add empty demodulator (for modes that don't produce any usable audio)

csdr/chain/analog.py

@@ -74,3 +74,14 @@ class Ssb(BaseDemodulatorChain):
             Agc(Format.FLOAT),
         ]
         super().__init__(workers)
+
+
+class Empty(BaseDemodulatorChain):
+    def __init__(self):
+        super().__init__([])
+
+    def getOutputFormat(self) -> Format:
+        return Format.FLOAT
+
+    def setWriter(self, writer):
+        pass

owrx/dsp.py

@@ -554,6 +554,9 @@ class DspManager(SdrSourceEventClient, ClientDemodulatorSecondaryDspEventClient)
         elif demod == "freedv":
             from csdr.chain.freedv import FreeDV
             return FreeDV()
+        elif demod == "none":
+            from csdr.chain.analog import Empty
+            return Empty()
 
     def setDemodulator(self, mod):
         self.chain.stopDemodulator()