ViewSonic VA550 Service Manual - Page 20

Service

Manual

VA550

ViewSonic

Corporation

THEORY

OF

CIRCUIT

OPERATION

Image

scaling

The

AM100

supports

several

different

input

modes,

and

the

input

image

may

have

different

sizes.

It

is

essential

to

support

automatic

image

scaling

so

that

the

input

image

is

always

displayed

to

the

full

screen

regardless

of

the

input

mode.

The

AM100

scales

the

images

in

both

horizontal

and

vertical

directions.

It

calculates

the

correct

scaling

ratio

for

both

directions

based

upon

the

LCD

panel

resolution,

and

the input

mode

and

timing

information

produced

by

the input

mode

detection

&

auto

calibration

function.

The

scaling

ratio

is

re

-adjusted

whenever

a

different

input

mode

is

detected.

The

ratio

is

then

fed

to

the

buffer

memory

read

control

logic

to

fetch

the

image

data

with

the

correct

sequence

and

timing.

Some

of

the

image

data

may

be

read

more

than

once

to

achieve

scaling

effect.

Image

Interpolation

The

AM100

supports

image

interpolation

to

achieve

better

image

quality.

A

basic

image

scaling

algorithm

replicates

the input

images

to

achieve

the

scaling

effect.

The

replication

scheme

usually

results

in

a

poor

image

quality.

The

AM100

implements

both

bi-linear

interpolation

and

a

proprietary

programmable

interpolation

algorithm.

The

programmable

interpolation

is

implemented

with

a

256

-entry

mapping

table

in

the

EEPROM

to

allow

the

system

user

to

adjust

the

bi-

linear

interpolation

parameters

to

control

the

sharpness

and

smoothness

quality

of

the

image.

In

the

default

setting,

the

mapping

table

contains

a

straight

line

of

slope

equal

to

1,

i.e.

the

data

in

entry

N

is

equal

to

the

value

N.

If

the

mapping

table

contains

a

line

of

slope

equal

to

2,

then

the

output

image

will

be

a

bit

sharper

than

the

image

generated

by

a

table

with

default

setting.

Dithering

The

AM100

supports

16.7

million

true

colors

for

6

-bit

panel.

Two

dithering

algorithms

are

implemented

and

again

users

can

choose

between

them

from

the

external

micro

-controller.

The

first

algorithm

is

area

-based

dithering,

and

the

second

algorithm

is

a

frame

-based

frame

modulation,

called

frame

rate

control.

Through

the

external

micro

-controller

users

can

choose

among

different

dithering

algorithms.

OSD

mixer

and

LCD

interface

At

the

output

stage,

the

AM100

performs

the

OSD

mixer

function,

and

then

generates

24

-bit

/

48bit

RGB

signal

to

the

LCD

panel

with

the

correct

timing.

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