There are many different wires for transmitting
video and this can be a very confusing topic, so lets talk about each
of them and give some recommendations on which to use. At the highest
level, there are two types of signals: analog and digital.
Analog Video Signals
There are many types of analog video signals
including: RF, Composite, S-Video, Component, Y/Pr/Pb (a.k.a., YUV, color
difference, YPbPr, Y/B-Y/R-Y, and YCbCr.), and RGB (and its two cousins
RGB H/V and RGBHV) - each has its own type of connector. In general, the
more wires the signal uses, the better the signal quality.
- RF is the worst quality of the group. It combines
audio and video on the same wire. The wire is a coax cable and typically
uses a screw-on connector called an F connector, but sometimes uses a
BNC connector (see below).
- Composite is better than RF. It has one wire just
for video (that is, the audio signal is not carried by the Composite video
wire). This wire usually uses RCA connectors (but in rare cases uses a
BNC connector which means you will probably need a BNC-to-RCA adaptor
to connect it to anything else).
- S-Video has 2 wires for video one for the chrominance
(color) and one for the luminance (brightness) thus producing better
color saturation and black levels. This wire uses a round connector with
multiple pins inside of it.
- Component has 3 wires one for luminance (brightness),
one for red color information, and one for blue color information thus
producing better colors. The Component cable almost always has 3 RCA connectors.
However, sometimes a display will use a 15-pin D-SUB connector. In this
case you will usually need a cable that is 3 RCA connectors on one side
and the 15-pin D-SUB on the other.
- Y/Pr/Pb (a.k.a., YUV, color difference, YPbPr, Y/B-Y/R-Y,
and YCbCr.) is Component. They
are one and the same. However, you often see the term Y/Pr/Pb used with
HDTV where as you see the term Component used with DVD players. Since
Y/Pr/Pb is Component, it uses the same connectors as Component.
- Ultimately, all other signals must end up as RGB
to feed the picture tube, plasma or LCD panel. However, video content
(e.g., pictures and movies) are never stored in this format, so, in reality
it is not any better than Component. RGB is available in 4 different signal/connector
types:
- Straight RGB in which the sync signal is embedded
in the green signal and the cable has 3 BNC connectors
- RGB H/V in which the sync signal is on a 4th
wire and it has 4 BNC connectors,
- RGBHV in which there are separate horizontal
and vertical sync wires for a total of 5 wires - all of which use BNC
connectors, and
- VGA which is equivalent to RGBHV but uses a 15-pin
D-SUB connector. This is by far the most common RGB format as it is the
format used to send the video signal from a computer to the computer's
monitor.
RF BNC
F Connector |
Composite |
S-Video |
Component
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Digital Signals
In addition to all of the different types of analog signals,
there are four digital signals that you may see used with video equipment.
- IEEE 1394 Firewire is a relatively old standard
that has appeared on some AV components. A major objection to this standard
by the broadcasters is that it does not inherently include any encryption
standard (that is, they cannot prevent you from copying the digital content).
- Digital Video Interface or DVI is a standard
for one-way digital communications from a source to a display. More importantly,
the standard includes an encryption format (High Bandwidth Digital Content
Protection or HDCP) that is transmitted and received by DVI-compliant
displays and is used to protect digital content from being recorded. DVI
ports appear on a number of high end video components.
- High-Definition Multimedia Interface or HDMI
is a standard that combines DVI video and multi-channel audio into the
same connector. It is relatively new, but is gaining in popularity with
high-end video components.
- TCP/IP over Ethernet or WiFi (or perhaps Bluetooth)
uses industry-standard networking protocols to transmit the video signal.
This method will become far more popular as Microsoft Media Center devices
begin to take off.
Interlaced vs. Progressive
Beyond all of the different wire types for video, there
is the issue of how to transmit the video: progressive scan vs. interlaced.
In progressive scanning, the whole frame of the video signal is scanned
onto the display at once. Where as, with interlaced, first the odd numbered
horizontal lines are scanned onto the display and then the even ones are
scanned onto the display. For a given frame rate, progressive is better
because it does not appear to flicker. Computer screens use progressive
scan where as traditional broadcast TVs use interlaced. One more slightly
esoteric issue is that when the industry talks about the interlace rate,
they actually talk about the rate of displaying just one half of the frame,
where as, when they talk about the progressive scan rate, they talk about
the rate of displaying the whole frame. This means that the actual frame
rate of interlaced is half its stated rate. Therefore, 1080 interlaced
(1080I) puts out 540 lines of resolution on each refresh, and this is
fewer lines per refresh than 720 progressive (720p) which puts out 720
lines of resolution per refresh. Interlaced can be transmitted over all
of the analog types of wires. Both progressive and interlaced are defined
over Component, RGB, DVI and HDMI. Where as RF, Composite, and S-Video
use interlaced.
Interaction of Video Signals
On most AV components the Composite
input is disabled when the S-Video input port of the same input connector
group has a connection going to it. For example, if you connect
the Composite output of a VCR to the "Video 1" input of a TV
and you connect the S-Video output of the same VCR to the same "Video
1" input of the TV (i.e., the same IR code is used to select the
group of connectors that includes both the Composite and the S-Video ports),
then it is likely that the Composite path will not exist. This can be
important if you try to connect a camera to the Composite input ports
on the front of the VCR because you will not be able to view the camera
on the TV until you transfer the video to a VCR tape and then play it
back (note that you may still be able to hear the audio even though you
cannot see the video).
We have also seen a few AV components (such as the EchoStar Dish 501
PVR) where the Composite output is disabled when
the S-Video output is connected to another AV component. This means
that the Composite output cannot be used for Video Sync, either.
Some AV sources that output a progressive signal,
will not output Composite or S-Video at the same time (because
Composite and S-Video are inherently interlaced signals). This is especially
true of satellite receivers and set top boxes, but less often true of
DVD players.
Summary
In summary, to get the highest quality signal, use DVI
or HDMI when you can. Otherwise, use Component. If you are using older
AV components such as a VCR, S-Video is likely to be the highest quality
signal you can get. Use 720p if you can, or in decreasing preference use:
1080i, 480p, or 480i. Note that 480i is the inherent signal quality of
a DVD disk. If you have a separate video processor or one in the display
or projector, you may want to send this signal to it and let the video
processor improve the signal.
If you need the signal to run a long way in a distributed
video application, you will probably want to use Composite video because
the video signal is all on one wire and thus does not suffer from skew
between different components of the video signal (and it is much cheaper
to run Composite wire). If you want to distribute live TV, recorded TV,
DVD, and/or photographs to PCs and TVs throughout your house using IP
over Ethernet or WiFi, then you may want to check out the Microsoft Media
Center architecture for networked AV content.
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