KERN CHECK / user manual

what CHECK does

CHECK shows you where your stereo mix falls apart in mono. it analyzes 40 frequency bands in real time using signed Pearson correlation, giving you a full picture of which frequencies are mono-safe and which have phase problems.

most correlation meters show a single number. CHECK shows you a curve across the entire spectrum – so you can see that your bass is safe even if your stereo reverb at 3 kHz is cancelling. it's the difference between "your mono compatibility is 40%" and "your bass is fine but your reverb tail is cancelling above 2 kHz."

CHECK is a pure analysis tool. it passes audio through unchanged with zero latency. no processing, no coloring, no side effects.

controls

four knobs control the analysis display.

SPEED

sets the EMA smoothing time constant for the correlation display. lower values (50 ms) show instant changes – useful for finding transient phase issues. higher values (up to 2000 ms) smooth out the curve to show the average correlation over time. default: 300 ms.

FOCUS

controls the frequency range shown in the display. three positions:

• LOW – zooms into sub-bass and bass (20 Hz - 500 Hz). use this to check if your low end survives mono.
• FULL – shows the entire spectrum (20 Hz - 20 kHz). the default view.
• HIGH – zooms into the upper range (2 kHz - 20 kHz). use this to check stereo effects, reverbs, and air.

BASS

controls how much the mono score emphasizes low frequencies. at 0%, all frequency bands count equally. at 100%, the weighting is:

• below 200 Hz: 3x weight
• 200 - 500 Hz: 2x weight
• 500 Hz - 2 kHz: 1x weight
• 2 kHz - 8 kHz: 0.7x weight
• above 8 kHz: 0.3x weight

the default is 70%. this weights the score toward bass because that's where mono collapse actually causes problems – club systems, phone speakers, and Bluetooth devices.

ZOOM

sets the vertical scale of the display. two modes:

• -1 / +1 – full correlation range. shows both positive correlation (mono-safe) and negative correlation (phase cancellation).
• 0 / +1 – zoomed into positive correlation only. use this for a more detailed view of your safe zone.

MONO / SIDE

two output monitoring buttons on the right side of the interface.

MONO

collapses the output to mono (L+R sum). click to toggle, or click and hold for momentary monitoring. this lets you hear exactly what your audience hears on mono playback systems. the display keeps updating normally.

SIDE

solos the side (difference) signal. this is everything that disappears when you sum to mono – the pure stereo content. if you hear important elements here (like your vocal or kick), they'll lose energy in mono.

score zones

the mono score in the bottom-left shows a bass-weighted compatibility percentage. four zones:

the score bar below the percentage fills and changes color to match the zone. a score of 55% (like the screenshot) means REVIEW MIX – some bands are safe but others need attention.

spectral display

the main display shows mono correlation across 40 ERB (Equivalent Rectangular Bandwidth) frequency bands. ERB spacing matches human perception – more resolution in the low-mid range where your ears are most sensitive.

reading the curve

• +1.0 – perfectly correlated (identical left and right). the signal is fully mono.
• 0.0 – uncorrelated (independent left and right). stereo content but no phase cancellation.
• -1.0 – perfectly anti-correlated. the left and right channels cancel completely in mono.

zone backgrounds

the display area is color-coded. green zone at the top (safe). amber in the middle (caution). red at the bottom (phase cancellation). when zone colors are enabled, you can read the display at a glance without checking numbers.

peak hold

the gray line behind the green curve shows the peak (minimum) correlation reached since the last reset. this helps you catch brief phase problems that resolve too quickly to see in real time. right-click the display to reset peak hold.

right-click menu

right-click the spectral display to access these options:

• Range – set FOCUS to LOW, FULL, or HIGH (syncs with FOCUS knob)
• Scale – toggle ZOOM between -1/+1 and 0/+1 (syncs with ZOOM knob)
• Peak Hold – toggle the peak hold curve on/off
• Zone Colors – toggle the background zone coloring on/off
• Reset – clear the peak hold curve and reset display state

what is mono compatibility

mono compatibility is how well your stereo mix translates when played back in mono – when the left and right channels are summed to a single channel. if your left and right channels have phase differences at certain frequencies, those frequencies lose energy (or disappear entirely) in the sum.

technically, it's measured by the correlation between your left and right channels at each frequency. correlation of +1.0 means the channels are identical at that frequency (perfectly mono-compatible). correlation of 0.0 means independent signals. correlation of -1.0 means they're perfectly out of phase and will cancel completely.

most stereo audio sits somewhere between 0.0 and +1.0. that's normal and fine. the problems start when correlation goes negative – that means active phase cancellation, and your audience will hear a quieter, thinner version of your mix.

why mono compatibility matters

more of your audience hears mono than you think:

• club systems – most club PAs sum to mono below 100-200 Hz. if your bass has phase problems, it disappears on the dancefloor.
• phone speakers – built-in phone speakers are effectively mono. streams, social media, voice messages – all mono.
• Bluetooth speakers – many portable speakers are single-driver mono devices.
• streaming services – some platforms apply mono summing for certain playback scenarios.
• radio – FM radio uses a mono-compatible stereo encoding (pilot tone). phase issues cause artifacts.

the fix is simple: check before you bounce. if CHECK shows problems, you can narrow the stereo width in those frequency ranges, adjust stereo processing, or fix phase issues in your effects chain.

installation

macOS

1. download the zip from the product page
2. unzip to get the plugin files
3. copy KERN CHECK.component to ~/Library/Audio/Plug-Ins/Components/ (AU)
4. copy KERN CHECK.vst3 to ~/Library/Audio/Plug-Ins/VST3/ (VST3)
5. restart your DAW

CHECK is a free utility – no license key needed. just install and use.

windows

Windows support coming soon. VST3 format.

quick start

1. insert CHECK on your master bus (or any track you want to analyze)
2. play your track and watch the correlation curve
3. green means safe. red means phase problems. the score gives you an overall number.
4. if you see dips below 0 in the bass, narrow your stereo processing below that frequency
5. click MONO to hear what your audience hears on mono systems
6. click SIDE to hear what disappears in mono

tips

1. check your master bus last. insert CHECK on the final output to see the overall picture.
2. check individual tracks too. put CHECK after stereo effects (chorus, phaser, widener) to catch problems at the source.
3. bass correlation matters most. keep BASS at 70%+ if your music needs to work in clubs. a bass score below 60% means your low end loses energy in mono.
4. use FOCUS LOW when mixing bass. zoom into the low end while you're working on your sub and kick.
5. don't aim for 100%. a score of 100% means perfectly mono – no stereo content at all. 60-80% is a healthy range for most mixes.
6. side monitoring reveals hidden problems. if you hear your vocal or kick in the SIDE signal, something in your chain is pushing those elements to the sides.
7. peak hold catches transients. leave peak hold on and play your full track. the gray line shows the worst-case correlation at each frequency.
8. A/B before and after stereo processing. use the A/B buttons to compare correlation before and after adding stereo effects.

troubleshooting

the curve stays at +1.0 everywhere

your input is mono. CHECK needs stereo input to measure correlation. make sure you're feeding it a stereo signal.

the score says PHASE CANCEL but it sounds fine in stereo

phase cancellation only matters in mono. click the MONO button to hear what mono playback actually sounds like. if you hear elements dropping out, it's a real problem.

the curve is jumping around too much

increase the SPEED knob to smooth out the display. 500-1000 ms gives you a stable average that's easier to read.

CPU usage seems high

CHECK uses FFT analysis at 2048 points. typical CPU usage is under 1% at 44.1 kHz. if you see higher numbers, check that you don't have multiple instances running.

the plugin doesn't appear in my DAW

make sure you copied the files to the correct directories. on macOS: AU goes in ~/Library/Audio/Plug-Ins/Components/, VST3 goes in ~/Library/Audio/Plug-Ins/VST3/. restart your DAW after installing.