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 a 10-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.
POST FILTER
After compressing or limiting, filters out artifacts that may have been introduced
by the compression or limiting. Mainly useful when agressive compression or limiting
is taking place. This costs some extra processing power.
HQ
High Quality mode. This mode results in better very deep bass sounds (if you have an on-board
sound card such as a SoundMax you probably won't notice any difference). Also, post filtering
(see above) and filtering after clipping (see below) is done more precisely, resulting in
an improvement in sound quality. This costs some extra processing power, and the sound is played
with a larger delay.
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.
In general, when the goal is to make the volume as constant as possible, use
limiting. If the goal is sound quality, use compression.
10-band 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.
10-band 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.
10-band 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 make the sound very flat,
too low values may cause the amplification to be
lowered too slowly, causing (probably unwanted) loud sounds
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.
10-band clipping
When the volume of a band gets higher than the value set in soft limit,
clipping can be used to cut off the sound that is too loud. The value of each
clipping slider determines at which volume clipping starts, for example when
the clipping slider is set to 1.50, clipping starts when the volume gets above
1.50 times the soft limit value.
Using clipping improves the sound quality
because very short very loud spikes that are left over after compressing or limiting
are removed. This makes the sound far less "jumpy", and the output volume more constant.
Clipping introduces artifacts, which are filtered out afterwards. This filtering
works better when HQ mode is enabled. This is especially important when a lot
of clipping occurs.
10-band equalizer
The 10-band equalizer can be enabled by checking the Equalizer checkbox.
This equalizer can be used before (enable Before Multiband) or after (disable
Before Multiband) the multiband compression/limiting.
If used afterwards, it acts as a normal equalizer. If used before compression/limiting,
it changes the input signal before any compression or limiting takes place. That way the
effect on the output signal is much smaller, because the compression/limiting removes
most of the differences that are introduced by the equalizer.
1-band limit output
This slider controls the maximum output volume of the multiband compressor/limiter.
The value that can be set is the percentage of the maximum output volume (based on the
total of the soft limit settings of all the bands) that may remain after adding the
results of the 10 bands.
If the total output volume is higher than the value set by this slider, the volume for
all the 10 bands is dropped, based on the 1-band down speed slider. Also, the volume
is clipped based on the 1-band clip slider.
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. Black bars inside these bars are displayed when clipping occurs.
Use these bars to visually check the effects of what you are doing.
