Television measurements
May 1, 2004 12:00 PM, BY DON MARKLEY
The onset of digital television has raised some new problems for TV stations. The sophistication of new measurement equipment for digital transmission makes the old analog equipment seem simple. The new equipment presents many challenges, even for measurements as simple as power output.
Sophisticated test equipment such as this Sencore monitoring system is necessary to measure, test and troubleshoot complex DTV signals.
DTV measurements are just more complex than the old analog waveform monitor/vectorscope can handle. So a station transitioning to DTV must buy new measurement equipment, and its staff must learn new measurement procedures. Start with the learning. Make a quick visit to www.tek.com/measurements and get a copy of “A Guide to Digital Television Systems and Measurements.” Another good reference is a textbook written by Walter Fischer called “Digital Television.” These two documents can help keep that new DTV system humming.
A hands-off posture
The FCC rules for measuring analog TV signals are extremely detailed. But, for digital television, things aren't so clear. You can find all of the applicable criteria at www.atsc.org/standards.html. The FCC simply requires DTV systems to comply with ATSC A/52. The commission maintains a hands-off posture on DTV and leaves it to the individual stations to determine what they need to do to operate in compliance with the ATSC standard. That doesn't mean that the station isn't held to a high standard of performance; it just means that the way in which stations meet that standard is their choice.
This arms-length regulation of DTV is a reasonable extension of the commission's move over the years toward deregulation and away from dictating exact terms. Some may argue that the commission has given too much slack in some areas, but the overall result has been to give stations the freedom to determine what they need to do to operate properly. Years ago, for example, the rules not only specified the frequency tolerance for radio stations, they required a specific type of frequency monitor. Even more specific rules governed the exact temperature variation allowed in the oven that housed the crystal in the frequency monitor. Now, however, the rules simply specify the frequency tolerance. If you want to measure operating frequency by how the tower feels when you put your arms around it, so be it. Stations can measure the frequency any way they want. But — and it's a huge but — the frequency had better be within the required tolerance when the commission checks it. The FCC doesn't hesitate to hand out fines if it finds that a station isn't operating in accordance with the commission's requirements.
After reviewing the two reference documents mentioned earlier, you might need some help to understand them. Test-equipment manufacturers offer an enormous amount of useful information. Again, www.tek.com/measurements has numerous application notes and technical papers that can help you understand the measurements, how to perform them and what equipment may be necessary. Other good sources are www.agilent.com and www.sencore.com. Each company has several pieces of test equipment in its product line designed specifically for the DTV industry. They aren't cheap. But then, the DTV signal is complex; breaking it down for analysis requires sophisticated equipment.
The right tool
Measuring DTV power is a particular concern for broadcasters. The old wattmeters and couplers designed for analog television or radio simply don't work for DTV. If you attempt to use them on the complex waveform of the 8-VSB DTV signal, they will produce large errors. To understand the problems involved in this area, check out the excellent paper at www.bird-electronic.com/products/pdfs/broadcast_rf_pwr_mes_techpaper.pdf. It describes the problems involved in measuring DTV signals and compares various measurement schemes.
Snake oil
The best way for a station's staff to handle monitoring is to work with the transmitter supplier. All transmitter manufacturers know what measurement equipment is suitable to use with their equipment. In fact, you can often buy a complete monitoring package with the transmitter at a considerable savings. But be careful. Whenever anything new comes along, you can always find a snake-oil salesman who wants to sell you a bottle of his magic elixir. And, like the tonics foisted on unsuspecting customers by those traveling peddlers, it is usually worthless. DTV has opened that bag of worms, and some people are making claims that are just plain wrong.
For example, some claim that you must measure signal strength to demonstrate that the digital signal coverage replicates the analog coverage. The truth is that one goal of the initial channel-allocation scheme was to duplicate the analog service. Once a station completed its allocation scheme, it could change its channel, antenna, height, power, etc., by showing the FCC that it would not create a new interference or increase interference by more than a de minimus amount. (De minimus is Latin for “It don't mean squat.”)
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
Send questions and comments to: don_markley@primediabusiness.com
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