RMS meters are useful when mastering to look at the relative average levels between tracks (for example). Correlation meters are useful when mastering to identify phase cancellation issues and mono compatibility. As an example, if you were mastering for vinyl and were only listening to the bass frequencies in your track and saw the correlation meter below 0, you know the bass is too stereo for vinyl...
Last night, I posted a simple RMS / Correlation Meter based off of "RMS Correlation Meter" by Hz37 found on Maxforlive.com.
The top two numbers show you the average level of the current single (versus the peak level Live shows you). Below that is a standard Correlation Meter which can be described this way:
A +1 correlation value indicates that the left and right channels correlate 100%. In other words, the left and right signals are in phase and are the same shape. (the more +1, the more Mono...)
Correlation values between +1 and 0 indicate that the stereo signal is mono compatible.
The middle position indicates the highest allowable amount of left/right divergence, which is often audible as an extremely wide stereo effect.
When the Correlation Meter moves to the left of the center position, out-of-phase material is present. This will lead to phase cancellations if the stereo signal is combined into a mono signal.
Unlike the original device, mine does not use gen~ and hence does not require Max 6. Though I created it using Live 9 and Max 6.1 beta it should work fine in older versions. It doesn't do much... but it is useful when mastering. Give it a try!