Q What's the longest distance that I can run an SDI cable?
Forest Park, Illinois
A When an analog cable run was too long, the picture quality would diminish. On the other hand, digital video cable runs give you a solid indication when you've gone too far: You won't get an image. The maximum distance before this occurs is usually well over 500 feet. But I'm taking your question at its face value. What you probably meant to ask is, "How long can I run an HD-SDI cable?"
SDI, or Serial Digital Interface, represents standard-definition digital video; HD-SDI (yes, the HD stands for high-definition) is what you use for HD signals that carry video at 1080p or 720p resolution.
In truth, there really isn't SDI or HD-SDI cable; there's simply coaxial cable (or coax) that can handle various bandwidths of signals. It's also good to know that a cable rated for transmitting HD-SDI can easily handle SDI signals. And as far as the cable is concerned, the signals are just at different bit rates. SDI runs at 270 Mb/s (megabits per second) while HD-SDI comes in at 1.5 Gb/s (gigabits per second).
So let me answer your question as it applies to HD-SDI. At the higher data rates, the maximum distance changes dramatically and is very dependent on the cable that's being used. For example, with some cable, the maximum distance comes in at around 100 feet; other cable stretches it out to about 300 feet.
Not only is the type of cable relevant to maximum distance, but the condition of the cable is important, too. Has the cable been run over by vehicles, thereby crushing the dielectric insulator and changing the performance of the cable? Has the cable been twisted or bent beyond the recommended tolerances? Has it been stretched or crimped by a too-tight wire tie?
Connectors are important, too. Are you using one long cable run, or are you using barrels to connect several pieces of coax? Each connector in the chain reduces the length. Were the connectors installed properly? Are they the right impedance? BNC connectors come in 75 and 50 ohm versions (75 ohm is the correct one to use).
So while an easy answer might be 300 feet, it really depends on a number of factors. Successful connections over longer distances require properly built, high-quality cables that are cared for and used properly.