Radio transmitters

[Bryson]

Hello all,

 

I have another strange one for you all. Can anyone help me with the practicalities and legalities of radio transmitters?

 

I'm doing a site-specific promenade performance based in a house. It's going to have multiple musicians wandering around the house with radio mics. So far, so good. But I need to get the sound redistributed back around the house. So, in keeping with the style of the piece (the house is quite decrepid, and the piece is about the "ghosts" of people who once lived in the house) I hit on the idea of having a suitable old-looking dodgy radio in each room, playing back the sound.

 

High fidelity sound is not an issue here, so I can consider most solutions. (ie: it doesn't need to be FM) And, being based in a single house, my need for range is very short. Say, 20m at the most (obviously, there are walls in the way.)

 

What I'd like to do, ideally, is have a matrix mixer, sending the feeds from each radio mic to a number of different transmitters, each transmitting on different frequencies, so I can control what goes to each room. (To try and avoid feedback problems.) So, my questions are:

 

Is there any means by which I can do this legally? I have no idea how radio licencing works in the UK, whatsoever.

Where can I get transmitters? I'm assuming (as you can make one with a kids electronic kit) that they aren't massively expensive...?

Is there any way to transmit multiple audio channels on different frequencies?

 

Many thanks in advance for the help.

 

Bryson

[bruce]

How about finding old radios - or "radiograms" etc - with a line input and using an IEM set for each one? Or even another lav radio mic pack running "backwards" with a line input from the desk rather than the mic?

[Bryson]

Could do...but all those IEMs are going to get pricey, methinks.

 

Perhaps camera-style radio mics could be an option. I would still like to investigate doing it with "real" radio, though.

[The Magus]

Have you considered using wireless IEM systems? I'm sure this would be the easiest solution. Having found your old radios, simply make up a lead using screened audio cable for each, consisting of a 3.5mm jack on one end (to plug into the headphone output of the IEM receiver), and wire the other end into the radio, with the screen going to ground, and the core to the wiper (centre contact) of the volume pot. Disconnect the wire from the wiper contact (on older radios where the rotary controls aren't PCB mounted), or if the volume pot is PCB mounted, unsolder the centre contact from the PCB and bend it up so that you can attach the signal wire from the IEM receiver (thus disconnecting the signal from the radio's tuner section). You may want to de-couple the signal with a capacitor (10uF at 16v would suffice). I hope I'm not teaching my grandmother to suck eggs! This seems such an obvious solution there may be a good reason why it won't fit the bill.

 

EDIT - I see Bruce just beat me to it!

[dbuckley]

If you were in some other regions of the world you would be allowed to use low power FM transmitters to deliver your audio to unmodified broadcast receivers (I'll try not to gloat!), but alas, in the UK, that is not permitted.

 

I think you'll find this quite hard to do legally and cheaply. Most of the spectrum space usable without draconian licencing is for specific purposes, so CB bands are not usable for your application. Your only legal options I can think of are the radio mics as suggested above, or short range TV/Audio senders, but you cant have too many of them together, or wireless networking sending audio, something like a Wireless Barix Exstreamer (this is an example, there are many others), but this then leaves you with an encoding problem, you'll need either yet more hardware or a PC.

 

What I would do would be to build a cabled RF distribution system. Run coax round the building to each point you need to place a receiver, and then use a standard cable TV coax tap to break out. The coax you run doesnt need to be thick heavy CATV cable, something lighter in duty and weight. It'll leak a little, but hey... A few FM encoders, a resistive mixer, job done.

[Ellis]

If you were in some other regions of the world you would be allowed to use low power FM transmitters to deliver your audio to unmodified broadcast receivers (I'll try not to gloat!), but alas, in the UK, that is not permitted.

 

I think you'll find this quite hard to do legally and cheaply. Most of the spectrum space usable without draconian licencing is for specific purposes, so CB bands are not usable for your application. Your only legal options I can think of are the radio mics as suggested above, or short range TV/Audio senders, but you cant have too many of them together, or wireless networking sending audio, something like a Wireless Barix Exstreamer (this is an example, there are many others), but this then leaves you with an encoding problem, you'll need either yet more hardware or a PC.

 

What I would do would be to build a cabled RF distribution system. Run coax round the building to each point you need to place a receiver, and then use a standard cable TV coax tap to break out. The coax you run doesnt need to be thick heavy CATV cable, something lighter in duty and weight. It'll leak a little, but hey... A few FM encoders, a resistive mixer, job done.

 

... and of course cheap coax 'leaks' RF - especially when not correctly terminated!

 

If you want to be legit, and can cope with a single channel, you could look at acquiring a short-term radio restricted services licence. As for equipment, low power FM transmitters that comply with the technical requiuirements are commercially available.

[beware]

or some form of surround sound amplifier with a small speaker wired to each box. this would give you at least 5 seperate outputs.

 

ie, http://www.argos.co.uk/static/Product/part...r%2Bsystems.htm , assuming it has 5 seperate inputs (I know argos used to do one like that)

[paulears]

If you want to do it legally, then you could always apply for a restricted service licence - the type normally used for special events. The snags are that paperwork and permission is very complicated and time consuming - and you need to plan early. Apart from this, then radio mic transmitters coupled to cheap receivers - the kind often advertised on ebay for airband and listening to ambulances - NOT scanners, old fashioned cheap things costing about a tenner with an real tuning knob - if you can get one that covers up to 175MHz it will do nicely. If you want decent quality, more reliable stuff, then cheap old scanners with a wideband facility work too.

 

Illegally, there are tons of transmitters available that will be picked up properly by any broadcast radio - but you'd be treated as a pirate radio broadcaster, apart from the fines, they can confiscate the equipment - which would include anything connected, so not just the transmitter - they could confiscate everything mixer, cd, mics - whatever they find connected and working.

 

So

Legal 88-108 MHz = expensive

Legal VHF radio mic band = not too expensive, but fiddly and likely to be either unreliable, or poor quality

Illegal 88-108 MHz = cheap, reliable and very, very risky!

[dbuckley]

... and of course cheap coax 'leaks' RF - especially when not correctly terminated!

No! Surely not!! What could I possibly have been thinking... Put a leaky feeder round the house?? :wacko:

 

OP needed several parallel paths, I would have thought that fact alone would rule out short term RSLs. If you could get licenced for same, and could afford it, it would be the most convenient way though. Here in NZ we have (I think) 16 frequencies in the extremes of the broadcast FM band which anyone can use, with minimal use restrictions (except power and technical) which is how I'd solve the problem here. There. I gloated. Thats very ugly of me...

 

Another thought is maybe WTs will do for what you need to do...?

[GUSTie]

I would forget broadcasting on FM, UHF or VHF. I would go for wires, its cheap, reliable, easy, not going to get you fined, and requires NO paperwork. I am constantly finding my self wishing that I had a broadcast licence but it is expensive and fiddly. coax would work but I would go for just running XLR and bodging it.

 

Matt

[Bryson]

Thanks all. I was hoping that there was some deregulated expanse of AM that I could use somewhere at a very low power, but it seems not. Shame really - it was a nice idea!

 

Looks like (as cabled solutions are absolutley no use) that IEMs inside old radio cases are the way forward. I thought I'd hit a nice easy solution, but it seems that's not the case. Oh well.

[djw1981]

If you can cope with Mono sound, you can use 2 IEM packs per transmitter frequency. You pan one hard left and the other hard right. You then make sure you mix your sends correctly and send say Aux 1 and Aux 2 into transmitter 1 which then feeds IEM 1 and 2. It works for bands as well, if they only wear one ear of IEM anyway. This halves the number of frequencies you need, which is handy!

[stuartglen]

I'm a lampie but have experience using radio control for other things ie model aircraft, your main reason for wanting seperate channels is for volume control, why not broudcast your audio on one channel and use seperate radio channels to control the audio on each radio, if these are old radios you are starting with then the volume controls will be based on a rotary potentiometer, just attatch a servo to this and use a cheap radio control for each radio.

 

You could use something cheap like this

http://www.alshobbiesstore.com/acatalog/jamara_compa_x4.jpg

£40 and 4 channels

http://www.alshobbiesstore.com/acatalog/Jamara_Radio.html

buy 4 recievers and 4 servos and this could do 4 rooms

 

expensive option but might be cheaper than radio mics

 

another idea if quality is not important then you could use cheap two way radios, walkie talkies set the one at the sound desk to vox and plus the headphone port of the one by the radio into the amplification circuit.

 

These are just a couple of ideas I had feel free to ignore them

[PoppaDom]

I have completed a similar project on a low budget utilising low power 2.4Ghz video/audio senders on different channels, broadcasting on multiple transmitters on different channels and then placing different receivers attached to small amplifier modules built into old radios. Worked a charm and was quite cost effective.

 

Video/audio transmitters were about 15squid and receivers a tenner.

 

PM me if you want more details.

 

Poppadom

[Peter Russell]

If you can cope with Mono sound, you can use 2 IEM packs per transmitter frequency. You pan one hard left and the other hard right. You then make sure you mix your sends correctly and send say Aux 1 and Aux 2 into transmitter 1 which then feeds IEM 1 and 2. It works for bands as well, if they only wear one ear of IEM anyway. This halves the number of frequencies you need, which is handy!

This doesn't work in practice unfortunately as you can only expect 10-20db of seperation between the left and right channel.

I found this out the hard way the other week on a show using Trantec S5000 IEM's.

On speaking with both Trantec & Sennheiser they confirmed that this is to be expected and not a fault with the kit I was using.

 

Peter