Last Updated on October 8, 2020
For the majority of the population, the concept behind broadcasting is pretty straightforward: you take good photos and videos and broadcast it online or through the television. But for anyone involved in the production itself, a successful broadcast is the culmination of a series of processes involving different technological advancements.
Serial Digital Interface (SDI) is considered the standard for digital video transmission as set by the Society of Motion Picture and Television Engineers (SMPTE). 270 megabits per second (Mbps) is the most common data speed of SDI signals, but it can go as high as 540 Mbps when a 75-ohm cable is utilized. But what about 3G and 4G signals? What do they mean? These terms are actually easy to understand. The number before the G refers to the bitrates of video that it’s capable of transferring.
For instance, 3G-SDI simply means that the port is capable of transferring 3 gigabytes of video data per second. If you relate this to shooting in 4K, you will need a stronger signal if you want to shoot higher quality videos. 12G SDI ports are especially designed for 4K videos while 3G SDI is optimized for HD video. If you want to stream in 4K, you need to use either 6G-SDI, 12G-SDI or Quad Link SDI. Although there are still no standardized rules for broadcasting in 4K, production teams are now becoming more creative in using different SDI ports to create a 4K signal that offers the best broadcast or streaming quality. Here, we’ll take a closer look at these different 4K signals and how they are created and utilized:
This 4K signal is almost similar to Quad-Link HD-SDI, the only difference being that the signal is transmitted through Dual-Link 3G-SDI. To create this type of port, you have to use 2 BNC cables supporting 3Gb/s of bandwidth for 3G-SDI. Dual-Link 3G-SDi is capable of transmitting up to 10-bit 4:2:2 4K at up to 30p and it is preferred by a lot of producers because they are less cluttered than Quad-Link ports because they only use two cables instead of four. Since Dual-Link 3G-SDI is not yet supported by 4K cameras, you’ll need to use a converter box to convert these outputs to and from different 4K formats.
This type of signal can also be converted to HDMI 1.4 using the Blackmagic Mini Converter SDI to HDMI, 6G-SDI using the Blackmagic Multiplex 4K and Quad-Link HD-SDI using the Blackmagic Multiplex 4K.
Employing the same concept as Quad-Link HD-SDI, this type of 4K signal is capable of transmitting 10-bit 4:2:2 4K for up to 60p or 12-bit 4:4:4 for up to 30p. The only difference between the two is that instead of HD-SDI, this port utilizes 4 3G-SDI signals. To create Quad-Link 3G-SDI, you will need to use 4 75-ohm BNC coaxial cables supporting at least 3Gb/s of bandwidth.
This type of signal is utilized mostly in post production because of the extra color information of a 4:4:4 signal that’s useful in this process. Although no cameras support the 4:4:4 signal yet, you can use a capture card to transmit it. Unlike Dual-Link 3G SDI, there is no way to convert a Quad-Link 3G-SDI signal just yet, but it can be used on any device that’s capable of doing Quad-Link HD-SDI.
This is the most popular SDI port used in transmitting 4K signals and it utilizes 4 1080p signals stitched together to create a strong 4k signal to ensure top quality 4K videos. HD-SDI ports are recognized by the SMPTE for transmitting 2k and 1080p videos, so they are the best cables to use for creating a Quad-Link HD-SDI port that’s capable of transmitting 4K videos.
To make this possible, 4 separate 75-ohm BNC coaxial cables need to be used and you have to ensure that they support at least 1.5GB/s of bandwidth for them to work on your Quad-Link HD-SDI. This port is also capable of supporting DCI 4K and UHD 4k signals, and it can transmit a maximum video quality of 10-bit 4:2:2 at 30p. Quad-Link HD-SDI is used by most professional broadcasting production companies because of its reliability in delivering excellent quality 4K videos.
The use of SDI ports in transmitting 4K signal still has a long way to go. As we wait for SMPTE to create a standard for using 4K signals in broadcasting, technical teams are relying on their creativity in making sure that high quality videos are transmitted for viewers to enjoy.