DTV Transition For Wireless Mic Users

We're right in the thick of the digital television transition which started officially on February 17 and continues through June 12. But that is just the beginning of it for many wireless microphone users. The fact is that the airwaves are changing and some of that change might just knock you off the air.

The DTV transition is actually a shorthand way to describe a complete reassignment of the VHF and UHF frequencies that have been the domain of over the air broadcast television for some 50 years. The stated impetus is an upgrade in television from analog to digital broadcasting. But that's just the tip of the iceberg. If TV stations were only going to scrap their analog transmitters and replace them with digital transmitters on exactly the same channels, only TV viewers would be affected. But there's much more in play.

A second and perhaps more powerful incentive to make the transition is that over the air TV channels are being consolidated below 700 MHz. The upper UHF channels were sold off to other wireless service providers in a government auction. In addition, even the compressed television broadcast band is set to be shared to make more efficient use of scarce spectrum. All the channels that aren't filled with TV stations in any given area are available for use by other entities on a non-interference basis.

So what does this have to do with wireless mics? Many wireless microphones use equipment that jumped the gun on making use of those "white spaces" between powerful TV signals. They've been effectively squatting in the 700 MHz band without much of a problem. Their transmission range is short and the TV band never did fill up with stations. So interference problems were few and easily rectified by moving the mic in question to a different frequency.

All that is about to change. More TV stations are being crowded into the truncated UHF band. That leaves less room for multichannel wireless microphone transmissions, especially in larger cities. As soon as a new generation of white space devices approved by the FCC start moving into the remaining open channels, established wireless microphone users may find it tough to keep their preferred frequencies clear of interference. That could be a big problem for everything from theater productions to church sermons.

So what's a wireless mic user to do? Sweetwater, a major supplier of professional audio gear, offers a new guide called "Navigating the White Spaces" by Mitch Gallagher. It's an online resource available to read at no charge. If you are potentially affected by the DTV transition or just want to keep up with what is happening on the airwaves, it's well worth a read. Mitch outlines which microphones are affected (they all aren't), assessing your ability to continue squatting in the white spaces, and what other solutions are available. There are also links to additional guides and white papers published by the microphone manufacturers.

White Spaces Debate Outs Wireless Mics

One perhaps unexpected consequence of the increasingly heated discussion over the "white space" TV band spectrum is the outing of "unauthorized" wireless microphone users, such as churches, theaters, corporate lecture rooms and so on. It turns out that many of them have been squatting on unused UHF TV channels for years with nobody being the wiser. Now they'll have to vacate or risk being crushed like bugs by the winners of the 700 MHz spectrum auction when new services start moving into the old analog TV channels next February. What's more, it's becoming increasing less likely that they can simply re-tune their transmitters and quietly slip down into gaps between the new digital TV (DTV) channels.

Wireless microphones are one of those clever technologies that have become insidious over the years. You see them everywhere there is any type of vocal performance. Those microphones you see on televised concerts that don't seem to have any wires connected? That's them. A lot of times you don't even notice the mics. Ever see a microphone pointing at someone on a TV news show? The host and the guests are just chatting like they were in your living room, but you never hear any background noise. They're wearing wireless microphone packs with tiny mics on their lapels. Broadway shows are also extensive users of wireless mics, so that you can hear every nuance of the performance without the actors having to scream. Even your pastor may be using a wireless microphone on Sunday to be able to move around during the sermon.

So if this technology is so insidious, what's the problem? It seems to work just fine. Yes it does, but what you don't realize is that the frequencies many of those wireless microphones are transmitting on are supposed to require a government license and are only for specific types of users. They are regulated as low power auxiliary stations under FCC rules Part 74, subpart H. The key word is "stations." These devices are considered to be lower power radio stations. This part of the rules covers broadcasters, and these auxiliary stations are supposed to be for the use of broadcasters, radio and TV networks, movie and TV program producers, local distribution networks, and cable system operators that produce their own shows.

Oops! No mention of churches, theaters, schools, concerts, lectures, dance clubs, performing arts centers or the like. Yet, they all use wireless mics. So why hasn't the long arm of the law come after them? Mostly because they haven't been causing a problem. There was so much unused space in the UHF TV bands and these devices run such low power that there just wasn't an interference issue. That's about to change.

First thing that will happen is that the new owners of the upper UHF TV channels from 52 to 69 will move in with high power equipment for mobile broadband and other services. Considering the billions spent to gain this valuable spectrum, the chances of large blocks staying quiet are about nil.

On top of that, Google is mounting a public relations campaign called "Free The Airwaves" to drum up support for turning the remaining white space spectrum between DTV channels into unlicensed usage across the board. That would legitimize the continued use of wireless microphones for all performers, but interject the new problem of interference and lots of it.

So what's a poor wireless mic user to do? There are some other frequencies already available for this use. But "broadcast" quality equipment, especially systems that can support many simultaneous microphones, is scarce and unused channels even harder to find. There's also the issue that thousands, perhaps as many as a million 700 MHz wireless microphone systems are already in the field and will be inconvenient and expensive to replace. That argues strongly for protecting at least some of the new DTV spectrum below 700 MHz for wireless microphone usage. Perhaps a good long term solution could be found like the creation of the Wireless Medical Telemetry Service for hospital equipment that might otherwise be a white space squatter.

Medicine in the White Spaces

The drooling is well underway for whatever bones of the old VHF and UHF analog television service aren't already picked clean. The telcos, AT&T and Verizon, have already gobbled up the juiciest morsels of the 700 Hz spectrum that weren't already allocated for digital TV broadcasting. No one is going near the frequencies reserved for public safety service. Instead, everybody now wants in the cracks and crevices known as "white spaces" between the powerful signals of commercial television transmitters. That includes all that wireless medical equipment they hook you up to in the hospital.

Medical telemetry, also known as biotelemetry, refers to medical equipment that uses wireless data links. One popular use is wireless cardiac monitors that keep track of patients' heartbeat activity without tethering them to a bedside monitoring unit. It can also include a wide variety of hospital equipment including imaging, scanning, ultrasound and more. Just about anything that isn't voice and video and is designed for medical application fits the definition.

As it turns out, medical telemetry has already staked out its claim in the UHF TV band. Channel 37, covering the frequencies between 608 and 614 MHz, has been used since 2000 for this purpose.

But how can that be? DTV is just getting going in a big way and all the old analog transmitters are still running full bore. I thought white space transmissions were something that would spring up when the analog channels go dark next February?

Turns out that the FCC established the Wireless Medical Telemetry Service (WMTS) on three bands in June of 2000. Two of those are in the microwave region of the spectrum that were originally assigned for government use but no longer needed for that purpose. The other is 608 to 614 MHz, also known as TV channel 37.

But what happened to the TV stations on channel 37? There weren't any. Channels 37 was reserved for radio astronomy work. Since radio telescopes are few and far between, the channel can be shared between radio astronomers and hospitals with only rare situations of interference. So, in a way, medical telemetry is already in the white spaces.

Medical telemetry is one of the existing applications threatened by proposals to turn the TV white spaces into wireless Internet broadband, cellular backhaul, or other services yet to be conceived. Since only channel 37 is assigned to WMTS, one way to avoid trouble is to have white space transmitters include a spectrum mask to prevent transmissions on channel 37, plus one channel to each side for additional protection. The proposed white space transmitters would have to avoid channels being used for TV transmissions in a particular location anyway, so this is just a slight further constraint.

Medical electronics is a rapidly advancing technology that gobbles up bandwidth of all types, including wireless and wireline and fiber optic connections. Medical imaging alone needs all it can get to enable transmission of large image files in seconds, not hours. Compared with the requirements of CAT scan and MRI equipment, patient monitoring is currently a fairly narrowband operation. But with future advances, even this activity may need all the white space or other bandwidth it can get.

Looking For Spectrum In All The White Spaces

Now that the winners of the recent 700 MHz spectrum auction are being announced, a cold chill is descending over the wireless industry. It's gone. It's all gone. Once the analog TV transmitters are shut down and the auction winners claim their new slices of the electromagnetic spectrum, there will be no decent place to go for over-the-air real estate. Like beach-front property, it's all bought up and they ain't makin' any more of it. Or is it?

Instead of a land-rush that's over with all property staked out and registered, think of the spectrum as more like choice territories in the Cretaceous period, some 65 million years ago. Those T-Rex's, the guys with the big teeth, each ruled their piece of territory to the exclusion of all others. Think of each powerful TV station as one of these T-Rex's. Once they've multiplied to the capacity of the land, there's really no room for any new predators. They'll be stomped-and-chomped as soon as they stick their heads up. Or will they?

While it's true that the spectrum seems full of thunder lizards, there is really lots of territory available if you are small and nimble like a furry little mammal. You have the world at your disposal as long as you are smart enough to stay out of the way of those ferocious big guys. This is what advocates of using the "white spaces" in the TV band are thinking. It should be OK to squat on any frequencies that aren't being blasted with RF from those thundering over-the-air digital TV stations.

Google is the latest to point out that the broadcast television bands are not fully occupied. Part of the reason is the legacy of analog television standards that divide the spectrum into 6 MHz channels. In any given area, you won't find TV stations on adjacent channels. There's always at least one blank channel separating TV transmissions. That's because early tuner designs based on coils and capacitors weren't so precisely selective. For a receiver to pull in a powerful station on one channel and completely reject an equally powerful station on the next channel over was too much to ask in the age of vacuum tubes and rabbit ears. Even today, you can sometimes see a TV station bleeding into an unused adjacent channel if you are close enough to the transmitter.

When channels 2 through 13 in the VHF band filled up, the FCC simply opened up a new UHF band with 56 more channels. It was no problem, because those ultra high frequencies weren't much in demand right after WWII. Over the decades, the UHF band also began to fill up. But it never filled up entirely. The construction, operating and programming costs for high power television stations became less attractive after cable and satellite delivery gained popularity. So much so, that the FCC felt justified in carving off the chunk of UHF spectrum from channels 52 to 69 and putting them up for auction to the wireless companies.

Even what's left of the broadcast spectrum won't be fully occupied with high power TV transmissions or even low power repeaters and translators or limited coverage stations. Right now it's as full as it's going to get, with each broadcaster spreading out with an analog transmitter on one channel and a digital transmitter on another. But half of those are going bye-bye next February 17 when the analog transmitters get shut down for good. The auctioned spectrum is given to the winners for other uses and the broadcasters settle into their assigned DTV channels. The channels that aren't assigned or are used as "guard" channels between local broadcasters will sit there empty. These are the white spaces that might be pressed into service.

What could this white space spectrum be good for? How about meshed networks of WiFi hotspots that could spread to provide broadband service in rural areas? WiFi currently has just a handful of channels available and they are located in the 2.4 GHz and 5 GHz bands where they don't carry or propagate very far. The VHF band from 54 to 216 MHz and reduced UHF band between 470 and 698 MHz can penetrate buildings much easier. Why not press those white space frequencies into service and essentially get a new "free" WiFi band?

Broadcasters are suspicious. They fear that thousands of unlicensed transmitters operating right next to their signals or, worse, right on top of their signals will ruin their over the air business completely. There are a couple of work-arounds to prevent this. First, load a database of all licensed and occupied channels into each white space transmitter along with a GPS receiver. The transmitter will then avoid channels known to be in use in a given area. Another approach is to use "spectrum sensing" to listen for transmissions on a channel before using it yourself. The sensing would have to be done a level that ensures any receivable digital TV signal would be left alone.

Early implementations of proposed white space transmitters failed miserably and were withdrawn from consideration. But that's more of an engineering development problem than a fatal flaw in the theory. With demand for new wireless applications increasing and available spectrum being locked up by carriers with deep pockets, it seems reasonable to at least seriously consider getting the most value from every MHz out there. With Google now advocating for white space usage, the idea has a good chance of picking up steam and getting the nod for deployment in the next few years.

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