P3D Re: Audio VS Video

["Gregory J. Wageman" <gjw@xxxxxxxxxx>]

Mark Dottle writes:

>SEPARATION- I was referring to the playback mix, and not channel
>separation. For example, in an audio mix we can increase or decrease the
>volume of a guitar, and move the guitar " forward or backward" in the
>mix without effecting the other instruments.

That's not "separation", that's "imaging".  Placement of a source in the
stereo image.  We still describe it in terms of 3-space: front,
back, left, right, center, etc.  It's also sometimes called the "soundstage".

>In photography we cannot move the tree forward, backward, left or right
>after the image is recorded. So the term as I intended does not apply to
>photography.

Can you move a guitar player around in space as he comes off your stereo
CD player?  I can manipulate the placement of sources in the stereo image
in my digital multitrack studio, but once I mix down to two-track, the
position of everything in the stereo image is fixed.  Same with stereo
slides.  With a tabletop scene, for example, you can move any element
anywhere.  Once you've pressed the shutter, you've "mixed it down" to
two perspectives, and frozen the location of everything on film.

>CROSSTALK- is very very rarely recognized in newer audio components and
>in most cases is not audible. Ghosting is very noticable and
>distracting. In audio crosstalk is often a measurable specification, and
>ghosting is a strange and distracting variable that is noticable, but
>not expressed as a spec.

Last I knew stereo tape decks were still in production.  Analog tape
decks like cassettes suffer from crosstalk, and it most definitely is
audible, although it may not be particularly disturbing since music is
rarely recorded with complete silence on one track.  Who cares if the
high-hat cymbal bleeds over into the rhythm guitar?  In the average
listening environment, stereo separation is quickly lost due to ambient
reflections anyway.  However, the analogy holds because the *intent* was
for the two elements to remain separate.

Ghosting in stereo projection could easily be expressed numerically.
The specifications for polarizing filters usually include their efficiency
or extinction percentage.  Real-world results could be spec'd, if anyone
bothered to measure them and quantify them that way.  As there is no
"specsmanship" going on in the stereo photographic market, which largely
consists of used equipment, there is no urgent need to "prove" that "my
extinction is better than yours" as there seems to be in the audio equipment
market.

>FREQUENCY RESPONSE- in audio we can select specific frequencies of
>different instruments and change them before or after the recording. In
>photography we can change color temperatures within limits but the
>entire photo is affected. For example we cannot use a filter to " warm"
>the sky and at the same time make the water appear " colder" (unless of
>course we use my gel effects:-) )

Sure you can.  A quick look at a recent B&H catalog lists a plethora of
Hoya filters that allow effects in parts of, rather than all of, a scene.
For example, they sell "Color Spot" (Grey, Green, Yellow or Red background
with hole in center), "Dual image" (Rotating half clear, half black for
double exposures), "Dual color" (orange/green, yellow/pink, red/blue
rotating) etc.  That last one would allow exactly the effect you mention.

And with digital manipulation in Photoshop or similar, virtually any
effect can be applied to any portion of an image, and in a precisely
repeatable way for stereo.

>I still think it's a s t r e t c h to compare audio and photo terms as
>equivalents.

Perhaps you need to limber up your mind a bit, if you think it's that much
of a stretch.  :-)

	-Greg


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