Add perceptual volume adjustment

The perceived loudness change of a change in volume depends
on the listening volume: at high volumes a 1 dB increment is
noticeable, but at low volumes a larger increment is needed
to get a comparable change in loudness.

Perceptual volume adjustment accounts for this fact, and
divides the hardware volume range into a number of steps.
Each step changes the dB volume by a variable amount, with
most of the steps concentrated at higher volumes. This
makes it possible to sweep over the entire hardware volume
range quickly, without losing the ability to finely adjust
the volume at normal listening levels.

Use "Volume Adjustment Mode" in the system settings menu
to select perceptual volume mode. The number of steps used
is controlled by "Number of Volume Steps". (Number of steps
has no effect in direct adjustment mode.)

It's still possible to set a specific dB volume level from
the sound settings menu when perceptual volume is enabled,
and perceptual volume does not affect the volume displayed
by themes.

Change-Id: I6f91fd3f7c5e2d323a914e47b5653033e92b4b3b

1 files changed, 17 insertions, 0 deletions

diff --git a/manual/configure_rockbox/system_options.tex b/manual/configure_rockbox/system_options.tex
index ba80a6e6e4..d4a282b445 100755
--- a/manual/configure_rockbox/system_options.tex
+++ b/manual/configure_rockbox/system_options.tex
@@ -137,6 +137,23 @@ This sub menu relates to limits in the Rockbox operating system.
       \item LAN party computer $\rightarrow$ \dap $\rightarrow$ human
       \end{itemize}
   }
+\opt{perceptual_volume}{
+  \subsection{Volume Adjustment Mode}
+  This setting selects the method used to adjust volume with \ButtonVolUp{} and
+  \ButtonVolDown{}. In \setting{Direct} mode each volume step changes the volume
+  by a fixed number of decibels (dB).
+
+  In \setting{Perceptual} mode, the hardware volume range is divided into a
+  number of steps, controlled by the \setting{Number of Volume Steps} option.
+  Each step changes the volume by a variable number of decibels (dB) so the
+  perceived loudness changes by about the same amount at each step. The dB
+  change is smaller at high volumes and larger at low volumes, so a large
+  range of low volumes are effectively compressed into a smaller number of
+  volume steps.
+
+  \setting{Volume Adjustment Mode} does not affect how volume is displayed by
+  themes.
+}
 \opt{quickscreen}{
   \subsection{Use Shortcuts Menu Instead of Quick Screen} This option
   activates the shortcuts menu instead of opening the quick screen when enabled.