Vizio VX42L HDTV10A Service Manual page 44

2. 8-VSB and Clear-QAM Reception
MT5112BD contains a 10-bit A/D converter, an 8-VSB/QAM demodulator, followed by
a trellis-code de-modulation (TCM) decoder and a Reed-Solomon forward error
correction (FEC) decoder. Moreover, an embedded 8-bit microprocessor intelligently
handles the acquisition and tracking to ensure the best receiving performance under
various channel conditions. The microprocessor communicates with the external host
controller via an I2C-compatible interface, and also provides direct control to the RF
tuner via another I2C-compatible interface.
MT5112BD accepts the tuner IF output centered at 44MHz or 43.75MHz, or the low IF
signals from a down-converter. With good adjacent channel immunity, additional IF
SAW filters for adjacent channel rejection can be saved. An on-chip programmable
gain-controlled amplifier (PGA) is designed to provide extra signal gain when the tuner
output level is low. The amplified IF signal is then sample and digitized for further
demodulation process.
MT5112BD keeps A/D input power level at a desired level so as to maximize the
received SNR. It measures the power level of the digitized samples and provide two
signals (both sigma-delta encoded; one delayed and one non-delayed) for front-end
gain control purpose. The signals is low-pass filtered before connected to tuner or IF
gain stages.
For the 8-VSB reception, the carrier frequency offset is estimated and compensated by
a fully digital synchronizer. It also controls the rate conversion in the digital re-sampling
device by estimating the sampling frequency offset; hence no external VCXO is
required. The digital synchronizer simultaneously offers very wide frequency
acquisition range and stable tracking capability. This makes MT5112BD robust work
under severe impairment conditions.
The MT5112BD is equipped with a powerful equalizer for mitigating the multi-path
effects due to terrestrial propagation of 8-VSB signals. The delicate equalizer design
makes the MT5112BD boast its ability for strong echo cancellation. With this powerful
equalizer, the MT5112BD can not only easily pass the tests of A74 equalization mask,
ATTC channel ensembles, CRC channel ensembles, but also provide superior
capability of live signal receptions.
For cable signal reception, the MT5112BD adopts the fully digital modules for timing
and carrier synchronization, with no external VCXO required. Specially designed
carrier synchronization module enables the MT5112BD passing the OpenCable ATP
burst and phase noise tests, while maintaining excellent reception performance under
normal reception conditions.
CONFIDENTIAL – DO NOT COPY Page 7-13
File No. SG-0211