Copper vs. Fiber in the Broadcast Space
十一月 11, 2014
Fiber optic cable was first offered for sales in the early 1970’s, and ever since its introduction, designers and integrators have said “THIS is the year for fiber! Copper is dead!” And every year since then, copper sales have gone up, and fiber sales have gone up too. So is copper done? Is it finally over?
First of all, there are the classic reasons why you would want to use fiber instead of copper (and by copper I mean coax cable or twisted pairs). The first is distance. There is no question that fiber will go farther than any copper cable. Even the short-run fiber (multimode) will easily go a couple of kilometers (more than a mile). Long-haul fiber (single mode) will easily do 20 kilometers (more than 12 miles) and really is unlimited in its effective distance.
If you’re a broadcaster, it’s pretty rare that you have to go over a mile so copper will often do the distance. But, as signals get higher and higher in frequency, like digital video, the effective distances get even shorter. So, for many signals, the line between copper and fiber is, at best, a moving target. Table 1 shows a list of the various signals that might be used in a broadcast facility and the limit of these signals on copper cables. If you pass these distances, you’re in fiber territory.
|Analog Audio||20 kHz||5,000||1,500|
|Analog Video||6 MHz||3,000||1,000|
|Digital Audio||25 MHz||1,000||300|
|Standard Definition - Digital Video||270 MHz||3,000||1,000|
|High Definition - Digital Video||1.5 GHz||1,000||300|
|1080p/60 Digital||3 GHz||600||200|
|4K (proposed)||12 GHz||500||150|
There are a number of factors not apparent on the table above. For instance, the longest distance for a video signal will be on the largest coaxial cable available. This may be expensive and hard to install. For analog audio, distance is determined by internal factors (the capacitance of the cable) and external factors (the source impedance). It’s often true that you would not want to run a cable to this maximum distance, but might pick a “half-way” point, a “safe” distance. The distances shown here are a “reasonable” maximum. Specialty cables might go somewhat farther.
You can see, as we approach 4K video (and 8K coming after that – not shown in the table) that the effective distances are getting dramatically shorter. Is this the end to digital video on coax?
Of course, if you are an installer or end-user, the box you buy will dictate how it is connected. If the box has fiber connections, then fiber is what you will use. If it has copper connectors, then appropriate copper cable is what you will use. To convert from electrons (on copper cable) to photons (on fiber optic cable) is an expensive proposition, for digital video, currently costing around $500 for each end of the cable. That is $1,000 to get a signal to work on a given cable. A thousand dollars will buy a lot of copper cables and even a lot of fiber. But the fanciest coax or twisted pair cable is probably the same price as, or pretty close to, a fiber optic cable itself.
This is why coax continues to live. It is a “known technology” having been around for almost 80 years. It is easy to put in, with many connector and tool choices. Its reliability and quality can be excellent and, if installed in a single building (such as a TV station) the inherent limitations are easily accommodated.
The second reason someone would use fiber is its immunity to noise. Since the signal is carried by photons and not electronics, that signal is unaffected by electromagnetic effects, sometimes called EMI or RFI, on that cable. This is especially valuable if your signal is running through something with a lot of radiated noise, such as a broadcast transmitter or antenna tower.
The third reason for fiber is the small size and light weight of these fibers. However, if you’re building a broadcast truck or something like that, you will have to add in the weight and bulk of the converters/adaptors. But if you’re looking for something thin or light, fiber is hard to beat, especially when you consider that a coax or twisted pair that is the same size and weight is very distance-limited.
Both fiber and copper cables come in various fire ratings, the most common of which are “riser”, which allows cables to go vertically between floors in a commercial building, and “plenum” intended for use in drop ceilings or raised floors. The purpose of plenum cable is to not be the fuel for a fire. And this also allows you to use these cables without the need for conduit. (Some areas, such as Chicago and Las Vegas maintain their own “fire code” where things like conduit requirements are different. Always check with your local Fire Marshal, Board of Permit Appeals or other AHJ – authority having jurisdiction- before you buy or install any cable in a commercial building.
And, for both copper and fiber cables, there is the issue of connectors. In both cases, your choice is most often dictated by the connectors on the back of the equipment you buy. For coax, the common connector is the BNC. For fiber cables there is a range of connectors, LC, FC, SC, ST and others. Some are easier to put on than other, some are cheaper, some are higher performance, some are more rugged, so be sure and do some research long before you buy or install any equipment.
Although fiber is increasingly utilized, the future continues to hold a place for copper as well. The key is to select the appropriate cable for the application. Copper is reliable and easy to install while fiber is light-weight and maintains signal integrity at much longer distances.
(Summary by IEWC)