GOING THE DISTANCE WITH SDI

GOING THE DISTANCE WITH SDI

The Serial Digital Interface (SDI) standard was first introduced back in 1989 and quickly became established as the digital video interface of choice for broadcast and professional use. One of the main reasons for the success of SDI was the fact that it used the same coax cable and BNC connectors as the analog video standard that had preceded it. This meant that broadcasters could make the upgrade to digital video without having to completely rewire their facilities or replace all their cables and patch panels, thus reducing the cost of entry.

The original SDI standard has since been joined by HD-SDI and 3G-SDI, which add HD capability, and more recently 6G-SDI and 12G-SDI, which add Ultra HD capability. Each of these new standards builds on the last while retaining the same BNC connectors and coaxial cable.

It would be easy to fall into the trap of thinking we could simply replace our SD equipment with HD or Ultra HD equivalents and keep all our cabling infrastructure the same, but in practice it is not always so simple. As video resolution has increased from SD to HD to UltraHD, the SDI bit rate has increased dramatically from 270 Mb/s to 11.88 Gb/s, which in turn places increased demands on the cabling infrastructure that we use. In practical terms this greatly reduces the cable lengths that can be used, and some older or lower cost cables may not be usable at all.

If we take a look at the technical specifications for a typical piece of broadcast equipment, we will usually find a table indicating the maximum length of cable that can be used for each supported video standard. For example:

 

Bit Rate

Max. Cable Length (Belden 1694A)

270 Mbit/s

250 m

1.5 Gbit/s

140 m

3 Gbit/s

80 m

 

The table lists the performance that can be expected with a particular type of cable, in this case Belden 1694A. However, even these figures do not tell the whole story and practical cable runs are often significantly shorter.

The actual length of cable over which SDI can be successfully sent and recovered depends on a number of factors including:

type and quality of the cable

number of joins in the cable run

kinks or damage to the cable

quality of the BNC terminations

quality of the equipment at each end of the cable

 

Any of these items can cause signal degradation and in combination they can cause a significant reduction in the usable cable length. There are two underlying issues at play here: cable losses and impedance mismatch.

 

CABLE LOSSES

An ideal video cable would transfer all of the SDI signal power from the source equipment to the destination with no losses, but in the real world, DC resistance and parasitic capacitance act together like a low pass filter. As the cable length increases, the signal amplitude is reduced and the higher frequencies roll off, causing significant degradation to the video signal. Because high frequencies are attenuated the most, HD and UltraHD signals are affected much more than SD. Cable quality really does make a difference here so high quality, low loss cables should always be used for HD and Ultra HD unless you only plan to go a few meters.

 

IMPEDANCE MISMATCHES

To efficiently transfer the SDI signal from one end of the cable run to the other also requires a uniform 75 ohm impedance along the cable length. Any variations in impedance result in a “mismatch” that causes some of the signal power to be reflected back down the cable in the opposite direction. This reflected signal power does not reach the destination equipment, so the usable cable length is reduced. Impedance mismatches can be caused by poorly designed equipment, badly terminated connectors or even a crushed, kinked or damaged cable.

 

MAXIMIZING USABLE CABLE LENGTH

Follow these simple rules of thumb to maximize usable cable length:

use good quality low loss cable

use good quality connectors

repair or discard poorly terminated, kinked and crushed cables

avoid joins in the cable

limit the use of patch panels

 

EXTENDING SDI TRANSMISSION DISTANCE

In some applications an SDI video signal just cannot reach far enough. You might have an Ultra HD camera on the far side of a sports field, or video monitors in far flung halls of a convention centre. In these instances another solution is required.

 

The range of an SDI signal can be doubled by inserting a good quality reclocking distribution amplifier(DA) in the middle of the cable run. The DA includes an equalizer, which compensates for cable losses at the input, and a reclocker which regenerates the video clock to remove jitter from the video signal. The output of the DA is a fully restored version of the degraded signal at its input.

 

For even longer distances the SDI signal can be transmitted over fiber optic cable. A small SDI to optical fiber media converter is used to convert from electrical to optical format without altering the video information in any way, and at the other end of th

e fiber another media converter is used to convert back to coaxial cable. The optical signal can be sent over several kilometers of fiber with no loss of quality.