Audio Developments AD149 - Manual (Page 11)

10
COINCIDENT MICROPHONE TECHNIQUES (Intensity Stereo)
A crossed pair of microphones is used with the capsules USUALLY arranged in a
vertical plane and as close to each other as possible; the angle included by the
capsules is symmetrical with respect to the central axis of the sound source. The
angular accuracy of the stereo image is independent of the distance from sound
source to microphone, but the resultant recording can seem rather dry and lacking a
sense of space.
There is a choice of capsules available when using this crossed pair or X-Y technique.
The earliest of these techniques were developed by Blumlein. ONE of his techniques
uses a pair of figure-of-eight microphones set at an included angle of 90°. Best used
in large spaces where reflections from boundary surfaces are minimal and where
strong signals are not presented to the rear quadrants, this technique produces a very
natural sound.
When cardioid or hypercardioid microphones are used, the included angle is usually
90°-120°. (Some authors reserve the term X-Y to refer only to a crossed pair of
cardioid microphones.) The included angle determines the width of the stereo image
and is selected with other factors in mind; distance from, and width of the sound
source and polar response of the chosen microphones. The polar and frequency
responses (together with sensitivities) should be matched as closely as possible,
otherwise considerable colouration and image shift can occur with changes in pitch.
When cardioid capsules are used they are usually placed fairly close to the sound
source giving a high ratio between direct and reverberant sound, and good rejection of
unwanted sound at the rear of the microphone pair. This means that in TV recording
the microphone appears on camera. A longer range system is required for picking up
dialogue in drama etc. Even listeners in stereo will have difficulty with text if an X-Y
pair is set out of sight and probably with neither microphone pointing directly at the
sound source.
Even with an optimally positioned X-Y pair of cardioid microphones there is no
guarantee that the resultant recording will fill the space between the loudspeakers.
Post-production techniques are available to overcome this and other problems, but
experience has shown that to take advantage of these techniques the capsules
should be CROSSED OVER and spaced apart at a distance of approximately 50 mm.
(FIG 1)
This spacing, which does not apply to other directional microphones, actually
improves stereo imaging.
FIG 1