Voxengo Correlometer by Sound-Guy
Aleksey Vaneev has done it again - a new, powerful audio analysis module that is free. If you don’t have some of his plug-ins, you are missing some excellent tools. There are 15 free products on his site, all with excellent audio quality (except those only used for measurement which affect audio not at all, such as SPAN and the new Correlometer). SPAN and MSED should already be in everyone’s audio toolbox, and now I’d add the Correlometer. If you haven’t purchased some of his incredible dynamic and linear phase EQs like GlissEQ and CurveEQ, dynamics processors like Marquis Compressor and Voxformer, harmonic exciters and saturation devices, you’re missing some excellent and unique tools at very fair prices.
What is a Correlation Meter?
A correlation meter shows the phase relationship of two signals. Used mostly with a stereo signal it indicates +1.0, meaning perfect positive correlation, when the left channel is perfectly in-phase with the right (dual-mono). Such a signal will play as mono. If the left and right channels have the same waveform, but one is inverted, the correlation will read -1.0, and the sound through speakers or headphones will be hollow and weak, kind of feeling like your head is being squeezed. Not a desirable condition in most cases. In between these extremes, signals in the positive range indicate stereo, from strong (zero reading) through moderate back to mono at +1. Below zero indicates channels out of phase which can have a number of causes from the effects of reverb and delay (usually desired) to mic placement issues, usually not desired.
But I Got a Correlation Meter
Yes, even the free SPAN “fast Fourier transform” audio spectrum analyzer includes a traditional correlation meter display, but the new Correlometer provides some new ways to view correlation.
Voxengo Correlometer with variable resolution across the audio spectrum
Based on the Voxengo PHA-979 phase-alignment plug-in (a fine phase and time correction tool), this new meter displays real-time correlation across the audio spectrum with from four to 64 bands, variable averaging time from one millisecond to five seconds, and a bandwidth/sensitivity control. Most common broadband correlation meters show overall phase issues, sort of an average, and can’t help identify problems in specific spectral bands. While this is useful when there are overall phase problems, normal meters will not show if only a particular frequency range has phasing issues. So the Correlometer shows the same correlation information as a conventional meter, but split into frequency bands.
Correlation actually shows the mix of mid and sides signals in a stereo audio stream. A reading of -1 is a fully uncorrelated signal, which is essentially the “sides”. A reading of +1 is the “mid” of a mid-sides configuration. This can be seen below where pink noise was run through the Correlometer using MSED to isolate the components; on the left is the “sides” signal of the noise, and on the right is the “mid”:
Sides signal only on left; mid signal only on right
Below is a snapshot of full stereo pink noise (not dual mono, but separate uncorrelated noise signals on left and right channels) which shows positive and negative excursions that bounce around the zero line:
Full stereo pink noise
This is a momentary snapshot since the plot is dynamic - in fact watching the dynamics while a mix is playing can be instructive. Note that reverb and delay tend to create negative correlation measurements since they create delayed versions of the original sound.
Below is a snapshot of the meter with a Latin Jazz mix showing bass frequencies are almost pure mono, mid-range is strongly stereo with some reverb/delay creating some "bouncing" negative measures, and above a few kilohertz is moderately stereo.
Latin Jazz Mix
Note that the pan position of tracks does not affect the Correlometer display - in fact one channel can be 30 dB, 40 dB or even 60 dB lower than the other and the phase information will remain the same. If you want to view levels versus frequency, use SPAN (also free - though the advanced SPAN Plus version adds some useful features for a reasonable price).
One good use of the Correlometer is in determining if tracks are true stereo or dual-mono. Of course if you created the tracks yourself you probably know, but if you mix for others and receive (huge) sets of files, you may have noticed that many recording engineers send only “stereo” track files, yet many are really mono (recorded as dual-mono) and have no need to be stereo which uses twice as much file space, and more processing power in most DAWs than a true mono track. Trying to decide which tracks are stereo and which are dual-mono can be time consuming. Viewing them up in a DAW with the time scale expanded works, but can take a lot of time. One suggested method is to flip the polarity of one channel and see if that nulls the sound - if so, it is dual-mono. That technique is also time consuming. Another way is to use a tool like Voxengo MSED and listen only to the “sides” signal - if there is no sound, it is dual-mono. However, you can’t audit those mono tracks unless you bus them separately to an output bus. With the Correlometer on the master bus, you can solo each track, listen to it and view if it is dual-mono. If it is, you can convert it to true mono.
A Good Deal
The Correlometer includes sidechain inputs so you can compare two different mono tracks. This can be handy for phase alignment of different tracks such as bass (miked cabinet and D.I.), drum mic pairs, miked guitar and D.I., etc. I found that some DAWs (REAPER) can use the sidechain inputs of either a VST or VST3 plug-in, but others (Studio One 4.1) only see the sidechains for VST3. However, with Studio One I could easily split tracks out to the left and right channel of a bus and use a VST version of Correlometer in the bus for measurement. But sidechain input can be handy.
And if you really need some sophisticated phase and time alignment, you can purchase the Voxengo PHA-979 phase-alignment plug-in which includes a version of the Correlometer along with some excellent audio processing functions.
The Correlometer is a very useful tool for anyone mixing or mastering audio. If you are a rare person who hasn’t found Voxengo, check out all the plug-ins at the site. Available for Mac (AU, AAX, VST and VST3) and Windows (VST, VST3 and AAX).
Thorough analysis of correlation over the entire audio spectrum.
Adjustable averaging time and number of frequency bands.
Sidechain inputs for more flexibility.
None I can think of.
Multi-Band Correlation Meter Plugin [VST, AU, AAX] - Correlometer | Voxengo