Microphone systems suffer interference issues

Users of wireless microphone systems are likely to experience more interference problems with the switchover to digital television

Analogue television begins its transformation to digital next month, freeing up the 700-800MHz band for sale. It means also that users of wireless microphone systems will experience more interference problems unless they are using the latest technology.

It’s been an issue since May 2007 when Freeview was launched. Freeview uses big chunks of the same UHF spectrum which had been used by the comparatively low-power wireless devices for a couple of decades. Wireless users reported problems such as increased signal drop out and unwanted noise as soon as Freeview trial transmissions began.

Not all brands of wireless microphone were affected because each works in different parts of the UHF spectrum. But the issue was sufficiently worrying for the industry to form a strategic plan to lobby government as the Wireless Users of New Zealand.

Key issues included ensuring that the wireless user industry was consulted on future UHF spectrum developments so it wouldn’t be caught out again.

Wireless microphone users currently use unoccupied UHF spectrum from 646MHz to 806MHz. They are permitted to operate within those frequency ranges used for television on a secondary basis. In other words, television has primary rights to the spectrum; secondary users have to find un-used space but don’t have guaranteed rights. They need to check out which channels are being used by television in their area and work around that.

With people now abandoning their analogue sets in favour of digital, the issue of interference has become more apparent.

Nowsound, a provider of digital wireless microphones, says it has received numerous calls from customers about the issue. “It seems that Freeview is the main culprit,” says southern region sales manager John Abbott. “Users are having to upgrade to the latest technology where the microphones can monitor multiple frequencies and jump to what is available. Older microphones have dedicated frequencies.”

Computerworld put a number of questions about the issue to the Radio Spectrum Policy and Planning Department of the Ministry of Economic Development.

In a written response, Radio Spectrum manager Len Starling says the television broadcast frequencies for Freeview high definition have been restacked in several regions.

“We are making these changes to make sure that we’re using the radio spectrum as efficiently as possible and to release the 700MHz spectrum for new uses (the digital dividend). Restacking is moving the digital television services into the 518 to 686MHz frequency range.

“The frequencies used by television vary from area to area and will evolve with time: for example, to accommodate new services such as Sky’s Igloo transmissions.

“The digital dividend spectrum (686 to 806MHz) is now clear of all digital television transmissions. Some further changes in the spectrum below 686MHz will be required after television becomes completely digital on December 1, 2013.”

Starling reaffirms the secondary basis of radio microphone use and says that as television services change frequencies, some radio microphone users may need to readjust the frequencies they use.

“Broadly speaking, spectrum in the 515MHz to 686MHz frequency range is expected to continue to be available for radio microphones in the long term. As television becomes completely digital, there will be some changes to the exact band edge frequencies. Once all the analogue services have been switched off, there should be more clear spectrum in the 518-686MHz frequency range which could be used for radio microphones.

“In the future, commercial drivers for television could cause change in the spectrum from time to time, including some increased use.

“Wireless microphones have been allowed temporary use of the 686-806MHz spectrum until the television digital switch-over in December 2013 when they will no longer operate in this frequency range.”