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Video Signals
Thursday, 2007 June 7 - 11:24 pm
A public service information article on video signals and cables.

Tim came by and asked me about video signals and cables. It's a fairly common question: how should I connect my game console, DVD player, cable box, etc. to my TV in order to get the best picture?

After browsing on the Internets a bit, it seems like there's a distinct lack of useful information out there. So in the interest of public service, here's a tutorial on the various video signal formats, from highest quality to lowest.

Digital. The highest quality picture is achieved with a pure digital signal. Digital signals are carried over a couple of different kind of cables: DVI, and HDMI. DVI is typically used on computer equipment; HDMI is used on some cable boxes and other HD equipment.

HDMI also allows for encryption of the video signal using a copy-protection protocol called HDCP. HDCP can also be carried on a DVI connection, but that's less common.

HDMI can also carry an audio signal; DVI only carries video.

Digital signals can achieve very high resolutions, up to 1920x1200 on a single-link connector, or even higher on a dual-link connector.

RGB. RGB stands for Red, Green, and Blue, and it refers to a video cable that carries the three different color signals over different wires. The signals are analog, but they're uncompressed, and it's possible to get very high quality signals using RGB. RGB is the most common signal for computer monitors; the cable connector is commonly called VGA (a bit of a misnomer), or DB-15.

Analog RGB signals are typically limited by cabling to 1600x1200 resolution... more expensive cables (capable of carrying higher bandwidth) are required to achieve higher resolutions.

Component. Component video refers to any form of video where the signal is separated into multiple parts, but usually, we're talking about "Y Pb Pr". "Y" is the overall black/white brightness of the picture; "Pb" is the difference between "Y" and the blue signal; and "Pr" is the difference between "Y" and the red signal. The difference between this and RGB is that the "Pb" and "Pr" signals are frequently compressed so that they're half the resolution of the "Y" signal. This is because the human eye isn't capable of distinguishing color detail as well as brightness detail. So for example, while your overall video signal ("Y") might be 1024x768, the color signals ("Pb" and "Pr") might only be 512x768.

Because of this compression, lower-quality cables can be used for component video versus RGB. You won't notice a difference between component video and RGB if the source material is already encoded in "Y Pb Pr" form (or the digital equivalent, "Y Cb Cr")... which is the standard for broadcast video and DVD.

Component video can usually achieve up to 1600x1080 resolution (1080p) on the "Y" signal. It's usually carried over three cables with RCA-style jacks.

S-Video. S-Video is a form of component video where the brightness is separated from the color signal. It is decidedly inferior to component video, for a couple of reasons: one, it is fixed at 480i (720x480 resolution, interlaced), and two, the color signal is encoded on a frequency-modulated signal that's subject to noise and interference.

S-Video is usually carried over a "DIN" cable with four pins.

Component video. Component video is almost universally supported on consumer electronics equipment. It's a signal that includes both brightness and color in one; the color portion is in a higher-frequency part of the video signal. The TV or monitor splits the color from the brightness using filters.

Because the signals are combined together, they can interfere with each other, causing noise and distortion. Component video is visibly inferior to S-Video, even on inexpensive equipment.

Component video is usually carried over a single cable with RCA-style jacks.

RF Modulated video. RF-modulated video is what carries analog television signals. It's component video that's transmitted over an FM band. It's the lowest-quality of the common video signal types; it's what you get when you connect your VCR player or cable box to your TV using a coaxial cable. (This is the kind of signal that requires you to tune your TV to channel 3 or 4.)

Feel free to post questions in the comments, and I'll answer them as best I can.
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Posted by Ken in: techwatch


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