DSP Compressor: Sidechain, Exponential Atk/Rls

This is an improvement to the current compressor which I have added
to my own Sansa Fuze V2 build.  I am submitting here in case others
find it interesting.

Features added to the existing compressor:
Attack, Look-ahead, Sidechain Filtering.
Exponential attack and release characteristic response.

Benefits from adding missing features:
Attack:
Preserve perceived "brightness" of tone by letting onset transients
come through at a higher level than the rest of the compressed program
material.

Look-ahead:
With Attack comes clipping on the leading several cycles of a transient
onset.  With look-ahead function, this can be pre-emptively mitigated with
a slower gain change (less distortion).  Look-ahead limiting is implemented
to prevent clipping while keeping gain change ramp to an interval near 3ms
instead of instant attack.

The existing compressor implementation distorts the leading edge of a
transient by causing instant gain change, resulting in log() distortion.
This sounds "woofy" to me.

Exponential Attack/Release:
eMore natural sounding.  On attack, this is a true straight line of 10dB per
attack interval.  Release is a little different, however, sounds natural as
an analog compressor.

Sidechain Filtering:
Mild high-pass filter reduces response to low frequency onsets.  For example,
a hard kick drum is less likely to make the whole of the program material
appear to fade in and out.  Combined with a moderate attack time, such a
transient will ride through with minimal audible artifact.

Overall these changes make dynamic music sound more "open", more natural.  The
goal of a compressor is to make dyanamic music sound louder without necessarily
sounding as though it has been compressed.  I believe these changes come closer to this goal.

Enjoy.  If not, I am enjoying it

Change-Id: I664eace546c364b815b4dc9ed4a72849231a0eb2
Reviewed-on: http://gerrit.rockbox.org/626
Tested: Purling Nayuki <cyq.yzfl@gmail.com>
Reviewed-by: Michael Giacomelli <giac2000@hotmail.com>

3 files changed, 26 insertions, 2 deletions

diff --git a/apps/lang/english.lang b/apps/lang/english.lang
index d0dc5c5fc4..dcad532f7a 100644
--- a/apps/lang/english.lang
+++ b/apps/lang/english.lang
@@ -11911,6 +11911,23 @@
   </voice>
 </phrase>
 <phrase>
+  id: LANG_COMPRESSOR_ATTACK
+  desc: in sound settings
+  user: core
+  <source>
+    *: none
+    swcodec: "Attack Time"
+  </source>
+  <dest>
+    *: none
+    swcodec: "Attack Time"
+  </dest>
+  <voice>
+    *: none
+    swcodec: "Attack Time"
+  </voice>
+</phrase>
+<phrase>
   id: LANG_COMPRESSOR_RELEASE
   desc: in sound settings
   user: core
diff --git a/apps/menus/sound_menu.c b/apps/menus/sound_menu.c
index fd192cb661..28cc257193 100644
--- a/apps/menus/sound_menu.c
+++ b/apps/menus/sound_menu.c
@@ -140,12 +140,15 @@ static int timestretch_callback(int action,const struct menu_item_ex *this_item)
     MENUITEM_SETTING(compressor_knee,
                      &global_settings.compressor_settings.knee,
                      lowlatency_callback);
+    MENUITEM_SETTING(compressor_attack,
+                     &global_settings.compressor_settings.attack_time,
+                     lowlatency_callback);
     MENUITEM_SETTING(compressor_release,
                      &global_settings.compressor_settings.release_time,
                      lowlatency_callback);
     MAKE_MENU(compressor_menu,ID2P(LANG_COMPRESSOR), NULL, Icon_NOICON,
               &compressor_threshold, &compressor_gain, &compressor_ratio,
-              &compressor_knee, &compressor_release);
+              &compressor_knee, &compressor_attack, &compressor_release);
 #endif
 
 #if (CONFIG_CODEC == MAS3587F) || (CONFIG_CODEC == MAS3539F)
diff --git a/apps/settings_list.c b/apps/settings_list.c
index 6ffb2b551b..bd2bfce36f 100644
--- a/apps/settings_list.c
+++ b/apps/settings_list.c
@@ -1653,7 +1653,11 @@ const struct settings_list settings[] = {
     CHOICE_SETTING(F_SOUNDSETTING|F_NO_WRAP, compressor_settings.knee,
                    LANG_COMPRESSOR_KNEE, 1, "compressor knee",
                    "hard knee,soft knee", compressor_set, 2,
-                   ID2P(LANG_COMPRESSOR_HARD_KNEE), ID2P(LANG_COMPRESSOR_SOFT_KNEE)),
+                   ID2P(LANG_COMPRESSOR_HARD_KNEE), ID2P(LANG_COMPRESSOR_SOFT_KNEE)), 
+    INT_SETTING_NOWRAP(F_SOUNDSETTING, compressor_settings.attack_time,
+                       LANG_COMPRESSOR_ATTACK, 5,
+                       "compressor attack time", UNIT_MS, 0, 30,
+                       5, NULL, NULL, compressor_set),                  
     INT_SETTING_NOWRAP(F_SOUNDSETTING, compressor_settings.release_time,
                        LANG_COMPRESSOR_RELEASE, 500,
                        "compressor release time", UNIT_MS, 100, 1000,