AD723 Analog Devices, AD723 Datasheet - Page 17

• Manufacturer Part Number: AD723
• Description: 2.7 V to 5.5 V RGB-to-NTSC/PAL Encoder with Load Detect and Input Termination Switch
• Manufacturer: Analog Devices

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.
AD723