Program-dependent Compression

Generally, when responding to the level of an input signal, the attack, release, and ratio of a compressor respond to a fixed value. Program-dependent compression means that rather than responding to a fixed value, the attack and release times (and ratio in the case of the LA-2A) of a compressor vary somewhat depending on the frequency and amplitude of the incoming signal. The advantage of a program-dependent release time, for example, would be as follows: To avoid extended dropouts after a transient, a short release is required. On the other hand, it’s better to have a longer release time during a prolonged state of heavy compression to reduce pumping and harmonic distortion (caused by repetitive attack-release cycles). The unique character of program-dependant compressors such as the Universal Audio LA-2A, Manley Labs EL-OP, and Fairchild 670, are claimed to be responsible for that classic “rock ‘n’ roll” sound.