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>

1 files changed, 3 insertions, 0 deletions

diff --git a/manual/configure_rockbox/sound_settings.tex b/manual/configure_rockbox/sound_settings.tex
index f06bfaf6e5..d2da07b983 100644
--- a/manual/configure_rockbox/sound_settings.tex
+++ b/manual/configure_rockbox/sound_settings.tex
@@ -603,6 +603,9 @@ non-compressed signal to a compressed signal.  Hard Knee means that the
 transition occurs precisely at the threshold.  The Soft Knee setting smoothes
 the transition from plus or minus three decibels around the threshold.
 
+The \setting{Attack Time} setting sets the delay in milliseconds between the
+input signal exceeding the activation threshold and acting upon it.
+
 The \setting{Release Time} setting sets the recovery time after the signal is
 compressed.  Once the compressor determines that compression is necessary,
 the input signal is reduced appropriately, but the gain isn't allowed to