DVI-Dual Link? Single Link? Which do I need?

Dual Link: Dual T.D.M.S. (transition minimized differential signaling) "links". DVI can have up to two TMDS links. Each link has three data channels for RGB information with a maximum bandwidth of 165 MHz, which is equal to 165 million pixels per second. Dual-link connections provide bandwidth for resolutions up to 2048 x 1536p.
Single Link: Single T.D.M.S. link. Each link has three data channels for RGB information with a maximum bandwidth of 165 MHz, which is equal to 165 million pixels per second. Bandwidth for a single-link connection supports resolutions of over 1920 x 1080 at 60 Hz (HDTV).
Dual link vs. Single Link: Don't believe the "hype" from some websites indicating that dual link cables are superior to single link cables. A single link cable is 100% as good as a dual link cable for single link equipment which covers about 99.5% of current equipment, including HDTVs, Projectors, Plasma Screens, and High Definition Set top Boxes. If your equipment does not specifically state it is Dual link in its specifications, it almost certainly is not. A better quality cable is a better quality cable, and single and dual link has nothing to do with quality. On the other hand, if both devices being connected support Dual links, then a dual link cable is the proper cable for the application, and you will have the capability of much greater resolutions and refresh rates. A properly designed Dual link cable should have no negative effects when used with single link equipment.
"Since it is Digital, Quality does not make a difference": This is false. DVI does not have any error correction. While you can get away with lower quality cables for short lengths, lower quality cables will have an increasing amount of errors with increasing length. Since the quality of implementation of DVI transmitters and receivers varies widely in equipment, the best reasonably priced cable should be used to ensure the best possible video quality.

Analog with DVI

Another benefit of the DVI specification is the ability to support both analog and digital connections on a single interface. The DVI connector can be seen below:
DVI Socket
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On the left you'll notice 3 rows of 8 pins each; these 24 pins are the only pins required to transmit both sets of three digital channels (Dual Link) and one clock signal. The crosshair arrangement on the right is actually a total of 5 pins that can transmit an analog video signal.

This is where the specification divides itself in two; the DVI-D connector features only the 24-pins necessary for purely digital operation while a DVI-I connector features both the 24 digital pins and the 5 analog pins. Officially there is no such thing as a DVI-A analog connector with only the 5 analog pins although some literature may indicate otherwise. By far, the vast majority of graphics cards with DVI support feature DVI-I connectors.
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