filter-graph: add dcblock and ramp plugins

src/daemon/filter-chain/36-dcblock.conf

@@ -0,0 +1,59 @@
+context.modules = [
+    { name = libpipewire-module-filter-chain
+        args = {
+            node.description = "DCBlock Filter"
+            media.name       = "DCBlock Filter"
+            filter.graph = {
+                nodes = [
+                    {
+                        name  = dcblock
+                        type  = builtin
+                        label = dcblock
+			control = {
+			   "R" = 0.995
+			}
+                    }
+                    {
+		        # add a short 20ms ramp
+                        name  = ramp
+                        type  = builtin
+                        label = ramp
+			control = {
+			   "Start" = 0.0
+			   "Stop" = 1.0
+			   "Duration (s)" = 0.020
+			}
+                    }
+                    {
+                        name  = volumeL
+                        type  = builtin
+                        label = mult
+                    }
+                    {
+                        name  = volumeR
+                        type  = builtin
+                        label = mult
+                    }
+		]
+		links = [
+		  { output = "dcblock:Out 1" input = "volumeL:In 1" }
+		  { output = "dcblock:Out 2" input = "volumeR:In 1" }
+		  { output = "ramp:Out" input = "volumeL:In 2" }
+		  { output = "ramp:Out" input = "volumeR:In 2" }
+		]
+		inputs  = [ "dcblock:In 1"  "dcblock:In 2" ]
+                outputs = [ "volumeL:Out" "volumeR:Out" ]
+            } 
+	    capture.props = {
+                node.name      = "effect_input.dcblock"
+                audio.position = [ FL FR  ]
+                media.class    = Audio/Sink
+            }
+            playback.props = {
+                node.name      = "effect_output.dcblock"
+                audio.position = [ FL FR ]
+                node.passive   = true
+            }
+        }
+    }
+]

src/modules/module-filter-chain.c

@@ -472,6 +472,28 @@ extern struct spa_handle_factory spa_filter_graph_factory;
  *
  * It has two input ports "In 1" and "In 2" and one output port "Out".
  *
+ * ### dcblock
+ *
+ * Use the `dcblock` plugin implements a [DC blocker](https://www.dsprelated.com/freebooks/filters/DC_Blocker.html).
+ *
+ * It has 8 input ports "In 1" to "In 8" and corresponding output ports "Out 1"
+ * to "Out 8". Not all ports need to be connected.
+ *
+ * It also has 1 control input port "R" that controls the DC block R factor.
+ *
+ * ### Ramp
+ *
+ * Use the `ramp` plugin creates a linear ramp from `Start` to `Stop`.
+ *
+ * It has 3 input control ports "Start", "Stop" and "Duration (s)". It also has one
+ * output port "Out". A linear ramp will be created from "Start" to "Stop" for a duration
+ * given by the "Duration (s)" control in (fractional) seconds. The current value will
+ * be stored in the output notify port "Current".
+ *
+ * The ramp output can, for example, be used as input for the `mult` plugin to create
+ * a volume ramp up or down. For more a more coarse volume ramp, the "Current" value
+ * can be used in the `linear` plugin.
+ *
  * ## SOFA filter
  *
  * There is an optional builtin SOFA filter available.