SDI

SDI

SDI is the choice of all professionals on set to send video signals. It is a well-established standard that has been enduring various developments that allowed it to offer lots of flexibility and features. 

SDI is preferred in production environments because of its longer range (up to 90m/ 300 feet) and reliability. It’s typically sent along BNC cabling that has specialized connectors on each end to lock into the devices they connect to.

SDI Types

Common SDI Standards

• HD-SDI
• Dual-Link HD-SDI
• 3G-SDI
• 6G-SDI
• 12G-SDI
• Quad Link SDI

Single-Link, Dual-Link, and Quad-Link

With so many common standards, it's easy to mix up which SDI signal does what. 3G-SDI, 6G-SDI, 12G-SDI, Dual-Link, and Quad Link SDI each have their own respective bitrates and are used to accomplish different tasks with different video formats. Let's try to break down the differences between each:

First, we need to understand the numbers before the letter G in the SDI types (3G-SDI, 6G-SDI, and 12G-SDI).  All the numbers before the G are just referring to bitrates of video. So 3G-SDI simply means it can transfer about 3 Gbit/s of video data. 6G can transfer roughly 6 Gbit/s and 12G can transfer roughly 12 Gbit/s of video data. The higher the bitrate, the more data we can send in a given timeframe.

That is why 6G and 12G SDI are better for 4k video, whereas 3G SDI is only optimized for HD video like 1080. The table above will show you what type of video quality you can expect out of each signal type.

However, suppose you only have 3G-SDI ports on your card. Are we just limited to 1080p? Well, some clever engineers came up with the idea to utilize several ports simultaneously which allows us to send more data through two or more SDI cables instead of one, and then combine all that data into one signal. Think of it as water pipes, the more pipes we have the more water can flow through them. And that is where the terms Single, Dual, and Quad link came from. 

Single-Link: using a single SDI cable at a time. 

Dual-Link: using two SDI cables at a time. For example, Dual-Link 3G-SDI means combining two 3G-SDI cables, giving us the ability to send 6 Gbit/s instead of 3. The same goes for Dual-Link 6G-SDI, being able to carry a 12Gbit/s signal. When we say Dual-Link, we refer to dual HD-SDI signals that combine into a 3G-SDI signal. Note that there are 3 variants of Dual-Link called Level A, Level B-DL, and Level B-DS, so you need to know which one you will be using. 

Quad-Link: using 4 SDI cables at a time. For example, Quad-Link 3G-SDI means combining 4 3G-SDI cables, giving us the ability to send 12 Gbit/s instead of 3.

So, if you're looking to stream 4k video at 60 frames per second, then you really only have three options: Dual-Link 6G-SDI, 12G-SDI, and Quad Link SDI.

Please note that currently Aximmetry does not support Dual-Link, and only supports Quad-Link for a few selected cards.

3G HD-SDI: Level A and Level B

3G HD-SDI uses the double data rate to combine the two Dual-Link cables into one cable. This is done in different ways called Level A, Level B-DL, and Level B-DS:

• Level A is a direct mapping of the video signal into a 3G SDI signal. It supports 1080p: 50, 59.94, 60.
  
• Level B-DL (Dual-Link) simply combines the two signals from the dual-link format into one 3G SDI signal. It supports 1080p: 50, 59.94, 60.
  
• Level B-DS  (Dual-Stream) combines two separate/independent 1.5G HD-SDI signals into one 3G SDI signal. It supports 1080p/1080i: 25, 29.97, 30.

Please note that all of your video production gear should support and be configured to the standard that you plan on using. If your camera output is sending a Level A signal, then your capture card should be configured to Level A as well. Otherwise, in case the capture card only supports Level B-DL for example, then a converter from Level A to Level B is needed.

SDI Limitations

SDI is known to have a longer range than HDMI, but it doesn't mean that you can roll out a 1 kilometer cable and expect it to work. You should also note the better the SDI standard you use the lower the maximum range is. 

Below is a table of the SDI coax cable lengths you can use before you need to boost the signal with a repeater. 

Audio

The SDI standard includes 16 channels of embedded audio with your video signal. That means you get up to 16 mono or 8 stereo channels synchronized with your video.