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11.4.2 DVB-T Part
The IF output of the tuner (TU401) is converted to Transport Stream signals
(multiplexed video, audio and data information) after some filtering, and COFDM (coded
orthogonal demodulation).
The digital demodulator (IC401) output is transported to the STI5518 MPEG–2 decoder
(IC100) with cm_clk (clock signal to decoder), cm_valid (indicates valid data bits of
transport stream (TS) and cm_sync (indicates start of a TS).
TU401 is controlled by demodulator (IC401) via sclk and sdata. Demodulator is
2
controlled via I
C by STI5518 decoder.
The CPU (ST20) inside STI5518 controls all processes. Demultiplexer of the CPU
provides the transmission of the desired channel's information from TS (Transport Stream)
to MPEG Decoder section.
The program that runs on STI5518 is in Flash memory (IC302). 64Mbits SDRAMs
(IC300 and IC301) are used for data memory of this program. IC300 is SMI SDRAM and
decoded data is stored here. It has a clock input of 121.5Mhz. Software is mostly run over
IC301 EMI SDRAM. Some program and status information is stored in EEPROM (IC304).
The CPU (ST20) inside STI5518 uses 32- bit data and 22- bit address buses for
access to flash, DRAM and MPEG decoder. It uses RAS, CAS etc. (read, write, enable)
signals to activate related IC while accessing them.
The main clock (which is needed by STI 5518), is generated at power on mode and at
STBY by 27MHz crystal (X112) and IC102 (74HCU04). The output of PWM outputs of STI
5518 is filtered to have a DC level via R128 and C133. At the output of IC103, the 27MHz
clock can be adjusted according to capacity of pins of 27MHz crystal to ground. This
capacity is related with DC value on D100 and D101 (BB133) at pins of crystal. This
operation maintains synchronization between audio-video that are coming from MPEG
transport stream and PCR clock.
MPEG decoder in STI5518, is responsible for decoding of MPEG video and audio
signals. The video, which is compressed using MPEG2 and audio, which is compressed
using MPEG1 Layer 1-2, are processed here. After decoding, CCIR 601 formatted 8-bit
video and PCM formatted audio, are generated by mpeg decoder.
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