SDI and Genlock: The Hidden Plumbing of Television

Two Black Boxes Labelled Genlock and SDI

Half buried under a lime green towel in the back of a Kona sit two small black boxes. One has the word GENLOCK printed on it. The other says SDI, next to a thin blue line. Around them lie round metal connectors with a twist-lock collar, the kind called BNC, and DVI breakout cables fanning out into more of the same.

Those two words, genlock and SDI, are the hidden plumbing of television. Every live broadcast you have ever watched, every news bulletin, every Eurovision final, every ice hockey match, ran through cables and boxes built around them. Almost nobody outside a TV station knows what they mean. And right now, all over the world, the gear built around them is being ripped out of studios and sold at auction, because television is in the middle of a quiet revolution.

This episode explains what that plumbing is, why it was built that way, and why it is ending up in cars heading north to Jämtland.

The Problem of Many Cameras

Start with a simple scene. A TV studio has three cameras, a graphics computer and a video playback machine. A director sits in the control room in front of a wall of monitors and a vision mixer, the big desk of buttons used to cut between sources. Camera one, camera two, a graphic, back to camera one.

Now here is the problem. Video is not a continuous flow. It is a series of still images, frames, sent one after another, 25 of them every second in Europe. Each camera produces its own sequence of frames. If camera one starts a new frame at a slightly different moment than camera two, then the instant the director cuts from one to the other, the picture glitches. In the analogue days the screen would roll or jump. Even today, cutting between unsynchronised sources causes a hiccup.

The solution is genlock, short for generator locking. The studio has one master clock, a sync generator, usually a box in a rack in the equipment room. It sends out a reference signal to every device in the building. Every camera, every graphics machine, every playback server locks its timing to that reference. They all start every frame at exactly the same instant. Now the director can cut between any source, at any moment, and the transition is clean.

In a studio, nothing is allowed to keep its own time. Everything listens to the house clock. That is the whole religion.

The reference signal itself has a lovely old name. In standard definition it was called black burst, because it was literally a black video picture with a short burst of colour timing information. For HD the industry moved to something called tri-level sync. But the idea never changed. One heartbeat, distributed everywhere.

So a box labelled GENLOCK in the haul is probably either a breakout that brings that reference into a capture card or a media server, so the server can stay in step with the cameras. Without it, a Hippotizer feeding graphics into a live broadcast would drift out of time with everything else.

SDI, the Cable That Carries Everything

The second word is SDI, which stands for serial digital interface. It is the standard way of moving professional video around a building.

In the 1980s, television was moving from analogue to digital. Analogue video cables were fussy. The picture degraded with cable length, colours drifted, and every connection added noise. When digital video arrived, the industry needed a way to send it over the coaxial cable that every studio already had in its walls, using the same BNC connectors engineers had trusted for decades.

SDI was the answer, standardised around 1989 by SMPTE, the Society of Motion Picture and Television Engineers. The first version carried standard definition digital video at 270 megabits per second, down a single coaxial cable. Then came HD-SDI in the late 1990s at about 1.5 gigabits per second for high definition. Then 3G-SDI, at 3 gigabits, which arrived in the mid 2000s for higher frame rates. Later still 6G and 12G for 4K. The same basic cable, the same BNC connector, carrying more and more over the years.

Engineers loved SDI for reasons that are easy to understand. It is uncompressed, so there is no quality loss and almost no delay. A frame goes in one end and comes out the other within a fraction of a millisecond. The BNC connector locks in place with a quarter twist, so it cannot fall out during a live show when someone trips over it. A single cable can run 100 metres or more. And it can carry embedded audio, many channels of it, inside the same signal. One cable per source. Clean, simple, reliable.

You never think about SDI when it works. You think about it for the rest of your life the one time it doesn't, live, at nine p.m.

A large broadcast facility could have thousands of SDI cables running between cameras, routers, mixers, recorders and transmission. The heart of such a building was the router, a giant switching matrix where any input could be sent to any output. Engineers spent their careers laying, labelling and maintaining that copper.

The Companies Who Made Cards for It

For a long time, SDI gear was expensive specialist equipment, sold to broadcasters at broadcaster prices. Capture cards that let a computer take in or send out SDI cost thousands of dollars. Then a few companies changed that, and they are the names the studio man spoke with affection.

AJA Video Systems, founded in 1993 in Grass Valley, California, made converters and later the Kona line of capture cards, which became standard in Mac edit suites. Blackmagic Design, founded in Melbourne a few years later, made its name with the DeckLink capture card and went on to sell broadcast tools at prices that shocked the industry, from converters the size of a cigarette packet to entire switchers and cameras. Datapath, from Derby in England, built capture and multi-display hardware for control rooms where uptime was everything.

Many of these capture cards are too dense to fit all their connectors on the card bracket itself. So they come with breakout cables or breakout boxes, which take one big multi-pin connector from the card and fan it out into separate labelled BNC connectors for each input, output, audio and reference. SDI in, SDI out, genlock, and so on. That is exactly what those labelled black boxes in the Kona look like. Somewhere in the pile, there is probably a card they belong to.

The Revolution That Emptied the Studios

So why is all this gear suddenly on auction tables?

Because television is moving from coaxial cable to computer networks. Over the last decade, the industry developed a set of standards called SMPTE 2110, which sends professional, uncompressed video, audio and timing over ordinary data networks instead of dedicated SDI cables. Instead of one coax per source and a giant router, a modern studio uses fibre and network switches, the same kind of equipment that runs data centres.

The advantages are real. A network can carry many signals on one fibre. Studios can be reconfigured in software rather than by recabling. Facilities in different cities can share sources. Production can happen remotely, with cameras at a stadium and the control room hundreds of kilometres away. Even the house clock has moved onto the network, through a protocol called PTP, precision time protocol, which distributes timing to every device over the same cables as the video.

When a broadcaster or production company builds a new facility around IP, the old SDI world goes. Racks of converters, routers, capture cards, breakout boxes, sync generators and, yes, older media servers that depended on SDI inputs. Some of it goes to smaller companies still running SDI, which is plenty of them, because SDI is far from dead. Much of it goes to auction in mixed lots, listed by someone who does not know what a genlock box is, and photographed badly.

Which brings us to eight hundred and thirteen kronor and a full Hyundai.

What You Can Do With Television Plumbing at Home

The good news is that SDI is wonderfully easy to play with, if you have the bits. Blackmagic in particular sells inexpensive converters between SDI and HDMI, so an ordinary camera or computer can feed into a SDI world, and an SDI signal can show up on a normal TV. Secondhand SDI gear is cheaper now than at any point in history, precisely because of the move to IP.

So here are a few ideas in the spirit of the haul, which means fun first and sense optional. A tiny village broadcast setup, with a couple of cameras feeding a capture card over long BNC cables, so you can run a camera at the far end of a hall without worrying about the signal. A genuine genlocked studio in a garage, just to say you have one, with a sync reference and the satisfaction of perfectly clean cuts. A live feed from a Game Fair arena into a media server, so the announcer's camera appears on a projected screen in real time. Or simply the joy of plugging a BNC connector in with that quarter twist and hearing the little click, which every broadcast engineer will tell you is one of the best sounds in the industry.

There is something quietly moving about this gear. For three decades, SDI and genlock carried almost everything people watched together. Election nights, moon landings replayed, royal weddings, cup finals, the evening news. The cables never got any credit. They lived behind walls and under floors, doing one job perfectly. Now they are being rolled up and sold by the kilo, replaced by fibre and switches that will do the same job differently.

Some of that copper and those little black boxes ended up in a Kona. They spent their working lives keeping television in time. In Kall, they get to keep a village in time instead.