Choosing a radio system for an assistive listening device - Page 1
Forum section: electronics / projects

author: tbj
author_rank: Contributor
timestamp: December 16, 2013, 08:37:55 am
content: Hello,I am currently working on designing a miniaturized version of the personal assistive listening devices you can get for hearing aids.My mother has a commercial version of this but she finds it unweildy and asked if I can design her something smaller. Challenge accepted.These basically consist of an induction loop you put round your neck, a receiver and an radio transmitter with microphone. You put the transmitter and mic in the middle of the room, and the receiver picks up conversations that you wouldn't otherwise be able to hear and sends them to your hearing aid(s) via the neckloop. This is useful because hearing aids are quite directional, and many people that wear them have issues with meetings or multi-person conversations.The induction loop bit I have already figured out, and I think I can drive it using a low-power class D amplifier for minimum power consumption. However, I am having trouble deciding on a type of radio system to use.The UK allocates the frequency bands 173.325 to 175.075 MHz for these systems at 2mW ERP, but I am a bit reluctant to send audio via FM due to privacy concerns (her line of work involves having conversations that are very confidential, and it doesn't seem right that someone with a scanner should just be able to pick them up).I have been looking at some of the ISM band radios that are available - 433MHz, 866MHz and 2.4GHz. Cost isn't really a problem because it will be a one-off unit, but I'd rather they didn't cost the earth. The main requirements are low power consumption, small size and ease of use.Power consumption is the biggie, because I would like the receiver to operate from a CR2032 coin cell.I'm planning to use a microcontroller or maybe a DSP to encode the audio to digital and send it over the radio link, so can anyone recommend some radio modules that will handle this? Data rate is an issue - these must be fast enough to be able to handle uncompressed audio.I am really just looking for some recommendations or ideas on the best way to proceed.Cheers.R

author: Rerouter
author_rank: Super Contributor
timestamp: December 16, 2013, 09:40:04 am
content: as its going to be a one off and you already have a micro controller then roll your own encryption on those designated frequency bands, doesn't have to be anything serious just something that prevents a Ham operator or radio from obviously playing back what it hears, e.g. reverse the frequency change from positive to negative and invert on the other end,if you wanted to get interesting you could probably phantom power the earpiece off of an AM carrier with a bit more punch behind it (neck loop would make a perfect antenna to pick it up) leaving the only power source in the transmitter, though arguably this will burn more power than just a battery,

author: fcb
author_rank: Super Contributor
timestamp: December 16, 2013, 02:36:13 pm
content: Nordic's excellent little nRF2460.

author: tbj
author_rank: Contributor
timestamp: December 16, 2013, 02:59:11 pm
content: Quote from: fcb on December 16, 2013, 02:36:13 pmThis looks perfect, except that I don't really feel comfortable soldering the 36-pin QFN package by hand (I have no hot air gear either).I am really limited to SSOP or DIP, or utilising prebuilt modules, to do this.Sorry, I should really have mentioned this before

author: tbj
author_rank: Contributor
timestamp: December 24, 2013, 06:08:14 pm
content: Quote from: Rerouter on December 16, 2013, 09:40:04 amI've been doing some thinking about this and I've decided that maybe analog is the way to go, so this is what I'm going to try - a simple FM transmitter and receiver on those bands.Just as a test, I made a little single-transistor FM bug like this:http://webshed.org/wiki/Simple_iPod_Txand the sound quality is OK, but it's unstable as hell and really difficult to tune into. Even moving your body in front of the transmitter changes the frequency or destabilizes it altogether.So I guess my question is this: how should I begin with building a stable, low-power FM transmitter that can operate at the prescribed frequency with good stability and ease of adjustment? I'm not asking anyone to design anything for me, I just need a good starting point and some reading material that would relate to this application. Maybe some existing designs to look at if anyone has done anything similar. I just don't know where to start.Thanks!

author: calexanian
author_rank: Super Contributor
timestamp: December 24, 2013, 06:24:55 pm
content: Quote from: tbj on December 16, 2013, 02:59:11 pmFor that sort of project I am sure we could find you somebody you could send the prototypes to and have them solder the i.c.'s on. I do it all the time for people who wander into our shop. Too bad I am in the U.S. or I would do it for you.

author: cwalex
author_rank: Frequent Contributor
timestamp: December 24, 2013, 11:53:15 pm
content: Quote from: tbj on December 24, 2013, 06:08:14 pmyou could try some of the circuits from 14 fm bugs to build by talking electronics. The ant is designed to be stable and can be carried. You can get a pdf of the book from their sitehttp://www.talkingelectronics.com/projects/14%20FM%20Bugs/14FM_Bugs.pdf