diff --git a/lib/dtsc.cpp b/lib/dtsc.cpp
index 90b901a0..9e2e8fd3 100644
--- a/lib/dtsc.cpp
+++ b/lib/dtsc.cpp
@@ -1038,3 +1038,13 @@ DTSC::File::~File(){
     F = 0;
   }
 }
+
+
+bool DTSC::isFixed(JSON::Value & metadata){
+  for (JSON::ObjIter it = metadata["tracks"].ObjBegin(); it != metadata["tracks"].ObjEnd(); it++){
+    if (!(it->second.isMember("keys") && it->second["keys"].isArray() && it->second["keys"][0u].isMember("bpos"))){
+      return false;
+    }
+  }
+  return true;
+}
diff --git a/lib/dtsc.h b/lib/dtsc.h
index 5e0c5278..ac07bcf1 100644
--- a/lib/dtsc.h
+++ b/lib/dtsc.h
@@ -50,6 +50,7 @@
 /// - nalu_end (int, if set, is a end-of-sequence)
 /// - offset (int, unsigned version of signed int! Holds the ms offset between timestamp and proper display time for B-frames)
 namespace DTSC {
+  bool isFixed(JSON::Value & metadata);
 
   /// This enum holds all possible datatypes for DTSC packets.
   enum datatype{