Earlier this month, our Beloved Commish (aka the FCC; hat tip to TV Technology’s Mario Orazio) issued a ruling that finally brings some clarity to the whole issue of TVBDs (TV Band Devices–formerly known as White Space Devices). Many of the wireless mic companies are applauding the ruling, and generally for good reason. It’s still a bit of a mixed bag for those of us working in fixed installations like churches, but it sure could be a lot worse.
Rather than go into a ton of detail of the ruling itself, I will try to parse out what it actually means for us.
First of all, the FCC has created a “Safe Haven” for wireless mics on either side of channel 37 (608-614 MHz, currently reserved for radio astronomy). The new ruling states that the first unused channel on both sides of channel 37 in any given market will be useable for wireless mics and off-limits for TVBDs. This does not mean that channels 36 and 38 are free zones for wireless mic use; both of those channels could be active DTV stations in your market. For example, in the LA basin, we have to go all the way to ch. 30 and ch. 40 to find the first unused channel on both sides of 37. Shure has a great tool—Wireless Frequency Finder—for determining the TV stations in your area.
Second, as portable TVBDs have been one of the great unknowns in this whole equation, we now know that they will only be allowed to operate from channels 21-51 (512-698 MHz). That’s a lot of bandwidth, but the good news for us is that there will be no portable TVBDs below 512 MHz. Also, portable TVBDs will be limited to 40 mW of power on any frequency that’s adjacent to a TV channel in the area. They will be allowed to operate at up to 100 mW of power in non-adjacent bands. That means if you are using wireless mics that operate above channel 21 (Shure H4 for example), you’ll be safer landing your wireless mics next to a DTV station as opposed to 2 channels away from it. It’s still possible you can be hit with up to 40 mW on a mic channel, but that’s better than 100 mW.
Fixed TVBDs will be allowed to operate anywhere between ch. 2-51 (which gets down into VHF territory and includes the UHF block 470-698 MHz that we use for wireless mics). While fixed TVBDs can operate at up to 4 W, they are prohibited from operating on channels adjacent to a DTV station. Again, another reason to look to adjacent TV stations for landing your wireless.
Finally there was talk of requiring TVBDs to include spectrum sensing technology that would avoid TV stations and wireless mics. Since every test of the existing technology failed miserably, the FCC dropped that requirement in exchange for a requirement to check in with a geo-location database. Still no word on how one might register with the database, who will get to register and what restrictions there will be once registered.
What does all this mean for you and your church? Well, let me walk you through a four step process to determine optimal frequency positioning for wireless mics. I’ll use our system in this example since I have to do it anyway. I’m using a spreadsheet that I built in Numbers to help me coordinate the process.
Step 1: Determine your wireless system’s tuning range.
In our case, we’re using Shure UHF-R and PSM-900. Our UHF-R is in the H4 band (518-578 MHz), and our PSM900 is in G6 (470-506 MHz).
Step 2: Determine your local TV stations
We’re a Shure house so I’m using the Wireless Frequency Finder listed above. It may be easier to use the web-based tool provided by your wireless manufacturer. Once you know where TV stations are in your area, you can block them out on the spreadsheet so you know where not to put your wireless.
Step 3: Determine your open frequencies
Now that you know where you can’t put your wireless, it’s time to see where you can. In a crowded metro area like LA, I have just 5 channels open for my mics and 2 for my IEMs. Based on what we know of the FCC order on TVBDs, we can start to look at what the best open channels might be. As I mentioned, the first open channel below ch. 37 in LA is ch. 30. Since that’s a Safe Haven from TVBDs, that’s the first place I will look for putting my critical mic channels. As chs. 23, 25 & 27 are adjacent to DTV stations, those are the next best places to put wireless mics. I’ll avoid ch. 22 because both fixed and portable TVBDs might show up there.
The same process will work for my IEM. Both 17 and 19 are good slots to work in as they are adjacent to other DTV stations (no fixed TVBDs) and they are all below ch. 22 (no portable TVBDs).
Step 4: Determine an optimal mix of frequencies
I’ll use Wireless Workbench to synthesize a new group of frequencies based on those criteria. Sennheiser, Audio Technica, AKG and others offer software to build frequencies that don’t interfere with each other as well. You can also use the recommended groups of frequencies provided by the manufacturer if you don’t want to build your own list.
If you have a hodgepodge of wireless, you may want to contact the manufacturer that makes the most of your wireless and have them help you put together a set of frequencies that will work for you. One thing to keep in mind is that wireless mic manufacturers want you to have a good experience; if you are having trouble, contact them and they will gladly help you get them dialed in.
As I said, this is a bit of a mixed bag for us. While we do have some key open frequencies that we can more or less count on, I wish I had known which ones would be open when I followed the rules and upgraded our system to vacate the 700 MHz spectrum. This ruling is not a disaster for us, but I may have split our system into half H4 and half G1 to get a better mix of safe channels.
This ruling also means we will have to be more strategic on where we place our wireless frequencies. Since we can typically get 6-8 mics in a TV channel, if we have more than that, we have to place them carefully. In our case, I want to be sure our teaching mics and worship leader’s mic end up in ch. 30—a channel free from TVBDs because it’s the first open channel below 37. Our systems don’t tune high enough to get to ch. 40 on the other side of 37, so we can’t utilize that one. If I can’t fit the whole rack of wireless in ch. 30, I will want to use channels adjacent to other DTV stations as there will be no fixed TVBDs and portable TVBDs will be limited to 40 mW.
So the good news is it’s not all bad. The bad news is that it’s not all good. We’ll have to work a little harder and be a little more selective but we will be able to make wireless mics work for the foreseeable future.
Download the Numbers spreadsheet.
Download the Excel version (not as cool, but it should work)
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Even more geeky discussion