Apple Logic Express 7 Informations techniques télécharger pdf (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.

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Apple Logic Express 7 Informations techniques Page 28