Thứ Sáu, 13 tháng 9, 2013

What is 3G SDI?

The 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.


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" 


How We Got Here

Initially, a standard HD-SDI signal had a data rate of 1.5Gb/s. Due to the data rate
limitation this meant signals had to be at a frame rate of 24, 25 or 30 frames per
second3 and a bit depth of 4:2:2.
There was a need for higher frame rates and also higher bit-depth, but since the
standard HD-SDI signal was maxed out, a new solution was needed. This lead to the creation of the Dual-Link specification4, which links two standard HD-SDI signals together -- essentially doubling the bandwidth. Each signal carries a separate 1.5Gb/s 4:2:2 signal.

This can be used several ways. First, higher frame rates can be achieved by sending alternating video frames down each HD-SDI cable. This works similarly to interlacing within a video image. The alternating frames are chained together on the receiving end into a 50 or 60 fps signal.

Alternately, the two HD-SDI signals can carry higher bit depth information. By splitting the data between the two cables, more information per frame can be carried. This is usually referred to as Dual-Link 4:4:4 and can be either YUV or RGB. The advantage of YUV 4:4:4 is that the primary channel is carrying a standard YUV 4:2:2 signal, which can be plugged into a standard Single Link device.

Soon after, the ability to handle up to 3Gb/s on a single cable (called 3G) was
introduced.5 This made the need for two cables obsolete, since the higher frame rate and bit depth signals could now be carried on a single cable. When used to carry higher frame rates such as 1080p 50/60, the signal is referred to as Level A. When used to carry higher bit depth signals, it is referred to as Level B.

In addition, since 3G could carry twice the information itʼs also possible to combine two separate 1.5Gb/s 4:2:2 image streams into a single 3G stream. This is referred to as
ʻDual Stream 3Gʼ. Dual Stream 3G is particularly useful in stereoscopic production
where separate left and right eye information is being captured.

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