What You Should Know about 6G-SDI, 12G-SDI and 24G-SDI

Let’s start from the beginning.

What is SDI?

SDI stands for Serial Digital Interface.

On set, its purpose is to help you move uncompressed data around between devices without sacrificing quality.

There are three parts to SDI that you should know about as a filmmaker:

1. The connector
2. The cable
3. HDMI to SDI, or SDI to HDMI convertors

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The BNC connector

The BNC connector stands for Bayonet Neill–Concelman after its bayonet mount locking mechanism and its inventors.

The BNC connector is a crucial component in SDI systems. They’re known for reliability and providing a secure connection.

It is designed for coaxial cable, and its sturdy locking mechanism prevents accidental disconnection. Typically, what they use in video SDI is the 75 ohm variety.

The Cable

The cool thing about the SDI standard is you can have two types of cables:

• Coaxial – 300 meters (980 feet). SDI goes to 100 meters (330 feet).
• Fiber – not limited

As the signal payload reaches 3 Gbps (in 3G-SDI), the strength of the signal drops by 40%! Fiber performs better than copper cables here, but are more expensive.

Still, the long runs possible with SDI is what makes it different from the HDMI standard. The quality of the signal via HDMI can degrade anywhere between 15-50 feet.

The maximum HDMI can run is about 15 meters, which is 50 feet.

SDI can transport your video feed farther, with integrity, in a more secure connection. That’s why they’re still around. Here are the main advantages in a nutshell:

• Long cable length
• Robust BNC connector
• Uncompressed video and audio

Where do you find SDI connectors?

You’ll find SDI connectors, BNC usually, on the following devices specific to filmmaking:

• High-end cameras and cinema cameras
• External monitors, viewfinders and recorders
• Broadcast and grading displays

What are the different types of SDI?

As technology progressed, SDI got better and better, while retaining the same connector. However, it is not backward compatible.

Here are the important differences:

Today, it is not useful to think of anything below 3G-SDI.

You could still use 3G-SDI if all you want to monitor is 1080p. It’s a perfectly acceptable resolution to monitor in.

However, if you’re investing in a monitor, camera or recorder, I highly recommend looking for 12G-SDI and better. The main reason, in addition to future proofing, is color.

Here’s how the color information and data is distributed:

If you haven’t noticed yet, the number before the “G” represents the maximum bit rate of the system. E.g., 2,970 is close to 3, so 3G-SDI. 6000 is 6G, 12,000 is 12G, 24,000 is 24G.

The more data can be transported through the system, the higher the resolution and frame rate. You can be sure there’ll be a 48G at some point to accommodate 8k UHD up to 60 fps.

RAW is a special case, but it has only one-third the data of uncompressed Y’CbCr or RGB. Companies can force RAW out an SDI connector but this isn’t within the official specification. This means, you’ll need a proprietary solution to read a RAW signal.

The beauty of the SDI protocol is you can use any device that complies with the standard. That’s not true of RAW.

The downside of SDI (which is sort of an upside, too!) is it takes time for a standard to be ratified and then take hold in the industry. The SMPTE takes its time.

The upside is that your investments will work for a longer time, giving you better ROI in a professional capacity.

Bottom line, to keep it simple, follow this rule:

Get 12G-SDI or better.

Look for SDI cables here (Amazon, B&H).