4K Video Over 6G-SDI Connections

We began seeing the first products supporting 6G SDI announced at NAB this year.  I heard this was coming, but didn’t fully grasp the significance of it until looking around the show floor.  Hardly anyone is using dual-link 3G connections, presumably for 1080p60 in full RGB, and basically all 4K work is done in RGB, so going from four cables to two isn’t that helpful.  But broadcast applications with QuadHD frame-sizes is where this new technology is going to be key.

When 3G SDI was released, I wasn’t too impressed with it.  Few people were even running dual link HD-SDI, so there wasn’t a very wide need for it yet.  It doubled the bandwidth of the existing standard, but that paled in comparison to the 6x increase of the jump to HD-SDI from the original SD version.  3G allowed 4:4:4:4 RGBA data in a single cable, or 60p frame-rates at 1920×1080.  While I use the RGB feature all the time, I still have yet to use anything at 1080p60.  Multi-stream 3G connection are frequently used for 4K video, over 4 separate 3G SDI connections.  Once you have more than one cable, they have to be well labeled to not cross connect them, and they are liable to get tangled.  The jump from two to four cables isn’t that big of a deal, in a relative sense.  I have never heard of anyone doing 4K in 4:2:2, over dual link 3G, because anyone currently shooting 4K is interested in maximum image quality.  But it would seem that is about to change.

Broadcast found 4:2:2 processing to be sufficient during the transition to HD, and they are about to do the same thing for QuadHD.  Quad HD is four times the resolution and data of HD, and 6G SDI is four times the bandwidth of standard HD-SDI.  So all of the rules that apply to HD-SDI, apply to 6G SDI at 4K.  A single link can run 4:2:2 at 24-30fps.  A dual link can run up to 60fps in 4:2:2, or up to 30 fps at 4:4:4. (Similar to 2K running at 24 anywhere HD runs at 30, 4K runs at 24 anywhere QuadHD runs at 30.)  Most television studios are wired to pass video data around over a single BNC cable.  The walls and racks are already wired for single link video connections.  With the advent of 6G SDI based gear, those same wires can now be used to run 4K data, as long as the devices on either end are replaced.  So all of those existing infrastructure items can still be used in a new 4K environment.

Blackmagic-Design is the first company to really run with the 6G SDI thing.  But with the wide variety of products they offer, one could conceivably run an entire studio on their upcoming gear.  The 4K “Digital Cinema Camera” doesn’t actually shout Cinema spec 4096, but broadcast spec QuadHD 3840 wide.  The 6G SDI output from those cameras can be sent to their new ATEM 4K live mixing switcher, mixed with graphics from their UltraStudio 4K, and then recorded on their Hyperdeck Studio Pro.  I would imagine that this complete solution will drive some of their products into places they never would have otherwise been considered.  It will be interesting to see if they ever get to the point of rivaling Sony and Panasonic’s complete solutions.

It should be noted that RAW images are single channel, requiring much less bandwidth.  RGBA signals have four full resolution channels in HD or 2K at 3G data rates.  So re-tasking those channels to 4 quadrants of a single channel QuadHD or 4K image, allows it to be transmitted without compression over a single 3G SDI connection.  This is how the Canon C-500 connects to external recorders at 4K over a single BNC cable.

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