Compression Basics
Wed, 1 Jun 2011
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BP0611_techbench1_nrThis waveform of a plucked note shows a strong attack, followed by a rapid decay.

  

BY E. E. BRADMAN

Have you heard the one about the party at Effects Central?

Tough guy Octaver shows up, a sweet little Chorus on his arm, and that funky character Envelope Filter—accompanied by Synth Bass and a strange dude everyone calls Pitch Shifter—saunters in, dressed to shock. Overdrive and his brother Fuzz buzz by, as Wah and Autowah squeak back on the sofa with Phaser and Flanger. Suddenly, Compression enters the room, and the guest of honor, Clean Boost, gives him a strange look. The party screeches to a halt as Looper, that night’s DJ, stops mid-song.

“What are you doing here?” says Envelope Filter, looking ready to throttle Compression. “You’re making all of us feel small!”

Corny jokes aside, compression really does get a bad rep. It doesn’t have the cool factor of its brawny, ballsy, or downright freaky effects kin, and its reputation as a tone killer is unparalleled. Besides, isn’t compression for suckers who can’t play evenly and speakers that aren’t tough enough to handle active basses?

Truth be told, compression is one of the most important tools for recording bass, and it can be quite useful in live situations, too. When used properly, it can help define a bass’s place in a live or studio mix while taming the instrument’s wide dynamic range. Because bass forms the foundation of a track, it’s important that the bass level doesn’t jump around in the mix, which can be tricky since bass players—especially those of us with mediocre technique—don’t always exercise precise control over our dynamics. Regardless of technique, though, every bass player who has ever played even a mid-size gig or recorded a bass line has been compressed. The secret is that a good engineer or sound person knows how to use just enough compression that you—and the listeners— can barely tell it’s there.

COMPRESSION, COMPRESSED

Simply put, compressors squeeze the dynamics of a sound so that the loud sounds are softer and the soft sounds are louder. A compressor uses a high-gain preamp and a control circuit that alters the preamp gain based on the compressor input. As you send more signal into the compressor’s input, the control circuit senses this and turns down the preamp gain to maintain a more consistent output level. The louder you play, the more your bass signal will get slapped down, while the soft parts will get turned up a bit. Limiters do the same thing, but they only restrict peaks, without touching the lowvolume parts; they can do a rather hamfisted job of processing dynamics. Luckily, most modern compressors are limiters, too.

BP0611_techbench2_nrCompression mellows the attack and amplifies the decay.

 

The best way to understand what a compressor does is to plug your bass into one, tweak the knobs one by one, and listen. It’s a good idea to bypass the compressor every so often to remind yourself what your natural bass tone sounds like, and A/B it with the compressed tone. Start by setting the compressor’s INPUT level. One thing you’ll notice is that compression lowers the overall average levels of your signal, but don’t be tempted to slam the compressor with more juice: The all-important OUTPUT GAIN knob will help you make sure you get the volume you need.

Next, set the THRESHOLD level, which determines how loud things can get before the compressor kicks in. A low threshold means you want the compressor to be very sensitive, whereas a higher threshold means you’ll have to really whack the string to get the compressor to tamp down. Next, use the RATIO (or SLOPE) control to tell your compressor how much compressing it needs to do. If you set the THRESHOLD to –10dB and the RATIO to 3:1, for example, for every 3dB of signal above –10dB that goes into the compressor, only 1dB comes out. Adjust your output gain accordingly. Now that you’ve decided when and how much your compressor needs to jump into action, use ATTACK to tell it how quickly it should react to your signal. The RELEASE knob (sometimes labeled DECAY) allows you to control how long it takes your compressor to stop compressing after it’s been triggered.

Every compressor is a little different, and it’s worth experimenting with different types to see what works for you. Some compressors, for example, have a HARD KNEE/SOFT KNEE switch, which controls how sharply the compressor does its job. If you choose soft knee compression, the compression ratio gradually increases as the input level approaches the threshold. With hard knee compression, the signal undergoes the specified amount of compression the moment it crosses the threshold. Hard-knee action creates more punch; soft knee may work well if the level variations are extreme, as it allows for more dynamics.

BP0611_techbench4_nrWith a compression ratio of 4:1, exceeding the threshold by 16dB at the input (here, going from –8dB to +8dB) yields an output increase of 4dB.

 

Other compressors or pedals have only two knobs: COMPRESS (or SUSTAIN) and LEVEL. If this is the case, consult the specs to see whether the COMPRESS knob is for setting threshold, ratio, or attack parameters, and use the LEVEL knob to control your output gain. If you’re using a studiograde compressor, you might have a GATE control, which turns off the audio when it goes below a defi ned threshold. It’s great for removing noise.

SQUEEZE BOXES

Most bass players choose between rackmount and pedal compressors. Rackmount units are usually either tube and solid-state; many people favor tube compressors, such as the Teletronix LA-2A and the Tube-Tech CL1B, because you can generally get more out of them before you start to sense the signal being squeezed. Some of the bestknown solid-state compressors include the dbx 160 and the UREI 1176. Models such as the Avalon VT-747SP can be switched between tube and solid-state. While there are dozens of stompbox compressors on the market, Aguilar, DigiTech, EBS, Electro- Harmonix, MXR, and Markbass are just a few manufacturers that have compressors aimed specifically at bassists.

If you’re a slapper or rocker who digs in— and especially if you play with a pick—compression can help tame those sharp peaks and rein in the dynamic variance between plucks and thumps. You love that tight Jaco Pastorius tone? Lots of people use compression to help them sound more punchy. If you go for a warmer, old school R&B or reggae tone, you’ll appreciate compression’s ability to tighten up the bottom end and make your bass sit better in a mix. If you prefer long notes and you’re going for that big, country bass sound, use compression to help get sustain: With the peak volume at the beginning of the note—the attack—snipped down, the listener doesn’t hear the natural decay of a plucked note, which can sound very legato.

Whatever style you play and whether you’re in the studio or onstage, compression can help you get the tone you want. Who knows—one day, compression could be your main squeeze.

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