Published 5 Oct 2013

Radiomicrophones after digital switchover
Spectrum availability in N.Z

The N.Z 'digital dividend' comes about because the government can sell a large swathe of spectrum which was previously used by analogue television broadcasts. They will auction the spectrum between 703MHz and 803MHz to telecommunications providers for them to use for 4G mobile services. Digital television continues to occupy the lower part of the UHF band which here, falls between 510MHz and 686MHz. However, radiomicrophones also use that same spectrum on a secondary basis. That means they can operate unlicenced, but must not cause interference to legitimate (licenced) services using the band; namely TV and shortly, 4G mobiles.

The problems for radiomicrophones

The main issue for radiomicrophone users is that any radio-mics operating between 698MHz and 806MHz may only continue until 11 March, 2015, after which time, they will not be permitted. Users of such equipment have the choice of having their microphones re-tuned (if possible) or replaced. There is still plenty of UHF spectrum available. The band between 502MHz and 606MHz plus the band between 622MHz and 698MHz will be permitted. The following graphic from the radio spectrum management group of the Ministry of Business, Innovation and Employment (MBIE) illustrates current and new spectrum availability: spectrum graphicThe upper GREEN blocks are what is being auctioned for 4G mobile, inclusive of guard bands, which RSM provides to assist adjacent users from interfering with each other. The ORANGE blocks are currently permitted radiomicrophone bands; due to expire. BLUE blocks are where radio-mics will be permitted in future, while the LIGHT RED block is the block that must be vacated before 11 March, 2015. YELLOW blocks show the TV channel assignments; mainly for reference. TV no longer exists above TV47; 686MHz.

Despite all the spectrum available, there is only one block that users could set their equipment to use on a national basis, without a risk of that frequency becoming used at some point, by a television service. This might affect nationwide users such as major broadcasters and other industries. This will be between 686MHz and 698MHz. In the past, users could make use of the un-used part of all UHF TV channels, space which existed right at the upper end of each channel. This amounted to 900kHz of vacant spectrum per TV channel and was due to the fact that here, we broadcasted the PAL-G standard in an 8MHz wide channel. Users could set their mics to frequencies in any and all the TV channels, knowing that the space was vacant everywhere in the country. This space no longer exists and is not specifically because of the 'digital dividend' but because of the switch from analogue to digital TV transmission. Digital TV services take up nearly all of each 8MHz assignment. Some more detail can be found at the end of this article.

GURL's and secondary users

Radio-microphones are permitted under a general user radio licence, which means that individual radio transmitters need not be licenced but are assigned operating bands which are shared with like users. The Wi-fi 2.4GHz band is an example of a GURL. In the case of radio-mics, they are permitted in the TV bands as per the previous para, as long as they do not interfere with TV reception.

I regard secondary use licencing as a two-edged sword. On the plus side, the secondary users do not have to licence and pay a fee for each and every radio-microphone. This would become unwieldy in administration as well as being a financial burden to some users, such as churches, community groups and charitable organisations. The regulatory authorities do not want the administration overhead either. However, the requirement for users to avoid interference to TV reception is entirely unworkable. Even if the radio-mic user cared and wanted to avoid interfering with TV reception, how would he know he was causing any? A TV watcher, adjacent to the venue of a charity fund-raising event, for example, and intermittently losing reception, is going to have no clue as to what was causing the interference and extremely unlikely to relate his problem to the use of radio-microphones. Even persons located near a permanent site where radio-mics are used, are not going to connect interference to them. Interference caused by a radio-mic will be rather intermittent. It will depend on mic orientation at any one moment, height above ground, variable proximity, reflections from nearby objects and whether it is on or off. Even experienced technicians will have trouble tracing such a problem.

Getting authorities to solve interference

The RSM has very limited resources to trace interference issues and does not generally want to spend time doing so, unless it is a safety matter. They have some self-help advice for radio and TV viewers on their webpages, but it is limited and much still relates to analogue TV. If you do want to involve them in a domestic interference problem, you need to complete their form. In practice, it will be more difficult for radio-mics to interfere with digital TV reception than it was for them to interfere with analogue TV. Unless an affected TV was right on the limit of viable reception anyway, a radio-mic would affect at most 300kHz of the 8MHz TV channel and normal error-correction algorithms in a TV would minimise the effect. Unless several radio-mics operated in the same TV channel at the same time, adjacent to a block of flats....

Options and cautions

Radio-microphone users need to take some action:

  1. Check the operating frequencies of all mics:

    If they are in the band from 698MHz to 806MHz, find out if they can be changed by March 2015. If the equipment appears to be able to go to the new permitted frequencies, then all is well. If not, consult the supplier, or agency for the microphone brand first. However, if suppliers and agents say that the equipment cannot be retuned, it may be because they want to sell new equipment. I am not saying that agents and suppliers are untrustworthy, because all those that I have dealt with are professionals and will be working in your interests. However there are always some out there who think otherwise. If 'no way' is their response and you desperately want to try and keep your mic, which is in good condition, it may pay to consult an independent service. Some equipment may be re-tuneable by an experienced technician, even if it means a small modification. You will be up for an small assessment fee, which of course, will be sunk funds if the mic actually cannot be retuned anyway.

  2. Choose your new frequency:

    There are plenty of free TV channels in any one region. The west coast of N.Z, for example, has no terrestrial digital TV and all channels are available. In all areas with DVBT, the channels follow an n+2 grid. That is, they are spaced two apart. Radio-mics could use the intervening channels. However, in Wellington and Auckland, there are also a number of in-fill stations. These are slotted adjacent to each main station. Wellington currently uses CH30,32,34,36 from Kaukau and the in-fills use CH31,33,35,37. It would be unlikely that you would be in the coverage area of both a main and in-fill, but it is possible. Future new TV stations will keep to this n+2 pattern. The channel sets do vary in each region, so you need to check which ones to avoid.

    My preferences for channels would be (a) Use 502-510MHz. There are no TV transmissions anywhere in the country on these channels yet, although new stations are potentially allowed in these slots. (b) Use 686MHz to 698MHz. RSM is going to permit mics right up to the 4G lower guard band and TV48 will remain free from TV. (c) Use 638MHz to 686MHz. There are no current national multiplexes in these slots although there are some regionals. You need to check transmissions in your area.(d)Use 510 to 534MHz.

    Bear in mind, if you do operate many microphones in one area, they will not operate on their immediate adjacent channels without interfering with each other. Just how close in frequency you can operate them depends on manufacture, but it is likely you will need at least 300-400kHz gap.

  3. Be careful when buying new equipment. There are pitfalls out there and care must be taken to ensure the new mics can operate on the new permitted frequencies.

Co-channel with digital TV?

Would a radio-mic work within a DVBT channel? Avoiding use of a mic on a working digital TV channel seems a given, but sometimes these things are assumed rather than proven. I have seen no research on whether a radio-mic could co-exist within an operating DVB-T channel. Some experimentation may be worth trying especially for use within large indoor venues.

As mentioned above, it is relatively difficult for a radio-mic to cause interference to DVBT reception unless circumstances conspire. Perhaps if several radio-mics were used within the same TV channel at the same venue, then blocking of the TV service may be possible. The main issue will be whether the radio-mic can deliver a satisfactory signal to noise ratio at the receiver in the presence of DVBT energy.

Bear in mind, digital TV is broadcast at a lower power than was analogue TV. Secondly the spectrum power of DVBT is spread over nearly 8MHz. The radio-microphone channel is around 300kHz wide and the spectral energy of the TV channel within any 300kHz band is 14dB down, or only 4% of the total TV power.

For any test, the first stage should be to use the opposite polarisation that is used by TV in the area. Unless you are unfortunately close to the TV transmitter, polarisation discrimination when added to the building penetration loss, may be enough.

A second test might be to use a frequency right at the edge frequency between two TV channels. All the DVBT services are actually 7.61MHz wide, and they are centred within their 8MHz channel. There is a 200kHz gap at each end. So, if the mic was centred at the channel edge there is a 400kHz wide space. Now, to be fair there are possible fishooks to the plan. First, the radio-mic receiver may not cope with the immediately adjacent DVBT energy in any case. Secondly, the 200kHz gap at the end of the TV channel will have some intermodulation products from the TV transmitter present. Normally, these reduce quickly and are further filtered in the station's channel combiner filters, but could potentially be a difficulty. That is why a trial is warranted before any commitment. However, if this proved viable it is a solution to the problem of not having a national set of frequencies. Be aware that the SKY Igloo TV channels occupy more of their channel (7.77MHz DVB-T2), so the Igloo channels are best avoided.

The future

The RSM group has left options open for further spectrum research. Ideally, radio-mics need their own GURL space however impractical that is, due to needing to use the same frequencies that radio-microphones are manufactured for. Some radio-mics internationally use VHF channels, but these are not authorised here. Now that the VHF TV band is entirely vacant (actually after 1 Dec this year), perhaps that is a potential use of some of that band. However I expect the radiation efficiency of a VHF mic is not as good as the UHF variants. Digital radio-mics are not yet a mature technology but are likely to become so. They will be generally lower in power output and ought to work in the presence of DVBT energy, so could make a good future option at least for the professional users. RSM is 'looking' at possible spectrum for these as well.

Axino-Tech Consulting & services; October 2013


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