Add 2 channel intermediate PCM

This allows us to use ALSA automatic mono -> stereo conversions.

config/asound.conf

@@ -85,57 +85,41 @@ pcm.dmixer {
 #   Attenuate to 28% to avoid amplifier clipping
 #   For now leaving at 64% to maximize SNR before the amplifiers, and to leave
 #   some headroom for per-zone attenuation settings
+# A 2 channel intermediate PCM layer is added to each plug output below
+#   this enables automatic mono -> stereo conversions since
+#   ALSA does not understand how to convert mono to 8 channels.
+#   In that case the mono input would be mapped to the first (left) channel,
+#   with nothing playing out the right channel.
 pcm.ch1 {
-    type            plug
-    slave.pcm       "dmixer"
-    slave.channels  8
-    ttable.0.6      0.64
-    ttable.1.7      0.64
+    type                plug
+    slave.pcm {
+        type            route
+        slave.pcm       "dmixer"
+        slave.channels  8
+        ttable.0.6      0.64
+        ttable.1.7      0.64
+    }
+    slave.channels      2
 }
 pcm.ch2 {
-    type            plug
-    slave.pcm       "dmixer"
-    slave.channels  8
-    ttable.0.0      0.64
-    ttable.1.1      0.64
+    type                plug
+    slave.pcm {
+        type            route
+        slave.pcm       "dmixer"
+        slave.channels  8
+        ttable.0.0      0.64
+        ttable.1.1      0.64
+    }
+    slave.channels      2
 }
 pcm.ch3 {
-    type            plug
-    slave.pcm       "dmixer"
-    slave.channels  8
-    ttable.0.4      0.64
-    ttable.1.5      0.64
-}
-
-# Duplicate mono streams to stereo
-# - `speaker-test` only plays one mono output at a time
-# - fmradio.py only proceses mono and alsa cannot automatically map
-#   to stereo for syntetic outputs ch1, ch2, and ch3
-pcm.ch0m2s {
-    type            route
-    slave.pcm       "ch0"
-    slave.channels  2
-    ttable.0.0      1
-    ttable.0.1      1
-}
-pcm.ch1m2s {
-    type            route
-    slave.pcm       "ch1"
-    slave.channels  2
-    ttable.0.0      1
-    ttable.0.1      1
-}
-pcm.ch2m2s {
-    type            route
-    slave.pcm       "ch2"
-    slave.channels  2
-    ttable.0.0      1
-    ttable.0.1      1
-}
-pcm.ch3m2s {
-    type            route
-    slave.pcm       "ch3"
-    slave.channels  2
-    ttable.0.0      1
-    ttable.0.1      1
+    type                plug
+    slave.pcm {
+        type            route
+        slave.pcm       "dmixer"
+        slave.channels  8
+        ttable.0.4      0.64
+        ttable.1.5      0.64
+    }
+    slave.channels      2
 }

streams/fmradio.py

@@ -71,7 +71,7 @@ def main():
   # alsa output stage
   # - convert mono output -> stereo using alsa routes chXm2s
   # - normally mono conversion happens automatically but only the left channel was playing for ch1, ch2, and ch3
-  aplay_args = ['aplay', '-r', '171000', '-f', 'S16_LE', '--device', f'{args.output}m2s']
+  aplay_args = ['aplay', '-r', '171000', '-f', 'S16_LE', '--device', f'{args.output}']
 
   rtlfm_proc = subprocess.Popen(args=rtlfm_args, bufsize=1024, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
   redsea_proc = subprocess.Popen(args=redsea_args, bufsize=1024, stdin=rtlfm_proc.stdout, stdout=subprocess.PIPE, stderr=subprocess.PIPE)