bus interface to RFR6500-RFT6150 I&Q data which
are input/output through demodulator of MSM6800A. Here power AMP control, AGC feedback, i.e. receiver AGC
adjustment, transmitter's AGC and frequency are controlled. It supports CDMA signal, demodulation, pilot signal
generation, data interference, non-interference, and local oscillating adjustment and lock state.
1.2. MICRO - PROCESSOR CORE
MSM is in ARM9TDMI and provides external peripheral devices, especially memory (FLASH MEMORY, SRAM, and
EEPROM) control interfaceARM9TDMI controlling most functions of phone provides possible general signals
(GPIOs). Parts of GPIOs are used to interrupt process and the rest support other interrupt functions or com-pose
KEYPAD interface. Microprocessor has interrupt controller, multiple timer
1.3. Variable Rate Vocoder
The key to efficiency in digital voice transmission is the conversion of an audio signal into binary numbers. This
allows the digital signal to be compressed and grouped into packets of information. The increased spectral
efficiency of digital cellular is partly due to the use of a vocoder to convert speech into digital code. The vocoder
samples the spoken word, but only encodes a skeleton of the actual word.
CDMA achieves performance near toll-quality through a unique variable-rate vocoder developed by QUALCOMM
which effectively achieves the same voice quality of other vocoders, but does so with an average of half the rate
of standard vocoders. While a standard vocoder is either on, transmitting background noise as well as blank
space at full rate, or off, QUALCOMM's variable rate vocoder outputs data at various rates between 0.8 kbps ( 1 ⁄ 8
rate) and 8.55 kbps (full rate) for the rate set 1 option, or various rates between 1.0 kbps ( 1 ⁄ 8 rate) and 13.3 kbps
(full rate) for the rate set 2, PureVoice™ option.
Background noise is encoded at the lower quality level of 1 ⁄ 8 rate. The vocoder samples speech every 20
milliseconds. As soon as it detects speech coming up, it doubles the encoding rate to 1 ⁄ 4 rate, then 1 ⁄ 2 rate as the
intensity increases, then full rate during high energy speech, while achieving full rate voice quality at all times. As
speech drops off the process reverses, falling first to 1 ⁄ 2 rate, then 1 ⁄ 4 rate, then 1 ⁄ 8 rate for the background noise.
In reality the actual average data rate is significantly less than 1 ⁄ 2 of full rate.
Variable rate vocoding cannot help TDMA because it is incapable of providing for variable rate packet sizes in a
way that would allow additional sharing of the channel.
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