Ghetto Silent Disco System

This Is A Test...

Many of the villages at DEFCON this year are sharing a single space at the Bally's Events Center. The Events Center looks to be a huge convention floor space. With all the villages in the same space I'm expecting a lot of noise from attendees. Additionally, villages will have talks and media displays going on in the same space. My thought was how to be able to give talks and play media while overcoming the background noise *and* not adding additional noise to the environment.

I though about "silent discos" and how I could set something like that up. For media, streaming from a server would be fine but for a speaker it wouldn't work. There would be too much lag. There are systems that tour guides and churches use that consist of a wireless mic and a set of receivers with headphones. These aren't cheap. I'm sure they work well but I didn't have +$500 to drop even on a small system. I decided to experiment with a Ghetto system.

The system I'm building consists of three major components

The FM Transmitter


Auto Drive Car FM Transmitter

The transmitter is one of those things that, if you don't have an AUX input on your car stereo system you can use to play audio over the FM car radio. This one has an audio jack input, Bluetooth and SD/USB to play MP3s. It also has a IR remote. I got this one at Walmart for $25

It runs off 12 VDC but I have an AC to 12 VDC adapter that has a auto power socket. You can set a FM radio station to broadcast to and then play your audio source through the device. You'd pick a channel that isn't being used. The transmitter is really weak so it is legal as long as you don't interfere with other radio stations.

The Radios


I ordered a bunch of (10) cheap AM/FM radios with headphones. Specifically headphones because we don't want people sharing ear buds (yuck).  Each of the talk attendees will use a radio tuned to the FM transmitter frequency to hear the audio. Thes GPX radios cost $8 at Walmart. (total of $80)

The wireless Mics 

Naxa NAM-984

The last major component was the wirless mic system. These don't broadcast on a frequency that can be picked up by the AM/FM radios (unless modified). I found a cheap two mic setup. I got this one for $10! Not much more than a glorified "Mr Microphone" (actually the whole system could be done with an old Mr Microphone)

Initial Results

It works! To a certain extent. I tested it with some old crappy radios not the final, GPX, radios (they haven't arrived yet). But I'll retest when they come in. I'm not sure how the whole system will hold up in the actual village but it's all an interesting experiment for about $120 that supports an audience of 10. The system can easily be expanded by adding more radios or having attendees use their own radios.

I'll report back as I do more tests.

--- Update ---

I got the AM/FM radios in and it works well! I was able to easily broadcast around 30 feet and that was across the apartment. This should work even better in a unobstructed environment. One problem encountered is that while the FM transmitter is great in that it supports Bluetooth, unless it is paired it is constantly looking for something to pair with. This will be a problem at DEFCON because someone will undoubtedly try to pair with it. I'll pick up one that only accepts an audio jack. 

Another concern is the RF environment in the conference space. Two things could happen.
  1. The conference room could block outside RF, meaning there would be little or no reception of FM radio stations. This would be good because I wouldn't be competing with them.
  2. The RF environment is extremely noisy. There is a lot of RF broadcasts going on from Ham and other radios. While not on the same frequency, a close transmitter could overpower either the wireless mics or the FM frequency. While Ham equipment shouldn't do this there still might be a chance.
The final test will be setting it up at DEFCON and actually seeing what the performance is. I suspect we'll get something but I'm not sure exactly what it will be,