libpsdxx [FEATURE]: provide mapping of linear frequency scale to logarithmic

src/libs/libpsdxx/function_log_frequency.cc

@@ -121,6 +121,22 @@ namespace psd {
      //                        const unsigned int& nsamples,
      //                        const unsigned int& n_per_decade)
 
+ /* ---------------------------------------------------------------------- */
+
+   TDseries log_frequency(const TDseries::Tcoc& flin,
+                          const unsigned int& n_per_decade)
+   {
+    // consistency checks
+    PSDXX_assert(flin.size()>1, 
+                 "input series must contain at least 2 values");
+    double df1=flin(flin.f()+1)-flin(flin.f());
+    double df2=flin(flin.l())-flin(flin.l()-1);
+    PSDXX_assert(std::abs((df2/df1)-1.)<1.e-8,
+                 "input frequency scale is not linear");
+    return(log_frequency(df1, flin.size(), n_per_decade));
+   } // TDseries log_frequency(const TDseries::Tcoc& lin_frequency,
+     //                        const unsigned int& n_per_decade)
+
 } // namespace psd
 
 /* ----- END OF function_log_frequency.cc ----- */

src/libs/libpsdxx/psd.h

@@ -268,6 +268,21 @@ namespace psd {
                           const unsigned int& nsamples,
                           const unsigned int& n_per_decade);
 
+   /*! \brief return a logarithmic frequency scale
+    *
+    * \param lin_frequency series of frequency values on linear scale
+    * \param n_per_decade number of samples per frequency decade
+    *                     for logarithmic sampling
+    * \return series of frequency values in Hz with logarithmic spacing and 
+    *         \p n_per_decade frequency values per decade
+    *
+    * This function is used to compute a series of frequency values with
+    * logarithmic spacing. The return value can be used as input parameter to
+    * the actual sampling functions.
+    */
+   TDseries log_frequency(const TDseries::Tcoc& lin_frequency,
+                          const unsigned int& n_per_decade);
+
    /*! sample complex values on a logarithmic frequency axis
     *
     * \param s spectral values with uniform sampling along frequency axis