Wireless Microphone channels

[AfroAlex]

Hi guys,

 

Am just doing some research into this subject just for personnel interest as I have heard a lot of different answers and am a little hazy, sorry if I sound incompetent.

 

Basically as far as I'm aware, wireless microphones use either channel 70 (free channel) and channel 39 (requires license). Now I have been told that you are only supposed to use 4 microphones maximum with channel 70 and my question is why is this?

 

Is it illegal it use more?

If so how do they monitor it?

Can you actually use more?

Do you need a license to use more than on channel 70?

 

The reason I ask is because I went and saw a am-dram show and they were using 8 separate wireless mics. I asked the sound guy in the interval about it as was curious and he said they were all on channel 70 and he had no problems with bandwidth crossing over and also that he didn't have a license.

 

As you can see confusing to what I had been previously told.

 

Hope you can shed some light.

 

Thanks in advance,

 

Alex.

[Shez]

Hi Alex,

 

It's a confusing area!

 

It's actually only a small chunk of ch.70 that is licence free - between 863.1 and 864.9MHz. You can easily fit four radio mics in that space; with better mics you can fit in six. It's a technical issue rather than a legal one that prevents you from using more than that. Using eight in that space just isn't possible - he was either lying or didn't understand which bit of ch.70 you can actually use. And he was breaking the law either way.

[paulears]

The physics says that channel 70 is too narrow for lots of channels to co-exist. 4 seems the limit the manufacturers suggest, although there is the occasional claim of 5.

 

In essence it's pretty straightforward. Each manufacturer designs their system to occupy a specific bandwidth - some a bit more, some a bit less. Frequency response, deviation and signal to noise ratio are the main design considerations. If they overlap, then you will get problems. The way a receiver acts when another transmitter is too close also varies. Some are better at making the situation less nasty. Trouble is, it's unpredictable. Where I live, we used to get major TV interference in the summer when Dutch TV transmissions very close to ours would sometime cause interference. Other people not far away had no trouble at all. A combination of equipment, signal strength and location. Don't forget the actual frequencies often specified as the ch 70 band are the edges of the band. Using the edge frequency as your operating frequency means you are spreading into unlicensed areas - but of course can squeeze in an extra channel. I have never, ever heard of anyone squeezing in 8 - so as a guess, I bet he was operating outside the channel 70 band - you just caught him out.

 

So there is no legislation saying only 4 mics - it simply allocated a band for people to use, and it's the design that sets the number that will work. If you limited the frequency response, you could squeeze in far more as the narrower band required for the limited frequency response would let you have more. Just maths and physics really.

 

As for monitoring it? It's an unpoliced bit of the spectrum - and totally unprotected. No way I can ever imagine the investigation service would be sent out to check channel 70 - unless it was interfering with essential services.

[AfroAlex]

Thanks that's made a clearer. Thanks for all your help.

[Bobbsy]

As others have said, it's a bit of a misnomer to call the licence-free band "Channel 70". It's only a 1.8MHz chunk of the total 8MHz channel. Anything venturing outside that range is illegal and, although you're not likely to be caught, the penalties if you are are strict and expensive. Further, we're not that far off a time when the adjacent frequencies are going to be used by big telephone companies who WILL make a fuss if any interference is detected.

 

At the risk of telling experienced users things they already know, FM transmissions (and radio mics are FM) work be deviating their frequency above and below the specified centre frequency. The higher the audio level, the wider the deviation.

 

Taking a Sennheiser G3 as an example, it specifies that the deviation at an audio peak is +/-48kHz. This means that, on a loud part of the audio, each channel is occupying just under 100kHz bandwidth--which is why you can't select the 863.1MHz centre frequency and use it legally--all you loud bits will extend 48kHz out of band.

 

It gets worse though. No electronic filter is perfect and, in the receivers, each channel will likely need around 200kHz free space--maybe more--to avoid receiving interference from the next channel. The exact number will depend on the quality of the gear--which is why the EW300G3 can cram in more channels than the EW100G3.

 

Beyond all the above, you can't just depend on bandwidth either. Every time you transmit a radio frequency, there are also harmonics generated on other frequencies at specific spacings above and below the main frequency. This means that, even if apparently separated far enough, certain frequencies interfere with other ones. This is basically what Intermod software like the Sennheiser SIFM system calculates for you.

 

So, that's the long way of saying "I'll eat my hat if the guy you met crammed 8 channels into the licence free area of Channel 70. He was cheating, either through a lack of understanding or deliberately.

[Shez]

each channel is occupying just under 100kHz bandwidth--which is why you can't select the 863.1MHz centre frequency and use it legally--all you loud bits will extend 48kHz out of band.

As you're outside the UK, I'll forgive you that one ;) I always refer to the licence free band as being between 863.1 and 864.9 simply because those are the outside most centre frequencies that you can ordinarily use. The actual band is 863 - 865 so using 863.1 as a centre frequency is fine. If I say to someone inexperienced that they can use anything between 863 and 865, then the first frequency they might set would be 863 - this of course is not acceptable because the deviation then goes down to 862.9 or thereabouts, which is outside the band.

 

which is why the EW300G3 can cram in more channels than the EW100G3.

Engaging pedantry mode for a moment, aren't the RF specs of each of the G3 family the same? It was that way with the G2s in that it was only facilities, channel presets, colour display etc that was different; the RF side was the same across the range.

[Brian]

Further, we're not that far off a time when the adjacent frequencies are going to be used by big telephone companies who WILL make a fuss if any interference is detected.

Or more likely their big and chunky mains powered transmitters will put out way more power then a puny battery powered radio mic and will trample all over anything straying into their airspace. You won't pick up any audio from them but your signals will be totally swamped.

[AfroAlex]

so when a wireless mic has 16 channels is that to be able to use 16 mics with a license or is it just 16 different channels in case someone else in the area is using wireless.

[Bobbsy]

Engaging pedantry mode for a moment, aren't the RF specs of each of the G3 family the same? It was that way with the G2s in that it was only facilities, channel presets, colour display etc that was different; the RF side was the same across the range.

 

The transmission spec is the same but I have to assume (dangerous, I know) that there is a difference in the quality of the filtering in the transmitters and receivers. Certainly there has to be some difference since (as evidenced by posts here) the more expensive G3 systems can cram extra channels into the same bandwidth.

 

Apologies for the confusing on the licence free frequencies. It's now almost six years since I left the UK and things I used to know are becoming distant memories. I just accepted the frequencies posted above as fact!

[Shez]

so when a wireless mic has 16 channels is that to be able to use 16 mics with a license or is it just 16 different channels in case someone else in the area is using wireless.

Good question - it's one some manufacturers muddy the waters a little with. If a system is advertised as having 16 selectable channels, it means that each transmitter can be set to any one of 16 different frequencies. That's pretty much the end of what you can read in to that.

 

On some systems (e.g. Sennheiser SK50), that also means that you can operate 16 systems together on those 16 frequencies (with the right licence of course). On others, it doesn't mean that. I've seen cheap ch.70 systems that offer 16 switchable frequencies but you can still only use four of them together (and even then only if you choose the right four frequencies).

Edit: clarification

[paulears]

16 channels is what it's capable of operating on. It's up to the user to determine where exactly these frequencies are.

 

People like Shure always designed their kit to display a channel number rather than frequency, and as a result it was difficult to blame the users who had no idea what frequency they were using?

 

Even 16 channels isn't that easy to fit in. It also doesn't guarantee in most cases that all 16 can be used at the same time - as Bob mentioned - there are harmonics outside of the main frequency which will inter modulate with other fundamental and harmonic frequencies and produce 'phantom' signals at new frequencies. Very complex and the reason why most manufacturers give you sets of channels to use that will work together. This usually means some of the available channels need to be avoided, because using them, even if they appear to be in a clear gap, can actually be destructive.

 

RF theory is pretty magical sometimes. Common sense won't work without some fairly clever maths - and often the figures you need are very difficult to get from the manufacturers.

 

What seems certain is that when 4G kicks off, they will have to pay compensation to people who need filters on their TVs - nobody has even mentioned radio mics.

[Bobbsy]

so when a wireless mic has 16 channels is that to be able to use 16 mics with a license or is it just 16 different channels in case someone else in the area is using wireless.

 

 

A bit of both. The radio mics I had back in the UK had way more than 16 channels--from Channel 69/70 down through Channel 67. My standard licence was for Channel 69 where I could use 12 channels (or sometimes 14 with care even though the numbers said it was a problem) and, if I needed more, it was possible to get temporary licences in the lower channels--for a cost.