Video Basics
IMAGE 4
IMAGE 3
IMAGE 2
IMAGE 1
TIME
VERTICAL
SCANNING
HORIZONTAL
SCANNING
...
FIGURE 6. Progressive
displays “paint” the lines
of an image consecutively,
one after another.
2. Consumer equipment that supports
composite video or
analog YPbPr video
usually uses tech-
nique 3. For digital video, either technique 1 is commonly
used or timing code words are embedded within the
digital video stream.
FIGURE 5. Video is composed
of a series of still images.
Each image is composed of
individual lines of data.
lines followed by the even-numbered lines (as
shown in Figure 7), the amount of information
sent for each image was halved. Given this
advantage of interlacing, why bother to use
progressive?
With interlace, each scan line is refreshed
half as often as it would be if it were a progressive display. Therefore, to avoid line flicker on
sharp edges due to a too-low refresh rate, the
line-to-line changes are limited, essentially by
vertically lowpass-filtering the image. A progressive display has no limit on the line-to-line
changes, so it is capable of providing a higher
resolution image (vertically) without flicker.
Today, most broadcasts (including
HDTV) are still transmitted as interlaced. Most
CRT-based televisions are still interlaced, while LCD,
plasma, and computer displays are progressive.
Digital Video
Interlaced vs. Progressive
Since video is a series of still images, it makes sense
to simply display each full image consecutively, one after
the another. This is the basic technique of progressive —
or non-interlaced — displays. For progressive displays that
“paint” an image on the screen (such as a CRT), each
image is displayed starting at the top left corner of the
display, moving to the right edge of the display. The
scanning then moves down one line and repeats scanning
left-to-right. This process is repeated until the entire
screen is refreshed, as seen in Figure 6.
In the early days of television, a technique called
“interlacing” was used to reduce the amount of information
sent for each image. By transferring the odd-numbered
The most common digital video signals used are RGB
and YCbCr. RGB is simply the digitized version of the
analog RGB video signals. YCbCr is basically the digitized
version of the analog YPbPr video signals and is the format
used by DVD and the various terrestrial, cable, and satellite digital television standards (ATSC, DVB, and ISDB).
Not too long ago, DVI was introduced to consumer
products for transferring digital RGB video between
components. In 2004, the trend has shifted to using
HDMI, which has the advantage of a smaller connector,
the ability to transfer digital audio, and the ability to
support both the RGB and YCbCr digital video formats.
Best Connection Method
There is always the question, “What is the best
connection method for equipment?” For DVD players and
digital cable/satellite/terrestrial set-top boxes, the typical
order of decreasing video quality is:
FIGURE 7. Interlaced displays first “paint”
one half of the image (odd lines), then the
other half (even lines).
1. HDMI (digital YCbCr)
2. HDMI/DVI (digital RGB)
3. Analog YPbPr
4. Analog RGB
5. Analog S-video
6. Analog Composite
F
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NUTS & VOLTS
E
v
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VERTICAL
SCANNING
HORIZONTAL
SCANNING
FIELD 1
...
HORIZONTAL
SCANNING
FIELD 2
...
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Some may disagree about the order. However, most
consumer products do digital video processing in the
YCbCr color space. Therefore, using YCbCr as the inter-connect for equipment reduces the number of color
space conversions required. Color space conversion of
digital signals is still preferable to D/A (digital-to-analog)
conversion, followed by A/D (analog-to-digital) conversion, hence the positioning of DVI above analog YPbPr.
JANUARY 2005
