ZEROLINE is a gain staging utility that measures each track's average level and sets trim values in a single pass before mixing.

1. Overview

In gain staging before mixing, leveling each track is essential yet time-consuming. ZEROLINE is a utility plugin that measures track audio (LEARN) and automatically calculates and locks the trim gain required to hit the target level (0 VU = -18 dBFS).

• One-shot measurement without continuous tracking: After measuring with LEARN, the trim value is locked (LOCKED). Unlike plugins that continuously track volume changes, it does not act like a slow compressor and will not flatten the natural dynamics and musical expression of a performance.
• VU-based (average level) leveling: By aligning trim to the average level (0 VU = -18 dBFS), which closely mirrors human hearing perception, it provides natural and consistent gain staging.
• Reliable peak warning: Because it levels average levels rather than peak values, when transients such as drums are expected to push output peaks near 0 dBFS (-0.2 dBFS or higher), it displays PEAK WARNING on screen to alert you.
• Intuitive interface: Input levels before trim and output levels after trim can be monitored at all times with the left and right VU meters, and post-measurement fine-tuning is effortless with the large central TRIM knob.
• Zero latency: Introduces zero processing delay (latency).

2. Requirements

Item	Details
Supported OS	Windows 10 / 11 (64bit), macOS 11 or later (Apple Silicon / Intel)
Plugin formats	Windows: VST3 / Mac: VST3, AU
Supported hosts	VST3 or AU compatible DAWs
Channels	Mono / Stereo
Latency	Zero (0 samples)
Window size	720 × 480 px (fixed)
Screen language	Japanese / English (follows the OS display language)

3. Installation

Installation (Windows)

1. Extract the downloaded ZIP file.
2. Copy the ZEROLINE.vst3 folder to the following location:
   C:\Program Files\Common Files\VST3\
3. Launch your DAW and perform a plugin rescan (if you copy while the DAW is running, it may not be detected immediately).

Installation (Mac)

1. Extract the downloaded ZIP file.
2. Copy ZEROLINE.vst3 to ~/Library/Audio/Plug-Ins/VST3/, and copy ZEROLINE.component to ~/Library/Audio/Plug-Ins/Components/ (where ~ represents your home folder. In Finder, open the "Go" menu, choose "Go to Folder...", paste the path, and press Return. If the folder does not exist, create it).
3. Open Terminal (Applications → Utilities → Terminal), paste the following 3 lines one by one, and press Return for each:

   cd ~/Library/Audio/Plug-Ins
   xattr -dr com.apple.quarantine VST3/"ZEROLINE.vst3"
   xattr -dr com.apple.quarantine Components/"ZEROLINE.component"

4. Launch your DAW and perform a plugin rescan (if you copy while the DAW is running, it may not be detected immediately. If AU does not appear in Logic Pro or other DAWs, restart your Mac).

Uninstallation

[Windows] Delete the copied ZEROLINE.vst3 folder.
[Mac] Delete the copied ZEROLINE.vst3 and ZEROLINE.component files.

4. The screen

Header bar (top)

• Logo & version: Displays the logo and version (v1.1.0) at the top left of the window.

InfoBar

• Located right below the header. When you hover over a knob or button, an explanation of its function is shown in English or Japanese, following the OS display language.

LEVEL panel

• IN meter: A VU meter showing the input signal level before trim.
• OUT meter: A VU meter showing the output signal level after applying trim.
• Target line (green line): The green line across the meters indicates the target level (0 VU).
• CLIP indicator: Located at the top of the meters, lights up red for 2 seconds whenever the signal exceeds -0.2 dBFS.
• Reference readout: Displays the reference level "0 VU = -18 dBFS" at the bottom of the meters.

TRIM panel

• TRIM knob: A large knob for adjusting trim gain (-24.0 to +24.0 dB, in 0.1 dB steps). After being automatically set by LEARN, it can also be fine-tuned manually.
• LEARN button: A button to start and stop measuring the input level.
• Status readout: Displays the current operating status and measurement results across 2 lines.
• Ø (Phase invert) button: Inverts the phase of the output signal (180° inversion).

TARGET panel / MATCH panel (Pro exclusive features)

• In the free edition of ZEROLINE, the TARGET panel and MATCH panel display a "PRO" badge and controls are disabled. These features are available in the upgraded edition, ZEROLINE Pro.

5. How to use LEARN

Plugin placement

Insert ZEROLINE at the very beginning of your effect chain. On software instrument tracks, insert it immediately after the instrument and before any other effects.

Measurement procedure

1. Start playback in your DAW.
2. Click the LEARN button on ZEROLINE (the status readout changes to LEARNING).
3. Play the loudest section of the track (such as the chorus).
4. Click the LEARN button again, or stop playback in your DAW.
5. The trim gain is automatically calculated and locked (the status readout changes to LOCKED).

6. Status readout

In the status readout area at the center of the TRIM panel, the first line displays the operating status (color-coded), and the second line displays supplemental information or the measured trim value.

• PLAY, THEN LEARN: Standby state. Start playback in your DAW before pressing the LEARN button.
• LEARNING: Measuring. The elapsed time in seconds since measurement started is shown on the second line.
• LOCKED: Measurement is complete and the trim value matching the target level is locked. The measured input average level (e.g., -5.0 VU) is displayed on the second line. The determined trim value can be checked on the TRIM knob.
• PEAK WARNING: The trim value has been locked, but leveling the average level is expected to push the output peak near 0 dBFS (-0.2 dBFS or higher). The estimated output peak value is displayed on the second line.
• OUT OF RANGE: The trim gain required to match the target level exceeded the adjustment range (±24.0 dB), so trim was clamped at the upper or lower limit. The clamped value is displayed on the second line.
• NO SIGNAL: No measurable audio signal was detected. The trim value remains unchanged. Check plugin placement, DAW playback status, and track routing.

7. Tips

• Leveling the foundation of your mix:
  What ZEROLINE levels is the signal level before entering your effect chain. Because downstream compressors and saturators receive a consistent level every time, presets and settings become much easier to reuse. Since all tracks are brought to the same average level, a hi-hat and a lead vocal would sound equally loud if left as-is. Set the final volume balance between tracks using your channel faders afterward.
• Why it does not track continuously:
  Plugins that continuously adjust gain according to volume act like a slow compressor, compromising the natural dynamics and performance intensity of the music. Because ZEROLINE locks (LOCKED) the value measured with LEARN, you achieve accurate gain staging while preserving the song's original musical dynamics.
• Handling peak warnings (PEAK WARNING):
  ZEROLINE adjusts levels based on average level (VU), which closely matches human auditory perception, rather than peak values. Therefore, on transient-heavy sounds like snare drums or percussion, leveling to 0 VU may cause peaks to jump near 0 dBFS (-0.2 dBFS or higher). Because modern DAWs process audio in floating-point format, exceeding 0 dBFS internally will not immediately cause harsh digital clipping. However, subsequent gear or plugins may distort, so if this concerns you, simply lower the TRIM knob slightly by hand.
• Using the phase invert (Ø) button:
  When layering multiple microphones or lines—such as multi-mic drums (snare top/bottom, kick in/out) or bass line/amp recordings—pressing the Ø button to flip phase is convenient for checking low-end punch and body.
• About bypass:
  ZEROLINE does not include a dedicated BYPASS button on its interface. To compare sound by bypassing the plugin, use the bypass button in your DAW's track insert slot.

8. About ZEROLINE Pro

For users seeking more advanced gain staging and mixing workflows, an upgraded edition, "ZEROLINE Pro", is available. ZEROLINE Pro adds the following features:

• TARGET panel (flexible target settings):
  • 3 measurement modes (MODE): Choose between standard VU (average level), LUFS (integrated loudness tailored for streaming and broadcasting standards), or PEAK (sample peak) to match by peak amplitude.
  • Customizable target level (LEVEL): Target values can be freely adjusted for each MODE (VU: -24.0 to -8.0 dBFS, LUFS: -36.0 to -10.0, PEAK: -24.0 to 0.0 dBFS). This allows flexible calibration to your working reference standards (such as -20 / -18 / -14 dBFS).
• MATCH feature (pre/post-effect level matching):
  • Insert two ZEROLINE Pro instances before and after effects on the same track (ROLE IN and ROLE OUT, Groups A to H). Measuring with the MATCH button on the OUT instance aligns and locks post-effect level to match the pre-effect level. By toggling the effect and the OUT instance on/off together, you can evaluate sound character differences without being fooled by volume changes.
• LEARN ALL feature (all-track batch measurement):
  • Simply clicking LEARN ALL on a single instance starts and finishes LEARN simultaneously across all ZEROLINE Pro instances inserted across your project (ROLE OFF / IN). Gain staging for your entire project can be completed in a single action.

BOOTH ↗

9. Notes

• This product is distributed free of charge (freeware) "AS IS", without support. Feedback and suggestions are welcome at info@tonecraft.blue-x.tokyo. We cannot promise individual support.
• The source code is proprietary and not public.
• All copyrights to this product belong to Tonecraft Tokyo.
• Modification, reverse engineering, redistribution, and resale of this product are strictly prohibited (please download from the Tonecraft Tokyo official website and BOOTH).
• Free for both commercial and non-commercial use.

10. Disclaimer

• This product is provided "AS IS", without warranty of any kind, express or implied, including but not limited to merchantability, fitness for a particular purpose, and non-infringement of third-party rights.
• Tonecraft Tokyo is not liable for any damages arising from the use of, or inability to use, this product, including but not limited to loss or corruption of project files or audio data, equipment malfunction, lost working time, or lost profits.
• Operation is not guaranteed for every combination of DAW, operating system, and hardware.
• Specifications may change, and the product may be discontinued, without notice.
• Use this product at your own risk. Back up your projects before important work.

11. Licenses

• VST is a trademark of Steinberg Media Technologies GmbH.
• This product uses JUCE.
• Noto Sans JP / JetBrains Mono — SIL Open Font License 1.1