Audio Developments AD149 User Manual

Page 19

overall level of the stereo signal. To steer the output of an M-S pair to any position
within the final stereo picture, the outputs of the matrix amplifier are mixed,
differentially, into the main L-R signal. [Refer to FIG 3(a) and FIG 3(b)]

It is essential that the relative phase of all stereo signals is maintained. If X or Y is
phase reversed, the stereo image will be imprecise with a lack of low frequencies.
A mono reduction will now be the S-signal - which is not a normal audio experience.

If the S-signal is phase reversed, the final presentation will be changed from L-R
to R-L. Reversing the phase of S is a useful technique if the orientation of an M-S
pair has been changed during a recording. For example, a boom may have been
changed from an 'over-the-top' position to an 'up-and-under' position.

In a situation where several M-S pairs cover the sound stage, the S-level control can
be used to reduce to narrow stereo (or even mono) in times of trouble or when
making difficult cross-fades between pairs of microphones.

MATRIX AMPLIFIER WITH WIDTH CONTROL

FIG 3 (a)

If a mono signal is to be derived from fully modulated left and right signals, simple
addition will result in over-modulation of the mono channel due to the increase in level
caused by centrally situated signals. Some manufacturers reduce this resultant level
by 3dB and some by 6dB.

Mixing M-S signals is a valid and very powerful technique, particularly when matching
sound to visual images. A mono signal is directly available; the stereo image can be
made narrower or wider; the position of dialogue can be changed within the stereo
image without affecting the stereo ambience and equalisation can be applied without
affecting the mono signal.

OUT

MATRIX AMP