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External DC powered radio mic tansmitter


cedd

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Hi all

Interesting project! I've been approached to build a teaching aid that'll essentially be a motorised radio control vehicle (think ride on kiddy car sort of size) with some on board cameras and audio kit to allow the operator to interact with children even though they're controlling it from another room.

The actual build of it is quite straightforward. 2.4GHz radio control system and a couple of on board cameras to relay FPV back to the operator (including one camera mounted on a stand away from the vehicle so the operator has a sense of what's happening around them).

They will also wear a headset (I'm thinking a comms system style one) that allows them to speak through an onboard speaker and listen to an onboard mic. The control equipment will all be packaged in a peli style case, powered by a local battery inside it with the option to plug it in to charge too. The kit on board the vehicle will be powered by on board batteries (12 or 24V - it needs both for other systems).

The obvious way to go is a pair of radio mic systems working in either direction, with a beltpack and receiver at each end. Using the pre amp from the "send" beltpack to amplify the operator's headset mic. The receiver is mounted in the vehicle and feeds an amplifier via its' line out. The "return" system has a small mic mounted to the vehicle, again using the beltpack's own pre amp and using the receiver's headphone out to drive the headset earphones.

I just can't help thinking there's a more elegant way of doing this. I'd happily strip the PCB's out of the beltpacks and receivers to reduce weight and space, and can probably provide external DC power by soldering to the battery terminals on the beltpacks (via a buck converter to give battery voltage).

It all just seems a bit Heath Robinson though. Using a dedicated audio link like the kind used to drive remote speakers is one way to go, but then I lose the mic pre amp and headphone amp, so will have to do them separately. I'm also conscious that I've got beltpacks and receivers built in to both ends, which not only means 2 aerials but also the chance of intermods and overloading the receiver front ends.

I highly doubt it, but do any products exist that fit the bill? I'd prefer to stay in the UHF spectrum if I can as I'd like to leave 2.4GHz free for radio control and video, and might need 5GHz for a video sending system if I try to put a built in screen on board to show video and stills, and maybe give it some animated eyes (still being decided if they want this functionality).

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What interesting projects you get to do!

 

I would use radio mic equipment but I'd be looking at using on-camera type receivers e.g. Sennheiser EK100 as these are much more compact.

One advantage of using radio mic systems is that you shouldn't have problems with intermod as they are designed to be used with multiple units close together. If you found other modules to do it (I am not aware of any) then I would be worried about overloading front ends.

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Does what you're using for video link support audio? Although, it would probably be going in the wrong direction compared to the video.

 

Is a PMSE radio mic license, or use of the license free fully compliant for use in this scenario?

 

Would a duplex (possibly even semi duplex?) PMR type or digital comms radio be a simpler setup? Base station and headset at operator end, PMR handset and adaptor cabling at remote end, you could probably hack the speaker drive from the handset if you wanted a better/bigger speaker.

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Would full duplex radios be a neater solution? Headset one end, and speaker/mic arrangement at the other.

 

Paul should know what's available.......

 

(Phone rang & Jon posted while I was talking!)

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Most fpv transmitters carry audio as well, so the operator can see and hear.

Why not use IEM ? They are designed to work in that direction, and it's relatively easy to connect to a powered speaker. Loads of room in a ride on car

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There are baby monitor systems that already combine video and 2-way audio - I'd be looking to hack one of those to combine all your coms in to one simple tried and tested unit rather than trying to merge conflicting systems together.

 

Sound quality and range is rubbish though (speaking as someone who has tried to use them for the intended purpose). 10m line of sight was a struggle with the ones I tried and the sound was telephone-grade or worse.

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Thanks folks.

I think, as I kind of expected, there isn't an off the shelf solution that's going to work out for me. Audio quality needs to be good enough to address a classroom full of school children (I'm also contemplating providing a line out from the peli case controller to feed external speakers if needs be for larger groups - that changes my needs for a pre amp in that direction as part of the mic system). Sound quality therefore needs to be as good as a radio mic system at least.

IEM's don't really do what I need - at least not in an "off the shelf" way. What I could really do with is a combination of a beltpack transmitter and an IEM receiver that are known to play well together (or an eng receiver as somebody else pointed out, though I don't think they have a headphone out that'd be suitable for driving the headset directly). I'll then just hack in some wires to the battery compartments (or I could 3d print some fake batteries with contacts on them and thin wires going to them - that'd be a fun way around the issue.

PMR would be good if I didn't need to simultaneously transmit and receive. I realise that the mic in the vehicle will pick up the amplified voice, but I'm hoping with some careful mic and speaker placement I can minimise problems. It'd actually be nice to have some of the transmitted audio get back to the operator as a bit of a sidetone-like confidence check.

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What I could really do with is a combination of a beltpack transmitter and an IEM receiver that are known to play well together (or an eng receiver as somebody else pointed out, though I don't think they have a headphone out that'd be suitable for driving the headset directly).

 

Sennheiser beltpack transmitters and ears receivers will do this - at least, SKP transmitters will transmit to ears beltpacks. I've done this in the past to hack together wireless comms.

 

<snip>

 

 

It'd actually be nice to have some of the transmitted audio get back to the operator as a bit of a sidetone-like confidence check.

 

Some kind of matrix mixer needed then - I used an old (green) Soundweb for my comms mixer.

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