Adds support for 14 bit HIFI mode as an alternative to STANDARD_PLUS mode; adapts the description.

Firmware/fmsynth/fmsynth.ino

 /*  `fmsynth` is a little instrument using frequency modulation for sound synthesis.
 
-    Sound synthesis happens digitally on an Arduino board using the [Mozzi sonification library](https://sensorium.github.io/Mozzi/) from Tim Barrass.
+    Sound synthesis happens digitally on an Arduino board using the
+    [Mozzi sonification library](https://sensorium.github.io/Mozzi/)
+    written by Tim Barrass.
     
     Features of `fmsynth`:
     * FM synthesis with 2 operators (carrier and modulator)
-    * ADSR envelope on modulator amplitude
+    * ADSR envelopes on both operators
+    * audio signal with 14 bit depth and 32768 Hz sampling rate
     * MIDI for monophonic note input
-    * knobs to control some parameters
 
-    For good quality AUDIO_MODE should be STANDARD_PLUS at least and
-    AUDIO_RATE set to 32768 in mozzi_config.h within the Mozzi library directory.
+    
+    For best audio quality, change Mozzi/mozzi_config.h:
+    * #define AUDIO_MODE HIFI
+    * #define AUDIO_RATE 32768
 
     This Arduino sketch is made by Josch 2020, CC by-nc-sa.
 
@@ -102,8 +106,8 @@ void updateControl(){
   
   carrierEnv.update();
   modulatorEnv.update();
-  carrier_gain = carrierEnv.next();
-  modulator_gain = modulatorEnv.next();
+  carrier_gain = carrierEnv.next(); // writing Q8n0 into Q24n8 -> values from 0 to 1
+  modulator_gain = modulatorEnv.next(); // writing Q8n0 into Q0n8 -> values from 0 to 1
 }
 
 
@@ -111,7 +115,11 @@ int updateAudio(){
   Q15n16 envelopedModulator = modulator_gain * static_cast<Q23n8>(modulatorOsc.next()); // Q0n8 * Q23n8 -> Q15n16
   Q15n16 mudulation = Q15n16_to_Q23n8(envelopedModulator) * mod_index; // Q23n8 * Q8n8 -> Q15n16
 
-  return Q23n8_to_Q15n0(carrier_gain * carrierOsc.phMod(mudulation)); // Q24n8 * Q7n0 -> Q23n8
+#if AUDIO_MODE==HIFI
+  return static_cast<Q15n0>(carrier_gain * carrierOsc.phMod(mudulation)) >> 2; // Q24n8 * Q7n0 -> Q23n8 of which Q7n8 are used; bring it to 14 bit signed
+#else
+  return Q23n8_to_Q15n0(carrier_gain * carrierOsc.phMod(mudulation)); // Q24n8 * Q7n0 -> Q23n8 of which Q7n8 are used; bring it to 16 bit signed of which lower 8 bit are used
+#endif
 }