noise-suppression: RNNoise virtual mic via PipeWire, enable on host

App-independent mic denoising as a PipeWire filter-chain exposing a
"Noise Canceling source" — replaces Discord's unusable Krisp and works
in every app. Imported into the nixos-pc host.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hosts/nixos-pc/default.nix

@@ -8,6 +8,7 @@
     ../../modules/nvidia.nix
     ../../modules/gaming.nix
     ../../modules/tailscale.nix
+    ../../modules/noise-suppression.nix
   ];
 
   networking.hostName = "nixos-pc";

modules/noise-suppression.nix

@@ -0,0 +1,74 @@
+{ pkgs, ... }:
+
+# App-independent microphone noise suppression via PipeWire + RNNoise.
+#
+# Replaces Discord's Krisp (which can't be enabled on current nixpkgs Discord:
+# the binary is patchelf'd, and even after bypassing Krisp's signature check the
+# newer "THz SDK" stage fails with KRISP_INIT_ERROR_GLOBAL_INIT). Instead we run
+# RNNoise as a PipeWire filter-chain that exposes a cleaned virtual microphone.
+#
+# This works in EVERY app (Discord, browser, OBS, games), survives Discord
+# updates, and touches no DRM. In each app's input settings, pick the source
+# named "Noise Canceling source" instead of the raw mic.
+
+{
+  # LADSPA plugin providing noise_suppressor_mono/stereo (werman RNNoise).
+  environment.systemPackages = [ pkgs.rnnoise-plugin ];
+
+  services.pipewire.extraConfig.pipewire."99-input-denoising" = {
+    "context.modules" = [
+      {
+        name = "libpipewire-module-filter-chain";
+        args = {
+          "node.description" = "Noise Canceling source";
+          "media.name" = "Noise Canceling source";
+          "filter.graph" = {
+            nodes = [
+              {
+                type = "ladspa";
+                name = "rnnoise";
+                plugin = "${pkgs.rnnoise-plugin}/lib/ladspa/librnnoise_ladspa.so";
+                label = "noise_suppressor_mono";
+                control = {
+                  # Higher threshold = more aggressive gating of non-voice.
+                  # 50% is a balanced starting point; raise toward 90–95 if a
+                  # noisy background still leaks through, lower if speech clips.
+                  "VAD Threshold (%)" = 50.0;
+                  "VAD Grace Period (ms)" = 200.0;
+                  "Retroactive VAD Grace (ms)" = 0.0;
+                };
+              }
+              {
+                # Makeup gain baked into the graph so the source volume is
+                # reproducible instead of living only in WirePlumber's runtime
+                # state. Mult = 1.53 matches the 153% level set in the mixer.
+                type = "builtin";
+                name = "gain";
+                label = "linear";
+                control = {
+                  "Mult" = 1.53;
+                  "Add" = 0.0;
+                };
+              }
+            ];
+            links = [
+              { output = "rnnoise:Output"; input = "gain:In"; }
+            ];
+          };
+          # The virtual mic apps select. media.class = Audio/Source makes it
+          # appear as a capture device throughout the desktop.
+          "capture.props" = {
+            "node.name" = "capture.rnnoise_source";
+            "node.passive" = true;
+            "audio.rate" = 48000;
+          };
+          "playback.props" = {
+            "node.name" = "rnnoise_source";
+            "media.class" = "Audio/Source";
+            "audio.rate" = 48000;
+          };
+        };
+      }
+    ];
+  };
+}