Circuit Explanations - Mitsubishi Electric LT-2220 Service Manual

Circuit Explanations

1. Video circuits
This section explains the video circuitry of models LT-2220/LT3020. Both models have almost the same
circuits. The explanation corresponds to the various inputs (PC, TV, VIDEO, Component, DVI).
The HD input has realized high definition with the resolution of WXGA.
(Refer to the LT-2220 / LT-3020 Video block diagram, for the flow of video signals.)
1-1. Video input circuit
1-1-1. PC Input video circuit
For PC input, video is inputted into IC400 of the signal processing IC via the following IC's.
(A) IC2A3 : Analog-to-digital conversion IC (AD9883AKST-110 : Analog devices)
The video signal (R, G, B) of PC input is changed into a digital signal (8 bits each) from an analog
signal. Besides digital conversion, this IC has the functions below:
Contrast adjustment for PC
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brightness adjustment for PC
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This IC is controlled by register control of IC 2A3 via 3.3V IIC-bus.
(B) IC2A2 : Inverting IC with Schmidt trigger (74LCX14 : Fairchild)
Input IC for Horizontal and Vertical sync signals from the PC input.
In case of noise on the leading and trailing edges or low amplitude of the sync signals outputted from a
PC, it is necessary to modify the waveform.
Improper waveforms are corrected by this IC in the following manner.
Noise on the leading/trailing edge : Rise-time and fall-time is carried out early.
-
Waveform is noisy
-
Low Amplitude
-
Since the IC is an inverter, it will let it pass twice and has united polarity.
1-1-2. Component and DVI Input circuit.
For Component and DVI inputs, video is inputted into IC400, signal processing IC, via the following IC's.
(C)IC204 : Analog-switch IC (BA7657 : Rohm)
In this IC, one of the 2 component signal inputs (Y, Cb, Cr) is chosen by the select signal. Select
signal CTL is controlled by the MCU.
MODELS: LT-2220 / LT-2240 / LT-3020 / LT-3040 / LT-3050
: False triggering prevented by the Schmidt circuit.
: If voltage threshold is satisfied, it will output at 3.3V.