Figure 1: The LFL matrix element from the ’89 patent and Dolby
Pro-Logic, scaled so the maximum value is one. Note the matrix
element consists of simple intersecting surfaces. The value is 0.71
in the center of the plane, and rises to one at the left vertex. The
values at Rear and Center are 0.5.
The function G(x) is described
in the 89 patent, and specified
in the ’91 patent. It varies from
0 to one as x varies from 0 to
45 degrees. When steering is in
the left front quadrant (lr and cs
are both positive) G(x) can be
shown to be equal to 1-|r|/|l|
where |r| and |l| are the right and
left input amplitudes. See
Figure 1 and Figure 2.
In the recent AES paper these
elements were improved by
adding the requirement that the
loudness of unsteered material
should be constant regardless of
the direction of the steering.
Mathematically this means that
the root mean square sum of the
lfl and lfr matrix elements
should be a constant. It was
pointed out in the paper that
this goal should be relaxed in
the direction of the steering –
that is, when the steering is full
left, the sum of the squares of
the matrix elements should rise
by 3dB. Figure 3 shows that the
above matrix elements do not
meet this requirement.
Figure 2: The lfr matrix element from the ’89 patent and Dolby Pro-
Logic, scaled similarly to figure 1. The minimum value and max
value is +-0.5. Note that at the center and rear vertexes the value
here when combined with the values in the matrix element in figure
1 will cause exact cancellation of a strongly steered signal.
Complete cancellation also occurs for signals that follow the
boundary trajectory for positive values of lr – in this drawing the
trajectory from center to right, and from right to rear.
