AD725 datasheet(14/20 Pages) AD

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AD725

Vertical Scaling

In addition to converting the computer generated image from

noninterlaced to interlaced format, it is also necessary to scale

the image down to fit into NTSC or PAL format. The most

common vertical lines/screen for VGA display are 480 and 600

lines. NTSC can only accommodate approximately 400 visible

lines/frame (200 per field), PAL can accommodate 576 lines/

frame (288 per field). If scaling is not performed, portions of

the original image will not appear in the television display.

This line reduction can be performed by merely eliminating

every Nth (6th line in converting 480 lines to NSTC or every

25th line in converting 600 lines to PAL). This risks generation

of jagged edges and jerky movement. It is best to combine the

scaling with the interpolation/averaging technique discussed

above to ensure that valuable data is not arbitrarily discarded in

the scaling process. Like the flicker reduction technique men-

tioned above, the line reduction must be accomplished prior to

the AD725 encoding operation.

There is a new generation of VGA controllers on the market

specifically designed to utilize these techniques to provide a

crisp and stable display for both text and graphics oriented

applications. In addition these chips rescale the output from the

computer to fit correctly on the screen of a television. A list of

known devices is available through Analog Devices’ Applica-

tions group, but the most complete and current information will

be available from the manufacturers of graphics controller ICs.

Synchronous vs. Asynchronous Operation

The source of RGB video and synchronization used as an input

to the AD725 in some systems is derived from the same clock

signal as used for the AD725 subcarrier input (4FSC). These

systems are said to be operating synchronously. In systems

where two different clock sources are used for these signals, the

operation is called asynchronous.

The AD725 supports both synchronous and asynchronous

operation, but some minor differences might be noticed be-

tween them. These can be caused by some details of the inter-

nal circuitry of the AD725.

There is an attempt to process all of the video and synchroniza-

tion signals totally asynchronous with respect to the subcarrier

signal. This was achieved everywhere except for the sampled

delay line used in the luminance channel to time align the lumi-

nance and chrominance. This delay line uses a signal at eight

times the subcarrier frequency as its clock.

The phasing between the delay line clock and the luminance

signal (with inserted composite sync) will be constant during

synchronous operation, while the phasing will demonstrate a

periodic variation during asynchronous operation. The jitter of

the asynchronous video output will be slightly greater due to

these periodic phase variations.

NONINTERLACED

ODD FIELD

EVEN FIELD

a. Conversion of Noninterlace to Interlace

NONINTERLACED

ODD FIELD

EVEN FIELD

b. Line Doubled Conversion Technique

NONINTERLACED

ODD FIELD

EVEN FIELD

c. Line Averaging Technique

Figure 21.

LUMA TRAP-THEORY

The composite video output of the AD725 can be improved for

some types of images by incorporating a luma trap (or Y-Trap)

in the encoder circuit. The basic configuration for such a circuit

is a notch or band elimination filter that is centered at the

subcarrier frequency. The luma trap is only functional for the

composite video output of the AD725; it has no influence on

the S-Video (or Y/C-Video) output.

The need for a luma trap arises from the method used by com-

posite video to encode the color part (chrominance or chroma)

of the video signal. This is performed by amplitude and phase

modulation of a subcarrier. The saturation (or lack of dilution of

a color with white) is represented in the subcarrier’s amplitude

modulation, while the hue (or color as thought of as the sections

of a rainbow) information is contained in the subcarrier’s phase

modulation. The modulated subcarrier occupies a bandwidth

somewhat greater than 1 MHz depending on the video standard.

For a composite signal, the chroma is linearly added to the

luminance (luma or brightness) plus sync signal to form a single

composite signal with all of the picture information. Once this

addition is performed, it is no longer possible to ascertain which

component contributed which part of the composite signal.

At the receiver, this single composite signal must be separated

into its various parts to be properly processed. In particular, the

chroma must be separated and then demodulated into its or-

thogonal components, U and V. Then, along with the luma

signal, the U and V signals generate the RGB signals that con-

trol the three video guns in the monitor.

A basic problem arises when the luma signal (which contains no

color information) contains frequency components that fall

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