diff --git a/owrx/dsp.py b/owrx/dsp.py
index eabe244..b268c6a 100644
--- a/owrx/dsp.py
+++ b/owrx/dsp.py
@@ -3,7 +3,7 @@ from owrx.wsjt import WsjtParser
 from owrx.js8 import Js8Parser
 from owrx.aprs import AprsParser
 from owrx.pocsag import PocsagParser
-from owrx.source import SdrSource
+from owrx.source import SdrSource, SdrSourceEventClient
 from owrx.property import PropertyStack, PropertyLayer
 from owrx.modes import Modes
 from csdr import csdr
@@ -14,7 +14,7 @@ import logging
 logger = logging.getLogger(__name__)
 
 
-class DspManager(csdr.output):
+class DspManager(csdr.output, SdrSourceEventClient):
     def __init__(self, handler, sdrSource):
         self.handler = handler
         self.sdrSource = sdrSource
diff --git a/owrx/fft.py b/owrx/fft.py
index cbb98a6..717c086 100644
--- a/owrx/fft.py
+++ b/owrx/fft.py
@@ -1,7 +1,7 @@
 from owrx.config import Config
 from csdr import csdr
 import threading
-from owrx.source import SdrSource
+from owrx.source import SdrSource, SdrSourceEventClient
 from owrx.property import PropertyStack
 
 import logging
@@ -9,7 +9,7 @@ import logging
 logger = logging.getLogger(__name__)
 
 
-class SpectrumThread(csdr.output):
+class SpectrumThread(csdr.output, SdrSourceEventClient):
     def __init__(self, sdrSource):
         self.sdrSource = sdrSource
         super().__init__()
diff --git a/owrx/service/__init__.py b/owrx/service/__init__.py
index a8e60d8..98cf557 100644
--- a/owrx/service/__init__.py
+++ b/owrx/service/__init__.py
@@ -1,5 +1,5 @@
 import threading
-from owrx.source import SdrSource
+from owrx.source import SdrSource, SdrSourceEventClient
 from owrx.sdr import SdrService
 from owrx.bands import Bandplan
 from csdr.csdr import dsp, output
@@ -59,7 +59,7 @@ class Js8ServiceOutput(ServiceOutput):
         return t == "js8_demod"
 
 
-class ServiceHandler(object):
+class ServiceHandler(SdrSourceEventClient):
     def __init__(self, source):
         self.lock = threading.RLock()
         self.services = []
diff --git a/owrx/service/schedule.py b/owrx/service/schedule.py
index 992757e..8cf4261 100644
--- a/owrx/service/schedule.py
+++ b/owrx/service/schedule.py
@@ -1,5 +1,5 @@
 from datetime import datetime, timezone, timedelta
-from owrx.source import SdrSource
+from owrx.source import SdrSource, SdrSourceEventClient
 from owrx.config import Config
 import threading
 import math
@@ -204,7 +204,7 @@ class DaylightSchedule(TimerangeSchedule):
         return entries
 
 
-class ServiceScheduler(object):
+class ServiceScheduler(SdrSourceEventClient):
     def __init__(self, source):
         self.source = source
         self.selectionTimer = None
diff --git a/owrx/source/__init__.py b/owrx/source/__init__.py
index fcb176f..defda6a 100644
--- a/owrx/source/__init__.py
+++ b/owrx/source/__init__.py
@@ -16,6 +16,19 @@ import logging
 logger = logging.getLogger(__name__)
 
 
+class SdrSourceEventClient(ABC):
+    @abstractmethod
+    def onStateChange(self, state):
+        pass
+
+    @abstractmethod
+    def onBusyStateChange(self, state):
+        pass
+
+    def getClientClass(self):
+        return SdrSource.CLIENT_INACTIVE
+
+
 class SdrSource(ABC):
     STATE_STOPPED = 0
     STATE_STARTING = 1
@@ -238,7 +251,7 @@ class SdrSource(ABC):
         clients = [c for c in self.clients if c.getClientClass() in args]
         return len(clients) > 0
 
-    def addClient(self, c):
+    def addClient(self, c: SdrSourceEventClient):
         self.clients.append(c)
         hasUsers = self.hasClients(SdrSource.CLIENT_USER)
         hasBackgroundTasks = self.hasClients(SdrSource.CLIENT_BACKGROUND)
@@ -246,7 +259,7 @@ class SdrSource(ABC):
             self.start()
             self.setBusyState(SdrSource.BUSYSTATE_BUSY if hasUsers else SdrSource.BUSYSTATE_IDLE)
 
-    def removeClient(self, c):
+    def removeClient(self, c: SdrSourceEventClient):
         try:
             self.clients.remove(c)
         except ValueError: