Skip to main content

Wire in broadcast

Wire is one thing every media facility has, and it usually has several different kinds. Fifty years ago, RG59 coax was all a broadcaster needed. Monochrome analog video didn't have frequencies above 4.5MHz, and the graceful roll-off of high loss cabling was usually hard to detect because facilities were much smaller, requiring shorter runs. For long runs, low loss cable was available, but it wasn't important until the early 1960s when NTSC and PAL began to take hold. To carry color information, subcarriers were layered on the composite signal at the upper end of their spectrum. It was important to control loss in cabling, so manufacturers designed cables for the application.

When the broadcast industry moved toward a digital future, that change was huge. The maximum frequency in a signal no longer made much sense due to the digital nature of the signal, but it is generally accepted that a cable needs to operate at twice the bandwidth of the fundamental of the signal. Initially, that meant that cables for SMPTE 259M had to support twice the composite digital bit rate of 143Mb/s (about 300MHz), or twice the component digital bit rate of 270Mb/s, which would require nearly 0.5GHz.

At the frequencies needed for SD signals, the cable is an RF transport. At HD bit rates (starting at 1.485Gb/s), the frequency is in the microwave band. 1080p signals transported on SMPTE 424 at 2.97Gb/s need systems capable of passing 6GHz. Satellites work at lower frequencies.

Frequency variations

Clearly some things are different at those frequencies. Using true 75V connectors is no longer a suggestion; it is a necessity. Properly handling cable is critical. Figure 1 shows what happens to the performance of a cable when a series of deformations are made at repeating 10ft intervals (return loss is shown).

At the frequencies involved in analog systems, all the same effects happen, yet in many cases, the results are nearly impossible to find. But now the impairments can cause an entire system to demonstrate bit errors that seem to be inexplicable and could affect all of the cables in a bundle.

How far one can send a signal in a facility depends on the signal and the type of wire. SMPTE 292 specifications show a loss of 20dB at half the clock frequency as the nominal acceptable value and a return loss value of 15dB. According to SMPTE 292M-2006, the return loss “shall be greater than 15dB over a frequency range of 5MHz to the clock frequency.” Modern hardware can equalize for considerably more loss, but if you leave headroom, it is wise to stick with this guideline. The actual distance a cable can perform is related to the loss of the cable, and different types of common cable are shown in Figure 2.

Note that miniature coax approaches the loss specification much quicker and is not even rated at HD frequencies. Neither is precision coax, which is suitable for carrying high-quality analog signals. But RG59 dual-shield coax is rated up to 3GHz. Carefully read the specifications on any cable to be sure it is suitable for the application. The options expand for short cable runs, but when it comes to long runs of more than 50 percent of the capability of the interconnect system, it is critical to select the right product and ensure it is carefully installed.

It's not just coax anymore

As IT-based systems invade broadcast facilities, it is important to learn all aspects of that technology. The instructions for installing coax are similar to those for installing high-quality, high-bandwidth data systems, but they are not the same. The cables are not as mechanically robust as thicker coax cables. Pull strength limitations of the cable make it important to use less force when pulling it into location. Bend radius is key. If cables make too sharp of a bend, the association between pairs can be disrupted, seriously affecting performance. Cables with bonded pairs help, but proper installation is critical.

With GigE becoming prevalent in broadcast plants and 10GigE increasingly being used, an engineer might wonder if he can reuse existing Cat 5 cabling. It depends on how long the cable is and how it was installed. Cat 6 cabling should be installed instead. Cat 5 cabling nominally is designed for about 100MHz, while Cat 6 should handle 250MHz. Cat 6a extends to 500MHz and is suitable for 10GigE. As with the coax discussed earlier, these applications push the interconnection into the domain once only used for RF transmission. The technology of both the wire and the receivers/transmitters has come a long way to make this possible.

We are approaching the point where the bandwidth of the signals transmitted around a plant will require fiber for transmission over anything more than short distances. SMPTE 424 (2.97Gb/s) works on the best coax for only 220ft (usable range of just over 100ft), and some coaxes will provide well under 100ft of practical interconnect. The next set of standards, for 4:4:4 coded 1080p signals among other signal types, carries 10Gb/s, a rate impractical for copper interconnection (see SMPTE 435). Our next jump will truly be to light speed.

John Luff is a broadcast technology consultant.

Send questions and comments