The FCC wants help in squeezing more wireless microphones into ever-shrinking spectrum.

Traditional wireless microphones – the kind you see on TV, big stage shows, and in lecture halls and churches – operate on locally vacant TV channels. But those channels are becoming scarce. The FCC has asked for comment on how to accommodate these microphones in the future.

In the old days of analog TV, there were a lot of TV channels, and the shortcomings of analog receivers meant a lot of those channels in each market could not be used for TV. That left plenty of room for wireless microphones. The picture began to change in 2009, when the last full-power analog stations went off the air. Because digital TV stations can be packed more tightly than analog stations, the FCC was able to free up 18 channels for other uses, which left fewer empty channels for wireless microphones. Then, a year ago, the FCC approved the first operation of “white space” devices that provide Wi-Fi-like service in some of the remaining vacant TV channels. The FCC reserved two channels in every market for wireless microphones, and provided for additional channels where needed in a complicated set of regulations; but there is no getting around the fact that a lot more devices will be trying to operate in a lot less spectrum. Then, last month, the FCC proposed “incentive auctions” designed to encourage broadcasters to give up still more channels.

Despite the squeeze on spectrum for wireless microphones, we can’t do without them – at least so long as we want good audio in our movies, TV, and stage shows. Even the FCC has implicitly acknowledged these devices are indispensable. For decades, it issued licenses for TV-band wireless microphones to just a few categories: broadcasters and broadcast networks, cable TV operators, and movie and TV producers. That’s all. Missing from the list are Broadway shows, concert venues, college lecture halls, and your local house of worship. Operation in all such facilities was commonplace, but illegal. Of course the FCC knew about those uses – it even had a wireless microphone in its own meeting room. But even the illegal operations were well managed, causing no interference to TV stations, so the FCC wisely left things alone.

The advent of white space devices, though, brought the need for better control over who uses microphones, and where. Bringing regulation into line with reality, the FCC considered broadening the list of eligible licensees. It also took the unusual step of proposing to legalize previously illegal operation by allowing lower-power wireless microphones to operate as unlicensed devices, under the same basic rules as Wi-Fi and cordless telephones. The power limit would be lower than for licensed wireless microphones, but higher than for most other unlicensed devices, and should suffice for good sound in most halls and churches. That proposed relaxation has not yet been adopted.

Now the FCC is hoping technological advances will solve the spectrum problem. After all, digital TV stations can fit four channels into one analog TV channel; digital cell phones carry twenty times the traffic in the same spectrum as the old analog cell phones. Why shouldn’t digital wireless microphones show similar improvement? 

But there is a catch. The increased spectrum efficiency in digital TV and digital cell phones comes not from digitizing the signal, but from compressing the signal once it is digitized. And compression adds delay. (To hear the delay, call your cell phone from your wireline phone, put one to each ear, and talk.) A performer using a wireless microphone can tolerate delay of only a few thousandths of a second, which limits the possible compression.

Still, digitizing can help. Analog microphones on the same TV channel have to be spaced well apart, allowing only about six to eight per channel, or else they interact to create unwanted signals. Digital microphones each take up about as much spectrum, but they can be squeezed closer together, so a TV channel can accommodate a dozen or so.

Before it moves further on these matters, the FCC wants to hear from wireless microphone manufacturers, installers, and users. The detailed request for information is here. Comments and reply comments will be due 20 days and 51 days after publication in the Federal Register. (Check back here with for updates.) Comments and reply comments on the proposed incentive auction rules, including rules applicable to wireless microphones, are due on December 21 and February 19, respectively.

In the meantime, the FCC has released a public notice on how wireless microphone users can register for protection from white space devices in certain east coast states. The details, which are complex, can be found here.