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VPC3230D

Comb Filter Video Processor

Micronas 

VPC 323xD

PRELIMINARY DATA SHEET

2. Functional Description

2.1. Analog Video Front-End

This block provides the analog interfaces to all video

inputs and mainly carries out analog-to-digital conver-

sion for the following digital video processing. A block

diagram is given in Fig. 2–1.

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

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

control loops are closed by the Fast Processor (‘FP’)

embedded in the decoder.

2.1.1. Input Selector

Up to five analog inputs can be connected. Four inputs

are for composite video or S-VHS luma signal. These

inputs are clamped to the sync back porch and are ampli-

fied by a variable gain amplifier. One input is for connec-

tion of S-VHS carrier chrominance signal. This input is

internally biased and has a fixed gain amplifier. A second

S-VHS chroma signal can be connected to video-input

VIN1.

2.1.2. Clamping

The composite video input signals are AC coupled to

the IC. The clamping voltage is stored on the coupling

capacitors and is generated by digitally controlled cur-

rent sources. The clamping level is the back porch of

the video 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 res-

olution. An integrated bandgap circuit generates the

required reference voltages for the converters. The

two ADCs are of a 2-stage subranging type.

2.1.5. 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.

2.1.6. Analog Video Output

The input signal of the Luma ADC is available at the

analog video output pin. The signal at this pin must be

buffered by a source follower. The output voltage is

2 V, thus the signal can be used to drive a 75 Ω line.

The magnitude is adjusted with an AGC in 8 steps

together with the main AGC.

Analog Video

Output

CVBS/Y

CVBS/Y

CVBS/Y

CVBS/Y/C

C

VIN4

VIN3

VIN2

VIN1

CIN

Fig. 2–1: Analog front-end

clamp

bias

reference

generation

AGC

+6/–4.5 dB

ADC

digital CVBS or Luma

gain

frequency

ADC

DVCO

±150

ppm

digital Chroma

system clocks

20.25 MHz

8

Micronas

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