THE BIG SQUEEZE

Compressors may seem like utilitarian tools designed to control dynamic range, but in the right hands, the use of compression can be an art unto itself.

BY RICH TOZZOLI

Audio compression has been around since the earliest days of record making, yet it can be one of the most abused and misunderstood tools in the studio. But with just a little basic knowledge and a thoughtful approach, it can become an invaluable friend. We'll take a look at just what compression is, how a few of the industry's top pros put it to use, and how you can apply it to make your productions sound better.

Dynamic Control

Simply put, a compressor is a device that reduces the dynamic range of an audio signal, so that there’s less of a difference between the loudest and softest sounds. It does this by attenuating any signal that exceeds a pre-determined level, known as the threshold. Anything below the threshold is left untouched. The more the signal goes over the threshold, the more it's reduced.

Compressors come in many forms. You'll find hardware units and software plug-ins, which can process either mono and stereo signals. On the hardware side, you'll find analog tube or solid state units, as well as digital compressors. Many—but not all—software compressors are designed to model their analog relatives. Yet with all this variety, most compressors use the same basic layout. Figure 1 shows the popular Waves Renaissance compressor plug-in; its controls are pretty standard: Thresh (Threshold), Ratio, Gain, Attack and Release. Let's take a look at each in turn:

Threshold: This sets the input signal level that will trigger the compressor. Threshold is measured in decibels (dB). The higher the threshold, the louder the signal needs to be before triggering the compressor.

Ratio: This setting determines how much the signal is reduced once it crosses the threshold: For example, if the ratio is set to 4:1, any input signal that goes above the threshold by 4dB will be reduced so that it's only 1dB above the threshold at the compressor's outputs.The signal does cross the level set for threshold, just not by as much as it would without the compressor in place.

Attack: Measured in milliseconds (ms), the attack determines how quickly the compressor will begin working once the threshold is crossed. Fast settings (low attack times) will cause the compressor to start working almost immediately. This can sometimes cut off attack transient—which isn't too desirable on sources like snare drums, etc. Setting a longer (slower) attack time lets the attack transient come through with little or no compression.

Release: Also measured in milliseconds, the release control determines how long the compressor keeps working after the input signal has fallen below the threshold. A quick release time (typically 20-40ms) will allow the compressor to quickly get ready for the next signal above threshold. Slower release times can possibly cause the compressor to 'pump,' or get stuck on a signal, while trying to get back to zero. Not every compressor lets the user control the attack and release times.

Some models are fixed based upon the design and/or the input level of the signal. Classic examples such as the Teletronix LA-2A are actually leveling amps—compressors with fixed ratio, attack, and release settings.

Gain (or Output): Often referred to as "makeup gain," this control governs the level at the compressor"s output, allowing you to make up for any gain loss caused by the compression itself. By raising the gain, the compressor can be used to boost the level of quiet signals that are below the threshold. So even though the processor is reducing the hottest peaks, it allows the overall audio to be louder if that's what you're looking for. Combined with the right settings for attack and release, makeup gain can help lengthen the audible sustain of a signal like a long bass or guitar note.

In Use

How much compression is enough? That's one of the most common issues faced by even experienced engineers. There is of course, no right or wrong answer. Pros will say that they usually apply compression only as needed—either to tame a loud instrument or to smooth out and help balance a track. But compression can also be used for effect, and many compressors are prized for the unique color they add to the sound.

Compression can be applied individually to a track, to a group of tracks, or to an entire mix. Typical candidates for single-track compression include vocals, bass, drums and percussion (snare and kick in particular), and guitars. Like equalizers, compressors are normally used as inserts on individual tracks and group buses (including the master bus), or are used between a preamp and an input while recording. Unlike reverb and other spatial effects, compressors are not used as send/return effects—unless they are being applied to the spatial effect itself.

Some engineers choose to include a compressor in the input signal chain (between the mic preamp and mixer/interface) when recording—especially on vocals and bass. However, if you choose to "print" with compression, proceed with caution: Remember, you cannot remove it once it has been recorded!

The appropriate setting can vary depending on both your ultimate sonic goal and on the nature of the signal. Is it smooth or percussive? Is the attack loud or soft? Does its unprocessed dynamic range vary much? With vocals for example, you might start with the threshold relatively high, the ratio relatively low (3:1 to 4:1), a moderate attack time, and quick release. Try setting the threshold so that the loudest part of the vocal (the peak) just barely causes the compressor to work. With most compressors, this will provide a natural sounding track that rides evenly within the mix—without sounding "squashed" or unnatural. Of course, the singer's dynamics will influence your settings. If you can hear the compressor click, or if the loudest peaks somehow sound quieter than the quiet passages, you're probably overdoing it.

Stereo or subgroup compression is used on such things as layered background vocals and drums—especially on room mics and overheads. Think about that incredible compressed room sound of John Bonham's drums on Led Zeppelins "When The Levee Breaks." By applying a compression setting to room mics with a low threshold, a 3:1 ratio, and slower attack (80-100ms) and (50-70ms) release times, you can actually emphasize the room ambience relative to the dry sound of the kit. Try compressing them extra hard and use makeup gain to increase the entire kit's level. This technique is a great way to spice up the sound of drum loops that were recorded in a live room.

Many software compressors have presets that can help you find a starting point. As a general approach, dense pop and rock productions may need more compression, which is used both to balance the mix and to help the track punch through the speakers. I've found that more open, acoustic-based music requires less compression, which helps the music "breathe" more.

The Signal Path

One thing to keep in mind is that a compressor will interact with other processors in a given signal chain. The most common question is whether it should come before or after an EQ. The definitive answer is… that depends! (Sometimes, it's before and after!) Grammy Award-winning engineer/producer John Holbrook (Natalie Merchant, Brian Setzer, B.B. King) says, "In some cases, I want to adjust the EQ curve before compression, so the compressor only has to deal with the frequencies that I want it to receive. Let's say you've got a vocal track that has too much low end from the proximity effect [the bump in lows that can occur when a source is close to a mic]. If you know for sure that you're going to EQ out some of those lows, it makes sense to do so pre-compression, so the compressor isn't responding to frequencies you never wanted in the first place.

"In the 'post compression' situation, we're using the compressor to do its thing on a fullrange signal and then use the EQ to emphasize or de-emphasize the result," Holbrook adds. "For example, some compressors tend to 'dullout' the signal a bit; a touch of high-frequency shelving boost can make it happy again. Quite a few compressors now have side chain curve options which effectively pre-EQ the level detection although the audio portion is left flat.

[Note: A side chain lets an independent signal trigger the compressor while it continues to operate on the signal at its inputs.] I find this especially useful for stereo mix compression, where you don't want your carefully crafted low end to over-trigger the compression and suck the whole mix down with it.'

Remember that if you apply your compressor after an EQ, you are amplifying any artifacts that the EQ placed into the track, including any additional noise, etc. Again, there is no right or wrong approach, and with today's digital audio workstations, it's easy to try both ways and see what sounds best. It's often best to use Save As to create a new version of your project when testing plug-in placements, just in case you want to get back to your original version quickly and easily.

Master Bus

As for applying compression and/or limiting across a final stereo mix, I checked in with another Grammy winner, Dave Glasser of Airshow Mastering in Boulder, Colo. "Compression often works to glue a final mix together," he says. "It can add punch, as well as smooth out elements that are poking out of a mix, or are not prominent enough in a mix. I usually use analog compression, but don't use it to increase volume. If I need to add a substantial amount of volume—which I am loathe to do—a digital compressor seems to work better, particularly hardware units such as the Weiss DS-1 Mk II.

"I use limiting to increase volume," Glasser continues. "With a good digital peak limiter, up to 2–3dB of level can be added pretty transparently without affecting the dynamics—something a compressor can't do. Compression tends to alter the balance of a mix."

So, what's the ideal amount of compression? "I think these days it tends to be overused because of the almost unlimited availability of plug-ins," Holbrook says. "Theoretically, you could have one or more compressors on every track of an 80-track mix. There is a law of diminishing returns though. I try to only use it where I think it really needs it.

I would like to mention that when I started out—back when dinosaurs roamed the earth—it was usual for a studio to have only a handful of outboard gear—let's say maybe one [Urei] LA2A, two 1176s, a pair of LA3s and maybe a couple of dbxs, and no bus compressors! So you had to pick your weapons carefully. I would like to encourage anyone reading this to try doing a mix with only those units listed (or plug-in versions thereof) and see how it turns out. Remember you can bus some things together and compress the bus. Now try and do a mix with no compression at all! (Oh OK, tape compression is allowed!)

Whether you use it for subtle dynamic control or as an obvious audio effect, the comporessor is an essential tool of the trade. And the more you get to know how compression works, the better your mixes will sound. "It can be used to affect a track in so many different ways, from increasing drama, to making drums pop, to leveling, to envelope modification. and on and on," Holbrook concludes. "I couldn't live without it!"

COMPRESSION VS. LIMITING

When you're looking for tools to control dynamics, you'll see devices known as compressors, limiters, and compressor/limiters. So, what's the difference?

Compression takes a signal that rises above a predetermined threshold and reduces it. The amount of reduction depends on the compression ratio. So at 4:1, if the signal jumps 4dBs above threshold, it will come out of the compressor only 1dB louder. A compressor's effect may be very subtle or very audible, depending on the relationship between the ratio, threshold, attack, and release times.

Limiting: When the compression ratio is set to around 10:1 and higher, you're entering the land of limiting. Limiting prevents the signal from going above the predetermined threshold. It's often used on the stereo mix, as it can tame any stray peaks in the audio, allowing the track to reach maximum loudness without causing an overload. A limiter's gain control can also be used to raise a track's overall loudness, though this should be applied sparingly. A "brick wall" limiter has a super high compression ratio (above 50:1) and it sets an absolute ceiling; once the signal reaches that level, it will go no higher. —RT