Radio Mics

[box1]

Hi all,

 

I am new to this website and am looking for a bit of advice. Im doing a show in a few weeks for a school and was looking into radio mics and trying to get an understanding of frequencies. Alot of it makes sense but I see switching bandwidths explained alot, and that some have narrower / wider widths. Can anyone explain this to me please and the benefits and results of using different bandwidths etc.

 

Also any other advice that you have in regards to stable frequencies within multiple systems would be great. I think we are using 16 shure ULX's

 

Thanks.

[Jon Wiles]

What ch are you using the ULX's on......ch 38/39/40 or ch 69

 

Jon

[paulears]

It's not too complicated. The switching bandwidth is the range over which the transmitter or receiver will operate. As RF gear relies on lots of tuned circuits, it's always been very difficult to design a transmitter or receiver that would be efficient across a large range. Back in the 80s and 90s, it was very common for the actual tuneable range to be very narrow, and the PLL chip - which locks the thing to the desired frequency would lose lock and the thing would stop. It didn't matter too much, when we had radios on fixed channels that were not user adjustable - you bought them pre tuned for the working channel. These were simply tuned circuits and didn't need to switch channels. The user switching we take for granted involved the 'new' PLL chips which could keep the oscillator running on a wider range of frequencies. Bit by bit the range has increased and is now very wide - enabling access to big chunks of bands. The actual switching bandwidth changes between brands and even between models - some of the newer Sennheisers have wider coverage than their predecessors. So we have one design parameter - top and bottom frequency limits. Next comes bandwidth. The audio signal applied adds to or subtracts from the centre frequency - the deviation is usually measured in KHz and sets the maximum audio frequency response. So a bigger deviation allows extended audio bandwidth, at the expense of less channels in a given space. Even worse is that the deviation if too narrow doesn't just limit the available frequency response, but also worsens the signal to noise ratio. To get around this problem, it's common for the audio to be compressed before transmission and then expanded in the receiver - a compander being the usual description. This helps improve the signal to noise ratio. Each manufacturer selects optimum specs for their system. Theoretically this should make the chances of one make of system working with another unlikely - in practice it often works. I routinely use a rack of Trantec receivers on Sennheiser transmitters - I tried it once and discovered that the combination actually works very well, and despite having the proper receivers, I still use the Trantecs because the compander doesn't match exactly and seems to leave the sound slightly compressed which I like. The other way around - Trantec transmitter and Sennheiser receiver is a tad over expanded - and not quite so pleasant for vocals.

 

The band plan for your system needs to use the recommended frequencies the manufacturers suggest. Every one has frequencies selected to produce good results on their own kit. These may even be different for different models in the range. As soon as you start using lots of channels, you cannot just pick frequencies by doing a bit of maths and leaving gaps based on the bandwidth specs. RF systems can be very unpredictable - You may be operating at 800+MHz, but internally there will be many other frequency divisions taking place in the receiver. All these oscillators can beat with each other - so certain combinations of operating channels simply won't work - there will be horrible interference. One channel may be fine, add the next and it still works, but add a third and the first two make nasty swirly noises! Some seem to work until two transmitters get close to the receivers and all hell breaks loose! Move away from the test frequencies at your peril. Some manufacturers will give you some software to allow you to test in advice for these problems.

 

The ULXs have a good reputation in multichannel systems - but if you're hiring them, the hire company will usually set the channels up for you in advance to a plan they know works.

 

When you see people cobble together systems from lots of different makes hoping to get a solid and stable multichannel system, it can be ok (luck) or simply unworkable (far more common).

 

That's a quick trip around the block - make sense? I've simplified a few bits but the guts is there.

[Bobbsy]

Paul's given a very good explanation there, but just to emphasise one point he was making: you're best off if you can obtain all the gear from a single source and working with frequencies they've worked out (and, if necessary, licensed) for you. There's a lot of interaction between frequencies (even if they seem a long way apart) and the only real way to be sure on this is a mix of experience and some good software that sorts through the various options. Any reputable hire company will have this all sorted out for you when you pick up the gear.

 

On the other hand, a recipe for problems is to borrow some mics from one place and rent others (probably a different make/model) then try to make them play nicely together.

 

One last thing, not to do with the mics themselves, is that you shouldn't underestimate the complexity of mixing a reasonable number of radio mics together. Make sure you rent early enough to give the operator enough time to set things up and practise.

[box1]

Hi Guys,

 

That has cleared a few things up. The only thing im still slightly confused on is where you say " The audio signal applied adds to or subtracts from the centre frequency" I understand how the deviation can effect the available spectrum space. Sorry if im being thick.

I appreciate your help

[paulears]

In the good old AM radio days we modulated the carrier wave with the audio waveform, and as the signal strength became weaker, so the volume dropped off - in FM radio we don't modulate the carrier wave at all - we push or pull it away from the centre frequency, so set to 864MHz, a transmitter that says in the spec, 45KHz maximum deviation, means that it extends out on signal peaks to 864.0225MHz one way and 863.9775 the other. If you allow a guard band so the next channel down doesn't bump into the other, you can see how fitting in wider deviation equipment eats up more space. It also shows why the license free channel 70 is NOT 2MHz wide - because if you set 863MHZ as your first channel centre point, when the system was running, half of your deviation would be downwards, into unlicensed territory. Does that make sense? Deviation has an impact on frequency response too - walkie talkies usually use 12.5KHz deviation - which is fine because they filter off the bass and treble, leaving just the speech frequencies. You can squeeze in a lot of comms into the bandwidth of an wider range audio frequency. The latest comms kit uses 6.25KHz wide channels because even these bands are pushed for space. Video, takes even wider chunks of band - hence why TV channels we're using for radio mics were originally designed for one TV channel!

 

useful info here.