pipewire: create a "effect_input.virtual" node which can become the "blast" audio source in the future

hosts/common/programs/pipewire/20-virtual.conf

@@ -0,0 +1,64 @@
+# Virtual stereo sink
+#
+# the main purpose is to provide a junction where audio can be selectively routed *to* this device,
+# and then consumers can tunnel it to some other place.
+# for example, `blast` can be used to cast this sink (source) to a TV.
+#
+# based on: <repo:pipewire/pipewire:src/daemon/filter-chain/source-duplicate-FL.conf>
+# but modified:
+# - duplicate both FL *and* FR
+
+context.modules = [
+  { name = libpipewire-module-filter-chain
+    flags = [ nofail ]
+    args = {
+      node.description = "Virtual Sink"
+      media.name       = "Virtual Sink"
+      filter.graph = {
+        nodes = [
+          {
+            name   = copyIL
+            type   = builtin
+            label  = copy
+          }
+          {
+            name   = copyIR
+            type   = builtin
+            label  = copy
+          }
+          {
+            name   = copyOL
+            type   = builtin
+            label  = copy
+          }
+          {
+            name   = copyOR
+            type   = builtin
+            label  = copy
+          }
+        ]
+        links = [
+          # we can only tee from nodes, not inputs so we need
+          # to copy the inputs and then tee.
+          { output = "copyIL:Out" input = "copyOL:In" }
+          { output = "copyIR:Out" input = "copyOR:In" }
+        ]
+        inputs  = [ "copyIL:In" "copyIR:In" ]
+        outputs = [ "copyOL:Out" "copyOR:Out" ]
+      }
+      capture.props = {
+        node.name         = "effect_input.virtual"
+        media.class       = Audio/Sink
+        audio.channels    = 2
+        audio.position    = [ FL FR ]
+      }
+      playback.props = {
+        node.name         = "effect_output.virtual"
+        node.passive      = true
+        audio.channels    = 2
+        audio.position    = [ FL FR ]
+      }
+    }
+  }
+]
+

hosts/common/programs/pipewire/default.nix

@@ -102,6 +102,7 @@ in
         default.clock.max-quantum = ${builtins.toString cfg.config.max-quantum}
       }
     '';
+    fs.".config/pipewire/pipewire.conf.d/20-virtual.conf".symlink.target = ./20-virtual.conf;
     fs.".config/pipewire/pipewire.conf.d/20-spatializer-7.1.conf".symlink.target = ./20-spatializer-7.1.conf;
 
     # reduce realtime scheduling priority to prevent GPU instability,