FXAlg #966: Stereo Image
Algorithm Reference-170
FXAlg #966: Stereo Image
Stereo enhancement with stereo channel correlation metering
Allocation Units:
1
Stereo Image is a stereo enhancement algorithm with metering for stereo channel correlation. The stereo
enhancement performs simple manipulations of the sum and difference of the left and right input channels to allow
widening of the stereo field and increased sound field envelopment. After manipulating sum and difference signals,
the signals are recombined (a sum and difference of the sum and difference) to produce final left and right output.
Block diagram of Stereo Image algorithm
The sum of left and right channels represents the mono or center mix of your stereo signal. The difference of left and
right channels contains the part of the signal that contains stereo spatial information. The Stereo Image algorithm
has controls to change the relative amounts of sum (or center) versus difference signals. By increasing the difference
signal, you can broaden the stereo image. Be warned, though, that too much difference signal will make your stereo
image sound ÒphaseyÓ. With phasey stereo, acoustic images become difficult to localize and can sound like they are
coming from all around you or from within your head.
A bass shelf filter on the difference signal is also provided. By boosting only the low frequencies of the difference
signal, you can greatly improve your sense of stereo envelopment without destroying your stereo sound field.
Envelopment is the feeling of being surrounded by your acoustic environment. Localized stereo images still come
from between your stereo loudspeakers, but there is an increased sense of being wrapped in the sound field.
The Stereo Image algorithm contains a stereo correlation meter. The stereo correlation meter tells you how alike or
how different your output stereo channels are from each other. When the meter is at 100% correlation, then your
signal is essentially mono. At 0% correlation, your left and right channels are the same, but polarity-inverted (there
is only difference signal). The correlation meter can give you an indication of how well a recording will mix to mono.
The meter follows RMS signal levels (root-mean-square) and the RMS Settle parameter controls how responsive the
meter is to changing signals. The ÔMÕ part of RMS is ÒmeanÓ or average of the squared signal. Since a mean over all
time is neither practical or useful, we must calculate the mean over shorter periods of time. If the time is too short
we are simply following the signal wave form, which is not helpful either, since the meter would constantly bounce
around. The RMS Settle parameter provides a range of useful time scales.
See also the Stereo Analyze algorithm (FXAlg #999) which allows you to experiment directly with sum and
difference signals.
L Output
R Output
L Input
R Input
Center Gain * 1/2
Diff Gain * 1/2
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