CONQUERING THE DTV CHALLENGE
Oct 1, 2002 12:00 PM,
By now practically everyone who uses wireless microphone and monitoring systems knows that the conversion to HDTV is causing great turmoil. It’s not hot news in 2002 that DTV allocations and activations have swallowed up once-unoccupied RF real estate (TV channels), the same portion of the RF spectrum that wireless systems operate within.
Users of wireless systems (radio mics) need a well-thought-out plan to successfully operate their systems on this new and ever-changing playing field.
In any contest in which victory over a challenge is the goal, you need to know the sources of opposition and the skills and tools for victory. Sizing up your opponent in the DTV challenge requires a bit of research. Luckily, the FCC licenses television broadcasts, and learning their whereabouts is relatively easy. The FCC publishes licensure information about all analog and digital TV broadcasts. You can request this information from the FCC, or you can gain it electronically at www.fcc.gov/mb/video/tvq.html. Another source is http://100kwatts.tmi.net/tv-listings.html.
DISCOVER THE POSSIBILITIES
Because not all DTV signals are up and running, the playing field is full of hidden possibilities. Some DTV signals are being tested intermittently, and it’s not uncommon for broadcasters to turn on their digital transmitters every now and then. Because broadcasters have until 2006 to have their DTV online, the activation of those signals is unpredictable. Many are active full time, whereas the majority will commence full-time transmission sporadically in the future. Further complicating matters, another possibility is that licensees have the option to petition the FCC for DTV channel reassignment. Given that DTV-assigned channel numbers may change over time, it’s possible (hypothetically) that an anticipated DTV signal on channel 44 in your area could be moved to channel 17. The FCC provides updated information about the petitions.
Developing the skills for victory involves interpreting the information you have collected on the opponent. TV channels are given a numerical identity. Knowing there’s a broadcast on channel 34 in your area is not much help without knowing where in the RF spectrum that signal resides. TV channels are 6 MHz wide, and they have a uniform beginning and ending frequency value (lower and upper edge). It is important to know and understand those bandwidths, as well as the operating bandwidth of your wireless system. The “VHF/UHF Frequency Ranges” table identifies both the bandwidths of TV signals and helps you to identify the TV channel orientation of your system’s bandwidth capabilities.
The proper interpretation and application of this information involves knowing the physical location of the source signal and its proximity to your location. A good rule to follow is to avoid operating on frequencies whose source is within a 75-mile radius of your location. With that information at hand, do not plan to operate your system on frequencies that fall within the bandwidth of active TV broadcasts that meet the 75-mile radius proximity criteria.
VICTORY AT HAND
The final strategy in conquering this challenge is possessing and applying the proper tools for victory. The RF spectrum is an ever-changing playing field. New TV broadcasts will continue to be licensed, and relying on the status quo may prove to be a costly error. Many modern wireless systems are designed to afford the user the ability to tune the system to a wide range of frequencies. This frequency agility provides the user the tools necessary to locate and operate on frequencies unoccupied by stronger TV signals. Knowing that low-output wireless systems cannot compete with TV signals that are quite often a million times stronger, the best strategy is simple: if you can’t beat them, avoid them.
Special consideration should be applied if you plan to operate multiple systems simultaneously. Frequency agile RF systems allow the user a variety of tuning options. Multiple system operation requires frequency compatibility between all members of the team, and many manufacturers design their systems with multichannel operation in mind. Manufacturers of high-quality RF systems provide a scheme of frequencies that will operate flawlessly together. It is recommended that you understand and follow their published sets of numerical intervals for best performance.
In the age of HDTV, the decision to go wireless must be accompanied with a solid plan for success. Whether your goal is one wireless system or dozens, your plan should include research to determine the inevitable sources of potential TV interference, a thorough understanding of the RF spectrum your system(s) will operate within, as well as choosing a brand and model of product with the frequency agility required to avoid interference from analog and digital TV transmissions.
Rick Belt is the RF wireless product manager for Sennheiser.