From: deanaj@elec.canterbury.ac.nz (A. J. Dean)
Newsgroups: rec.audio.high-end
Subject: Re: Laser Illusions Spatial Filter
Date: 17 Dec 1992 05:58:05 +0000 (GMT)
Organization: Electrical Engineering, University of Canterbury, New Zealand
 <01GSFLEA9TU8ANBTG7@csc.canterbury.ac.nz>; Thu, 17 Dec 1992 18:58:13 +1300
 17 Dec 92 18:58:09 NZD
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CHRISTIAN LANT (ctlant@lims01.lerc.nasa.gov) wrote:
: I am interested in why Laser Illusion's spatial filter works at all. 
: For an optical system with a high numerical aperture (N.A.) such as 
: a CD optical system, one needs a very small pinhole (on the order of 
: 10 micrometers) to block the spatial frequencies containing any 
: appreciable power.  It's like amplifier distortion - most of the 
: distorted power appears in the first few harmonics.  From the post 
: describing one reader's problems with the fit of the Laser Illusions 
: filter, the pinhole actually has a diameter of 60 mils!  This is more 
: than two orders of magnitude larger than the ideal, which should let 
: virtually all of the light's bandwidth through the filter, if 
: traditionally designed.  
: 
: I suspect that the filter isn't placed at the laser waist (focus) since 
: this would require additional focusing optics ( or placing the filter 
: .....

I don't quite understand all the finer (and not so fine...) points but I'm
sure the original poster said that the filter was placed directly over the
lens :. no where near the focus. I can't see a 60 mil pinhole over a lens
reducing the ability to focus to a fine poi hmm, sorry, it just doesn't gel
for me - perhaps I should go and do more in optics!

But I'm pretty sure about this: The lenses in CD players are diffraction
limited, ie they are optically as good as they are going to get. You can
never focus a beam of light down to an infinitely small spot - instead it
will have a 'size' dependant on the wavelength of the light (near IR for CD)
and the numerical aperture of the lens. Placing a hole over the lens
effectively giving a smaller lens is only going to increase the 'size' of
the spot (increasing crosstalk and blurring signal transitions, increasing
jitter) and reduce the signal power (also intorducing noise/jitter). I say
'size' because the spot is not in fact a spot but a diffraction pattern,
with a central fuzzy blob and concentric rings. The CD optical system's NA
(= 0.45 I think) is that needed to maximise the reduction of crosstalk from
adjacent tracks by placing the tracks (as far as possible) in the dark area
of the diffraction pattern (ie between the centre spot and the first ring).
We all know you can stop down a camera lens (closing the iris) to improve
its focussing performance for the 'image' use of light, but with the CD's
wave-based application this sort of of solution would be based on an
oversimplified understanding of what is really going on. I hope Laser
Illusions have a better understanding than me anyway!

But as far as the effect all this stuff is going to have on the sound goes,
I have to take the "engineer's" point of view (after all, I am one!) - that
digital data will be as close to perfect as you could want as long as it
does not have gross data errors, and this does not really matter when the
error correction can do its job perfectly (as it almost always will). I
would far rather concentrate on the OUT end than the IN end and all its
associated green pens, etc. Of course, a digital IN end always has its
subtle connections to the OUT end, and if anything can be proved to be
audible then it _is_, and no other valid argument can be made.

>From the few cases I've heard (as in by mouth, screen, etc) this spatial
filter doesn't look as if it does _anything_ on most players. Laser
Illusions (that name gets me!) can sue me once they explain to me how their
device works and I then proceed to neglect to retract my flaky theories!  

Antony. (deanaj@elec.canterbury.ac.nz)
[going away for the next 2 weeks]

PS/BTW: an easy to read (and understand) treatment of the subject of CD's
optics (as well as the rest of the player) is given in a book by John
Watkinson - "The Art of Digital Audio". If I could spell "thoroughly" and
"reccommend" properly I would do both!