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Synchronous vs. Asynchronous Operation

The source of RGB video and synchronization used as an input

to the AD723 in some systems is derived from the same clock

signal as used for the AD723 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 AD723 supports both synchronous and asynchronous

operation, but some minor differences might be noticed between

them. These can be caused by some details of the internal cir-

cuitry of the AD723.

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.

LUMA TRAP THEORY

The composite video output of the AD723 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 AD723; 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 ampli-

tude modulation, while the hue (or color 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 orthogo-

nal components, U and V. Then, along with the luma signal, the

U and V signals generate the RGB signals that control 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

within the chroma band. All signals in this band are processed

as chroma information since the chroma processing circuit has

no knowledge of where these signals originated. Therefore, the

color that results from the luma signals in the chroma band is a

false color. This effect is referred to as cross chrominance.

The cross-chrominance effect is sometimes evident in white text

on a black background as a moving rainbow pattern around the

characters. The sharp transitions from black to white (and vice

versa) that comprise the text dots contain frequency compo-

nents across the whole video band, and those in the chroma

band create cross chrominance. This is especially pronounced

when the dot clock used to generate the characters is an integer

multiple of the chroma subcarrier frequency.

Another common contributor to cross-chrominance effects is

certain striped clothing patterns that are televised. At a specific

amount of zoom, the spatial frequency of vertical stripe patterns

will generate luma frequencies in the chroma band. These fre-

quency components will ultimately be turned into color by the

video monitor. Since the phase of these signals is not coherent

with the subcarrier, the effect shows up as random colors. If the

zoom of a TV camera is modified or there is motion of the striped

pattern, the false colors can vary quite radically and produce an

objectionable “moving rainbow” effect. Most TV-savvy people

have learned to adapt by not wearing certain patterns when

appearing on TV.

An excellent way to eliminate virtually all cross chrominance

effects is to use S-video. Since the luma and chroma are carried

on two separate circuits, there is no confusion as to which cir-

cuit should process which signals. Unfortunately, not all TVs

that exist today, and probably still not even half of those being

sold, have a provision for S-video input.

To ensure compatibility with the input capabilities of the majority

of TVs in existence, composite video must be supplied. Many

more TVs have a composite baseband video input port than

have an S-video port to connect cameras and VCRs.

However, still the only common denominator for virtually all

TVs is an RF input. This requires modulating the baseband

video onto an RF carrier that is usually tuned to either Channel

3 or 4 (for NTSC). Most video games that can afford only a

single output use an RF interface because of its universality.

Sound can also be carried on this channel.

Since it is not practical to rely exclusively on S-video to improve

the picture quality by eliminating cross chrominance, a luma

trap can be used to minimize this effect for systems that use

composite video. The luma trap notches out or “traps” the

offending frequencies from the luma signal before it is added to

the chroma. The cross chrominance that would be generated by

these frequencies is thereby significantly attenuated.

The only sacrifice that results is that the luma response has a

“hole” in it at the chroma frequency. This will lower the luminance

resolution of details whose spatial frequency causes frequency

components in the chroma band. However, the attenuation of

cross chrominance outweighs this in the picture quality. S-video

will not just eliminate cross chrominance, but also will not have

this notch in the luma response.

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