These two figures show something mix engineers are often aware of – namely that a mix
prepared for playback on a Dolby Pro-Logic system can need more center loudness than a
mix prepared for playback in stereo. Conversely, a mix prepared for stereo will lose vocal
clarity when played over a Pro-Logic decoder. Ironically, this is not true of a passive
Dolby Surround decoder.
Creating two independent rear outputs
The major problem with both the ‘89 elements and the Dolby elements is that there is
only a single rear output. The ’91 patent disclosed a method for creating two independent
side outputs, and the math in that patent was incorporated in the front left quadrant in the
AES paper of 1996. The goal of the elements in this quadrant was to eliminate the output
of a signal steered from left to center, while maintaining some output from the left rear
channel for unsteered material present at the same time. To achieve this goal we assumed
that the LRL matrix element would have the following form:
For the left front quadrant
LRL = 1 – GS(lr) – 0.5*G(cs)
LRR = -0.5*G(cs) – G(lr)
As can be seen, these matrix elements are very similar to the ’89 elements, but with the
addition of a G(lr) term in LRR, and a GS term in LRL. G(lr) was included to add signals
from the B input channel of the decoder to the left rear output, to provide some unsteered
signal power as the steered signal was being removed. We then solved for the function
GS(lr), using the criterion that there should be no signal output with a fully steered signal
moving from left to center. The formula for GS(lr) turned out to be equal to G^2(lr),
although a more complicated representation of the formula is given in the ’91 patent. The
two representations can be shown to be identical.
In the AES paper these elements are
corrected by being given a boost of
(sin(cs)+cos(cs)) to make them closer
to constant loudness for unsteered
material. While completely successful
in the right front quadrant, the
correction is not very successful in the
left front quadrant. See Figure 14.
Figure 14
Figure 14: The square root of the sum of the squares of LRL and LRR, using the elements of March 1997.
Notice that in the front left quadrant there is a 3dB dip along the line from the middle to the left vertex, and
nearly a 3dB boost in the level along the boundary between left and center. The mountain range in the rear
quadrant will be discussed later. This drawing includes an “unsteered” dip of 3dB at the center of the plane,
which is hard to see in this projection.
