SDI is the acronym for Serial Digital Interface. It was first standardized by The Society of Motion Picture and Television Engineers (SMPTE) in 1989.
It’s a way to connect video devices to each other via coaxial cables and what’s called BNC (Bayonet Neill–Concelman) connectors to move digital signals from one point to another. The SDI standard provides for video signals as well as multiple channels of embedded audio on a single core wire using bit serial transmission.
Since its standardization in 1989 updated versions of SDI have emerged to accommodate growing video resolutions and corresponding data rates.
Here is a timeline of the current standards being implemented:
- SD-SDI (SMPTE 259M) ——————1989 Standard TV resolution
- HD-SDI (SMPTE 292M) ——————1998 HD resolution both 720p and 1080i
- Dual Link HD-SDI (SMPTE 259M) —2002 HD resolution 1080p
- 3G-SDI (SMPTE 424M) —————–2014 HD resolution 1080p (higher data rates)
- 6G UHD-SDI (SMPTE ST-2081) —–2014 4K 30p resolution
- 12G UHD-SDI (SMPTE ST-2082) —2014 4k 60p resolution
- 24G-UHD-SDI (SMPTE ST-2083) —2015 4k 120p resolution
So we have a progression from standard definition video, to ultra-high definition video all using the same connectivity standard of BNC.
The common coaxial cable versions run anywhere in length up to under 300 meters (over 900 feet). Fiber optic cable versions used with the 297M standard as an example, can have cable runs only limited by cable length, or the use of fiber optic cable repeaters.
There is a great deal of technical information about the standard that involves resolution and color bit depth and more. Images either uncompressed or compressed. All that data gets parsed by engineers into packets of digital information and depending on how much data needs to be transmitted, is what dictated the development of a standard to accommodate that.
Cameras using SDI are connected to recorders and or switches that have SDI built-in. Whether you’re in a small conference room with a single camera on one end of the room and a recorder/switcher on the other end, or you’re in a giant football stadium with many cameras running a live signal to a control room or mobile truck the same SDI standard is implemented to make the happen.
The many other parts used outside of just the cables and connectors are what your video professional is trained and experienced to manage for you. It can be simple or very complicated depending on the requirements of each job. SDI is simply the standard that connects all the pieces needed to make it all work.
There are other standards like Video over IP (NDI) that are waiting in the wings that have been proposed. It’s already been rolled out in limited situations. The new standard is supposed to overcome limitations of SDI and accommodate more flexibility in the workflow. In theory it’s all good but the ability to implement new technology to accommodate all that’s required to do the many jobs there are to do will make it a slow going process. One of the main things to consider is that a video professional is not just an entity unto himself, he/she/they will on occasion work with clients who themselves have video elements that must be interfaced with.
One of the main limiting factors that can slow the adoption of new standards is that broadcasters and people who create content at the highest levels of the business in places like NY and LA is budgets. Not just in major media markets but in smaller markets as well. As a professional video organization the idea of re-wiring your whole set-up in the face of shrinking budgets is a non-starter, technical arguments not withstanding. In the meantime SDI works very well. That’s the good news of a standard that’s been tried and tested and improved over time.