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[mathutils] increase ln(2) precision in freqtomidi/miditofreq

--- a/src/mathutils.c

+++ b/src/mathutils.c

@@ -522,7 +522,7 @@

   if (freq < 2. || freq > 100000.) return 0.; // avoid nans and infs

   /* log(freq/A-2)/log(2) */

   midi = freq / 6.875;

-  midi = LOG (midi) / 0.69314718055995;

+  midi = LOG (midi) / 0.6931471805599453;

   midi *= 12;

   midi -= 3;

   return midi;

@@ -534,7 +534,7 @@

   smpl_t freq;

   if (midi > 140.) return 0.; // avoid infs

   freq = (midi + 3.) / 12.;

-  freq = EXP (freq * 0.69314718055995);

+  freq = EXP (freq * 0.6931471805599453);

   freq *= 6.875;

   return freq;

 }