Airwindows AutoGain shim

A drop-in loudness-matching wrapper for Airwindows VST2 plugins. It proxies any Airwindows .dll (or .so/.vst) without modifying it, forwards every parameter and the full plugin interface to the host, and inserts an automatic gain-matching stage so the processed signal sits at the same level as the dry signal — honest A/B, no "louder sounds better" bias.

It needs no Steinberg VST2 SDK to build (it uses a clean-room minimal ABI header), so you can ship a prebuilt binary that users just run.

How a user uses it (no building)

1. Download the release: airwrap (generator) + awautogain (shim template).

2. Run the generator against your Airwindows VST2 folder:

   airwrap --shim awautogain.dll --in "C:\VST\Airwindows" --out "C:\VST\Airwindows AG"
   

   This creates one wrapper per plugin: ToTape6 AG.dll, Density AG.dll, … each with a small .cfg beside it.

3. Add only the output folder to your host's VST2 scan path (keep the original Airwindows folder out of the scan path so you don't see duplicates), or use --copy to bundle the originals into <out>/_airwin for a self-contained folder.

4. In your host, each wrapper shows up as <Name> AG with the plugin's normal controls plus extra parameters:

   • AG Hold — the wrapper always auto-matches: it continuously derives a BS.1770 gated, integrated dry-vs-wet loudness offset and applies it. Leave AG Hold off (the default) and it keeps adapting; set it above 50% to freeze the offset at its current value. The display reads "Auto ±x.x dB" / "Hold ±x.x dB".
   • AG Window — how much recent audio the rolling gated measure integrates, ~2 s to ~30 s. Shorter reacts faster; longer is steadier (and better for dynamics, where you want a representative span).
   • AG Trim — manual offset, ±12 dB, applied on top of the learned offset. Displays just its value (e.g. +0.0 dB); the loudness readout lives on AG Level below.
   • AG Level — a readout, not a control: its value tracks the output loudness, and its display shows In/Out LUFS (e.g. -14/-14). A parameter whose value changes is the only thing a host like Bitwig will re-read continuously, so this is the one that updates in realtime as the level moves. The plugin also notifies the host (~12 Hz) so non-polling hosts keep it fresh. Writes to it are ignored. A record-armed automation lane on it would capture the movement — harmless unless you deliberately arm it.

Bitwig (and other caching hosts): the parameter layout is cached at scan time. After updating the wrappers, force a rescan — remove and re-add the plugin, or clear/rebuild the plugin cache — or Bitwig will keep showing the old parameter set and AG Level won't appear.

How it behaves. Because the offset is gated and integrated over the window, it handles both static-character processing (EQ, saturation, consoles) and dynamics (compressors, limiters) without pumping — a continuous instantaneous match would fight a compressor's gain changes, but an integrated one settles on the right long-term level. Use AG Hold to lock a value once you're happy with it (e.g. after playing a representative section), and the held offset is saved with the project.

Boost is limited. The match attenuates freely but boosts at most +12 dB, and it ducks faster than it lifts. This keeps it from "restoring" the loudness of plugins that legitimately remove broadband energy — lowpass/highpass filters, band cuts, mid/side and mono utilities, dither — where loudness matching isn't meaningful. On those, Hold near 0 dB or just use Trim.

Set AWDEBUG=1 in the environment before launching the host to get a once-per-second readout in the log (see below) of dry/wet RMS, the computed match, and the gain actually applied — handy for confirming matching is active.

Building from source

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build --config Release

Produces awautogain.{dll,so,vst} (the shim template) and airwrap (the generator). On Windows build 64-bit to match Airwindows' 64-bit VST2s.

How it works

airwrap copies the single compiled shim once per plugin and writes a sidecar .cfg (target=, name=, id=). At load the shim reads its own filename, finds the matching .cfg, LoadLibrarys the real plugin, and returns an AEffect that:

• copies the inner plugin's I/O counts, flags and version verbatim;
• forwards dispatcher, get/setParameter, both processReplacing and processDoubleReplacing (Airwindows' double-precision path is preserved);
• translates the audioMaster callback pointer so host automation still resolves to the wrapper, not the hidden inner plugin;
• appends the four AG parameters (Hold, Window, Trim, Level) and reports numParams = inner + 4;
• wraps the state chunk (AGW1 header) so the AG settings survive save/reload — Airwindows set programsAreChunks(true), so without this the extra params would be silently dropped;
• measures dry vs. wet mean-square per block, integrates with a one-pole filter, and applies a per-sample-smoothed gain to match levels.

Set AWDEBUG=1 in the environment to get load diagnostics on stderr.

Caveats worth knowing

• Dynamics processors. Because the offset is gated and integrated over the window rather than instantaneous, it settles on the long-term level without fighting a compressor's own gain changes. For these, a longer AG Window (and Hold once it's settled) gives the steadiest result.
• Loudness, not flat RMS. Matching uses ITU-R BS.1770 K-weighting (the same weighting LUFS meters use), so it tracks perceived loudness. This matters for saturators, tape, and consoles whose harmonics read louder than their flat RMS suggests; a plain-RMS match would leave those a couple of dB hot.
• Latency alignment. Dry is measured before the inner plugin, wet after. For the rare Airwindows plugin reporting latency this is unaligned within the integration window; with the default slow window it's negligible. (A dry delay line matching initialDelay would tighten it — easy to add if you hit a case that needs it.)
• Legal. VST2 is Steinberg's deprecated format; this is a personal-use utility built against a clean-room ABI header (no Steinberg SDK source). "VST" is a Steinberg trademark.

Airwindows is MIT-licensed work by Chris Johnson. This wrapper is independent and does not include or modify Airwindows source.