Nigel Seth-Smith
Gennum

For the last 25 years, video has been transported in the broadcast and professional environment using the Serial Digital Interface (SDI) over coaxial cable. The success of this technology has been phenomenal, with data rates up to 3 Gb/s transported for many tens, or even hundreds, of meters over cable that looks no different from that designed for 4.2 MHz of NTSC. But there are always requirements for longer runs, and digital signals over cable have a very well defined quality cut-off point with cable length.

The reason for this sharp threshold is that coaxial cable has an attenuation characteristic which is frequency-dependent. This attenuation is compensated for using adaptive equalizers, but these have a limit to their capability. Once the limit is reached, the high frequency loss in the cable cannot be equalized, the eye starts to close, and signal quality degrades very rapidly. For longer runs, a different technology is needed. This paper explains why fiber optics are an excellent alternative.