VDP3108B datasheet(7/72 Pages) MICRONAS

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PRELIMINARY DATA SHEET

VDP 31xxB

Micronas

2. Functional Description

2.1. Analog Front-End

This block provides the analog interfaces to all video in-

puts and mainly carries out analog-to digital conversion

for the following digital video processing. A block dia-

gram is given in Fig. 2–1.

Most of the functional blocks in the front-end are digitally

controlled (clamping, AGC, and clock-DCO). The con-

trol loops are closed by the Fast Processor (‘FP’) em-

bedded in the decoder.

2.1.1. Input Selector

Up to five analog inputs can be connected. Four inputs

are for input of composite video or S-VHS luma signal.

These inputs are clamped to the sync back porch and

are amplified by a variable gain amplifier. One input is

for connection of S-VHS carrier-chrominance signal.

This input is internally biased and has a fixed gain ampli-

fier.

2.1.2. Clamping

The composite video input signals are AC coupled to the

IC. The clamping voltage is stored on the coupling ca-

pacitors and is generated by digitally controlled current

sources. The clamping level is the back porch of the vid-

eo signal. S-VHS chroma is also AC coupled. The input

pin is internally biased to the center of the ADC input

range.

2.1.3. Automatic Gain Control

A digitally working automatic gain control adjusts the

magnitude of the selected baseband by +6/–4.5 dB in 64

logarithmic steps to the optimal range of the ADC. The

gain of the video input stage including the ADC is 213

steps/V with the AGC set to 0 dB.

2.1.4. Analog-to-Digital Converters

Two ADCs are provided to digitize the input signals.

Each converter runs with 20.25 MHz and has 8 bit reso-

lution. An integrated bandgap circuit generates the re-

quired reference voltages for the converters.

2.1.5. ADC Range

The ADC input range for the various input signals and

the digital representation is given in Table 2–1 and Fig.

2–2. The corresponding output signal levels of the

VDP 31xxB are also shown.

2.1.6. Digitally Controlled Clock Oscillator

The clock generation is also a part of the analog front

end. The crystal oscillator is controlled digitally by the

control processor; the clock frequency can be adjusted

within

±150 ppm.

2.1.7. Analog Video Output

The input signal of the Luma ADC is available at the ana-

log video output pin. The signal at this pin must be buff-

ered by a source follower. The output voltage is 2 V, thus

the signal can be used to drive a 75

W line. The magni-

tude is adjusted with an AGC in 8 steps together with the

main AGC.

DVCO

±150

ppm

clamp

bias

frequency

20.25 MHz

gain

ADC

AGC

reference

generation

ADC

VIN3

+6/–4.5 dB

Fig. 2–1: Analog front-end

VIN2

VIN1

CIN

digital Chroma

digital CVBS or Luma

system clocks

CVBS/Y

CVBS/Y/C

Chroma

Analog Video

Output

VIN4

CVBS/Y

VDP3108B Datasheet(PDF) 7 Page - Micronas

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