Introduction

This article provides an introduction to the fundamental principals and uses of signal compression and limiting in the audio recording, mixing and mastering processes. You will need to be aware of some basic terminology ...

• Signal / soundwave = the audio to be processed
• Transient = the beginning of a sound, its attack
• Dynamic range = the difference between the loudest and quietest parts of a signal, or the range of volumes a device or technology can accommodate
• Attenuation / Gain reduction = lowering the signal level
• Gain = signal level or a control to alter signal level
• Volume = signal level
• Loudness = the perceived volume (usually defined as the overall perceived volume of a signal/performance)
• Device/technology = in this context, a compression technology

Note: File compression and encoding (to reduce file size) is discussed here.

What is compression and limiting?

Compression and limiting are processes which employ technology (both analogue and digital) to automatically control the dynamic range of a signal. In a nutshell, they are automatic volume controls. They do this by turning down the volume when it exceeds a preset level (threshold). Because the process turns down, or attenuates, peaks in the signal, the overall level can then be raised (gain makeup) to maximise the dynamic range of the record or playback system. Therefore, compression and limiting increases the average loudness of the signal by reducing its dynamic range. NOTE: Compression can also widen dynamic range if signal peaks are permitted to pass with a slow attack setting.

In "popular" music recording it is common to to use compression during the following processes ...

• Recording and mixing bass
• Recording and mixing electric guitar
• Recording and mixing acoustic instruments
• Recording and mixing vocals
• Mixing drums and percussion
• Mastering

These are by no means the only uses.

Limiting and clipping

Limiting is compression at its most extreme ratio settings (typically greater than 20:1). The limiting process prevents almost any signal from exceeding the threshold level.

Clipping is limiting at its most extreme and allows no signal beyond the threshold. This causes audible distortion that may go unnoticed if only occasional peaks are processed. A "brickwall" limiter will operate in extreme limiting and clipping territory.

Side effects of compression

It is important to understand that very few, if any, compression technologies can reduce dynamic range without introducing audible side effects. These become more pronounced at more extreme settings. Whether these side effects are desirable or undesirable for a given job is for you to decide. Possible side effects include ...

• Pumping (unnatural and fast changes in loudness)
• Loss of transient detail
• An unnatural dynamic range (eg softly performed elements sound as loud as aggressively performed ones)
• Distortion (subtle effects are sometimes called "warmth" or "colouration")
• Tonal changes (as frequency balance differences are emphasised)

Creativity vs commerce - the loudness wars

Together with EQ and effects, compression is regarded by many, but not all, as one of the essential creative process of "non-classical" music production (classical music recording is preoccupied with recording as faithfully as possible a live performance, so any processing is frowned upon).

However, there are still some who regard compression and limiting as tools of the profit seeking recording and broadcast industries, so pre-occupied with ensuring their artists sound louder than their competitors. The trend towards over compression and limiting has led to many recordings which are both fatiguing to listen to and distorted. Read more here ... www.PleasurizeMusic.com/

 

Why compress and limit?

Although first conceived to solve technical problems inherent in recording and broadcast technologies, compressors and limiters are now widely regarded as both important technical and creative tools, and a multitude of models and technologies are employed by sound engineers not only to control dynamic range and "shape" the envelope of sounds but also to impart a pleasing distortion ("colour" or "warmth" etc).

The following are some of the primary applications ...

Compression/limiting applications	Example applications
To prevent a signal with an unpredictable dynamic range from overloading a recording system	When recording a singer.
To reduce dynamic range in order to give a performance a more consistent overall volume.	When mixing a bass guitar part.
To change the dynamic envelope of short sounds.	To make a snare drum "fatter" or "snappier"
To change the tonality of a sound, to colour it.	To overdrive a valve compressor to add "warmth" to a vocal performance during mixing
To reduce dynamic range in order to safely maximise average volume during broadcast.	To limit all music broadcast from a radio station to ensure differing recordings all sound as loud as each other, and ensure each individual recording has more consistent loudness.
To help 'bind" a mix together.	Bus compression being used to process an entire mix.
To isolate and compress individual frequency ranges.	Multi-band compression isolating and compressing just the bass of a completed mix.

 

Example scenarios

1. Vocal mixing

Consider a lead vocal performance. It is common for a performance to contain wide variations in volume (wide dynamic range). Sometimes these are intended, such as when a singer must use power to reach a high note, sometimes not, such as when a singer loses power/volume towards the end of a long phrase. Backing instrumentation is less likely to exhibit these variations in volume, so during mixing it is possible that the vocal will sometimes overpower the backing whilst at other times become in-distinct. The answer is to "ride" the volume during mixing either by altering the level with fader movements and/or by employing a compressor to create a more even overall loudness (reduce the dynamic range). In practice it is common for both these techniques to be employed. Many engineers employ compression to reduce dynamic range and then use fader movements to raise the overall level during louder passages such as choruses and lower it for verses.

2. Drum shaping

Many sounds have changing frequency content over time and compression can alter this. Compression can have a dramatic effect on the tonal qualities of drum hits, as well as controlling the overall loudness of a performance. By altering attack and release settings it is possible to transform the sound of a snare drum by altering the volume relationships between the transient (attack) and decay parts of the sound. Some kick drums have greater low frequency in the transient, so a fast attack may reduce its punch. It is well worth experimenting to gain an understanding of what is possible. (See Suggested Settings later).

3. Multi-band compression

Mastering engineers often find that the finished stereo mixes they receive need correcting. One of the tools they can employ is a multi-band compressor. These devices use a filter to isolate a part of the frequency range (eg bass between 20Hz and 2 00Hz) which can then be compressed without altering the mid range and treble.

 

How do I operate a compressor?

Using compressors effectively requires theoretical knowledge and experience. The effect of compression on individual elements can be subtle to the untrained ear, but have a dramatic effect on the cohesiveness and effectiveness of a mix. Many experienced engineers may be embarrassed to admit that they use the same devices and settings again and again because experience has showed them they work. One famous mix engineer never alters the settings on his compressors. Amongst his collection he has multiples of the same model/device each with a different setup that he favours. If he discovers a new setting on an existing device, he buys a duplicate device for it!

With the best (expensive!) professional devices, it is harder to produce poor results and easier to emulate the sounds of past successful productions. I think it is true to say that along with microphones and mic-pre-amps, attitudes towards the choice of compression technologies, and their settings, are among the most conservative in music production. This may reflect the fact that even professionals sometimes have trouble discerning the best devices and settings for a given performer, so they fall back on what they know has worked for them and other successful engineers in the past. Devices with minimal controls are particularly popular (eg The Teletronix LA2A).

Parameters & controls

Even though some of the most revered technologies have very few controls, every audio engineer needs to understand the following concepts.

Threshold

Compressors turn down the signal when it exceeds the threshold level. Control of this level is implemented in several differing designs ...

• Threshold control. This functions as you would expect. As you turn the threshold level down, compression occurs at lower and lower signal levels and the signal is compressed more often. Therefore the peak level, and overall perceived loudness, of the signal becomes quieter.
• Input gain control. With this arrangement there is a pre-fixed threshold level and you determine the amount of compression by altering the gain of the signal entering the compressor stage of the device. In this case the overall perceived level of the signal increases. For novices this can give the false impression that the sound is improving and its easy to apply inappropriate amounts of compression.
• Compression / Peak Reduction control. The simplest of controls compresses the signal more as you turn it up. It may simply be lowering the threshold level and applying pre-set ratio, attack and release settings, or employ a combination of threshold level, increasing ratio, and sometimes changing attack and release characteristics which change as the control is turned. Again, with this arrangement, the peak level, and overall perceived loudness, of the signal becomes quieter.

Ratio

This is the ratio of gain reduction. Ratio is usually expressed thus ... "3:1". In this example any input signal which exceeds the threshold level will be turned down by two thirds. In other words, a signal which exceeds the threshold level by 3dB will be turned down by 2dB, and only 1dB of signal level above the threshold level will pass.

Some devices have a means to change the ratio manually, some have fixed ratio and some change the ratio automatically as you adjust the input gain or peak reduction control.

Ratio	Effect
1.5:1	Gentle compression
3:1	Typical vocal compression
6:1	Strong compression
12 to 20:1	Aggressive compression
18 - 20+:1	Limiting
Infinity:1	Clipping

 

Knee characteristics (also known as slope)

This control determines how the compressor reacts as the signal approaches the threshold level.

A soft knee setting will gently increase the ratio as the signal approaches the threshold level until the ratio is reached usually at some point just beyond the threshold level. This produces a gentle curve (soft knee). This is the preferred setting when natural and transparent compression is required as it produces a less pronounced effect and less unwanted side effects.

A hard knee setting will produce a more aggressive and pronounced change in ratio as the signal reaches the threshold and is often preferred for sounds with faster attacks such as drums.

Some compressors have a hard/soft knee switch whilst others automatically change the knee characteristics as more compression is applied.

Attack

Set in milliseconds (ms) this sets how quickly the compressor will react once the threshold level has been exceeded. For very short sounds (eg a rimshot) a long attack time may prevent compression completely. A fast attack time will attenuate the transient. A medium time will allow the transient through and attenuate the decay or body of the sound, thus emphasising the transient.

Release

Set in milliseconds (ms) this sets how quickly compression will cease once the signal has fallen back below the threshold level. If you set this too long, you may find that the signal is always being compressed. If you compress a snare drum with a fast attack and release times, the transient will be reduced and the decay/body will be left unaltered, producing a less clicky and fatter/bigger snare.

Gain makeup

Once the overall loudness of the signal has been reduced by the compression process, this control allows the overall volume of the signal to be turned back up to peak level (maximising dynamic range) before it leaves the device. Some devices without such a control may have an automatic make-up gain circuit.

Side chain

In order to determine when to apply gain reduction, a compressor must analyse or "look" at the signal. In most compressors this is achieved internally by taking a copy of the input signal (side chain) and using this to determine the action of the gain reduction circuitry. Some compressors allow an external signal to be used for this purpose so you could, for example, apply compression to a guitar part according to the signal from a bass drum, or perhaps feed the side chain signal through an EQ before it is analysed (so called frequency conscious compression).

Metering

It is useful, and common, for a compressor to have a meter which can be switched between showing ...

• the input level - the meter moves up (LED) or left to right (VU) to show increasing signal level. Red will indicate distortion (analogue) or clipping (digital) of the gain reduction. circuitry.
• amount of gain reduction (GR) taking place - the meter moves down (LED) or right to left (VU) to show increasing compression. When no compression is taking place the meter will rest at 0dB. Green will indicate gain reduction and Red will indicate limiting and then clipping.
• overall output level - (the meter moves up (LED) or left to right (VU) to show increasing signal level. Red will indicate distortion (analogue) or clipping (digital) of output amplifier.

Hook up

Unlike effects processors which may be shared between sounds via an auxiliary send/return setup, it is usual practice to route a single entire signal through a compressor.

Recording

During recording it is common to place the compressor inline between the pre-amplifier and recording device (commonly the A to D converter of an audio interface). This may involve using the insert send and return of a mixers channel strip, or by chaining a stand alone pre-amp and compressor together in line. A ratio of between 1.5:1 and 3:1 might be chosen with the threshold set high to catch peaks. Such as setup will protect the recording device from clipping and allow it's dynamic range to be exploited to the fullest.

Mixing

Compression is usually applied by using a channel (or bus/group) insert send/return loop to route the entire signal through the compressor.

Chaining

You may wish to apply more than one setting to deal with different dynamic range problems. This can be accomplished by chaining compressors together. For example you may wish to even out differences in volume between kick drum hits with a compressor set to attenuate just the hits which are too loud (high threshold, fast attack, slow release) and then shape the transients of all hits by applying a lower threshold, medium attack and fast release settings.

Splitting

If you find a single compression setting is unsuitable for every part of a performance, either automate setting changes, or duplicate the signal, insert a different compression on each signal and use mutes to switch between them.

Bus compression

You can compress an entire mix, or groups of sounds (stems) such as the rhythm section, brass section, backing vocals, guitars etc, by inserting a compressor into the master output or group buses. If you are going to have your mixes professionally mastered, you should avoid these processes and leave them to the mastering engineer.

Parallel compression

This is a process in which the signal is duplicated, compression is applied to one of the signals only and then they are mixed back together again. This can make the perceived average loudness of the sound more consistent and punchier, and the tone "warmer"without spoiling the transients or sacrificing clarity and air.

A version of parallel compression favoured by Nashville studios is "invisible compression". Instead of being duplicated, a copy of the signal is sent down an auxiliary bus to the compressor which is treated like an effect processor (send and return).

Suggested applications and settings

The best way to use any compressor is to experiment with it. There are absolutely no rules for achieving creatively pleasing results. However, the following are some tried and tested applications and methodologies which may serve as some sort of starting point. NOTE: It is almost impossible to recommend exact settings because every situation, performance and signal differs.

Application	Notes	Threshold	Knee	Ratio	Attack	Release
Bus compression for mixes	Setting up effective bus compression can be challenging because signals containing combinations of sounds/frequencies are more likely to produce unwanted side effects. Start by experimenting with a gentle compression ratio and set the threshold to catch peaks only. 2-3dB of gain reduction with moderate attack and release times will allow you to increase the overall loudness of your mixes without producing pumping or unwanted distortion. Experiment with release settings.	For peaks only	Soft	2:1	Med	Med
Increasing the click (transient) of a kick or snare	Let the transient pass and attenuate the decay/body.	Med	Hard	8:1	Med	Med
Decreasing the click (transient) of a kick or snare to create a fatter sound	Attenuate the transient but let the decay/body pass. NOTE: This method can reduce punch by attenuating bass frequencies which may mostly reside in the transient.	Med	Hard	8:1	Very fast	Fast
Making a kick performance consistent (for dance music)	Set a threshold which catches any hit which is too loud and the attack and release to turn down the entire hit.	Med	Hard	5:1	Very fast	Slow
Mix balancing. Transparent gain reduction to improve average loudness consistency (vocals, guitars etc)	Attenuate any peaks and use gain make-up to raise the average loudness.	For peaks only	Soft	3:1	Fast	Med
Compressing basses	Basses often have wildly varying dynamic ranges in the lower frequencies. Don't be afraid to experiment with settings that add warmth and distortion.	Med	Soft	6:1	Med	Slow
Toms	To tighten and make more punchy	Med	Soft	5:1	Fast	Med
Acoustic guitars	Tighten dynamic range	Peaks only	Soft	3:1	Fast	Slow
Drum overheads	Tighten dynamic range, soften transients	Peaks only	Soft	2:1	Fast	Med

 

Technologies/devices

Most compressor/limiter technologies fall into 1 of 3 categories ...

1. Technologies that can compress a signal (compressors)
2. Technologies that can limit a signal (limiters)
3. Technologies that can compress and/or limit a signal (compressors/limiters)

These technologies can take several forms ...

• Analogue hardware outboard devices such as the Teletronix LA2A electro-optical compressor and Dbx-160 VCA compressor
• Analogue hardware channel strip devices such as the Focusrite ISA 430
• Analogue and digital guitar pedals
• Software plug-ins/emulations in hardware devices such as digital mixing desks
• Software plug-ins/emulations in outboard hardware devices such as the Focusrite Liquid Channel
• Software plug-ins/emulations in audio interfaces (to help control levels when recording)
• Software plug-ins/emulations in DAWs (such as Pro Tools, Logic and Cubase)
• Software plug-ins/emulations in FireWire and PCI DSP processors (such as the UAD PC1e cards or TC Electronics PowerCore devices)

The precise design of compression technologies is critical to its ability to produce a musically pleasing result. If you wish to emulate the sounds of many mainstream commercial "professional" recordings, you may need to use professional (expensive!) compressor technologies.

Affordable devices

Unlike effect processors and synthesisers (for example), for some reason there are few, if any, affordable compressors capable of achieving good results with ease, and many which can make things worse. At best it may be challenging to get the results you want, at worst it will be impossible.

Compressor designs

We have already discussed the side chain signal path but the exact electronic design of the signal sensing circuitry and gain reduction amplifier is what gives compressors their individual character and tonality.

There is no one design that is considered superior. Some devices employ valves whilst others use transistors (FET). The LA2A was the first compressor to use an electro-optical cell in the side chain to control the gain reduction amplifier. Voltage controlled amplifiers (VCA's) are the most common gain reduction device but no analysis of circuit board topography will tell you how a compressor will sound or whether you are going to like it. As ever, you should read what the experts have to say and then audition, experiment and decide for yourself.

Classic compressors

Some classic compressor models have achieved legendary status and are widely thought to be essential for producing that "professional sound". This maybe the result of a mix of good engineering and good marketing on the part of manufacturers, but there is no doubt that a well designed model will make it easier to achieve effective transparent control of dynamic range, and impart a pleasing colouration (distortion) to the sound. Cheaper models often produce less pleasing distortion and require skill and effort to use effectively.

Here are some classics and some of the applications they have become well know for ...

Device	Popular applications
Empirical labs Distressor	Widely used to apply large amounts of compression to drums in order to change their tonal qualities. Vocals, when extra character and enhancement is required. This device emulates a number of classic valve compressors and has extensive side chain options.
SSL bus compressor	Entire mixes but also sounds great on bass and electric guitar, and snare drums.
Urei 1176	Adds sparkle and excitement to vocals. Helps basses cut through a mix.
Fairchild 670	Favoured for the tonal enhancement qualities it imparts to vocals (lots of valves!) as much as for its gain reduction circuitry. Imparts presence.
Teletronix LA2A	Warm musical vocal compression. Also great on guitars and bass.
Dbx-160 series	Clean transparent vocal compression. Much favoured for drums, great for transient tweaking.
Summit TLA100A	Musical vocal compression that retains the high end
Neve 33609	Effective bus compressor offers smooth and transparent gain reduction. Also works well on guitars and almost everything else too.
Avalon VT737SP	Great for lead vocals. Smooth transparent warm compression.
Focusrite Red 3	Fat and warm vocal compression.

 

This is by no means a comprehensive list and remember, every compressor has its advocates and its detractors.

 

In conclusion

In researching this article I experimented with my own compressors, talked to engineers and referred to reference material from the last 15 years. I was surprised to find that 1990s editions of authoritative and comprehensive books on sound recording practice, gave up no more than one or two pages to the subject, and covered only the basic technical concepts. This I think was not the fault of those authors but a reflection of how compression has come to be regarded as an important creative tool over the last decade. Some of us were aware of Joe Meek's and Geoff Emerick's groundbreaking work in the 1960s, but it seems that only recently, perhaps because of the popularity of the project studio, that creative compression techniques have been widely embraced by mainstream music makers.

Nevertheless, it is not always desirable or necessary to use compression. It's perfectly possible to create wonderful, transparent and dynamic recordings without it. Who knows, no compression might be the fashion in a few years time.

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