November 1992

5

Philips Semiconductors

Preliminary specification

Advanced monitor video controller

TDA4881

FUNCTIONAL DESCRIPTION

the TDA4881 (pins 2, 5 and 8) from a low ohmic source

and are clamped to an internal DC voltage (artificial black

level). Composite signals will not disturb normal

operations because an internal clipping circuit cuts all

signal parts below black level. Channels 1 and 3 have a

maximum total voltage gain of 6 dB (maximum contrast

and maximum individual channel gain), Channel 2 of 4 dB

(maximum contrast and nominal channel gain). With the

nominal channel gain of 1 dB and nominal contrast setting

DC voltages are used for brightness, contrast and gain

control.

Brightness control yields a simultaneous signal black level

shift of the three channels relative to a reference black

level. For nominal brightness (pin 1 open-circuit) the signal

black level is equal to the reference black level.

Contrast control is achieved by a voltage at pin 6 and

affects the three channels simultaneously.

To provide the correct white point, an individual

gain

control (pins 3 and 11) adjusts the signals of Channels 1

and 3 compared to the reference Channel 2. Gain setting

effects contrast and brightness to achieve correct grey

scale tracking. Each

output stage provides a current

output (pins 20, 17 and 14) and a voltage output (pins 19,

16 and 13). External cascode transistors reduce power

consumption of the IC and prevent breakdown of the

output transistors. Signal output currents and peaking

characteristics are determined by external components at

the voltage outputs and the video supply. The three

channels have separate internal feedback loops which

ensure large signal linearity and marginal signal distortion

The

clamping pulse (pin 10) is used for input clamping

only. The input signals have to be at black level during the

clamping pulse and are clamped to an internal artificial

black level. The coupling capacitors are used in this way

for black level storage. Because the threshold for the

clamping pulse is higher than that for vertical blanking (pin

10) the rise and fall times of the clamping pulse have to be

faster than 75 ns/V (1 V to 3.5 V).

The

vertical blanking pulse will be detected if the input

voltage (pin 10) is higher than the threshold voltage for

approximately 300 ns but does not exceed the threshold

for the clamping pulse in the time between. During the

vertical blanking pulse the input clamping is disabled to

avoid misclamping in the event of composite input signals.

The input signal is blanked and the artificial black level is

inserted instead. Additionally the brightness is internally

set to its nominal value, thus the output signal is at

reference black level. The DC value of the reference black

level will be adjusted by cut-off stabilization.

During

horizontal blanking (pin 9) the output signal is set to

reference black level as previously described and

output

clamping is activated. If the voltage at pin 9 exceeds the

switch off threshold the signal is blanked and switched to

ultra black level for screen protection and spot

suppression during V-flyback. Ultra black level is the

lowest possible output voltage (at voltage outputs) and

does not depend on cut-off stabilization.

For

cut-off stabilization (DC coupling to the CRT)

respectively

black level stabilization (AC coupling) the

video signal at the cathode or the coupling capacitor is

divided by an adjustable voltage divider and fed to the

feedback inputs (pins 18, 15 and 12). During horizontal

blanking time this signal is compared with an internal DC

voltage of approximately 5.8 V. Any difference will lead to

a reference black level correction by charging or

discharging the integrated capacitor which stores the

reference black level information between the horizontal

blanking pulses.