In article ,
ScottW wrote:
On Apr 24, 5:07*am, "Gary Eickmeier" wrote:
KH wrote:
Actually no, I wasn't referring to solid geometry. *As Donald states,
I'm talking about time, amplitude, and direction. *Hence all the
discussion surrounding vector information. *A 2-D signal does not
contain directional information that is not just required, but is an
intrinsic property of a vector. *Acoustic signals comprise
multitudinous vectors. *It's the vector summation, filtered and
transformed via the HRTF that allows identification, aurally, of
three dimensions. *It's the angular component that is excised during
transduction.
Perhaps this is some of the confusion. *I thought vectors would be
well understood in the discussion.
Keith
So Keith Howard says there is no directional information in a stereo
recording? Just time and amplitude? Then what, pray tell, is the difference
from mono?
I think directional information is not captured by a single
microphone or a single channel but it can be captured (or at least
estimated) from multiple mics and recreated from two or more channels
in playback.
ScottW
As has been pointed out previously, the only directional information
that can be captured by a microphone is intensity for omnidirectional
mikes and on-axis and off-axis response for cardioids and time delay for
both. I.E. If put two omni microphones in front of an orchestra about 15
ft apart. Each mike pick-up its half of the orchestra with greater
intensity than it will pick up the other half because it's closer to
it's half of the ensemble. Each mike will also pick up its off-side
delayed slight from the other mike which is closer to its side of the
ensemble. There will also be phase differences between the soundfields
picked up by each respective mike. It's really not down to any actual
directionality at all. Now with cardioids , they have a directional
attribute in their pick-up pattern. They are designed to be the most
sensitive to sounds directly on axis with the microphone's front . I.E
they are designed to be pointed, like a flashlight, at whatever one is
trying to capture. Just as a light beam from a flashlight is tight at
the source but is cone shaped as it leaves the flashlight and the
further one points the beam away from the source, the circle of
illumination gets larger. That's sort of (in a very crude way) how
cardioid mikes work. Imagine, if you will, being able to mount two
flashlights on a foot-long bar, and suspending that bar (from a tripod,
perhaps) so that both flashlights shine on a wall, say, 30 feet away.
The beams will each be a large, but pretty dim, circle of light and if
the flashlights are aimed straight ahead, the two circles of light will
intersect. Now, move the two flashlights at an angle to one another and
stop when the two pools of light overlap just enough to form the largest
common area of illumination before the two pools separate into two
distinct circles. You can now see how cardioid mikes work in a closely
spaced, coincident pair. The left mike "illuminates" the left side of
the ensemble and the right mike "illuminates" the right side, with some
overlap in the middle. Where the flashlight analogy breaks down is that
a pool of light thrown by a flashlight is pretty even from edge to edge
and goes dark altogether outside of the cone of illumination. A cardioid
mike, OTOH, would be brightest in the center, with illumination falling
off as you get further from the center. So flashlights and cardioid
mikes are really the crudest of analogies but the conceptual view of
both is pretty valid.
Stereo takes TWO microphones. If you are using widely spaced ominis,
then it might be useful to use a third in the center and blend that
third mike equally into both right and left channels. This is called a
center-fill mike. If you use more than two (or three spaced omnis)
mikes, then you are pan-potting the mikes into whatever position
laterally across the stage you want them to be. This is NOT stereo.
Multiple mikes create two channels that have a group of instruments
artificially blended in a right-to-left ratio that allows each
instrument to be placed along a lateral line, from right-to-left. If you
mike each instrument separately, you can produce two or three groups of
instruments which sound as if they are located on the right, with
another group on the left, and perhaps a third group in the center
(called a phantom center channel. This "channel" is monaural). This is
the way most jazz ensembles have been recorded for decades. Rock is
similarly produced. It is two channel sound, but it is NOT stereo
because it registers only left and right information. No depth is
captured, no height. It is merely two dimensional, not three.
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