diff --git a/include/grgsm/receiver/clock_offset_control.h b/include/grgsm/receiver/clock_offset_control.h
index a6e2972..c4ea8b1 100644
--- a/include/grgsm/receiver/clock_offset_control.h
+++ b/include/grgsm/receiver/clock_offset_control.h
@@ -49,7 +49,7 @@
        * class. gsm::clock_offset_control::make is the public interface for
        * creating new instances.
        */
-      static sptr make(float fc, float samp_rate, unsigned int osr);
+      static sptr make(float fc, float samp_rate, unsigned int osr=4);
       virtual void set_fc(float fc) = 0;  
       virtual void set_samp_rate(float samp_rate) = 0;
       virtual void set_osr(unsigned int osr) = 0;