Chapter 6. Sound Settings
53
Dithering adds low-level noise to the signal prior to throwing away the surplus bits,
which gives the resulting signal a uniform noise floor which is independent of the sig-
nal. Most people find this noise preferable to the time-varying noise heard when not
performing dithering.
After dithering, noise shaping is performed. This basically just pushes the dithering
noise to the parts of the frequency spectrum humans cannot hear so easily. In Rockbox’
case, some of the noise is pushed up to above 10 kHz.
This setting will be put to its best use when listening to dynamic music with frequently
occuring quiet parts, classical music being a typical example. It is worth noting that the
effects of dithering and noise shaping are very subtle, and not easily noticable.
Rockbox uses highpass triangular distribution noise as the dithering noise source, and
a third order noise shaper.
6.11. Timestretch
Enabling
Timestretch
allows you to change the playback speed without it affecting
the pitch of the recording. After enabling this feature and rebooting, you can access
this via the
Pitch Screen
. This function is intended for speech playback and may
significantly dilute your listening experience with more complex audio. See section
4.3.3
(page
32
) for more details about how to use the feature.
6.12. Compressor
The
Compressor
reduces, or compresses, the dynamic range of the audio signal. This
makes the quieter and louder sections closer to the same volume level by progressively
reducing the gain of louder signals. When subsequently amplified, this has the effect of
making the quieter sections louder while keeping the louder sections from clipping. This
allows listening to the quiet sections of dynamic material in noisy environments while
preventing sudden loud sections from being overbearing.
There are several settings associated with the compressor. The first, and most impor-
tant, is the
Threshold
. The threshold is the audio input level at which the compressor
begins to act. Any level louder than the threshold will be compressed to some extent.
The maximum amount of compression, or the quietest level at which the compressor will
operate, is -24 dB. The default of Off disables the compressor.
The
Makeup Gain
setting has two options: Off and Auto.
Off means that the
compressed audio will not be amplified after compression. The default of Auto will
amplify the signal so that the loudest possible signal after compression will be just
under the clipping limit. This is desirable because the compressed signal without makeup
gain is quieter than the input signal. Makeup Gain in Auto restores the signal to the
maximum possible level and brings the quieter audio up with it. This is what makes it
possible to hear the quieter audio in noisy environments.
The
Ratio
setting determines how aggressively the compressor reduces gain above
the threshold. For example, the 2:1 setting means that for each two decibels of input
The Rockbox manual
(version rUnversioned directory-141216)
Iaudio M3
