SpectrumTalk

I wrote the following article for the March 2007 issue of PolicyTracker, a London-based European spectrum newsletter and am using it here with their kind permission. So if the spelling seems a little unusual ... The wireless microphone issue is now getting a lot of attention at both FCC and the UK's Ofcom.

Wireless mics can enter the digital era

Complaints from the theatrical community in both the US and the UK are unjustified, says consultant Michael Marcus. He argues the problem is not an engineering one, but an issue of policy and economics.

A friend of mine in the US recently mentioned that he heard Sir Andrew Lloyd Weber on National Public Radio, the closest thing in the U.S. to BBC Radio 4, talking about the U.K.’s wireless microphone problem and the resulting threat to live theatre in London. I don’t know if any comparable U.S. figures have gone to London to give media coverage about their wireless microphone problem with FCC which allegedly threatens Superbowl coverage. How did this all start? In the days of analog TV, limitations of receiver technology made “whitespace”, i.e. vacant TV channels in a given area inevitable. Use of FM wireless microphones in this whitespace by broadcasters and theatres was a simple way to recycle this idle spectrum for something useful. Limiting eligibility to a small set of users meant they could mutually agree on its use. In this era, there was relatively little demand for spectrum anyway and no viable alternative for manual coordination. A lot has changed. DTV can use a higher fraction of available spectrum in a given region since receivers are more robust than analog receivers. This more intensive use of spectrum/ channels creates the “digital dividend” issue: one can either have more TV signals in the same amount of spectrum or pack the existing signals into a smaller TV band and ‘recycle” the remaining spectrum In the U.S. the TV stations of channels 2-69 will be packed into channels 2-51 in February 2009 as directed by Congress. There is also much more demand for wireless spectrum today than there was when the current wireless microphone use first developed. New cognitive radio technology now offers a more flexible alternative alternative to manual coordination of limited users for whitespace use. Thus a major policy challenge in both countries is how to address the needs for wireless microphones in today’s environment. In the U.K. Ofcom has proposed reserving one TV channel exclusively for wireless microphones. The broadcasters and theatres, ably represented by Sir Andrew, have stated that this will not be enough because they can only use today’s analog technology since digital technology will lead to audio delays that are unacceptable in live performance. ( See Digital Dividend Review: Ofcom fails to placate PMSE sector PolicyTracker 20.12.06) Now as a great admirer of Sir Andrew’s music, I am disappointed that he has taken to making poor engineering judgments. Does analog-to-digital conversion (DAC) always lead to large delays? No! No fee spectrum is part of the problem Linear predictive coding (LPC) is a DAC technology developed in the late 1970s for highly efficient compression of voice to binary data and is widely used in cell phones. Like other types of “source coding”, e.g. .zip and .jpg files, LPC involves a trade-off between delay and compression efficiency. But there are LPC parameter choices that trade-off delay for output data rate. There are also traditional DAC technologies without source coding where the delay is limited to the sampling interval, a small fraction of a millisecond. In wireless microphone applications distances are short so low data rates are not critical since the total amount of spectrum resource consumed in terms of bandwidth times area is small. 3G cellular technology can handle arbitrary digital streams including DAC from wireless microphones. Does the 3G industry offer a high quality wireless microphone today? No. There is no demand and hence no product development for spectrum efficient wireless microphones as long as low tech FM units have access to free spectrum use. Free spectrum is always cheaper than efficient wireless technology! Ofcom has proposed to dedicate one TV channel to wireless microphone use. In the U.S. FCC has proposed in Docket 04-186 to allow new unlicensed “TV band devices” to use TV whitespace alongside wireless microphones. The wireless microphone interests in the U.S. have strongly objected to the FCC proposals citing the technical complexity of protecting wireless microphones, which in theory are licensed, from the new unlicensed devices. They conveniently ignore the fact that most wireless microphone use in the U.S. appears to be by users who are not eligible for licenses under present FCC rules. Wireless microphones have real societal value as their proponents on both sides of the Atlantic argue. But the exclusive dedication of whitespace spectrum to them as in the U.S. status quo and the Ofcom proposal is clearly an inefficient use of spectrum since the spectrum use of this narrow application in time and space is quite limited. In view of the growing demand for all spectrum uses it makes more sense to accommodate the wireless microphone requirements in bands that are shared with other users and can give the wireless microphone systems the quality they need. The U.K. wireless microphone anti-digital claim is either a smoke screen to avoid increased costs for spectrum and equipment or arises from technical confusion. In any case, it is unlikely that there will be any development of more efficient alternatives for wireless microphones as long as regulators in the U.S. and U.K. allow indefinite use of today’s low tech models and don’t consider the opportunity cost of denying the whitespace spectrum to other users who could use it more effectively. •