diff --git a/pyrelacs/dataio/circbuffer.py b/pyrelacs/dataio/circbuffer.py
index bdb64d3..2e1fa40 100644
--- a/pyrelacs/dataio/circbuffer.py
+++ b/pyrelacs/dataio/circbuffer.py
@@ -8,9 +8,9 @@ class CircBuffer:
     def __init__(
         self,
         size: int,
-        mutex: QMutex,
         channels: int = 1,
         samplerate: int = 40_000,
+        mutex: QMutex = QMutex(),
     ):
         self.__size = size
         self.__channels = channels
@@ -123,7 +123,7 @@ class CircBuffer:
         # easy case first, we can spare the effort of further checking
         if index >= 0 and index <= self.write_index(channel):
             if self.has_value(index, channel):
-                return (self.__buffer[channel, index], self.__time[index])
+                return (self.__buffer[channel, index], self.__time[channel, index])
             else:
                 raise IndexError(
                     f"Invalid index {index} on ring buffer for channel{channel}"
@@ -132,7 +132,7 @@ class CircBuffer:
         if index < 0:
             index = self.write_index() - 1
         if self.has_value(index, channel):
-            return (self.__buffer[channel, index], self.__time[index])
+            return (self.__buffer[channel, index], self.__time[channel, index])
         else:
             raise IndexError(
                 f"Invalid index {index} on ring buffer for channel{channel}"