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NTE7138

Integrated Circuit Advanced Monitor Video Controller for OSD

NTE Electronics 

Functional Description:

General

The RGB input signals 0.7V(P–P) are capacitively coupled into the NTE7138 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 7dB (maximum contrast and maximum individual chan-

nel gain), channel 2 having 4.4dB (maximum contrast and nominal gain). With the nominal channel

gain of 1dB and nominal contrast setting the nominal balck–to–white output signal is 0.79V(P–P).

Brightness, contrast and gain control is by DC voltage.

Brightness Control

Brightness control yeilds a simultaneous signal black–level shift of the three channels relative to a

reference black level.

For normal brightness (Pin1 open–circuit) the signal black–level is equal to the reference black level.

Contrast Control

Contrast is voltage controlled to affect the three channels simultaneously. To provide the correct white

point, individual gain controls adjust the signals of channels 1 and 3 relative to the reference channels

2. Gain setting also changes contrast to achieve correct grey scale tracking.

Output Stages

The output stages provide both voltage and current outputs. 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 channels have separate internal feedback loops which ensure large signal linearity

and marginal signal distortion irrespective of output transistor thermal VBE variation.

Input Clamping

The clamping pulse is for input clamping only. The input signals are at black level during the clamping

pulse and are clamped to an internal artificial black level. The coupling capacitors provide black–level

storage. The threshold for the clamping pulse is higher than that for vertical blanking, thereofre, the

rise and fall times of the clamping pulse need to be faster than 75ns/V during transition from 1 to 3.5V.

Vertical Blanking

The vertical blanking pulse will be detected if the input voltage is higher than the threshold voltage

for approximately 320ns 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.

Also the brightness is set internally to its nominal value, thus the output signal is at reference balck

level. The DC value of the reference black level will be adjusted by cut–off stabilization.

Horizontal Blanking

During horizontal blanking the output signal is set to reference black level and output clamping is acti-

vated. If the voltage 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 channel output voltage and is not dependent on cut–off stabilization.

Cut–Off and Black–Level Stabilization

For cut–off stabilization (DC coupling to the CRT) and 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 channel feedback inputs. During horizontal blanking time this signal is compared with an internal

DC voltage of approximately 5.8V. 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.

On Screen Display

For OSD, fast switching of control Pin6 to less than 1V (e.g. 0.7V) blanks the input signals. The OSD

signals can easily be inserted to the external cascode transistor.

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