In SDI systems, what type of cable is typically used to carry clocking signals such as wordclock and tri-level sync?

• A. Twisted pair
• B. Coaxial cable
• C. Fiber optic
• D. Ribbon cable

Answer: (B)

In SDI systems, timing references like wordclock and tri-level sync must be carried with a transmission path that has a consistent impedance and strong shielding to keep the timing stable and free of noise. Coaxial cable provides a controlled 75-ohm impedance and good shielding, which helps preserve the integrity of high-frequency clocking signals over typical distribution runs. It also matches the standard connectors and gear used in SDI setups (BNCs and 75-ohm cables), making it the practical choice for looping and distributing timing references alongside video.

Twisted pair lacks the consistent impedance and shielding needed for clean, low-jitter clock signals over longer runs. Fiber can carry timing as well, but in traditional SDI infrastructures it’s more common for video distribution itself, and it adds complexity and cost for clock distribution. Ribbon cable doesn’t offer the shielding or impedance control required for high-frequency timing signals, making it unsuitable for this purpose.