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6G or 12G? The Next Format War

By: Chip Bobbert II


For those that work in broadcast or production, the Serial Digital Interface or SDI for short, is king.  It’s the professional standard for AV connections.  SDI has been around since the 90’s and has been updated along they way as the world transitioned from 480i to 480P to 720P to 1080i to 1080P from copper to fiber.  The spec title changed along the way, from 259M to 292 to 424.  259 could carry about 270Mb/s of data with 292 (for 1080i) carrying about 1.5Gb/s.  424 upped the game again to about 3Gb/s.  SMPTE has been the governing body of the standard so this transition has always been smooth.

With 4K on the horizon we’re now seeing two flavors of SDI.  4K and UHD video requires four times the bandwidth of HD video, which itself is almost-2k.  So in theory, for 4K to work, you should need 12Gb/s, after all, 1080P needs 3Gb/s.  The answer to feeding the massive bandwidth requirements of 4K has been to use four existing 424M SDI (informally called 3G SDI) to create 12G SDI.  For single-camera productions this might not be a big deal, just hook up four cables from your camera to your recorder or confidence monitor and call it a day.  But for multi-camera productions, people doing live switching, and the increasing number of engineers in the integration space who are taking advantage of the benefits of SDI, running four cables to each device quickly creates a mess.  So what to do?  Enter 6G SDI.  Rather than relying on four cables (8 conductors) to create 12G, 6G SDI relies on technical improvements in the spec to double the bandwidth.  But 6 is still half of 12, how to they make up the rest of the bandwidth?  Well, they have been cutting into the color sampling to do that.  The result is a little sacrifice in quality for improved integration and simplified engineering.

Who cares?  The issue at the moment is which spec will win, 12G or 6G SDI.  Neither spec has been officially ratified by SMPTE yet but gear using 6 and 12G are already shipping.  Blackmagic for instance, is heavily invested in 6G.  AJA, the other major production capture card manufacturer, is heavily invested in 12G.  So if you’re trying to buy into 4K right now which standard is the most future proof?  Time will tell.

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