Manual

How to use ZL Spectrum Equalizer

About

ZL Spectrum Equalizer is a dynamic spectrum equalizer plugin with the following key features:

Unmatched Versatility: Sculpt sound with 9 filter types, 5 stereo modes, 7 variable slopes, and up to 24 linear-phase frequency bands.
Spectrum Dynamics: Experience precise frequency-domain dynamic processing that tracks and controls resonances across the spectrum, with adjustable threshold, attack, release, and knee width.
Powerful Engine: Tailor behavior and latency with different spectrum resolution settings, side-chain smoothing, and frequency-dependent attack/release skewing, all powered by a high-performance FFT engine.
Intuitive Workflow: Achieve fast, fluid equalization with a carefully designed interface featuring an interactive spectrum graph, smart collision detection, and smooth animations.

Top Panel

You can open the UI Setting Panel by double-clicking the logo.

Analyzer

You can open the Analyzer Setting Panel by clicking the text.

Spectrum Resolution

Choose the spectrum processing resolution:

Extreme Low: around 12 ms latency
Very Low: around 23 ms latency
Low: around 46 ms latency
Medium: around 93 ms latency
High: around 186 ms latency
Very High: around 372 ms latency
Extreme High: around 743 ms latency

Press: open the Spectrum Setting Panel

Spectrum Smooth Type

Choose the spectrum processing side-chain smooth type:

OCT: Octave
ERB: Equivalent Rectangular Bandwidth (smoother at low frequency)

Spectrum Smooth Value

Control the spectrum processing side-chain smooth value. See more info in Spectrum Dynamic.

Output

You can open the Output Setting Panel by clicking the text. It shows the current filter gain scale & total output gain.

Press: use the external side-chain
Release: use the internal side-chain

Release: bypass the plugin

Center Panel

The center panel consists of a spectrum analyzer, filter response curves, filter buttons, a floating window, and a decibel scale.

Spectrum Analyzer

The spectrum analyzer shows the spectrum of the input/output/side-chain signal and collision areas (optional). You can control it via the Analyzer Setting Panel.

Filter Response Curves

The filter response curves show the magnitude response of each single band and the magnitude response of each stereo placement.

Filter Buttons

The filter buttons are attached to filter response curves. You can drag them to change filter parameters. See more info in UI Controls.

When you double-click (with/without Ctrl/Command down) on the spectrum, a band will be added at the corresponding position (with/without dynamic behavior enabled) with a filter button.

Floating Window

The floating window is attached to the filter button of the selected band. You can control some filter parameters through this window.

Icon	Filter Type	Icon	Stereo Placement
	`Peak`		`Stereo`
	`Low Shelf`		`Left`
	`Low Pass`		`Right`
	`High Shelf`		`Mid`
	`High Pass`		`Side`
	`Notch`
	`Band Pass`
	`Tilt Shelf`
	`Flat Tilt`

Invert Gain

Click: invert gain of the selected band

Split L/R

Click: set selected band to Left and add a band with same parameters but in Right

Split M/S

Click: set selected band to Mid and add a band with same parameters but in Side

Copy

Click: copy all selected bands’ filter parameters to the clipboard

Paste

Click: get filter parameters from the clipboard and create those bands

Decibel Scale

You can choose the decibel scale of magnitude response curves (through a combobox at the top-right) and the decibel scale of the spectrum analyzer (through a combobox at the bottom-right).

Bottom Panel

Left Panel

Release: bypass the band.

Band Selection

Select the current band.

Filter Type

Choose the filter type: Peak, Low Shelf, Low Pass, High Shelf, High Pass, Notch, Band Pass, Tilt Shelf and Flat Tilt.

Slope

Choose the filter slope: 6 dB/oct, 12 dB/oct, 24 dB/oct, 36 dB/oct, 48 dB/oct, 72 dB/oct, and 96 dB/oct. A higher slope will make the filter’s response curve change more steeply. Peak, Notch, and Band Pass don’t support 6 dB/oct. Flat Tilt doesn’t support slope setting.

Stereo Modes

Choose the stereo mode: Stereo, Left, Right, Mid, and Side.

FREQ

Control the frequency. The maximum value of the frequency is affected by the sample rate.

GAIN

Control the base gain and the target gain. See more info in Spectrum Dynamic.

Q

Control the quality factor.

Press: turn on the dynamic behavior of the band

Click: turn off the band

Right Panel

Release: bypass the dynamic behavior. See more info in Spectrum Dynamic.

Dynamic Mode

Choose the dynamic mode. See more info in Spectrum Dynamic.

Threshold

See more info in Spectrum Dynamic.

Knee

See more info in Spectrum Dynamic.

Attack

See more info in Spectrum Dynamic.

Release

See more info in Spectrum Dynamic.

Spectrum Setting Panel

Tilt

Control the spectrum processing side-chain tilt slope. See more info in Spectrum Dynamic.

Attack Skew

Control the spectrum processing side-chain attack skew. See more info in Spectrum Dynamic.

Release Skew

Control the spectrum processing side-chain release skew. See more info in Spectrum Dynamic.

Gate

Control the spectrum gate of relative loudness. See more info in Spectrum Dynamic.

Output Setting Panel

GAIN

Control the additional output gain.

SCALE

Control the scale of all filters’ base & target gain.

Press: turn on Static Gain Compensation
Release: turn off Static Gain Compensation

SGC estimates the amount of compensation from the filters’ parameters. SGC is inaccurate. However, it will NOT affect the dynamic of the main-chain signal.

Press: start to measure the integrated loudness of the input signal and the output signal
Release: update the Output Gain to the difference between the two loudness values

Analyzer Setting Panel

Pre/Post/Side

Press: turn on the Pre/Post/Side analyzer
Release: turn off the Pre/Post/Side analyzer

Decay Speed

Slope

affects the spectrum display (not the actual signal)
0 dB/oct: no tilting, white noise displays as a horizon line
3 dB/oct: pink noise displays as a horizon line
4.5 dB/oct: default value, which represents perceived loudness better

Smoothing

Control the smoothing value, larger value indicates more smooth spectrum
OCT: smooth the spectrum in octave
ERB: smooth the spectrum in ERB (Equivalent Rectangular Bandwidth)

Press: turn on the FFT freezing feature. When you hover the mouse over the analyzer for 2 seconds, the Post/Side analyzer will be frozen until you move the mouse.

FFT Analyzer Stereo Placement

Icon	Stereo Placement
	`Stereo`
	`Left`
	`Right`
	`Mid`
	`Side`

Press: turn on the collision detection.

Strength

Control the collision detection strength.

UI Setting Panel

The UI setting panel controls analyzer colors, slider operations, etc. Components will be introduced in the order from top to bottom.

Color

You can adjust the color by clicking on the left color block and change the transparency by dragging the right slider.

Text Color

Background Color

For better accessibility, please set Text/Background to colors with high contrast.

Shadow Color

Glow Color

Pre Color

Post Color

Side Color

Collision Color

Grid Color

Color Map 1

The color map of the curves of each single filter.

Color Map 2

The color map of the curves of Stereo/Left/Right/Mid/Side.

Import Colors

Import color settings (.xml file)

Export Colors

Export color settings (.xml file)

Control

Wheel Sensitivity

Rough: mouse-wheel sensitivity when Shift is not pressed
Fine: mouse-wheel sensitivity when Shift is pressed
Menu: mouse-wheel sensitivity when adjust combobox items
Reverse: whether to reverse the direction of mouse-wheel when Shift is pressed

Drag Sensitivity

Rough: mouse-drag sensitivity when Shift is not pressed
Fine: mouse-drag sensitivity when Shift is pressed

Rotary Slider Style

Circular: A rotary control that you move by dragging the mouse in a circular motion, like a knob
Horizontal: A rotary control that you move by dragging the mouse left-to-right
Vertical: A rotary control that you move by dragging the mouse up-and-down
Horiz + Vert: A rotary control that you move by dragging the mouse up-and-down or left-to-right
Distance: the relative distance that the mouse has to move to drag the slider across the full extent of its range. It does not apply to the Circular style.

Slider Double Click

Return Default: when you double-click the slider, it returns to the default value; when you double-click the slider with Ctrl/Command, it opens the value editor.
Open Editor: when you double-click the slider, it opens the value editor; when you double-click the slider with Ctrl/Command, it returns to the default value.

Short-cut settings for following actions:

Enter Solo

Exit Solo

Context Menu

Toggle Dynamic

Toggle Bypass

Delete Band

Import Controls

Import control settings (.xml file)

Export Controls

Export control settings (.xml file)

Other

Refresh Rate

In general, 30 Hz is enough for an equalizer plugin.

For a better analyzer display, set this to 1/n of your monitor refresh rate. For example,

If your monitor refresh rate is 120 Hz, set it to 120 Hz, 60 Hz (1/2), or 30 (1/4) Hz. DO NOT set it to 90 Hz.
If your monitor refresh rate is 90 Hz, set it to 90 Hz or 30 Hz (1/3). DO NOT set it to 60 Hz.

FFT Setting

Tilt: the extra tilting slope of the FFT
Speed: the extra decay speed of the FFT

Curve Thickness

Control the thickness of the curve of each band & each stereo mode.

Tooltip

Choose the tooltip language. It will take effect when the plugin window is reopened.

UI Scaling

Choose the font size mode.

Scale: the font size scales with the window size. Control the relative ratio.
Static: the font size is fixed. Control the actual font size.

Curve dB Scale

Min: minimum decibel scale of magnitude response curves
Default: default decibel scale of magnitude response curves
Max: maximum decibel scale of magnitude response curves

Window Size Fix

Choose whether to turn on Window Size Fix.

Off: plugin window size adjustment will be stored
On: plugin window size adjustment will NOT be stored, but open as it is currently every time the plugin is opened. The size can still be changed, but the changes will not be stored.

UI Controls

Generally, you can enable fine-adjustment with Shift and enable special adjustment with Ctrl/Command. If the direction of the mouse wheel is reversed when Shift is pressed, you can reverse it again (in the UI Setting Panel) to put it back to normal.

Sliders

You can enable fine-adjustment with Shift when using the mouse to drag / the mouse wheel to adjust sliders.
You can use the left/right mouse button to control the first/second value when there are two values on the slider.

Filter Buttons

You can drag a button to change the frequency/gain value.
You can use the mouse wheel to change the Q value.
You can use the mouse wheel + Ctrl/Command to change the slope.
You can enable fine-adjustment with Shift when using the mouse to drag a button.
You can drag with the left mouse button + Ctrl/Command down while fixing the frequency.
You can drag with the right mouse button + Ctrl/Command down while fixing the gain.
Other controls can be set via UI Setting Panel, such as enter/exit solo, toggle dynamic/bypass, etc

Combobox

You can use the mouse wheel to change the selected item.

Window Size

You can drag a dragger at the bottom-right corner to adjust plugin window size.
Recommend way to set Windows Size
1. Set Window Size Fix to OFF and press the store button.
2. Adjust the plugin window to the size you prefer.
3. Close the plugin window.
4. Reopen the plugin window.
5. Set Window Size Fix to ON and press the store button.
6. Close the plugin window.
7. Then, the plugin will be opened with the window size set in the Step 2 every time.

Spectrum Dynamic

How Spectrum Dynamic Works

Unlike ZL Equalizer 2 that applies broad filter curves based on wide-band audio triggers, ZL Spectrum Equalizer operates directly in the frequency domain using Fast Fourier Transforms (FFT). Audio is split into discrete frequency bins, allowing dynamic gain adjustments to be applied independently to each bin based on the level of the corresponding side-chain bin.

Spectrum Resolution

The Spectrum Resolution setting determines the FFT frame size used for spectral analysis and dynamic processing, establishing a direct trade-off between frequency precision and latency:

Lower Resolutions (Extreme Low to Low): Offer minimal processing latency (~12 ms to ~46 ms) and quick time-domain response, making them well-suited for live tracking or transient-heavy material.
Medium Resolution (Medium): Balances time and frequency resolution (~93 ms latency) for general mixing tasks.
Higher Resolutions (High to Extreme High): Provide exceptionally fine spectral resolution (~186 ms to ~743 ms latency) for exceptional low-frequency precision and surgical notch correction of narrow resonances.

In general, if you do not process the low-frequency part heavily (e.g., a high-pass below 100Hz), choose medium or lower resolutions to save processing power, reduce latency, and make dynamic processing more reactive. However, if you want heavy & precise low-frequency control, you may need to choose higher resolutions. Please watch out potential pre-ringing & CPU-spikes when you choose higher resolution.

Core Dynamic Behavior

When dynamic behavior is enabled for a band, each FFT bin covered by the filter curve continuously modulates its gain between two target values: Base Gain and Target Gain.

Threshold: The trigger level for dynamic action per bin.
- When the side-chain bin level crosses above the Threshold, the main-chain bin gain transitions from Base Gain toward Target Gain at a rate determined by Attack.
- When the side-chain bin level falls below the Threshold, the main-chain bin gain recovers back toward Base Gain at a rate determined by Release.
Knee: Controls the transition curvature around the threshold.
- A hard knee (low value) acts strictly: gain modulation begins only after the side-chain bin level crosses the threshold.
- A soft knee (high value) creates a gradual curve, smoothly transitioning toward the target gain as the side-chain bin level approaches the threshold for transparent, unobtrusive spectral shaping.

Note: Because processing occurs independently per FFT bin, parameters like Threshold and Knee operate on individual bin levels rather than the full-band signal envelope found in conventional dynamic processors.

Dynamic Mode

You can change how the threshold is calculated by selecting different dynamic modes:

Absolute: Dynamic processing uses a static threshold.
Band: Dynamic processing uses a dynamic threshold related to band side-chain loudness.
Relative: Dynamic processing uses a dynamic threshold related to total side-chain loudness.

Spectrum Processing

In addition to the per-band parameters, you can adjust the following parameters that affect the overall side-chain analysis to further customize dynamic behavior:

Smooth Type: Choose the spectrum processing side-chain smooth type.
Smooth Value: Control the spectrum processing side-chain smooth value. A higher value makes the dynamic processing smoother on the frequency domain.
Tilt: Control the spectrum processing side-chain tilt slope.
Attack Skew: Control the spectrum processing side-chain attack skew. A higher skew makes the attack faster at high frequency and slower at low frequency.
Release Skew: Control the spectrum processing side-chain release skew. A higher skew makes the release faster at high frequency and slower at low frequency.
Gate: Control the spectrum gate of relative loudness. Under Band mode, if band side-chain loudness is smaller than Gate, dynamic will NOT be triggered. Under Relative mode, if total side-chain loudness is smaller than Gate, dynamic will NOT be triggered.