Volume compression (A.K.A. audio level compression) reduces the dynamic
range of a sound. This means that loud sounds become softer, and soft
sounds become louder.
Multiband volume compression means that the audio is separated into seperate
frequency bands, and each frequency band is compressed separately. This means
that for example bass and hi-hats are treated separately, and a loud peak
of one of the two does not affect the other. This greatly reduces pumping,
and it makes different recordings sound more equal. (If one recording
has a loud bass and the next has a loud hi-hat, after using the multiband
compressor the loudness of the bass and the hi-hat is lowered.)
See
Wikipedi: Audio level compression
for a lengthy description.
Stereo Tool offers an 8-band multiband compressor/limiter, which is especially
designed to reduce common volume compression problems such as pumping.
The following settings and sliders are available:
ON
Turns the multiband compressor on or off.
DIFF
If ON, plays the difference between the input
and output signal. What you hear is the sound
that the compressor would throw away. Useful
for analysing the effects of different settings.
For example, if it sounds distorted, the
settings are probably too agressive.
limit/compress Limiting means that audio below a certain volume is left untouched. If
the volume gets above this volume (A in the graph below), the output
volume is lowered such that the resulting output stays at the set maximum volume A. Compression affects all audio: Low volume sounds are amplified, high volume
sounds are de-amplified.
soft limit
For each band, how much of the output volume range may be used
before the amplification must be turned down.
This is the value A in the graph above.
up speed
For each band, when the output volume is lowered due to too loud sounds, this slider
determines how fast the output volume can be increased again.
A higher value means that the amplification can increase faster.
Too high values can cause a cracking sound,
too low values can cause pumping.
down speed
For each band, when a sound that is louder than the set maximum occurs, this value
determines how fast the output volume is lowered.
Too high values can cause a cracking sound,
too low values may cause the amplification to be
lowered too slowly, causing (probably unwanted) loud sounds
sound when an instrument starts playing suddenly.
When this value is 1, the output volume never gets above the values
in the graph above. Lower values mean that sudden peaks can cause the
volume to get shortly higher than the lines in the graph.
Up speed and down speed must match each other to get a good sound.
If the up speed and down speed are high, the output sound will be flat,
but the output volume is very constant.
If the up speed and down speed are low, the output sound will be much more
dynamic, but the volume is not that constant anymore.
If the up speed is high and the down speed is low, sudden peaks will still
sound loud, but most of the sounds will get the same volume, and the sound
is much more powerful than with both values high.
If the up speed is low and the down speed is high, the sound is almost
identical to that of both values low.
The bars at the bottom display the amplification. If a bar is filled
completely, no compression is being performed. If it goes down, compression
is taking place. Use these bars to visually check the effects of what you
are doing.
