VDP3108 Просмотр технического описания (PDF) - Micronas

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VDP3108

Single-Chip Video Processor

Micronas 

VDP 3108

ADVANCE INFORMATION

2.4.4. Brightness Adjustment

The DC-level of the luminance signal can be adjusted by

–30 ... +100 % with 8 bit resolution. It is desirable to keep

a small offset with the signal to prevent undershoots

from the peaking from being cut. The brightness adjust-

ment is separate for main and side picture.

LPF

IN

HPF

LIM1

var. notch

LIM2

OUT

Loop

Filter

2.4.5. Soft Limiter

Fig. 2–19: Block diagram of the soft limiter

The dynamic range of the processed luma signal must

be limited to prevent the CRT from overload. An appro-

priate headroom for contrast, peaking and brightness

can be adjusted by the manufacturer according to the

CRT-characteristic. All signals higher than above this

limit will be ‘soft’-clipped. The soft limiter can support or

even replace an analog beam current limiter. Aliasing

due to signal limitation is avoided by using a filterbank

with individual limiter circuits.

A block diagram of the soft limiter is shown in figure

2–19. The signal is split into high and low frequency

bands. The low frequency part represents the average

picture level; if the average level is too high the picture

tube will overheat and produce coloration. The high fre-

quency part represents the peak picture level which can

be considerably higher than the average picture level.

Due to this characteristic of the picture tube, both com-

ponents are treated individually and are later recom-

bined. For the low frequency band a limiter with adjust-

able threshold is used. The high frequency components

produced in the limiter are below the nyquist frequency,

therefore no disturbing alias frequencies are generated.

For the high frequency band, the limiting is done by a

variable gain notch filter, effectively bounding the peak

to peak amplitude of the signal. In this way the signal is

limited without generating unwanted aliasing.

When the high and low frequency bands are added to-

gether again a second limiter sets the exact signal ampli-

tude range. The state of this limiter is used to control the

attenuation of the variable notch filter.

2.4.6. Chroma Interpolation

A linear phase interpolator is used to convert the chroma

sampling rate from 10.125 MHz (4:2:2) to 20.25 MHz

(4:4:4). The frequency response of the interpolator is

shown in fig. 2–20. All further processing is carried out

at the full sampling rate.

dB

0

–10

–20

–30

–40

–50

MHz

0

2

4

6

8

10

Fig. 2–20: Frequency response of the chroma

interpolation filter

2.4.7. Chroma Transient Improvement

The intention of this block is to enhance the chroma res-

olution. A correction signal is calculated by differenti-

ation of the color difference signals. The differentiation

can be adjusted according to the signal bandwidth, e.g.

for PAL/NTSC/SECAM or digital component signals re-

spectively. The amplitude of the correction signal is ad-

justable independently for the Cr/Cb signals. Small

16

MICRONAS INTERMETALL

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