05.1 - DVI
The Digital Video Interface, or DVI, came from the computer world as a way to get RGB images from the computer to the display device in an all digital form. The three color channels of Red, Green, and Blue typically start as 8 bits per color inside the computer. With all the PC displays going digital, it makes sense to keep the bits digital for the short trip over to the display, rather than converting to analog RGB (VGA). To do this, DVI takes the 8 bits from a color channel, and serializes them for a high speed trip over a balanced line twisted pair cable. Each color gets it’s own twisted pair, and a fourth pair is a clock that ticks once per pixel. As far as the major data path is concerned, DVI is a one way data bus. The DVI connector looks like a computer D-sub style, with thumbscrews on either side. It has the nice advantage of being a connector that can be positively locked down to the equipment it attaches to.
DVI also specifies optional analog RGB lines. This was nice for the computer video card manufacturers so that could now support both digital and analog RGB interfaces with a single connector.
You may see references to DVI-D, DVI-A, and DVI-I. DVI-D is the normal digital DVI signals, which is the most common form. DVI-A means only the analog RGB signals are present, which is rarely, if ever seen. DVI-I means that both are present, and what is common on PC video cards. In terms of cables, if you have a DVI-I cable, you can do any of the above with it. Most of the time you can use a DVI-D only cable. As far as I have seen there are no DVI-A only cables.
In addition to the high speed twisted pair data lines, there is a low speed data line so the source device can query the display device for information such as make and model number, max resolution, etc. This is called the Display Data Channel (DDC) line.
DVI was brought into the consumer electronics world around 2003 to serve the same purpose, but for HDTV video modes. I’ll save you the math, but for 480p, there are 27 million pixels per second, 720p and 1080i both are about 74 million pixels per second, and 1080p is about 145 million pixels per second. Since DVI needs to send 8 bits per pixel, the actual data rate on the twisted pair is 10x those figures (there is an 8 to 10 bit conversion). So, bottom line is DVI at 1080p has a data rate of up to 1.45G bits/second (per channel).
Also keep in mind that twisted pair are not as good of a transmission medium as coax cable. It has more loss, and has balance issues if not constructed carefully. Many DVI cables use cheap twisted pair cores, and can cause problems at longer lengths. Actual length performance depends highly on the equipment at both ends, as well as the cable. So while analog video needs only about 75MHz of bandwidth of a coax cable, DVI needs in excess of 2GHz of bandwidth from a twisted pair cable to do the same thing.
Lengths of 15 to 30 feet is usually not a problem. 50 feet is usually achievable. Beyond that there are some signal restoration type of products that work well to extend the range out to 100 feet, in addition to fiber optic solutions that can go up to 300 feet. Fiber works, but can be cost prohibitive.
DVI cables cannot be made in the field, so if they are to be preinstalled in the walls, they should be tested thoroughly before installation, and again before drywall. Best way to test is with actually displays and equipment. The testers out there do test for many faults, but typically don’t test at the 1.45Gb/s speeds. Too tight of a bend radius can ruin a cable install.
Failures of DVI video typically start out as sparklies or random snow on the screen. If the cable was damaged during installation, the usual result is no picture at all.