VCT3811A datasheet(21/172 Pages) MICRONAS

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ADVANCE INFORMATION

VCT 38xxA

Micronas

2.10.4.Digital Brightness Adjustment

The DC-level of the luminance signal can be adjusted

by adding an 8-bit number in the luminance signal path

in front of the softlimiter.

With a contrast adjustment of 32 (gain+1) the signal

can be shifted by 100 %. After the brightness addition,

the negative going signals are limited to zero. It is

desirable to keep a small positive offset with the signal

to prevent undershoots produced by the peaking from

being cut. The digital brightness adjustment works

separately for main and side picture.

2.10.5.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 TV manufacturer according to

the CRT characteristics. All signals above this limit will

be ‘soft’-clipped. A characteristic diagram of the soft

limiter is shown in Fig. 2–15. The total limiter consists

of three parts:

Part 1 includes adjustable tilt point and gain. The gain

before the tilt value is 1. Above the tilt value, a part

(0...15/16) of the input signal is subtracted from the

input signal itself. Therefore, the gain is adjustable

from 16/16 to 1/16, when the slope value varies from

0 to 15. The tilt value can be adjusted from 0 to 511.

Part 2 has the same characteristics as part 1. The sub-

tracting part is also relative to the input signal, so the

total differential gain will become negative if the sum of

slope 1 and slope 2 is greater than 16 and the input

signal is above the both tilt values (see characteris-

tics).

Finally, the output signal of the soft limiter will be

clipped by a hard limiter adjustable from 256 to 511.

2.10.6.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). All further processing is carried out

at the full sampling rate.

Fig. 2–15: Characteristic of soft limiter A and B and hard limiter

2.10.7.Chroma Transient Improvement

The intention of this block is to enhance the chroma

resolution. A correction signal is calculated by differen-

tiation of the color difference signals. The differentia-

tion can be selected according to the signal bandwidth,

e.g. for PAL/NTSC/SECAM or digital component sig-

nals, respectively. The amplitude of the correction sig-

nal is adjustable. Small noise amplitudes in the correc-

tion signal are suppressed by an adjustable coring

circuit. To eliminate ‘wrong colors’, which are caused

by over and undershoots at the chroma transition, the

sharpened chroma signals are limited to a proper

value automatically.

100

200

300

400

500

600

700

800

900

1023

Calculation Example for the

Softlimiter Input Amplitude.

(The real signal processing in

the limiter is 2 bit more than

described here)

Y Input

16...235 (ITUR)

Black Level

16 (constant)

Contrast

Dig. Brightness

BLE

off

Peaking

off

Limiter input signal:

(Yin-Black Level)

⋅Contr./32 + Brightn.

(235-16)

⋅ 63/32 + 20 = 451

slope 2

[0...15]

Hard limiter

range=

256...511

slope 1

[0...15]

tilt 1 [ 0...511]

tilt 2 [ 0...511]

100

200

300

400

511

Output

Part 1

Part 2

Limiter Input

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