Understanding Compressors and Compression

Jan 1, 1999 12:00 PM, Barry Rudolph

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STUDIO COMPRESSOR CLASSES Most full-featured modern compressors can be set to emulate three classes of compressors: regular compressors, peak limiters and leveling amplifiers. These compressor classes all differ primarily in attack, release, ratio and threshold settings.

Regular Compressors * Though there are no rules and many exceptions, for most music recording engineers tend to use a regular compressor with medium attack and release settings. Most of the time the engineer is looking for a modicum of level control with higher average loudness and some peak protection. Some producer/engineer teams prefer compression that is smooth, transparent and truer to the source sound's fidelity. This is achieved by using a quality compressor with a higher threshold and lower ratio with medium attack and release settings. This team is looking for a natural sound that doesn't sound compressed and retains most of its dynamic range properties. Another crew may opt for more severe and noticeable compressor action for a more "controlled" or tight sound. This would call for a compressor set with fairly fast attack and release times, higher ratio and lower threshold and low distortion under maximum gain reduction. With pop record productions, threshold and ratio preferences, along with compressor choice and usage, contribute to a certain production style.

Peak limiters * A peak limiter is a compressor with a very fast attack time, medium to fast release time, a high ratio and a high threshold. The task for a peak limiter is to stop or control only the very quick, sudden peak levels that will overload the succeeding audio stage. Peak limiters are used in all broadcast transmitters, satellite transponder uplink audio feeds and in many digital audio systems where digital "overs" are not tolerated. Recording engineers sometimes chain a peak limiter after a compressor so they get the best of both worlds: They maintain a higher average loudness with the first compressor, and then protect the digital recorder from flash peaks with the peak limiter. This arrangement is good for recording a widely dynamic singer who also has poor mic technique-a nasty combination for the recording engineer. Aphex Systems makes a unit called the Compellor that combines a compressor, a leveling amp and a peak limiter all in one box.

Leveling amplifiers * The leveling amplifier is a compressor with a medium attack time, a medium to slow release time, a high ratio and a low threshold. The purpose of a leveling amp is to be constantly leveling the signal, always in gain reduction, holding the audio signal down in a smooth way (ergo the name). The average loudness of the program audio becomes higher since lower-level sounds are amplified, and louder sounds are leveled off. The slow release time ensures that sound level doesn't drastically change or "pump" up and down as it would with a faster release time setting. Early leveling amps had few controls: Ratio, attack and release settings were all internally fixed. Tube leveling amps are popular for bass, guitars, program mixes and vocals because they exert a smoothing action that works well for many production styles. The Teletronix LA-2 is the classic tube leveling amp.

USING COMPRESSORS IN MUSIC Trying to arrive at proper recording level can be like chasing a moving target, especially if you are working with inexperienced musicians or singers. It is easy to just "pack" the signal from a widely dynamic singer into a compressor, crank it way up and call it a day! On the other hand, the creative and caring engineer may use an equalizer in the sidechain of a compressor to selectively compress certain problem frequencies of the singer's voice. The sidechain input is a direct path to the compressor's detector circuitry, where an external signal can also trigger compressor action. Using an equalizer in the sidechain makes the compressor more (or less) sensitive to sounds within the EQ's frequency passband. Other sidechain compressor applications are de-essing and ducking. De-essing is accomplished by connecting an equalizer to the sidechain of a peak limiter and boosting, with a medium to high Q, the "s" frequencies anywhere from 1.5 to 6 kHz (depending on the singer). Strong "s" sounds are quickly reduced without (one hopes) too much consequence to the rest of the vocal sound's signal. Ducking is gain reduction that is triggered from a different audio source altogether. The most common usage is to "duck" or lower a music bed (track) whenever a narrative voice-over is active.

SOME PRACTICAL EXAMPLES Bass guitar * Generally, bass guitars seem to sound better when using some compression. I sometimes rely on the compressor to pull up some additional bottom end and warmth. I like to use a leveling amplifier for bass guitar such as an LA-2, Anthony DeMaria Labs or a Tube-Tech CL-1B compressor set up as a leveling amp. Based on the song's tempo and/or the bass player's style of playing, I might start with ratios of 5:1 with a medium threshold setting, medium attack and a slower release time. Unless you are looking for a pumpy effect or have a very wild bass player that the producer would like to squash, I rarely compress bass guitar more than about 5 dB max VU.

Vocals * A lot of anguish and pain surround the recording of vocals. Maybe some singer insecurities, producer apprehensions and overall great expectations weigh on the arrival of a stellar vocal sound. It's about preferences, and vocal sound is subjective and contextual within the backing track. It does not exist on its own unless you are recording an a cappella performance. What I might think is a great vocal recording on a certain CD may be not be what the singer and producer are looking for to convey the emotional import of the artist, the song's lyric or the vibe of the song. At the first vocal recording session, you will want a starting point vocal sound that fits the singer, the song and the production. This starting point sound will work well to capture the immediacy of the moment and hold up later in mixdown. After mic selection, mic preamp selection and EQ setting, a compressor type is decided. Although there is much interaction between all these components, my starting point settings are predicated upon a +4dB level coming from this mic/preamp/EQ chain. For vocals, I tend to use compressors with adjustable attack and release time controls. I like tube-based compressors, although for more difficult control problems I would go to a VCA-based unit. If you were to set the threshold at about 0 dB, the ratio at 4:1, the attack and release at middle positions and adjust the output level for a good recording level, you'd have about 3 to 6 dB of compression and probably make most people happy. This is just a good starting point, and you should get in there and change those settings to your own taste.

Stereo mix compression * A popular place to apply compression is on the entire stereo mix as it is going to the master tape. Stereo compression is also applied "after the fact," such as in mastering, but there is a big difference. If you are mixing your record into a compressor ahead of the master tape recorder (or DAW or whatever), then you are mixing many sources (tracks or elements), each with its individual dynamic content. Making mix moves (especially big gain changes) directly affects the compressor's action, which, of course, affects the relationship of all the other mix elements, that is, the whole mix! If you do not use a stereo compressor at mix and compress at mastering, then you have just the dynamics of a 2-channel stereo mix for the compressor to react to. Many engineers and producers mix into a compressor and then have the mastering engineer compress again, while others prefer to wait until mastering to compress digitally. A good starting place for the stereo mix compressor is a lower ratio with medium attack and release time settings. Set the threshold (and output level) so that you can hear an increase in level (average loudness) when you A/B the compressor in and out of circuit. Remember, this is just a starting point! You may want to use a higher ratio to control peaks better and/or a lower threshold for more compression and denser overall finished sound.

As with all audio processes, learn to recognize the effects of compression (the pitfalls and advantages of using it), and, as always, use your own ears to go for sounds that you like and that improve the sound of the music.

Gain reduction could be divided into five types based on the electronic method used. Knowing how they each basically work will help you in proper selection and in knowing why certain units always seem to excel in certain applications.

Optical isolators * The optical isolator section of compressors uses a light bulb (or an LED) to glow brighter or dimmer in response to incoming audio. A photocell (or phototransistor) is used to track the varying brightness of the bulb and change gain accordingly. This is a good example of an average responding detector. The inherent lag time that the bulb/photocell has in response to audio is factored into the attack and release time performance. The Teletronix LA-2 and the transistorized UREI LA-3 leveling amps are examples of this type of compressor. Compressors using this method are used a lot for bass guitar, vocals, program mix and drums. These compressors offer simple, natural sounding control (unless pressed hard).

FET * Field Effect Transistor compressors use a special transistor to vary gain. FETs were the first transistor to emulate tubes in the way they worked internally. Inherently a high-impedance device, the FET compressor sounds like no other box, and not many examples exist because of the expense of the extra attendant circuitry required. FET compressors are extremely fast, clean and reliable. I like to use these on vocals where a good amount of compression sounds good or better than the same amount with another unit, and on drums for room mics or individual drum mics. UREI's 1176LN peak limiter and LA Audio's Classic II stereo compressor/limiter are examples of FET-based compressors.

VCA * Voltage Controlled Amplifier compressors are the most versatile of all and so are the greatest in number. The VCA can quickly change gain in response to many different detectors looking at the same signal. VCA compressors are for the really tough cases where you want strict control over level and dynamics. However, they can be as gentle as any other compressor or anywhere in between. I like a good VCA compressor on vocals, drums, guitars, synths, bass, mix-basically anytime I need a compressor.

Vari-gain compressors * Variable Gain compressors include all units that incorporate discrete circuitry other than VCAs, FETs or opto-isolators. I put the Manley Variable-Mu tube unit in this category, and I like using these types for vocals, drums and stereo mix.

Computer based/digital compressors * Computer-based or digital compressors are now more prevalent, offering the ability to process audio in the digital domain under precise computer control. Digital audio stored in a computer has the advantage of being "predictively" processed. You can have zero attack time gain reduction as well as nearly infinite control of all parameters of compression on a moment-to-moment basis. The Waves L1-UltraMaximizer TDM plug-in for Pro Tools is one example of a software-based compressor. TC Electronic makes the Finalizer Express hardware unit for studio mastering. This is a single-rackspace, stand-alone, digital multiband compressor.

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