SDI cable length: coax limits and when to move to fibre

Why SDI cable length depends on the signal

An SDI cable length is not one number. Serial digital interface (SDI) is a family of video signals, and each step up in resolution or frame rate raises the bit rate. Coaxial cable attenuates high frequencies more than low ones, so the same cable that runs 300 m for standard definition may run under 100 m for 12G.

The bit rates in the table are published values (they are the nominal figures from the standards, before any rounding by vendors). Everything below follows from them. A broadcaster who asks "how long can my SDI run be?" has to say which format, which cable and which receiver, because all three change the answer.

Planning figures for coaxial SDI

Reach is set by the cable's loss at half the clock frequency, and by how much loss the receiver's equaliser can recover. Cable and equipment vendors publish their own figures, and they differ with cable grade, connector quality and receiver design. The ranges below are common planning values for a good 75 ohm precision video coax. They are not guarantees, and the data sheets of your cable and your equipment come first.

Notice how little margin 12G leaves. A truck with a 50 m cable run, two patch panel passes and a router can already be at the edge of the budget before a single cable is damaged. Thicker, lower-loss coax extends the reach but it also makes a heavier, stiffer cable, which matters for anyone who has to carry it. Our SDI cable page lists what Radix makes in copper.

What fibre does differently

Optical fibre does not have a frequency-dependent loss that scales with the signal rate the way coax does. A single-mode link at 1310 nm loses roughly 0.35 dB per kilometre for a typical fibre, and the same figure applies whether the signal is HD or 12G. The practical limit is the optical budget of the transmitter and receiver, which is usually many kilometres. SMPTE ST 297 is the standard that defines the optical interface for SDI.

That changes the comparison. With coax, you read the reach from a table that depends on bit rate. With fibre, you read it from the optical module's data sheet, and common SDI optics are specified for distances measured in kilometres. A 12G signal that cannot go 70 m on copper can go 10 km or more on single-mode fibre, with the same converter and a duplex cable.

The fibre cable does not carry SDI directly. At each end, a converter or SFP module turns the electrical SDI signal into light, and the receiving end turns it back. This is the job of the units described on the SDI convertor page, and the selection of converters is discussed in the guide to choosing SDI fibre converters. For the formats themselves, see the guide to SDI over fibre.

Side by side: copper SDI and fibre

The weakest line for fibre is the active equipment at the ends. Copper SDI needs nothing powered between the camera and the router. Fibre needs two powered converters, and both have to be working. For a 20 m studio run, that is complexity without a benefit. For a 400 m run between a stadium camera position and a truck, it is the only practical way.

The length where fibre starts to win

There is no sharp line, but a practical rule helps. Take the planning reach for your format and the real route length, including slack and the path around obstacles. If the route is less than about two thirds of the planning reach, coax is comfortable. If it is between two thirds and the full reach, coax is possible but has no margin for patch points and ageing. If it is beyond the reach, only fibre or a reclocking device will do. Reclockers and distribution amplifiers can extend a coax chain, but each one is another active unit, and the saving over a fibre pair disappears quickly.

Other reasons to choose fibre have nothing to do with distance. Venues with several power systems can create ground loops between a camera and the truck, and fibre breaks them. A long run along a stage rig lit by dimmers picks up noise on copper. A hybrid cable that carries power for the camera, such as the hybrid constructions covered in the SMPTE hybrid cable guide, carries fibre for the video and copper for power on the same sheath.

Worked example with assumed values

These figures are assumed to show the method. Replace them with the numbers from your own equipment and cable data sheets.

A production needs eight 12G-SDI camera feeds from a stage to an outside broadcast truck parked 250 m away along the cable route. Assume the cable data sheet supports 12G over 60 m with the receivers being used, and that the route needs 10 m of slack at each end.

1. Route length including slack: 250 + 10 + 10 = 270 m.
2. Copper reach at 12G: 60 m. The run is 4.5 times that reach (270 / 60), so coax alone cannot do it, and reclocking would need about four stages per feed.
3. Cable count: eight coaxes. At 7 mm each, the eight cables side by side take up 56 mm, and at about 60 g per metre per coax the weight is 8 x 60 x 270 / 1000 = 129.6 kg, which is two people's work to roll out.
4. Fibre alternative: eight converters at each end, or a multiplexer system, and one duplex or multi-fibre cable. Assume a 4-fibre tactical cable at about 8 mm and 60 g per metre. Weight: 60 x 270 / 1000 = 16.2 kg.

The weight values are assumptions for the arithmetic. On the figures given, the fibre cable is roughly one eighth of the copper weight, and the reach problem is gone. The cost of that gain is sixteen converters, their power supplies and a spare, so the decision depends on whether the production owns that equipment already.

Reading a converter data sheet for the fibre side

Once fibre is chosen, the converter data sheet decides most of what follows. Five lines matter.

• Supported rates. A converter that handles HD and 3G may not handle 6G or 12G. List every format you will send, including any future 4K work, and check each one.
• Wavelength and fibre type. Optics at 1310 nm on single-mode fibre are the common choice for long runs. Multimode optics at 850 nm cost less and cover shorter distances. A converter built for one cannot be used with the other.
• Stated reach. The figure is for a specified fibre loss. A run with several connectors or a damaged section may fall short of it.
• Connector on the converter. LC is common on SFP-based units and SC on older ones. Your cable assembly needs the matching connector at that end. See the LC vs SC comparison if the choice is open.
• Power and monitoring. Field converters usually run from a DC supply or a rack frame. Check the power source available at the camera position before ordering, because a unit with no power is a dead end.

Write all five onto the order for the cable, and give the cable supplier the converter model. That lets the connector, polish and fibre type be checked against it before anything is made.

Buyer checklist

• Write the signal format for every feed: SD, HD, 3G, 6G or 12G.
• Measure the route, not the straight-line distance, and add slack at both ends.
• Count patch points. Each BNC or optical pair adds loss.
• Check the receiver specification for supported cable length at your format, and not just the cable vendor's table.
• For fibre, choose single-mode or multimode to match the optics in the converters. Single-mode (OS2) suits long runs. Our single-mode vs multimode guide sets out the difference.
• Choose connectors that suit repeated field use, such as the ruggedised types described on the opticalCON connectors page.
• Decide whether the cable must be field-deployable, with a tough jacket and a reel, as in the guide to field-deployable fibre cable for outside broadcast.
• Plan spares: one spare converter pair and a spare cable length per site.

Common mistakes

Quoting a reach without a format. A vendor who says "SDI goes 100 m" may be thinking of HD. At 12G the same cable can fall short at half that figure.

Using 50 ohm connectors or cable. SDI is a 75 ohm system. A 50 ohm connector in the path creates a reflection that shortens the reach and makes errors that look intermittent.

Forgetting that fibre has a minimum bend radius. A coax that survives being folded may only get a dented jacket. A fibre cable that is kinked can lose light. The bend radius guide gives the rules.

Skipping end-face cleaning. A dirty optical connector fails in ways that look like a bad converter. The procedure is in the end-face cleaning and inspection guide.

Mixing up the two directions. A duplex SDI-over-fibre link usually carries one direction per fibre, or one signal on each fibre in a pair. Check that the transmit of one converter reaches the receive of the other.

When copper is the better choice

Keep coax for short runs inside a studio or truck, where cables are a few metres, signals are not at the limit and the extra converter adds nothing. Coax is cheaper per channel for those lengths, needs no power and survives rough handling. A fibre run is not a reason to remove every coax in the building. Many productions use both: coax within a rack and fibre between buildings or between a camera position and the control room. The broader pattern is covered in the guide to hybrid SDI and IP cabling, and the broadcast industry page collects the Radix product range for this work.

To get a quote, state the format, the number of feeds, the route length and the connector type, and send the request through the request a quote page. Radix makes SDI cable and fibre-SDI converters, and builds fibre assemblies to the buyer's drawing in Pune.