Clarifying SDI

May 30, 2013, 22:45 by dliu

Chris Perry, TelVue’s Director of Systems Engineering, gets a lot of questions in the course of his work with many stations across the country.  In an attempt to address some of the more frequently-asked questions, here is his Tip of the Day, on understanding SDI:

Although most of my day is spent dealing with IP video, SDI and analog video transport still plays a very important role in both control rooms and airpath distributions at many facilities.  There are however a few misconceptions when it comes to terminology.

SDI is an acronym for “Serial Digital Interface.”  But saying something is “SDI” doesn’t mean much more than that.  There are actually subsets of standards that fall under SDI, that are standardized by SMPTE (Society of Motion Picture and Television Engineers).

Misconception #1: All “SDI” means Standard Definition, “HD-SDI” means High Definition.

Clarification #1: While we call HD video that is traveling via the SDI standard “HD-SDI,” SDI could refer to both SD or HD video.

Misconception #2: SDI can be run a lot further than analog.

Clarification #2: While there is some truth that you can get away with longer runs without losing quality you can experience other issues.  Over long cables you encounter a higher “jitter“.  Jitter, simplified, is a “delay” in the cable.  This can cause breakup or loss of video.  One way that we fix issues with jitter over long runs is to use a “reclocker” which can fix signals that are having clocking issues.  When buying a router, DA, or other piece of SDI gear make sure that it has reclocking as a feature, especially if you are going to be using it in the field where cable runs can be unknowns.  In terms of distances that you can run SDI, a rule of thumb: 100 meters over RG-6 Cable or 50 meters over RG-59.

Misconception #3: SDI is a standard for video transport only.

Clarification #3:  The SDI standard has made provisions for audio to run embedded in the data stream.  What does this mean for you?  It means that you don’t have to run separate audio cables if you don’t want to.  Real world example: You have a shoot where a camera is 300′ from the switcher.  Instead of running 300′ of video cable, and 300′ of audio cable to that location, you could use an embedder/dembedder pair to carry the audio on the same cable.  Most ENG cameras these days already have the embedding portion taken care of so you can run a microphone right into the camera and have audio embedded on the SDI cable.  With a dembedder on the other end, you could run SDI into your video switcher, and audio into your mixer- all from the same cable.

Here’s a breakdown of the common SMTPE standards for SDI:

SMPTE 259M: “SD-SDI” Standard for SD video, 480i/576i.  This standard is capable of carrying up to ~370Mbps

SMPTE 292M: “HD-SDI” HD standard for 1080i/720p video.  This standard is capable of carrying up to ~1.5Gbps

SMPTE 424M: “3G-SDI” HD standard for 1080p.  This standard is capable of carrying up to ~3Gbps.

Most of us in the industry are working with 1080i over SDI so that would be the 292M specification.  The bitrate measurements are the important part here.  Remember, SDI, being a digital standard, is transmitting video as a series of zero’s and one’s.  Much like your network or internet connections we measure the standards in how fast data can be sent across that link.  HDMI, although a very different standard, can also be measured in terms of data speeds (technically HDMI is capable of carrying up to 10Gbps over short distances).

So what about 6G SDI?  Black Magic Design made a big splash at NAB this year with 4K and 6G SDI.  While the standard has yet to be ratified, what does it mean for you?  In all likelihood, nothing in the short term.  I’m not saying that 4K won’t EVER happen, but there are some distribution issues with 4K.  First of all it takes a ton of horsepower to encode 4K to a transmission format, and a similarly large amount of computational power to decode.  Secondly, 4K TV’s are not widely available yet, and those that are tend to be very expensive.  Overall, I’d put 4k and 6G SDI into the “wait and see category.”

Having done a number of installations in the field where I’m working with SDI here are a couple of notes:

-Although it’s not ideal, you can run SDI over analog cables.  If you’re testing or need a quick fix, you could give your analog cables a shot.

-Look for cables that have the “3G” on them.  I work exclusively with Belden cable, but Canare and Gepco also make great cables.  Belden part number 1505a and 1694a are two common cable types; RG-59 and RG-6 respectively.  The 3G label means they have been frequency swept and proofed for high bandwidth use.

Finally, a few notes about “HD Analog.”  While the two seem contradictory, it’s possible to carry HD video over analog cables!  This standard is slowly fading away, but there are still some examples of it in the field with encoders, DVD players, and TV’s.  HD is sometimes carried on three analog cables, Y, Pb, Pr, or RGB.  This is where they breakout the chroma (RGB colorspace) from the luminance (Y, Pb, Pr), Where Y is luma, Pb is the difference between blue and luma, and Pr is the difference between red and luma.  When these three signals are combined you have the full RGB colorspace and thus a complete color picture. 

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