ADV7120

REV. B

–6–

TERMINOLOGY

Blanking Level

The level separating the SYNC portion from the video portion

of the waveform. Usually referred to as the front porch or back

porch. At 0 IRE units, it is the level which will shut off the pic-

ture tube, resulting in the blackest possible picture.

Color Video (RGB)

This usually refers to the technique of combining the three pri-

mary colors of red, green and blue to produce color pictures

within the usual spectrum. In RGB monitors, three DACs are

required, one for each color.

Sync Signal (SYNC)

The position of the composite video signal which synchronizes

the scanning process.

Gray Scale

The discrete levels of video signal between reference black and

reference white levels. An 8-bit DAC contains 256 different lev-

els while a 6-bit DAC contains 64.

Raster Scan

The most basic method of sweeping a CRT one line at a time to

generate and display images.

Reference Black Level

The maximum negative polarity amplitude of the video signal.

Reference White Level

The maximum positive polarity amplitude of the video signal.

Sync Level

The peak level of the SYNC signal.

Video Signal

That portion of the composite video signal which varies in gray

scale levels between reference white and reference black. Also

referred to as the picture signal, this is the portion which may be

visually observed.

CIRCUIT DESCRIPTION AND OPERATION

The ADV7120 contains three 8-bit D/A converters, with three

input channels each containing an 8-bit register. Also inte-

grated on board the part is a reference amplifier and CRT con-

trol functions BLANK, SYNC and REF WHITE.

Digital Inputs

24-bits of pixel data (color information) R0–R7, G0–G7 and

B0–B7 are latched into the device on the rising edge of each

clock cycle. This data is presented to the three 8-bit DACs and

is then converted to three analog (RGB) output waveforms.

(See Figure 2.)

Three other digital control signals are latched to the analog

video outputs in a similar fashion. BLANK, SYNC and REF

WHITE are each latched on the rising edge of CLOCK to

maintain synchronization with the pixel data stream.

The BLANK and SYNC functions allow for the encoding of

these video synchronization signals onto the RGB video output.

This is done by adding appropriately weighted current sources

to the analog outputs, as determined by the logic levels on the

BLANK

and SYNC digital inputs. Figure 3 shows the analog

output, RGB video waveform of the ADV7120. The influence

of SYNC and BLANK on the analog video waveform is

illustrated.

The REF WHITE control input drives the RGB video outputs

to the white level. This function could be used to overlay a cur-

sor or crosshair onto the RGB video output.

Table I details the resultant effect on the analog outputs of

BLANK

, SYNC and REF WHITE.

All these digital inputs are specified to accept TTL logic levels.

Clock Input

The CLOCK input of the ADV7120 is typically the pixel clock

rate of the system. It is also known as the dot rate. The dot rate,

and hence the required CLOCK frequency, will be determined

by the on-screen resolution, according to the following

equation:

Dot Rate = (Horiz: Res)

× (Vert Res) × (Refresh Rate)/

(Retrace Factor)

Horiz Res

=

Number of pixels/line

Vert Res

=

Number of lines/frame

Refresh Rate

=

Horizontal scan rate. This is the rate at

which the screen must be refreshed, typi-

cally 60 Hz for a noninterlaced system or

30 Hz for an interlaced system.

Retrace Factor

=

Total blank time factor. This takes into ac-

count that the display is blanked for a cer-

tain fraction of the total duration of each

frame (e.g., 0.8).

CLOCK

ANALOG OUTPUTS

(IOR, IOG, IOB, I

DIGITAL INPUTS

(R0-R7, G0-G7, B0-B7;

SYNC, BLANK,

REF WHITE)

DATA

Figure 2. Video Data Input/Output