Audio Developments AD149 - Manual (Page 15)

14
SPOT MICROPHONES
Very great care must be taken when using highlighting microphones within a stereo
sound picture. When directional microphones are used, problems include blasting and
proximity effects, and image shift as the sound source (soloist) moves.
A 'spot' microphone can adversely affect the balance of the surrounding part of the
sound stage and an insensitive engineer can easily make the microphone appear out
of context with the overall stereo image. As the signal from the soloist's microphone
arrives in the mix ahead of the basic pickup, one solution is to delay this
signal 10-15ms with respect to the main signal or even to fall in the region of early
reflections; another is for just sufficient of this signal to be added to the mix to
increase the presence of the soloist. Precise positioning of the 'spot' microphone
within the overall picture is essential in order that the image does not shift with
changing frequency and intensity.
When using the X-Y format, a crossed pair of omnidirectional microphones with an
included angle of 60°-90° provides a very stable image of the soloist without blasting,
or proximity and directionality effects, but with a well-defined sense of space. When
using the M-S technique, further M-S pairs can be used as soloists' microphones;
each subsidiary pair will be steered to its correct position within the sound stage, and
its included angle will be less than that of the main pair.
So that maximum flexibility is maintained during post-production, these spot
microphones will be recorded on separate pairs of tracks on the tape.
For ENG/interview work it is perhaps legitimate to adopt a less purist approach without
making too many compromises. A mono microphone may be mixed into
the
M-channel ONLY, with the result that its image will take up a central position in the
final L-R stereo presentation. During post-production, the width and ambience of the
background may be changed, but the balance between the M-S pair and the mono
microphone remains fixed.
If a single M-S pair is used, the M-microphone should be pointed between the
interviewer and the interviewee. This technique will provide a more stable and
coherent image than mixing into the M-channel a microphone moving
between
sources - and consequently with a changing background - unless this microphone is
so close to the source that its background pickup is negligible compared with the
pickup of the M-S pair.
If an M-S microphone is too close to the sound source, small movements of the
source lead to large changes in the angle of incidence and thus excessive movement
in its position within the sound picture. This is particularly true when the S-signal has
a high gain. Conflict will occur when the situation demands a close mic position for
the sake of clarity.