Regarding Logic's Compressor, Tedness writes:
> One thing I didn't get ... is how the auto release functions in relation to the manual release setting...
With manual Release the compressor releases (stops compressing) at the set Release rate; with an auto-release the actual Release time is dependent on the audio signal, and so it varies with the audio—in the case of the Logic Compressor, apparently varying around the manual Release setting.
Auto-release was often a feature inherent in the design of many older (vintage) compressors, due to the circuitry that controlled gain reduction. Probably the best-known example is the Teletronix LA-2A—the hardware unit that inspired the Vintage Opto model in Logic's Compressor—which employed a light-based circuit to control compression. That circuit didn't offer manual control of the attack or release; those rates were based on the response of the optical circuit—the electro-luminescent panel and optical attenuator—that determine the LA-2A's compression response. This kind of audio-based response is referred to as program-dependent, and it's a feature of many classic compressors.
In the case of the LA-2A, the auto-release time is on the longer side, and is a staged response—faster at first, then slowing down, with the exact rate/response based on the audio signal. It's kind of like having the compressor optimize its response based on the particular audio signal, and this program-dependent response is generally acknowledged to be a significant component of the "smoothness" the LA-2A is known for.
A compressor may have a program-dependent release but still provide a manual Release control—in some cases the actual response may be based around the manual settings, but still vary with the audio signal, while in others enabling auto-release may override any manual controls, for a completely automatic release behavior. Logic's Compressor models all seem to take the former approach—the auto-response appears to vary based around the manual setting of the Release control, even if the original had no such control. If the Logic Compressor offers a manual control that wasn’t present on the original—like the Vintage Opto model—for more classic release behavior I’d set it at something close to the nominal release rate of the original—maybe around a second or so for the Opto model—and fine-tune by ear.
Generally, if you're looking for smoothness and transparency, enabling auto-release would probably be a good choice, while if you were going for more of a compression "effect"—like deliberate pumping to fatten up drums—then you might be able to dial up a more pronounced effect with auto-release off. With Logic's Compressor, since the controls often exceed the range of the controls on the original units being modeled, a little experimentation would be in order.