onhometheater.com's Guide to
Video Connections
Anyone who’s recently glanced at the back of an
audio/video receiver, preamp-processor, or video monitor can attest to the fact that
there's a baffling array of connection possibilities. It can be confusing, but
don't give up in frustration. There's a simple explanation for each of those options, and
over the next several Features, www.onhometheater.com
will help you sort them out. In upcoming columns, we'll tackle analog audio and digital
audio connections. This time out, we'll take a look at video hookup.
It would be great if there was a simple, standard
connection for all video products -- that's what the new HDMI connection is supposed to be
-- but at the moment, you probably have several different video products, each of which
has at least one, if not several, of the following connection systems. If so, there's not
much you can do about it, but as you buy new video products, pay attention to the way they
connect to the ones you already own -- it's a detail that can pay huge dividends in video
quality.
RF
Really ancient televisions (say, more than 10 years old)
used RF (radio frequency) connections, such as those from an over-the-air (OTA) antenna.
If your TV has two small screws (usually labeled "antenna"), RF connectors are
what you have.
An RF input combines the audio and video signals, leaving
it to the television to divide the signal into its component parts of audio, luminance
(video brightness and contrast), and chrominance (color video). Because RF doesn’t
deliver a high-quality signal, it’s not used these days.
If you buy a receiver or DVD player these days, you'll
never see an RF connector, but you may still have a monitor around that uses one. If so,
you can buy an RF converter at an electronics store to tide you over until your next TV
purchase.
Composite video
Composite-video connectors, like most of the ones that
follow, are video-only connections -- only the new kid on the block, HDMI, combines audio
signals into the video connection.
With composite video, the audio portion of the signal is
carried by other cables, but both the luminance and the chrominance video information is
contained in a single cable (hence the name "composite"). Composite connections
typically use RCA-style plugs and are customarily coded yellow to distinguish them from
the red and white, which designate the right and left connections used for stereo audio.
That's all you need to know in order to hook things
up. But if you're reading this you’re probably a bit more curious than that, so
here's the why to go with the how. The composite cable looks like any audio
interconnect, but it should be a coaxial cable designed for a 75-ohm signal. That signal
contains the red, green, and blue (RGB) signals that make up the video image. Those RGB
signals contain the luminance (Y) and chrominance (C) elements we first met in the section
on RF connections. The Y and C elements are separated within the television by a comb
filter, which passes them along to a color decoder, which reconstructs the original image.
Most mass-market video products have composite-video
connections, but the S-video connection is almost as universal and offers some performance
advantages.
S-video
S-video uses a DIN connection -- DIN stands for Deutsche
Industrie-Normen, which is the German Industrial Standards committee. That's just a
fancy way of saying that because S-video is a "standard," you won't see two or
three variations on it. The cable has male plugs consisting of a circular rim surrounding
four flat pins and an orientation "bump" that helps you align the plug properly.
You can think of S-video as "separated video,"
because the Y and C signals are carried by separate wires inside the cable. That means the
TV does not have to perform the separation with its comb filter -- which means the picture
will be sharper and the color better than with a composite connection.
Component
Component connections carry the concept of separating the
video signal into its component (!) parts even further. Better-quality video sources, such
as digital TV (DTV), digital satellite TV (DirecTV and Dish), and DVD players, frequently
use component-video signals, which require three cables. Component-video signals
have a luminance (Y) component and two color-difference components, which make up
the chrominance (C) portion of an S-video signal.
Why use component-video connections? You get better color
detail, finer shading, and fewer distortions, such as dot crawl and colors bleeding into
one another. If you have this option, it will almost certainly produce a better image than
S-video, although some viewers feel the differences are not as profound as those between
S-video and composite.
RGB, RGB H/V, and RGBHV
Another cuppa alphabet soup -- this time because there are
three variations on a theme. The cables look the same as component-video cables,
but the signal is split differently. In addition, the connectors terminating the cables
are generally a securely locking bayonet plug called a BNC (British Naval Connector).
RGBHV uses five cables, splitting the video signal
into red, green, and blue, as well as vertical and horizontal synchronization. RGB H/V
uses four cables, splitting the color into red, green, and blue, and combining the
horizontal and vertical sync on a single cable. Then there's "plain" RGB, which
employs three cables, splitting the signal again into red, green, and blue
elements, and carrying the sync information on one of the three, typically the green
(sometimes this is called RGB sync on green).
If you use an HTPC (home-theater personal computer) or a
game box and a projector, RGBHV is incorporated into a 15-pin VGA connection -- five of
those pins carry the RGBHV information. Some HDTV set-top boxes and monitors also use a
VGA connection, while others -- and many pro-style video components -- use the five-cable
RGBHV system.
DVI
All the preceding connections are analog, but newer
video sources, such as DVD players, DirecTV and Dish satellite services, and DTV can
deliver digital video signals when equipped with the right connectors.
DVI (Digital Visual Interface) connections look somewhat
like VGA connections. There are two types of DVI: DVI-D, the smaller of the two, supports
only digital signals; DVI-I is slightly larger and accommodates both digital and analog
signals. Which do you have? DVI-D is a 24-pin connection (three rows of eight pins each);
DVI-I has 24 pins plus another five for the analog RGBHV signals.
Most consumer DVI products incorporate something called
HDCP (High-Bandwidth Digital Content Protection), an encryption scheme designed to prevent
the digital copying of protected material.
HDMI
HDMI (High-Definition Multimedia Interface), an extension
of DVI, is the newest connection system. HDMI uses a small multipin connector that carries
multichannel digital audio data (up to eight channels) and A/V component control
data. That means a single cable can handle all of your A/V signal transfers.
HDMI is very new -- it was approved only in December 2003
-- but it has the support of a wide range of consumer-electronics companies, cable
companies, and satellite broadcasters, so it will probably be the connection of choice for
some time to come.
Fortunately, HDMI is back-compatible with DVI through the
use of an adapter. You don't gain the audio or control data transfer with the adapter
plug, but it does mean you can use DVI and HDMI components in the same system.
A single, simple connection that takes care of everything
-- what a concept!
...Wes Phillips
wes@onhometheater.com
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