No, I wasn’t planning to discuss Ronald Reagan’s Strategic Defense Initiative, also known by the acronym SDI. Instead, I’ll discuss a more contemporary use of the acronym as it applies to our industry, and rest assured, it has nothing to do with ballistic missiles.
SDI stands for Serial Digital Interface, and as defined by two important SMPTE (Society of Motion Picture and Television Engineers) standards, SDI is the most prevalent video interface used for high quality video transmission today.
• The SMPTE 259M standard defines the specs for SD-SDI, the standard definition serial digital interface.
• The SMPTE 292M standard defines the specs for HD-SDI, the high definition serial digital interface.
In brief, both of these SMPTE standards define how to transmit digital video signals from point to point. The signal itself is digital component video, comprised of a luminance channel (Y) and two chrominance channels (Cb and Cr). These three components are digitized, packetized, and shot down a coax cable, carrying a payload of pictures within the packets. When you’re sending digital video between devices, for example, from the camera to the video switcher or from the media server to the projector — more times than not, SDI is the preferred way to go.
Welcome to the Cliff
For all of us in the biz, the more important question is — what’s inside that coax cable, and what do I need to know about it? Is it an SDI signal, with ones and zeros heading down the cable? Is it an analog signal, with smooth waveforms traveling back and forth?
There’s no way to tell, unless you look at the target devices themselves, and even then, most modern digital devices output analog signals alongside the digital signals — typically for purposes of monitoring or displaying a “burned in” timecode window. Only the label on the input or output connector itself is the definitive answer.
So, you grab the coax, uncoil it, and connect the digital device to a monitor. After turning on power, you get — nothing. What gives? Doesn’t troubleshooting 101 apply to SDI signals? Yes, it does, but there are certain key differences, one of which is the “cliff.”
Take a given length of coax, plug it into the output of an analog VTR, for example, and connect an analog monitor at the other end. If you lengthen the cable, adding more resistance, the signal level gets progressively worse and worse — but it’s still visible (somewhat).
Now, take the same length of coax, plug it into the output of a digital media server and connect a digital monitor at the other end. Here, as you lengthen the cable, the signal remains pristine — after all, weak ones and zeros are still ones and zeros. However, at a certain point, lovingly called the “cliff,” the SDI signal vanishes. In fact, your image will start to sparkle the last foot or so, and about a millimeter beyond that, it’s toast.
The reason is that, after the distance rating of the cable is reached, the “clocking” pulse that essentially drives the digital signal down the cable isn’t strong enough to generate ones and zeros. This rating varies from cable manufacturer to manufacturer, but for SD-SDI (using Belden 1694A Brilliance® coaxial cable) this distance is approximately 300 meters, while for HD-SDI, the cliff is reached at approximately 100 meters. For a longer cable run, you’ll need to re-clock the signal.
Cables, Barrels and Connectors
For the health of the video signal, higher quality video cable yields a better distance rating and less resistance than inexpensive cable. For digital devices, you will not be able to use that old 50-ohm cable from the warehouse — the same cable with which you’ve always hooked up your VHS decks. In addition, you’ll need to be 75-ohm throughout the path — including the cable itself, the connectors on each end (rated for > 1.485 GBPS) and any BNC “barrels” that you might need to connect two or more lengths of cable together.
High quality cable is also less prone to electrical interference, enabling you to run it alongside power cables, if required. You won’t have (as many) problems with hum bars and ground loops, as with lower quality cables.
Professional audio/video cable manufacturers such as Belden (www.belden.com) provide very detailed specs on all of their coaxial cables — and if you have the budget, pay the extra bucks for the company’s digitally rated Brilliance cable. In addition to being more durable, it provides a better center copper conductor, better shielding and a far better distance rating.
Something Else Is Hiding In That Cable
Another useful fact about SDI signals is that both the SD and HD signals can carry up to 16 channels of embedded audio within the serial stream — essentially, eight stereo pairs of audio traveling in sync with the video.
Certain digital devices such as servers and DVRs handle the embedding and de-embedding of the digital audio themselves, but you can also use external embedders and de-embedders, such as those manufactured by AJA Video Systems (www.aja.com) to inject audio into an SDI stream.
The use of embedded audio is a mainstay at television stations worldwide, enabling facilities to eliminate the cost of miles of audio cables. In addition, because professional broadcast routing switchers also pass the embedded audio, the stations don’t need to purchase a separate audio router for embedded signals.
But the use of embedded audio is not strictly limited to broadcast. In fact, many rental and staging companies are currently making use of the technology for shipping audio (plus video) from point to point at an event. For example, you could use one SDI cable to send video and up to 16 audio channels from FOH to the video world backstage.
Or, perhaps, you could send digital audio and video from a main ballroom to one or more satellite rooms on site. Provided that the additional gear is on hand (such as digital embedders and de-embedders), the use of SDI with embedded audio could be a very handy addition to your tool box.
A Final Note
The beauty of the digital signal is that it is purely ones and zeros and it’s either received perfectly or it’s gone. The same signal that leaves a digital device enters the next device, for example, from server to projector. And you can’t get any more efficient than a single cable, eliminating the need for three-wire and five-wire complexity.
For SD-SDI and HD-SDI, keep in mind that since SDI is a 4:2:2 signal and not a 4:4:4 signal, the two color components are compressed. It comes down to your application. If you’re dealing with high resolution graphics, DVI is still the way to go. If you’re dealing with motion video, PowerPoint, camera feeds and server playback, SDI is an excellent option.
One more note — if you’re at all unsure about what’s in that video cable and where it should be connected, leave it to the video guys at the event.
If you’re a major video geek and interested in a more in-depth explanation of the bit rates, synchronization packets, data formats and other incomprehensible high-tech jargon related to SDI, you can visit www.smpte.org/standards, or search for Serial Digital Interface on Wikipedia.
