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What is 3G SDI?

By Helmut Lottenburger, DI Specialist
November 2010

3G SDIThe term "3G SDI" is now frequently seen in video production environments . Video equipment manufacturers like AJA and Blackmagic Design have recently released not only "3D" but also "3G" versions of their products and utilise the "3G SDI" term.

Generally spoken 3G-SDI is a successor technology to the well known and popular HD-SDI interface. It uses the same connectors and coaxial cables and is also completely downwards compatible. In a nutshell, it provides an increase in bandwidth which allows both higher resolutions and frame rates through a single connection. 

"Traditional" HD-SDI is a global standard, specified in SMPTE292M and allows HD video data to be transmitted as 10bit YUV (correctly: YCbCr) at 1.485 Gbit/s. Ancillary data such as time code and audio are embedded in the signal.

Higher picture quality and frame rates such as RGB 4:4:4 or 50/60p can be realized through dual-link SDI connections. Dual link is also used for Stereo3D playback and record of two separate video streams in full resolution.  

For broadcasters, standard HD-SDI has disadvantages when working with Stereo3D and high frame rates. The signals need to have two connections (dual-link SDI) which increases requirements for video routing, cabling, monitoring and ingest.

For post-production, an additional disadvantage is the limitation to a resolution of up to only 2048x1080 at 24fps. If full aperture 2K (2048x1556) needs to be transferred, a non-video data connection known as "High Speed Data Link" or HSDL is used which commonly runs at about 15 fps. Although HSDL uses the same dual-link interfaces it is generally non-realtime and not suitable for display purposes.

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3G-SDI (SMPTE424M) effectively doubles the bandwidth to 2.97 Gbit/s. Video data that previously needed two SDI connections can now use "only one cable" and images which couldn't be handled in realtime at all can now use dual-link 3G-SDI.

A lot of applications interface with a video card and support whatever resolution and frame rate that specific card provides. The ability of Final Cut Pro to capture HD at 50fps for example is determined by the utilised video board.

If you are looking into upgrading your existing editing workstation, you should consider your future need for higher frame rates and resolutions. This is especially true for SDI converters and routers which are generally more expensive if "3G enabled"