channelmix: make up/downmix levels configurable

Add channelmix.center-level, channelmix.surround-level and
channelmix.lfe-level to control how center, surround and LFE is
up/downmixed.

Fixes #5198

doc/dox/config/pipewire-client.conf.5.md

@@ -80,6 +80,9 @@ stream.properties = {
     #channelmix.fc-cutoff  = 12000.0
     #channelmix.rear-delay = 12.0
     #channelmix.stereo-widen = 0.0
+    #channelmix.center-level = 0.707106781
+    #channelmix.surround-level = 0.707106781
+    #channelmix.lfe-level = 0.5
     #channelmix.hilbert-taps = 0
     #dither.noise = 0
     #dither.method = none # rectangular, triangular, triangular-hf, wannamaker3, shaped5

doc/dox/config/pipewire-props.7.md

@@ -783,6 +783,15 @@ more to the center speaker and leaves the ambient sound in the stereo channels.
 This is only active when up-mix is enabled and a Front Center channel is mixed.
 \endparblock
 
+@PAR@ node-prop  channelmix.center-level = 0.707106781
+The level of the center channel when up/downmixing.
+
+@PAR@ node-prop  channelmix.surround-level = 0.707106781
+The level of the surround channels when up/downmixing.
+
+@PAR@ node-prop  channelmix.lfe-level = 0.5
+The level of the LFE channel when up/downmixing.
+
 @PAR@ node-prop  channelmix.hilbert-taps = 0
 \parblock
 This option will apply a 90 degree phase shift to the rear channels to improve specialization.

doc/dox/config/pipewire-pulse.conf.5.md

@@ -93,6 +93,9 @@ stream.properties = {
     #channelmix.fc-cutoff = 12000.0
     #channelmix.rear-delay = 12.0
     #channelmix.stereo-widen = 0.0
+    #channelmix.center-level = 0.707106781
+    #channelmix.surround-level = 0.707106781
+    #channelmix.lfe-level = 0.5
     #channelmix.hilbert-taps = 0
     #dither.noise = 0
     #dither.method = none # rectangular, triangular, triangular-hf, wannamaker3, shaped5