updated: 11 Oct 11
Vanishing Spectrum - why you might worry about radio frequencies...
To keep informed visit www.wunz.co.nz regularly.
The area of radio spectrum used for radio microphones is being reassigned between now and late 2013, as NZ switches to a fully digital television service. The frequencies commonly used for radio microphones will change—and before you think it’s not something that will affect you—think how widely wireless mics are used... From schools to lecture theatres, boardrooms and gymnasiums through screen and stage to broadcasters, the changes will be profound.
It’s not surprising that usage of the radio spectrum has grown exponentially over the last few decades. Senior soundies will remember when they had to buy their first two radio mics for feature film work. Now commonly, a soundie getting into basic television work needs at least that many.
Where are radio frequencies going and what exactly is happening?
The radio spectrum isn’t really vanishing, it is being reassigned. The frequencies themselves aren’t going anywhere, but bands of spectrum will be made unavailable to general users or used with different technology (digital television, and in the future ‘long-term evolution devices’).
How has use of the radio spectrum worked till now?
All smaller users operate under a General User Licence (GUL): A default state, not actual licence, for those outside the big fee paying users (radio, television and telecommunications). Operating under a GUL has always meant accepting limits to transmitter output and the areas or bands of spectrum that can be used. It also means an expectation that you will not interfere with use by fee-paying broadcasters.
Until WUNZ, people who used the estimated 100,000 radio mics in New Zealand had no input into the Government’s thinking. WUNZ has sought to improve the situation for general users by informing the ministry, ultimately to alter the declared government position (reiterated recently by the minister) that: “radio microphone users operate on the fringes of the spectrum, they don’t pay for a licence, therefore their rights are minimal” and the assumption that “there’ll [always] be sufficient spectrum for them to operate.” WUNZ has campaigned to be recognised as an affected party, is now actively involved in dialogue and gaining attention (for developments see: www.wunz.co.nz). Uncertainty remains as scheduled changes meet with technical delays, and updates come in on an adhoc basis.
How does digital tv affect radio mics?
The majority of radio mics operate somewhere between 518-806 MHz band. Analogue TV usage of the UHF spectrum leaves gaps “white space” (see first figure) – where typically all radio mics operate. The ‘white space’ is obvious on the 55 MHz example where black peaks and troughs are tv channels. Even within the smaller 8 MHz chunk taken up by an actual TV channel (video and audio) there’s still white space that can be used for radio mics and there’s other spectrum outside of that.
When we look at digital TV we’re faced with quite a different picture (see second figure). A digital TV transmission is totally different from an analogue signal, it occupies the entire 8 MHz chunk of bandwidth: leaving no white space at all. If your radio mic operates in that band of spectrum you have nowhere to go.
Goodbye 700 MHz
The Government has directed all users to clear the band of spectrum from ~686 MHz all the way up to 806 MHz (the entire 700 MHz band, more-or-less) by March 2015. Digital TV in this band will be re-channelled to a lower part of the spectrum, all analogue transmissions will be switched off and the free space created to be put up for auction - the ‘digital dividend’. WUNZ unsuccessfully fought the
‘digital dividend’ to retain this band of spectrum for general use. Unfortunately the government sees it as a valuable commodity (something that remains to be seen, see below) and radio microphones that operate in that spectrum will therefore be redundant by early 2015.
Managing the spectrum
In the USA the 700 MHz band has been closed off to general users since June 2010. Despite this, activity in this range is yet to be detected. So for an entire year this band of spectrum has gone unused begging two questions:
1) Is it really a valuable commodity at this point in time, or has this been misjudged?
2) why can’t it remain available for radio mics until it’s actually required for other use?
WUNZ outlined concerns about other situations requiring thoughtful management. For example where many radio mics are used simultaneously – how can conflicting use be managed? Under the existing GUL there is no means of resolving conflict as all users have equal rights. WUNZ has pointed out that such situations require management, and will likely support future models like that applied to the Rugby World Cup where an individual is responsible for administering the spectrum for maximum efficiency.
Looking into the crystal ball: predicting future allocation of spectrum.
WUNZ has made some educated guesses about what might happen in the future with the spectrum based what is currently known about how digital tv uses the spectrum, and how many future options on the digital tv might be taken up (for an explanation). The upshot is that broadcasters use double the spectrum in certain locations to allow for infill coverage. Given Maori TV would occupy 16 MHz (two adjacent bands) for one channel, and Sky 80 MHz (10 bands) for five channels, digital tv looks set to occupy a fair amount of the spectrum, leaving slim pickings, and major challenges ahead for general users (see the full article on nztecho.com, and www.wunz.co.nz for full graph of projected use).
Short term silver lining and the unknown future
Until 2013 it will be unclear how much spectrum will be utilised by digital tv. As the 700MHz band vacates, availability will actually improve for a while ...An Indian summer before the post–digital-tv–winter sets in. In the meantime, all radio mic users need to keep informed and manage their use carefully.
Thanks to Haresh Bhana and Stephen Buckland of WUNZ upon whose presentation this article is based and Grant Cummuskey of NZ Video News who provided a transcription of the presentation.