avoid demodulator concurrency

* this frees up resources used by the current demodulator before
  starting a new one
* this addresses an issue where users of single-channel AMBE sticks
  could not seamlessly switch between digital modes

csdr/chain/dummy.py

@@ -0,0 +1,14 @@
+from pycsdr.types import Format
+from csdr.chain import Module
+
+
+class DummyDemodulator(Module):
+    def __init__(self, outputFormat: Format):
+        self.outputFormat = outputFormat
+        super().__init__()
+
+    def getInputFormat(self) -> Format:
+        return Format.COMPLEX_FLOAT
+
+    def getOutputFormat(self) -> Format:
+        return self.outputFormat

owrx/dsp.py

@@ -7,6 +7,7 @@ from csdr.chain.demodulator import BaseDemodulatorChain, FixedIfSampleRateChain,
 from csdr.chain.selector import Selector, SecondarySelector
 from csdr.chain.clientaudio import ClientAudioChain
 from csdr.chain.fft import FftChain
+from csdr.chain.dummy import DummyDemodulator
 from pycsdr.modules import Buffer, Writer
 from pycsdr.types import Format
 from typing import Union, Optional
@@ -121,6 +122,19 @@ class ClientDemodulatorChain(Chain):
         if self.metaWriter is not None and isinstance(demodulator, MetaProvider):
             demodulator.setMetaWriter(self.metaWriter)
 
+    def stopDemodulator(self):
+        if self.demodulator is None:
+            return
+
+        # we need to get the currrent demodulator out of the chain so that it can be deallocated properly
+        # so we just replace it with a dummy here
+        # in order to avoid any client audio chain hassle, the dummy simply imitates the output format of the current
+        # demodulator
+        self.replace(1, DummyDemodulator(self.demodulator.getOutputFormat()))
+
+        self.demodulator.stop()
+        self.demodulator = None
+
     def _getSelectorOutputRate(self):
         if isinstance(self.demodulator, FixedIfSampleRateChain):
             return self.demodulator.getFixedIfSampleRate()
@@ -544,6 +558,7 @@ class DspManager(SdrSourceEventClient, ClientDemodulatorSecondaryDspEventClient)
             return FreeDV()
 
     def setDemodulator(self, mod):
+        self.chain.stopDemodulator()
         try:
             demodulator = self._getDemodulator(mod)
             if demodulator is None: