Dave Moulton /
10.09.2009 11:30 AM
Attack and Release
In my last column, part of a series on the basics of compressors, I discussed the Make-Up Gain and Ratio controls on compressors. This month, as promised, I want to discuss the Attack and Release controls, which address some of the time-based issues that are encountered with compressors.

IF YOU RECALL…

All compressors work by sending the incoming audio signal through an active gain stage (usually a voltage-controlled amplifier [VCA] or its digital equivalent). At the same time, the signal is also sent (in parallel) to a so-called level detector, which studies the signal and converts its amplitude into a control voltage (or digital equivalent). This, in turn, is used to regulate the gain of the signal via the VCA. That level detector and the resulting control voltage have some pretty tricky aspects, having to do with time.

Three things often happen, and you need to know about them. The first one is a distortion-like sound that can be created by the control voltage being changed too quickly. It’s called amplitude modulation. The second one is a change in spectrum, due to the attack and release times emphasizing one portion of the spectrum of the program, or attenuating another portion. The third one is called “pumping,” a gasping quality that relates to the level being returned to its uncompressed state too quickly or too often.

All of these have to do with time. In most cases compressor designers have included two special time controls called “Attack” (to control how rapidly the compressor “attacks” the level-over-threshold to turn it down,) and “Release” (to control how rapidly the compressor stops reducing the gain after the level-over-threshold has gone away).

Compressor designers usually include two special time controls called "Attack" (to control how rapidly the compressor "attacks" the level-over-threshold to turn it down) and "Release" (to control how rapidly the compressor stops reducing the gain after the level-over-threshold has gone away).
AMPLITUDE MODULATION

You’d think, intuitively, that we’d like the compressor to track the “level” of the signal just as quickly as possible, right? That way, we could compress just the offending elements and nothing else. It’d be the most accurate, right?

Unfortunately, the “level” of the signal is actually the wave trace itself. If we track it very closely, the control voltage will become an audio signal itself (because it goes up and down within the audio range), and cause the voltage-controlled amplifier to also generate an audio signal. This will modulate the actual audio in the low-frequency realm. It sounds just like fairly nasty, low-frequency harmonic distortion.

So, we use two strategies to head off this nasty. Set either the attack or the release control slow enough so the compressor can never modulate in the audio realm. “Slow enough” means a time GREATER THAN 50 ms.

Just so you know, 50 ms. is the period for a 20 Hz. tone, and if we keep things longer than 50 ms. then they’ll never get into the audio range above 20 Hz. Clever, eh?

If we want a fast attack to catch some sudden spikes of energy, we need to make sure the release is a good bit slower than 50 ms. If we want a fairly fast release, we may want to set the attack slower than 50 ms. As always, USE YOUR EARS!

CHANGING THE AUDIO SPECTRUM

This happens as a function of the specific program material, and is more of a problem with music than voice.

Often, for example, we’ll have extremely loud bass or kick drum signals. They can be (and often are) the loudest component in the program. They can trigger the compressor’s gain reduction, turning down the bass, but then (as the release occurs) let the higher frequencies through with little or no attenuation. Occasionally, the behavior will be just the opposite, depending on the settings and the program.

The problems also occur as a function of the attack time and/or the release time changing the envelope (shape of loudness) of individual sounds, and thereby changing their timbre. Solving these problems takes some practice and careful listening to the effect the attack time is having, and then the effect the release time is having.

And remember, it varies with the program material. Heavy metal will suffer differently than will a nice female jazz ballad. But they both will suffer from compression abuse.

PUMPING

Pumping is a real ENG problem, where we have both a voice and background noise. We feel the need to limit the level of the voice, to make it consistent and distortion free. So we compress with a fairly low threshold and large ratio. Also, we use a fast attack, so that the beginning consonants of words don’t spit at the listener.

If we use a slow release, once the compressor is invoked, the level stays down, maybe too far. So we speed up the release time, and as a result, during the spaces between words or sentences, the release of the compressor pulls the background noise back up, maybe by 15–20 dB, kind of a sudden sucking “sheeeuppp!” leading into the next word spoken, which then punches the level back down. When this happens over and over, it is very fatiguing and annoying.

The answer here is very judicious balancing of attack time and threshold together, against the release time, so that the attack of the compressor is fairly mild and not overdone, while the release is slow enough that the background noise just begins to start pulling up before the next word (and attack) arrive. Sometimes just reducing the total amount of gain reduction (via threshold adjustment) can solve the problem best.

Taken together, these compressor issues suggest a little more about the complexity of compression. Much of what we need to do is more concerned with time than level.

Compressors go back and forth between the realm where the level change can occur to each individual sound (fast attack and release, which changes the envelope of each sound, a key determinant of timbre) and the realm where we are changing the overall level for a while (which is what happens with a slow release).

Getting so you can hear the difference between compression as envelope and compression as level control is a big step in developing your hearing skills.

IN OUR NEXT EPISODE…

There’s one other aspect of compressors that we need to discuss: the level detector itself. Next column I’ll take a look at the various ways the level detectors are constructed or programmed, and what it all means. In many respects, it is the most important part of the compressor, the part that has the biggest effect on the sound of the compressor behavior.

Thanks for listening.

Dave Moulton is getting ready for another beautiful New England autumn. You can send him sympathy cards as well as complain to him about anything, at his Web site www.moultonlabs.com.



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