Induction loop audio

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a.mlw.walker
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Posted by a.mlw.walker: Mon. Nov 19, 2012 - 06:36 PM
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Hi everyone.
I am outputting audio from OCR1A (pwm) on a mega1284, passing it through a resistor and then to a 3.5mm jack for headphone connection.

I have been reading around and seen people making home induction loops for impaired hearing, and I wanted to make a portable mp3 player, for my grandad, who already wears an induction capable hearing aid.
So currently everything works on the mp3 side of things - i.e I can output the sound but I want to swap the ack for an induction loop, but not sure about the mathematics, and what is necessary.

I understand the more coils the better, but lets say I wanted to "induce" the audio over a metre, what would I need to think about in terms of circuitry and battery/power source. Could someone give the example calculation please? I assume I can buy an inductor rather than coil the wire myself...

Thanks
Alex

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Learning and Information, General Electronics, ATmega128, ATmega1284
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jgmdesign
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Posted by jgmdesign: Mon. Nov 19, 2012 - 07:31 PM
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COuldn't Grandad just use a set of NICE headphones instead?

Then again you could always make a wireless headset and 'induce' from even greater distances!! ;)

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js
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Posted by js: Mon. Nov 19, 2012 - 08:17 PM
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Careful if grandad has metal implants, they will pick up the sound. :-)

You can try with a few turns (4??), under the floor or in the ceiling, and feed it from a small audio amplifier (10W?)

In theory you would want the loop (2m in diametre or more??) to measure about 4-8 ohms as it it were a speaker, some car amplifiers will easily drive into 1 or 2 Ohms. Then adjust the volume to suit.

You will need some form of fast override from the music if the hearing aid is also to be used for the phone.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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ka7ehk
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Posted by ka7ehk: Tue. Nov 20, 2012 - 05:29 PM
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I had thought that these induction loop audio systems used a carrier, but I read that they do not:

http://en.wikipedia.org/wiki/Aud...

There must be some sort of pre-emphesis (for low frequencies) since the dI/dT drops as the frequency drops. To quote WikiPedia:

Quote:
The simple form of AFIL using a general purpose amplifier suffers from some disadvantages. The loop driver amplifier requires some additional circuits to overcome these. Using anything other than a correctly designed loop driver amplifier is not only unsatisfactory, but may result in a loop installation that can generate harmonics when driven into distortion,

Unfortunately, this articles does not say what constitutes a "correctly designed loop driver" but I would intuitively expect it to be a current drive, not voltage.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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a.mlw.walker
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Posted by a.mlw.walker: Tue. Nov 20, 2012 - 06:05 PM
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Hi guys,
thanks for responses althoguh Id kind of seen the opportunity to learn a bit about induction, at the same time as building a small mp3 player. Going about it that way, I'd feel I was cheating...

Basically I want to build on e of these in to my product:
http://compare.ebay.co.uk/like/2...

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js
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Posted by js: Tue. Nov 20, 2012 - 09:42 PM
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Quote:
There must be some sort of pre-emphesis
That's why one would use an Audio Power Amplifier with tone controls or better still an equaliser.
Quote:
I would intuitively expect it to be a current drive, not voltage.
That's why one feeds them from an Audio Power Amplifier. :)

Anyway a pestilent company in the UK named themselves almost after my company AMPTRONICS, there is some website speaking about the AMPERTRONICS induction loops etc. so I get several calls a year for quotes and I need to redirect people to the AMPTRONICS distributors in Australia. :roll:

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Chuck-Rowst
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Posted by Chuck-Rowst: Wed. Nov 21, 2012 - 04:59 PM
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Walker,

You stated: "Id kind of seen the opportunity to learn a bit about induction". I like your healthy curiousity and willingness to expand your knowledge of electronics.

My suggestion is to buy one of the t-coil lanyard products on ebay or elsewehere, use it to learn its performance limits (distance, frequencies, bandwidth, fidelity, etc). Then take it apart to see how it ticks and see if you: A: Can duplicate it yourself from scratch (yes, wind your own coil), then B: Improve the performance in some way thru your own innovations.

In the real world many products are designed this way - i.e. starting with a competitor's product and adding one's own improvements and additional features, if not taking a fundamentally different approach which may not have been at first apparent without actually using and examining the competitive product first.

Good luck.

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ka7ehk
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Posted by ka7ehk: Wed. Nov 21, 2012 - 07:43 PM
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I would bet that the post-loop audio processing would need a strong frequency response notch at the power line frequency (and maybe at 2X power line).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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a.mlw.walker
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Posted by a.mlw.walker: Thu. Nov 29, 2012 - 08:23 PM
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HI Guys, I took your advice and bought one of those loops I linked to. Turns out whats inside is very basic indeed.
As far as I can see from the audio source (Im talking mono audio) the signal travels through a 10uF capacitor, and into a toroidal inductor (about 15mm in diameter) and the to ground. Then the loop of wire that is supposed to hang around your neck is quite thick wire, and that loops through the hole in the inductor, and back to itself - no "real" connection. Ive tested its range - its about 20cm.
So I suppose the only thing I'd need to do to replicate it is find the inductance of the toroid.
Ive attached a picture and welcome any comments.

Attachment(s): 

Image icon The black wire goes to a 3.5mm jack and is the input, the beige colour is a loop of wire that you are supposed to hang around your neck so that the hearing aid picks up the induced signal
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