diff --git a/src/sink.cpp b/src/sink.cpp
--- a/src/sink.cpp
+++ b/src/sink.cpp
@@ -25,6 +25,7 @@ void Sink::connectFollower(Sink* sink)
     Q_ASSERT(!followers.contains(sink));
 
     followers.append(sink);
+    sink->setNumChannels(numChannels(), hasX());
 }
 
 void Sink::disconnectFollower(Sink* sink)
diff --git a/src/sink.h b/src/sink.h
--- a/src/sink.h
+++ b/src/sink.h
@@ -38,10 +38,14 @@ public:
 protected:
     /// Entry point for incoming data. Re-implementations should
     /// call this function to feed followers.
-    void feedIn(const SamplePack& data);
+    virtual void feedIn(const SamplePack& data);
     /// Is set by connected source. Re-implementations should call
     /// this function to update followers.
-    void setNumChannels(unsigned nc, bool x);
+    virtual void setNumChannels(unsigned nc, bool x);
+    /// Returns true if sink has X data
+    virtual bool hasX() const = 0;
+    /// Returns number of channels
+    virtual unsigned numChannels() const = 0;
 
     friend Source;
 
diff --git a/src/source.cpp b/src/source.cpp
--- a/src/source.cpp
+++ b/src/source.cpp
@@ -43,7 +43,6 @@ void Source::feedOut(const SamplePack& d
     }
 }
 
-/// Updates "number of channels" of connected sinks
 void Source::feedNumChannels(unsigned nc, bool x) const
 {
     for (auto sink : sinks)
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -23,7 +23,11 @@ find_package(Qt5Widgets)
 include_directories("../src")
 
 add_executable(Test EXCLUDE_FROM_ALL
-  test.cpp ../src/samplepack.cpp)
+  test.cpp
+  ../src/samplepack.cpp
+  ../src/sink.cpp
+  ../src/source.cpp
+  )
 add_test(NAME test1 COMMAND Test)
 qt5_use_modules(Test Widgets)
 
diff --git a/tests/test.cpp b/tests/test.cpp
--- a/tests/test.cpp
+++ b/tests/test.cpp
@@ -21,6 +21,7 @@
 #include "catch.hpp"
 
 #include "samplepack.h"
+#include "source.h"
 
 TEST_CASE("samplepack with no X", "[memory]")
 {
@@ -47,3 +48,74 @@ TEST_CASE("samplepack with X", "[memory]
     REQUIRE(pack.numSamples() == 100);
     REQUIRE(pack.xData() != nullptr);
 }
+
+class TestSink : public Sink
+{
+public:
+    int totalFed;
+    int _numChannels;
+    bool _hasX;
+
+    TestSink()
+        {
+            totalFed = 0;
+            _numChannels = 0;
+            _hasX = false;
+        };
+
+public:
+    void feedIn(const SamplePack& data)
+        {
+            REQUIRE(data.numChannels() == numChannels());
+
+            totalFed += data.numSamples();
+
+            Sink::feedIn(data);
+        };
+
+    void setNumChannels(unsigned nc, bool x)
+        {
+            _numChannels = nc;
+            _hasX = x;
+
+            Sink::setNumChannels(nc, x);
+        };
+
+    virtual unsigned numChannels() const
+        {
+            return _numChannels;
+        };
+
+    virtual bool hasX() const
+        {
+            return _hasX;
+        };
+};
+
+TEST_CASE("sink", "[memory, stream]")
+{
+    TestSink sink;
+    SamplePack pack(100, 3, false);
+
+    sink.setNumChannels(3, false);
+    REQUIRE(sink.numChannels() == 3);
+
+    sink.feedIn(pack);
+    REQUIRE(sink.totalFed == 100);
+    sink.feedIn(pack);
+    REQUIRE(sink.totalFed == 200);
+
+    TestSink follower;
+
+    sink.connectFollower(&follower);
+    REQUIRE(follower.numChannels() == 3);
+    REQUIRE(follower.hasX() == false);
+
+    sink.feedIn(pack);
+    REQUIRE(sink.totalFed == 300);
+    REQUIRE(follower.totalFed == 100);
+
+    sink.setNumChannels(2, true);
+    REQUIRE(follower.numChannels() == 2);
+    REQUIRE(follower.hasX() == true);
+}