Subject: Re: [linux-audio-dev] off topic, optical protocols
From: D. Stimits (stimits_AT_idcomm.com)
Date: Tue Mar 05 2002 - 05:00:24 EET
Taybin Rutkin wrote:
>
> On Mon, 4 Mar 2002, D. Stimits wrote:
>
> > make a small microphone pre-amp, and wanted it to create optical output
> > instead of analog, how big of a mess would I be getting into?
>
> I'm not much of an engineering guy, but does switching to optical
> necessarily mean switching to digital? Couldn't you use the frequency of
> the light much like an electrical wire? I guess it depends on the
> available hardware.
This is part of the question. I am very familiar with RF forms of
modulation, but not optical. Obviously it would be a problem trying to
frequency modulate an optical signal produced by a laser. I imagine it
is some sort of pulse code modulation (PCM), but I'm not sure. Maybe it
even involves merging two signals that are at slightly different phases
(done with mirror splitting?), as a form of digital on or off
modulation. So this is part of the question, what form of modulation is
used, and how difficult is it to create a modulator for this optical
signal on a common audio card optical input?
>
> Are you trying to eliminate electrical interference?
No, not electrical interference. It would be a side effect though. An
optical means of detecting a controller can be far more stable as
mechanical components wear out, whereas electrical means such as
variable resistors can be misaligned in a number of ways. Some optical
schemes can be used such that center and center drift does not care
about some of the component wear. Add to this that eliminating any D/A
or A/D controller and using direct optical means is a guaranteed 100%
linear signal, no temperature drive ever. No noise, no drift, no wear,
no A/D, no D/A.
As far as a microphone goes, I am thinking of two things. One is that my
mini-disc uses optical in, and if I had a preamplifier that plugged into
that, I could later extend it to a direct optical microphone. A long
time ago I had an idea for a microphone that can handle positional audio
from a single mic, through a scheme similar to the joystick scheme I am
looking at. The chips I needed did not exist back then, but my local
university ended up inventing my missing chip about 10 years ago. By now
I think they are available on the market. A microphone like this would
have a suspended element that has its position detected optically, not
just in compression in one direction, it could figure out what direction
the signal was coming from (at least on average; fourier could be used
to separate out locations of individual frequencies if really desired,
but think of it as a poor man's stereo, done with a single pickup).
D. Stimits, stimits_AT_idcomm.com
>
> Taybin
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