SW-546: Add comments describing how the offset is calculated

RTCP/Cobalt/OutputProc/src/TBB_Dipole.cc

@@ -316,14 +316,33 @@ namespace LOFAR
        * first received frame.
        */
       uint64_t offset = frame.header.time - itsDumpInfo.itsTime0;
-      // Convert the offset in [s] to bytes.  sampleFreq is in MHz.
+      /**
+       * Convert the offset in [s] to samples containing a complex numbers of
+       * two 16 bit integers.  sampleFreq is in MHz.
+       */
       offset *= (frame.header.sampleFreq * 1000000U);
       /**
-       * Divide the offset by the size of an FFT block.  The result is an index
-       * for the current data frame.
+       * Divide the offset, i.e. the number of samples by the size of an FFT
+       * block.  The result is the index of the raw voltage sampling window
+       * (size 1024 values) that got fed into the FFT.  This not a sample within
+       * the window but just a rough indicator which window we are in!
+       * [   1024   ][   1024   ][   1024   ]
+       *         ^
+       *         |  Start recording
+       *  ^
+       *  | Window number of the 0th TBB data frame
+       *                         ^
+       *                         |  Window number of frame N
+       * This is also then the index of the TBB frame for this frequency because
+       * the begin of a voltage window is also the begin of a frequency window.
        */
       offset /= SPECTRAL_TRANSFORM_SIZE;
 
+      /**
+       * Now try to figure out where in the raw voltage data window our spectral
+       * frame exactly begins.
+       */
+
       /**
        * NOTE:
        * untested body: when can I have enough test data from a working
@@ -343,6 +362,10 @@ namespace LOFAR
              * second:
              * 1 extra slice fitted in that second
              */
+            /**
+             * TODO
+             * Is this still needed?  Sander/Thomas need to look into this.
+             */
             offset += 1;
             // avoid duplicate offset compensation next
             time0 += 1;