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On 06/04/07, George Brock-Nannestad wrote:

> From: Patent Tactics, George Brock-Nannestad
> 
> Hello, just a few words on stereo:
> 
> 1) stereoscopy
> I am a not-very-active collector of stereoscopic representations, and
> I regard it as the poor man's holograms and extremely fascinating. I
> have a number (puny as collections go) of the old standard
> stereoscopic images, such as Underwood & Underwood, and I built my own
> Holmes-type viewer with variable inter-ocular distance for them. I
> also have a number of examples of the uses of stereo photography,
> scientific, mapping, and of the methods used for recreating the
> impression, i.e. the coding of the images.
> 
> There are several factors involved in good stereoscopy. First of all,
> each image must obey the rules of perspective, which means that the
> angle subtended by an object must be close to the natural angle of
> observation. For an ordinary telephoto image that means that you must
> see it from such a long distance that you really do not draw it
> closer, and a wide-angle image must be seen with an eye so close to
> the image plane that you may not be able to focus and hence you need
> to use a lens - usually called a magnifier. All of this makes for some
> simple geometrical calculations. In other words, for each image you
> come across (obviously without barrel or cushion distortion) there is
> only one particular viewing distance where the eye will regard it as
> natural. This is how a trompe l'oeuil is made. A famous example is
> used in Dorothy Sayers' short story "The Haunted Policeman".
> 
> A good stereo image pair is taken with an inter-objective distance
> corresponding to the average distance between the eyes, but that also
> has only one correct viewing distance for each enlargement. Otherwise
> your eyes tell you that something is wrong, the illusion partly
> disappears, and you get the cardboard cutouts that were mentioned.
> Aereal photography is like that: the roofs of houses appear much
> steeper than they are, because the base between photographs is so much
> wider (hundreds of feet) than the inter-ocular distance.
> 
> Now, if I look at many stereo pairs (it comes in bursts) I have until
> now been able to train my eyes to be wide-eyed, and then I can see
> without a viewer. However, John R.T. Davies once taught me that by
> crossing your eyes, which is considerably easier and may be trained to
> painlessness, you can instantly see many stereo pairs in 3D. However,
> a pair of zoom lenses would be ideal for a Holmes or Brewster viewer.
> 
> In 1997 I gave 3 papers at the AES Convention in New York, and one of
> these was on acoustic recording, with a lot of documentation. As an
> inlay to the preprint (as they were called then) I had prepared a
> sheet with three stereoscopic pairs of original VTMC/GC recording
> horns, exactly the size to fit an old viewer. I had made the images by
> controlled sideways shifting of the camera between exposures. To my
> knowledge, they are the only stereoscopic representations of recording
> horns, but I would like to hear about any others.
> 
> The best stereoscopic illusion I have ever had was watching an
> Underwood & Underwood picture from the Eiffel tower in Paris. It was
> taken straight down from the top, and holding the viewer vertical
> while standing up almost gave me vertigo. Even though it was in b/w.

Have you ever seen an Ives Chromskop image? There are six
black-and-white images, giving red, green and blue for left and right
eyes. Mirrors inside the viewer combine the colors.

> 
> 2) stereophony
> mono recordings made with one omnidirectional microphone and in a
> reasonable hall will have depth - there are many 78s that display
> this. 

That was what I had in mind. Unfortunately, the effect is easily lost in
a bad or over-processed transfer, and in early digital recordings.

For example, JRT Davies's transfer of the Fats Waller track "Minor Drag"
(with Eddie Condon) preserves the depth clearly. Ted Kendall's more
recent transfer does not. (Both in box sets on the JSP label.) 

> Stereo with just a crossed pair, and in particular headphones,
> provide width. The first time I ever heard a stereo LP on headphones I
> had the distinct feeling that the sound was coming from behind me, and
> I have been told that that is not unusual. I have mentioned a musician
> aquaintance before who cannot hear stereo at all, all she hears is two
> loudspeaker sources. Close-miking in a mix destroys the illusion of
> both depth and stereo, because there is an inconsistency with a
> natural sound. 

As in Rudy van Gelder's recordings for Blue Note etc.


> But then people who predominantly listen to mixes will
> perceive mixes as natural. 

If a recording was constructed in the studio from samples and overdubs,
then that was the kind of sound that the musicians wanted.

But for recordings of live acoustic instruments, it is nice to have some
depth. Especially for eighteenth century music.


Regards
-- 
Don Cox
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Regards
-- 
Don Cox
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