introducing FLUID_PEAK_ATTENUATION macro. (#397)

avoids hard-coding the magic 96 dB everywhere in the code

src/rvoice/fluid_rvoice.c

@@ -51,7 +51,7 @@ fluid_rvoice_calc_amp(fluid_rvoice_t* voice)
     fluid_real_t amp_max;
 
     target_amp = fluid_cb2amp (voice->dsp.attenuation)
-      * fluid_cb2amp (960.0f * (1.0f - fluid_adsr_env_get_val(&voice->envlfo.volenv))
+      * fluid_cb2amp (FLUID_PEAK_ATTENUATION * (1.0f - fluid_adsr_env_get_val(&voice->envlfo.volenv))
 		      + fluid_lfo_get_val(&voice->envlfo.modlfo) * -voice->envlfo.modlfo_to_vol);
 
     /* We turn off a voice, if the volume has dropped low enough. */
@@ -507,7 +507,7 @@ fluid_rvoice_noteoff_LOCAL(fluid_rvoice_t* voice, unsigned int min_ticks)
     if (fluid_adsr_env_get_val(&voice->envlfo.volenv) > 0){
       fluid_real_t lfo = fluid_lfo_get_val(&voice->envlfo.modlfo) * -voice->envlfo.modlfo_to_vol;
       fluid_real_t amp = fluid_adsr_env_get_val(&voice->envlfo.volenv) * fluid_cb2amp(lfo);
-      fluid_real_t env_value = - ((-200 * log (amp) / log (10.0) - lfo) / 960.0 - 1);
+      fluid_real_t env_value = - ((-200 * log (amp) / log (10.0) - lfo) / FLUID_PEAK_ATTENUATION - 1);
       fluid_clip (env_value, 0.0, 1.0);
       fluid_adsr_env_set_val(&voice->envlfo.volenv, env_value);
     }
@@ -569,7 +569,8 @@ DECLARE_FLUID_RVOICE_FUNCTION(fluid_rvoice_multi_retrigger_attack)
         volenv_val to achieve equivalent amplitude during the attack phase
 		for seamless volume transition. */
 		fluid_real_t amp_cb, env_value;
-		amp_cb = 960.0f * (1.0f - fluid_adsr_env_get_val(&voice->envlfo.volenv));
+		amp_cb = FLUID_PEAK_ATTENUATION * 
+		        (1.0f - fluid_adsr_env_get_val(&voice->envlfo.volenv));
 		env_value = fluid_cb2amp(amp_cb); /* a bit of optimization */
 		fluid_clip (env_value, 0.0, 1.0);
 		fluid_adsr_env_set_val(&voice->envlfo.volenv, env_value);

src/rvoice/fluid_rvoice.h

@@ -41,7 +41,6 @@ typedef struct _fluid_rvoice_t fluid_rvoice_t;
  */
 #define FLUID_NOISE_FLOOR 2.e-7
 
-
 enum fluid_loop {
   FLUID_UNLOOPED = 0,
   FLUID_LOOP_DURING_RELEASE = 1,

src/synth/fluid_synth.c

@@ -305,7 +305,7 @@ fluid_synth_init(void)
 		       );
   fluid_mod_set_source2(&custom_breath2att_mod, 0, 0); /* No 2nd source */
   fluid_mod_set_dest(&custom_breath2att_mod, GEN_ATTENUATION);  /* Target: Initial attenuation */
-  fluid_mod_set_amount(&custom_breath2att_mod, 960.0);          /* Modulation amount: 960 */
+  fluid_mod_set_amount(&custom_breath2att_mod, FLUID_PEAK_ATTENUATION); /* Modulation amount: 960 */
 
   /* SF2.01 page 53 section 8.4.1: MIDI Note-On Velocity to Initial Attenuation */
   fluid_mod_set_source1(&default_vel2att_mod, /* The modulator we are programming here */
@@ -317,7 +317,7 @@ fluid_synth_init(void)
 		       );
   fluid_mod_set_source2(&default_vel2att_mod, 0, 0); /* No 2nd source */
   fluid_mod_set_dest(&default_vel2att_mod, GEN_ATTENUATION);  /* Target: Initial attenuation */
-  fluid_mod_set_amount(&default_vel2att_mod, 960.0);          /* Modulation amount: 960 */
+  fluid_mod_set_amount(&default_vel2att_mod, FLUID_PEAK_ATTENUATION); /* Modulation amount: 960 */
 
 
 
@@ -381,7 +381,7 @@ fluid_synth_init(void)
 		       );
   fluid_mod_set_source2(&default_att_mod, 0, 0);                 /* No second source */
   fluid_mod_set_dest(&default_att_mod, GEN_ATTENUATION);         /* Target: Initial attenuation */
-  fluid_mod_set_amount(&default_att_mod, 960.0);                 /* Amount: 960 */
+  fluid_mod_set_amount(&default_att_mod, FLUID_PEAK_ATTENUATION);  /* Amount: 960 */
 
 
 
@@ -409,7 +409,7 @@ fluid_synth_init(void)
 		       );
   fluid_mod_set_source2(&default_expr_mod, 0, 0);                 /* No second source */
   fluid_mod_set_dest(&default_expr_mod, GEN_ATTENUATION);         /* Target: Initial attenuation */
-  fluid_mod_set_amount(&default_expr_mod, 960.0);                 /* Amount: 960 */
+  fluid_mod_set_amount(&default_expr_mod, FLUID_PEAK_ATTENUATION);  /* Amount: 960 */
 
 
 
@@ -464,9 +464,9 @@ fluid_synth_init(void)
 		       | FLUID_MOD_POSITIVE                      /* D=0 */
 		       );
   fluid_mod_set_source2(&custom_balance_mod, 0, 0);
-  fluid_mod_set_dest(&custom_balance_mod, GEN_CUSTOM_BALANCE);                  /* Destination: stereo balance */
+  fluid_mod_set_dest(&custom_balance_mod, GEN_CUSTOM_BALANCE);     /* Destination: stereo balance */
   /* Amount: 96 dB of attenuation (on the opposite channel) */
-  fluid_mod_set_amount(&custom_balance_mod, 960.0);
+  fluid_mod_set_amount(&custom_balance_mod, FLUID_PEAK_ATTENUATION); /* Amount: 960 */
 }
 
 static FLUID_INLINE unsigned int fluid_synth_get_ticks(fluid_synth_t* synth)

src/utils/fluid_conv.c

@@ -71,7 +71,7 @@ fluid_conversion_config(void)
      implemented according to the pictures on SF2.01 page 73. */
 
   for (i = 1; i < FLUID_VEL_CB_SIZE - 1; i++) {
-    x = -20.0 / 96.0 * log((i * i) / (fluid_real_t)((FLUID_VEL_CB_SIZE-1) * (FLUID_VEL_CB_SIZE-1))) / M_LN10;
+    x = (-200.0 / FLUID_PEAK_ATTENUATION) * log((i * i) / (fluid_real_t)((FLUID_VEL_CB_SIZE-1) * (FLUID_VEL_CB_SIZE-1))) / M_LN10;
     fluid_convex_tab[i] = (fluid_real_t) (1.0 - x);
     fluid_concave_tab[(FLUID_VEL_CB_SIZE-1) - i] = (fluid_real_t) x;
   }

src/utils/fluid_conv.h

@@ -23,6 +23,37 @@
 
 #include "fluidsynth_priv.h"
 
+/*
+ Attenuation range in centibels.
+ Attenuation range is the dynamic range of the volume envelope generator
+ from 0 to the end of attack segment.
+ fluidsynth is a 24 bit synth, it could (should??) be 144 dB of attenuation.
+ However the spec makes no distinction between 16 or 24 bit synths, so use
+ 96 dB here. 
+ 
+ Note about usefulness of 24 bits:
+ 1)Even fluidsynth is a 24 bit synth, this format is only relevant if
+ the sample format coming from the soundfont is 24 bits and the audio sample format
+ choosen by the application (audio.sample.format) is not 16 bits.
+
+ 2)When the sample soundfont is 16 bits, the internal 24 bits number have 
+ 16 bits msb and lsb to 0. Consequently, at the DAC output, the dynamic range of
+ this 24 bit sample is reduced to the the dynamic of a 16 bits sample (ie 90 db)
+ even if this sample is produced by the audio driver using an audio sample format
+ compatible for a 24 bit DAC.
+
+ 3)When the audio sample format settings is 16 bits (audio.sample.format), the
+ audio driver will make use of a 16 bit DAC, and the dynamic will be reduced to 96 dB
+ even if the initial sample comes from a 24 bits soundfont.
+
+ In both cases (2) or (3), the real dynamic range is only 96 dB.
+
+ Other consideration for FLUID_NOISE_FLOOR related to case (1),(2,3):
+ - for case (1), FLUID_NOISE_FLOOR should be the noise floor for 24 bits (i.e -138 dB).
+ - for case (2) or (3), FLUID_NOISE_FLOOR should be the noise floor for 16 bits (i.e -90 dB).
+ */
+#define FLUID_PEAK_ATTENUATION  960.0f
+
 void fluid_conversion_config(void);
 
 fluid_real_t fluid_ct2hz_real(fluid_real_t cents);