So you set up your rig and the projector tells you that no signal is detected. Better to fail now than in the middle of the event. You check your source selections; they’re good. You check your outputs; they’re good. You cable test the lines; they’re good. You realize you are trying to send HD-SDI down a 350-foot run of regular RG-59. That could be the problem. The variety of video signals we work with today provide us with unlimited opportunities for Mr. Murphy to rear his ugly head and remind us how much he really doesn’t like us.
So here are some tips to remember so that when Señor Murphy gets himself on the call list you can send him packing.
Setting The Standards
A typical analog composite video signal will be approximately 4.2MHz or below. Analog HD signals are about 25MHz. An XGA signal will be around 40.7MHz. So there is about a 10 fold difference between a composite video and an XGA signal. That also means that the impedance of the cable will be considerably higher for the XGA signal.
So what happens when we go to digital signals?
One of the precepts behind today’s formats was ease of upgrade and integration by using existing cables and connectors to make the jump to digital. But this kind of came back on us. Yes, you can use regular RG- 59 for HD-SDI runs, but you had better make them short or line loss will haunt you. (BTW, RG stands for Radio Guide. It is the part of the military specification for transmission lines. The current spec is MilSpec-C-17 if you want to go read it!)
SDI will be either 270Mbs (4:3 aspect ratio) or 360Mbs (16:9 aspect ratio). HD-SDI will be 1.5Gbs for its variations. For exact numbers and variation, please refer to SMPTE-292M.
There is also a standard referred to as Dual Link that is typically 3Gbs and is being used most commonly in digital cinema. If you can set up to cover this format, you should have few, if any, problems transporting signal.
Personally, I think we have reached a plateau at 1080p. I know we can get higher resolutions and faster clock speeds, but I believe the industry will hold here and wait for everyone to catch up. I am also holding out for the day that we transfer all this data via fi ber optic cable like the audio guys do. Lighter cables, less line loss – it’s a good thing, right?
Encountering Resistance
Meanwhile, back at the ranch we have our switcher set up at FOH, we have our 150- foot run to get backstage, and we have another 100-foot run to get up in the air and across to the projector. So how much line loss do we have? As an example, the impedance of a typical RG-59 cable is about 10 to 12 ohms per 100 feet with a digital signal at 1GHz. We have a 250 foot run with one break. We have run video 350 feet in the past with no problems, so why are we having problems today? Most likely because of return loss caused by the connectors in the line at the break. If this was one continuous run, we might get away with it.
We could also get by using RG-6 instead. At 1GHz, RG-6 is typically 7-9 ohms per 100 feet. We are able to sneak through enough signal to make pictures. The signal will be down 27dB because of the losses but hopefully we can pull it up and have decent images.
Better still would be to use RG-11. At 1 GHz, RG-11 is typically less than 5 ohms per 100 feet. We’ll have pretty pictures and no grief. But if we’re going to stick to RG-59, we would typically install some kind of buffering amp with an equalizer to help compensate for the line loss. Or we could install a booster amp on the front end and simply force the signal down the line. All of these options involve more connectors and active electronics that add noise to the signal path. Your best bet for the best image is short runs without breaks.
Wouldn’t it be great if we could just stick with plain vanilla RG-59 and not have to worry about line losses? It might be possible. The impedance of the line depends on a number of factors other than frequency. I have heard rumblings about manufacturers coming out with RG-59 rated for HD-SDI or 1GHz RG-59. If I fi nd out more or I can get a sample, I will let you know.
Whatever Can Go Wrong. . .
In the meantime, remember Mr. Murphy. He likes to eat connectors. Keep lots of spares on hand along with strippers and crimpers. I am stunned at how many people think that you can just pull the shielding off and squeeze on a new connector. That’s like saying you can solder with 50 pounds of lead and a blowtorch, which pretty much describes how I solder! Leads need to be cut to the proper length. Ends need to be clean. Crimps need to be set to the right depth using the right dies. And none of that guarantees that the connector will behave properly. RF signals are sensitive to all of these factors. A bad crimp can leak RF and now you and the audio guys (face it, there is very little sound about these creatures!) are fighting it out.
Cable manufacturers will have a connector or can at least recommend a connector to go with their cable. Many often sell the crimps, dies and strippers as a kit to go with the wire. Having these on a jobsite has saved my fat from the fire on more than one occasion.
It’s really a case of the right tool for the job. With this rig set up back stage, we probably will not have any issues. Intercom is easy to run if needed. And make sure there are spares on hand.
