As far as the wheelchair sender, I would suggest you either work out a way to
send the sound in an analog manner, or you only do one A/D conversion at the
wheelchair, send the data digitally directly into the computer (rather than
doing another D/A, at which point the sound card does a final A/D - very messy
and low quality if not done well)

As far as the sender/receiver with multiple channels, use regular FM.  Pretend
the LED is an antenna, and transmit at, oh, say 400kHz and above in multiples of
40kHz.  You should be able to find a single chip FM transmitter and single chip
receiver that will run at those frequencies.  Just make sure you aren't
overdriving the LED.

I've seen these in various multi-lingual theatres.  There are LED arrays in the
ceiling pointing downward (every 20ft or so?), and each person who wants to
listen in a different language gets a receiver.  I didn't get a good look at the
receiver, but it has a thumbwheel with either 10 or 16 positions.  They were on
cords worn around the neck so the receiver was always within line of sigh of one
or more transmitters.

At this point, you could use the same circuitry for both systems, and that would
be adviseable anyway, as you might want to enable multiple channels on the
receivers when multiple computers are used in one room.

-Adam

Russell McMahon wrote:
>
> I wish to implement two different shortish range highish quality udo systems
> using infrared.
> While there are both commercial designs and various circuits that do this
> someone here may have useful input.
>
> Application 1:
>
> Microphone extender allowing a wheelchair confined user to connect to a
> computer microphone input.
> Allows use of eg Dragon naturally Speaking voice control without linking
> wires.
> range needs only be a few meters.
> Quality of rendition must be excellent.
> Sounds easy enough.
>
> Application 2
>
> >From a number of sources (up to 4) take input audio and rebroadcast it via
> infra-red such that any of the 4 channels can be selectively received by a
> portable receiver. It is envisaged that the audio sources would originate in
> selected hospital rooms and the audio signals would be able to be received
> by nurses wearing receivers. Transmission would be by wire connected IR
> transmitters which "spray" infrared over rooms / corridors concerned from
> suitable points.
> The aims here are lack of RF radiation in a controlled environment and as
> long a battery life as possible for receivers.
> RF would be much easier but is frowned on.
> Voice quality need only be say telephone level (3 KHz ish BW).
>
> One way transmission OK (fixed to mobile)
> Doesn't sound too hard. Getting OK coverage may be annoying.
>
> Anyone have any useful suggestions.
>
>       Russell McMahon
> _____________________________
>
> What can one man* do?   Help the hungry at no cost to yourself!
> at  http://www.thehungersite.com/
> (* - or woman, child or internet enabled intelligent entity :-))
> >From other worlds:
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>
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