FXAlg #999: Stereo Analyze
Algorithm Reference-178
FXAlg #999: Stereo Analyze
Signal metering and channel summation utility algorithm
Allocation Units:
1
Stereo Analyze is a utility algorithm which provides metering of stereo signals as its primary function. In addition
to metering, the gains of the two channels are separately controllable, either channel may be inverted, and sum and
differences to the two channels may be metered and monitored. If you use this algorithm with Live Mode, you can
obtain a significant amount of information not only about your own mix, but of any recording you have in your
library.
There are separate meters for the left and right output channels. Two types of meters are provided: peak and RMS.
Meter display units are decibels relative to digital full scale (dBFS). The peak meters display the levels of the
maximum signal peak that occurred during the meter update period (every 40ms). The RMS meter displays the
average power of the input signal. RMS is an abbreviation for root-mean-square, so the signal is squared, averaged
and a square root is taken. For a real-time meter, we do not take an average over all time, but rather average past
signals with a stronger weighting to signals in the recent past than the far past. The RMS Settle parameter controls
how strong the weighting is for recent signals over much older signals. RMS Settle is expressed in units of dB/s
(decibels per second), meaning how fast the RMS meter can rise or fall with changing signal levels.
You can choose to meter and monitor normal left (L) and right (R) stereo signals, or with the Out Mode parameters,
you can select normalized sum and/or differences between the left and right channels. The Out Mode parameters
control the signals being passed to the outputs and to the meters: what you see on the meters are the signals to which
you are listening. The Invert parameters provide a polarity reversal to the input signals. This polarity reversal
occurs before sum and differences. The Invert parameters are actually redundant since Out Mode provides signal
inversions as well. The left and right Out Mode parameters may be set to any of the following:
L
left channel
R
right channel
(L+R)/2
normalized sum of left and right
(L-R)/2
normalized difference of left minus right
-L
polarity reversed left channel
-R
polarity reversed right channel
-(L+R)/2
polarity reversed and normalized sum of left and right
(R-L)/2
normalized difference of right minus left
You may well ask why you would want to meter or monitor reversals or sums or differences of your stereo channels.
One important case is to determine if your final mix is mono compatibleÑvery important if your mix is ever going
to be broadcast on radio or television. Set both the left and right Out Mode parameters to (L+R)/2 to listen to the
mono signal. If you find that parts of your mix disappear or start to sound metallic (comb filtered), you may have
to go back and do some work on your mix.
The difference signal (L-R)/2 provides a measure of the stereo content of your mix and can be very indicative of
mixing style. Listening to the difference signal of someone elseÕs recordings can often demonstrate interesting
techniques (and mistakes!) in stereo production. The difference signal contains everything that doesnÕt make it into
the mono mix. Out of phase signals will appear only in the difference signal. Panned signals will appear in both the
sum and difference signals to varying degrees. A delay between left and right channels will sound metallic (comb
filtered or flanged) in both the sum and difference channels. If the entire mix seems to have a relative left/right
delay, you can use the L/R Delay parameter to attempt to correct the problem. Positive values delay the left channel,
while negative values delay the right channel.