SDI Over Coax: Cable and Connector Discipline in Broadcast

SDI is the professional video world’s answer to a simple problem: get uncompressed digital video from one box to another over the coaxial cable that is already installed everywhere. It works, and it is unforgiving in ways that domestic video is not.

The cable matters more here than anywhere else in coax work, and the connector matters almost as much.

Quick answer

Question	Answer
What cable does SDI use?	75 ohm coaxial cable
What connector?	BNC, and it must be the 75 ohm version
Does cable length have a limit?	Yes, and it falls as the data rate rises
Why so strict?	A digital signal has a threshold, and the receiver can only equalize so much
Does a generic coax work?	Sometimes, but broadcast practice is to use cable with a published specification
What about very long runs?	Fiber, usually, with converters at each end

What SDI is, and why the rates matter

Serial Digital Interface carries uncompressed digital video as a serial data stream on a 75 ohm coaxial cable. The standards family steps up in data rate with picture resolution: the older standard-definition rates, the high-definition rates, and then the faster tiers used for higher frame rates, deeper color and higher resolutions.

The detail that governs installation is that the data rate and the achievable cable length are linked. A higher rate occupies more of the cable’s usable frequency range, so the same cable carries it a shorter distance before the signal arrives too degraded for the receiver to recover.

That is why a cable that works perfectly in an SD installation can be marginal at a high-definition rate, and why "what length can I run" has no answer without knowing the rate.

There is a second-order effect worth knowing about. Receivers include adaptive equalizers that compensate for the cable’s frequency-dependent loss, which is what makes long runs possible at all. The equalizer has a limit, and the cable’s loss at the upper end of the signal’s spectrum is what exhausts it. Cable with lower loss and better impedance control extends the reach.

The connector is not a formality

Every joint in a coaxial line is a place where the impedance has to stay constant. In an analog video system, a mediocre joint produces a small visible artifact. In an SDI system, a joint with a poor return loss produces bit errors, and the failure appears as sparkles, block artifacts or a dropped signal.

Two consequences follow.

Use 75 ohm BNC connectors. BNC exists in 50 and 75 ohm versions that look identical and mate with each other, and a 50 ohm part in a video chain is a mismatch by construction. The impedance difference is covered here, and this is the application where it bites hardest.

Count your joints. Every barrel, every patch panel and every wall plate adds a discontinuity and a small amount of loss. Broadcast installations keep the count low and use panels designed for the purpose rather than general-purpose hardware. Connector selection is covered here.

Cable choice in practice

Broadcast practice is to specify cable that publishes what matters: a consistent 75 ohm impedance, a documented attenuation curve, and a return-loss figure. That specification exists because the application needs it, not as a marketing position.

Generic coaxial cable may meet the impedance requirement and still vary enough in construction that some lengths perform noticeably worse. For a short link the difference is unlikely to matter. For a run that is already near its practical limit, it is the difference between locking and not.

The design decision that pays best is none of these: keep the run short. If a camera position can be reached with a shorter route, that is worth more than any cable upgrade.

Where fiber takes over

Beyond a certain length, which depends on the rate and the cable, no practical coaxial cable carries the signal. The answer is a fiber link with converters or an SDI-over-fiber extender at each end.

That is a normal part of broadcast design, not a failure of the cable. The same pattern applies in the network world, where the twisted-pair channel has a defined limit and fiber continues beyond it. The coax and twisted-pair trade-off in CCTV has a similar shape at a much lower data rate.

Scenarios

A studio with a router and a patch bay. Precision cable and purpose-designed patch panels, because the whole point of the installation is that connections move without introducing defects.

A camera 100 meters from the control room at a high data rate. Work the loss budget first. If the rate and the distance do not close, fiber is the answer rather than a different coax.

A venue with existing generic coax. Test it at the rate you intend to use. Cable that passes at a lower rate may not at a higher one.

A short rack interconnect. Length is on your side here, so cable choice is less critical than the connectors, which are still 75 ohm BNC.

FAQ

What cable does SDI need? 75 ohm coaxial cable, with a published attenuation and return-loss specification for serious work.

How far can SDI run on coax? It depends on the data rate and the cable. Higher rates reduce the reach, and the figure comes from the cable’s loss against the receiver’s equalization capability.

Can I use a 50 ohm BNC on an SDI cable? It will mate, and it introduces a mismatch. Use 75 ohm parts.

Does SDI work over Cat6? Not over a passive twisted pair. Converters exist, and they turn SDI into a different signal for transport.

Does an extra barrel connector matter? It adds a discontinuity and a small loss. In a chain that is already marginal, yes.

Is fiber better than coax for SDI? For long runs, it is the only practical option. For short runs, coax is simpler and cheaper.