Apple Logic Express 7 - Manual (Page 28)

28 Chapter 4 Dynamic
Logic’s Compressor was designed to emulate the response of the finest analog
compressors. It follows the following principle: When a signal exceeds the defined
Threshold level, the compressor actually alters the response, so that it is no longer
linear. What happens is that all levels that exceed the Threshold are attenuated by the
value set with the Ratio slider. A ratio of 4:1 means that an incoming level that is 4 dB
louder than the Threshold level is dampened, so that it comes out the other end of the
compressor with a level that is just 1 dB above the Threshold level. On the flip side, if
you route in a signal that is loud enough to double the output level of the compressor
(
+
6 dB), the input signal would need to have a level 24 dB greater than the Threshold
level. This tells us that a compression ratio of 4:1 is a fairly drastic manipulation of the
original signal’s dynamics. Given that the compressor lowers levels, the volume of its
output signal is normally lower than that of the input signal.
To compensate for this decrease in levels, the output of the compressor is equipped
with a Gain slider. Auto Gain automatically sets the level of amplification to a value
equivalent to the “sum of the threshold value minus the threshold value divided by the
ratio” or put less confusingly T—(T/R). This function ensures that a normalized input
signal is amplified so that the output signal is also normalized, regardless of the values
set for Threshold and Ratio—provided you are dealing with relatively static signals. Use
the Attack and Release knobs to shape the dynamic response of the compressor. Attack
determines the amount of time it takes for the compressor to react to signals that
exceed the Threshold. At higher values, the compressor does not fully dampen a signal
until it runs through its Attack phase. This type of setting ensures the original attack, for
example the sound of a pick or finger striking a guitar string, remains intact or clearly
audible. If, on the other hand, you want to maximize the level of a master signal, set the
Attack knob to low values, ensuring that the compressor responds more swiftly. Release
determines the amount of time it takes for the compressor to stop dampening louder
passages, once the signal level falls below the Threshold level. If the compressor
generates an ugly pumping sound, adjust the Release knob accordingly.