Drive-in cinema radio transmitter

[Stuart91]

Some customers of ours have decided to hold a drive-in cinema event in a couple of weeks time. It's being done by an established youth group, they are basically going to park up four or five of the leaders' cars and fill them with children, then watch a movie projected onto a wall.

 

The projection side will be OK, the challenge is the audio. Since it's December in the west of Scotland, they are going to have to keep the windows closed and probably engines idling to run the heaters. This means that we can't just set up a conventional PA system for them, by the time it's loud enough to be audible inside the cars it'll be annoying the neighbours.

 

I know the traditional way of doing this has been to use an FM transmitter, and have each car tune their radio in to get the soundtrack. Amazon and eBay are full of small FM transmitters, but they all seem to be designed to work inside one vehicle only, so I'm not sure that they'd have the range.

 

The other solution I can think of is to use IEM systems, and use a minijack lead between the headphone socket and the aux-in on the car stereos. Which would work as long as everyone has an aux port, and I suspect at least some won't.

 

Is there an FM transmitter available anywhere that can do, say, 10m distance, without costing a fortune or infringing any laws?

[Cerbertech]

If you go the IEM route you could use an FM transmitter as used for IPods etc in the cars without a line in.

[adam2]

If only four or five vehicles are involved, might it be simpler and more reliable to simply provide a small extension loudspeaker for each vehicle ? to be placed inside the vehicle. No reliance on unknown equipment installed in each vehicle, and no discharge of the vehicle battery if the engine is not run.

 

 

 

 

 

[pmiller056]

Using FM, even over a short distance is legally dubious. I know for a long time the small ipod devices were illegal, but I think it is now the case that a blind eye is turned, provided there is no interference with anything else. I have used some of the small transmitters and the audio quality has generally been mediocre to poor, even with a decent FM receiver. I'd go along with adam2's suggestion - even if it forces the user to open a car window a bit so the car doesn't steam up! I do remember seeing an old picture of a drive-in where a portable speaker was plugged in to a post at the side of the parking bay and the speaker had a bracket that hooked over the window glass. I doubt if the kids would notice or care if it was glorious mono instead of full surround sound.

[bruce]

The “in-car” devices are required to be less than 50 nW ERP to remain legal. I suspect many of them are higher.

If you have a clear frequency spectrum in the adjacent bands, you might get 10m out of that. Or maybe not...

 

Using FM, even over a short distance is legally dubious. I know for a long time the small ipod devices were illegal, but I think it is now the case that a blind eye is turned, provided there is no interference with anything else.

 

Not quite true - it’s not a “blind eye” - they used to be illegal, but are now specifically permitted under the Wireless Telegraphy (Exemptions) Regulations 2006, but the devices must be CE marked, and have an ERP of less than 50 nW.

[timsabre]

Using FM, even over a short distance is legally dubious. I know for a long time the small ipod devices were illegal, but I think it is now the case that a blind eye is turned, provided there is no interference with anything else.

 

If you can find a relatively unused frequency and put it close to the cars, you should get enough range to do 4 cars. I've got one and it does work from outside the car for about 10 metres, but only if it doesn't coincide with an actual station - it struggles to work inside the car then.

[Shez]

I do remember seeing an old picture of a drive-in where a portable speaker was plugged in to a post at the side of the parking bay and the speaker had a bracket that hooked over the window glass.

A very old picture

 

http://i1219.photobucket.com/albums/dd434/Sheriton/flintstoneCinema_zpshwyryrus.jpg

[bruce]

[

 

If you can find a relatively unused frequency and put it close to the cars, you should get enough range to do 4 cars. I've got one and it does work from outside the car for about 10 metres, but only if it doesn't coincide with an actual station - it struggles to work inside the car then.

 

There are (cough) ways to increase that range...

[paulears]

How about hiring some of the systems they use on the headphone silent discos?

[bruce]

That’s not a bad idea Paul - in fact, googling this suggests that at least one silent disco supplier (https://www.sdksales.co.uk/blog/wireless-audio-for-drive-in-movies) has boxes which are used for exactly this purpose.

 

Going for a “proper” FM transmitter with reasonable power really needs a RSL license, which is ridiculously expensive. Although if you were to fire up a half-watt transmitter out in the country, for one night only, the chances of an Ofcom enforcement officer visiting (especially in a country which doesn’t to my knowledge have any Ofcom enforcement engineers permanently based there) are vanishingly small. But of course I could not recommend that.

[dbuckley]

The "traditional" drive-in transmitter is the BW Broadcast TX5, which can be dialled back down to almost zero output. The Mk2 also also has a multi band compressor in there, which vastly improves reception, particularly given the engine will be running. It doesn't cost a fortune, but it's rather more than the little 1W transmitters one can get on AliExpress.

 

If you've got any sort of fancy audio coming out of the playback system, ie anything other than stereo, then you need to do a mixdown for transmitter; mix the centre channel into the left and right channels, at a level of about 6dB higher to create a centre weighted mix. If you're mono, the same applies; mix L&R together, and then add centre at +6dB.

[bruce]

The "traditional" drive-in transmitter is the BW Broadcast TX5, which can be dialled back down to almost zero output. The Mk2 also also has a multi band compressor in there, which vastly improves reception, particularly given the engine will be running.

... and there are several places in the UK that will hire one of these to you - although the legit ones would be asking about your RSL license.

 

We have a couple of spare units (including the 25W version of that BW system, and an Aareff/Veronica system) at the community radio station that I am involved with.

[dbuckley]

We have a couple of spare units (including the 25W version of that BW system...

 

Perhaps Stuart should PM you!

 

Edited to note: I tend to forget that Joe Public doesn't have any legal way to broadcast on the FM bands in the UK, unlike in the USA, many countries, and here in NZ, where you can have 1W eirp in the edges of the broadcast band. In the UK, you're, well, a pirate, and if caught, will feel the full force of the law.

[J Pearce]

Any transmission that can be easily decoded is almost certainly beyond the license terms of the film. You'd be fine with wifi or Bluetooth speakers, but transmitting the soundtrack of a film over FM would definitely be questionable.

 

Bluetooth speakers might not be a bad idea actually - the youth probably have enough between them too.

[dbuckley]

Any transmission that can be easily decoded is almost certainly beyond the license terms of the film.

 

Using FM for movie audio has been custom and practice for drive ins, and has been since the 1980s, when car radios got transistorised and good, and sound became important in film. Prior to that, it was speakers on poles in the field that you lifted into the car, but they were just mono, and fairly horrible. Going to FM stereo improved the audio for drive-ins a lot, and reduced the maintenance requirements.

 

It's where the pirates get the soundtrack from to add to their camcordered movies.

 

I've just checked our licence for a major distributor, and there is nothing in there specific to audio or pirating, other than not to "allow any person to infringe copyright in the Film, Trailers or Advertising Materials;".

 

This could be interpreted to keep the transmitter level to such that the broadcasts cannot be received over an excessive area.