An Introduction to Compression Basic Compression

An Introduction to Compression: Basic Compression Important information on how to use this e-Learning Module Congratulation on purchasing and downloading An Introduction to Compression: Basic Compression. We want you to receive maximum benefit from your purchase and so we ask you to consider the following. An Introduction to Compression: Basic Compression is an e-Learning Module, which has several advantages over conventional printed books and audio magazines. You may have noticed that in printed books there is often a lot of information, but somehow you can’t seem to find the exact information you need; important things somehow seem to have been ‘glossed over’. The reason for this is that few people who have actually worked in professional sound engineering and music have the time or patience to write an entire book. Therefore, nearly all are written by professional writers or academics. Yes, they generally do know what they are writing about, but they don’t necessarily have the practical experience to put the information across in a way that the reader can readily understand. Audio magazines often contain a lot of useful information. However, they have a certain ‘page count’ to produce every month, so you will rarely find that the information is conclusive or complete. They need you to buy next month’s issue too. In comparison, the information in all Audio Masterclass e-Learning Modules is sourced directly from people in the industry - equipment An Introduction to Compression: Basic Compression

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manufacturers and people actually doing the work. Audio Masterclass e-Learning Modules are concise and to the point, telling you the things you really need to know to improve your sound engineering and music recording. Please take time to absorb the information in this e-Learning Module fully. It is exactly what you need to know to get started in understanding basic compression.

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An Introduction to Compression: Basic Compression © Audio Masterclass MMVII

An Introduction to Compression: Basic Compression

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Contents Important information on how to use this e-Learning Module

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Viewing this e-Learning Module

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Copyright information

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An Introduction to Compression: Basic Compression

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Connecting the Compressor

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The Basic Action of the Compressor

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Compression Ratio

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Gain Reduction

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Setting the Threshold Control

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Fixed-Threshold Compressors

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Setting the Gain Make-Up

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Setting the Release Control

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Setting the Attack Control (Basic)

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The Stereo Link Button

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Noise and the Compressor

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Breathing and Pumping

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Artistic Considerations

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Conclusion

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An Introduction to Compression: Basic Compression The compressor is a magical and mysterious component of the recording studio. But it is practical too. Compression was invented decades ago to solve a particular problem. It can still solve that problem now, but at the same time it can add allure and sparkle to music, whether instruments or voices. This text will prepare you to operate the compressor effectively, and also use it to the full to improve and enhance your sounds. Firstly, we need to understand why the compressor was invented, and to do that we need to examine the sounds around us in the real world. Imagine it’s autumn season, or fall as it is known in some parts of the world. A leaf drops from a tree branch and gently floats down to join its fellow golden-brown leaves that are already covering the ground. You watch it fall in completely silent surroundings, and you can just, but only just, hear the tiny sound it makes when it lands. You have just experienced the quietest sound it is possible to hear. If the sound had been any quieter, you would not have heard it at all, even in those perfectly silent surroundings. There is a special phrase for this – the threshold of hearing. It is the level where sound is just audible. Of course, whether or not you hear the falling leaf also depends on how far you are away from it. So we must measure sound from the listener’s perspective. In almost all circumstances we take into account not only how much sound is produced, but the effect of distance and surroundings leading to a measurement of how much sound is heard.

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We have other more technical ways of describing the threshold of hearing… A sound that is just on the threshold of hearing is said to be at 0 dB SPL. ‘dB’ stands for decibels. ‘SPL’ stands for sound pressure level. Also, we can say that the sound pressure that corresponds to the threshold of hearing is 20 micronewtons per square meter. That is the same as 20 micropascals. One newton per square meter is the same as one pascal. Thirdly, we can say that the threshold of hearing corresponds to a sound power of 1 picowatt per square meter. One picowatt is a millionth of a millionth of a watt. That’s pretty small – and yet we can hear it! If you don’t feel comfortable with sound pressure and sound power, micropascals and picowatts, then concentrate on decibels. They are more relevant and easier to understand. Let’s return to dB SPL. As I said, the threshold of hearing is 0 dB SPL. The scale goes all the way up to the threshold of pain where the sound is so loud it actually hurts. This occurs at around 120 dB SPL. There are of course even louder sounds. Naturally they are best avoided by human beings. It is quite common to come across decibel scales that show the SPL for a range of sound sources. Often a jet engine is quoted as producing a level of 120 dB SPL. Of course this depends totally on distance. A plane in the sky produces a much lower level as heard on the ground. Very close up to a jet engine would be much more than 120 dB SPL. Here we have therefore the range of sound levels it is possible to hear in real life – from 0 dB SPL to 120 dB SPL. To make an accurate recording, the equipment used should be capable of handling this range of levels. An Introduction to Compression: Basic Compression

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Let me introduce another technical term here – dynamic range. We say that human hearing has a dynamic range of 120 dB between the quietest audible sound and the loudest sound that can he heard without pain. But there is a problem. Firstly, even with the latest cutting-edge technology it is difficult to achieve a dynamic range of 120 dB. So the human ear is in fact better in this respect than our latest and most wonderful equipment! Secondly, when we enjoy recordings, broadcasts and movies, we subjectively prefer the range of sound levels to be controlled. Yes, in a movie an explosion should be considerably louder than background woodland atmosphere, for instance, but not 120 dB louder. Thirdly, background noise is present in almost all places where recordings, broadcasts and movie soundtracks are heard. A domestic living room might have a background noise level of 30 to 40 dB SPL. Even the quietest recording studios struggle to achieve background noise levels lower than 15 dB SPL or so. So, because of background noise in the listening environment it is impossible to hear very quiet sounds in a recording. There is a problem at the other end of the scale. If you played back a recording in your living room at such a level that the peaks reached 120 dB SPL, your music would almost certainly be heard by your neighbors, unless you live in a desert. Even if they share your taste in music, this intrusion will certainly be unwelcome. In practice therefore there is a window between about 40 dB SPL and 90 dB SPL that all sounds on a recording, from the quietest to the loudest, should stay within when played on loudspeakers

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And this, quite simply, is why we need compression. We like compression for other purposes, but we need it to control dynamic range.

dbx 266XL Dual Compressor with Gate

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Connecting the Compressor There are a number of ways in which a compressor can be used… • To compress an individual signal as it is being recorded. • To compress an individual signal as it is being mixed. • To compress the entire stereo mix during the mixing process. It is also possible to compress groups of instruments together – the drums of a drum set for instance. However, this is an advanced topic that will not be covered here.

ART Pro VLA Two-Channel Compressor

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To use a hardware compressor in conjunction with a microphone preamplifier and a digital audio workstation (DAW). • Connect the microphone to the input of the preamplifier. • Connect the output of the preamplifier to the input of the compressor. • Connect the output of the compressor to a line input of the DAW, or to the line input of the audio interface if it is a software DAW.

ART Pro Channel Microphone Preamplifier and Compressor Model 215

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To use a hardware compressor in conjunction with a mixing console, using the mixing console’s internal microphone preamplifier. • Connect the microphone to the microphone input of one channel of the console. • Connect the channel insert send of that channel to the input of the compressor. • Connect the output of the compressor to the channel insert return of the same channel. Note that in some consoles the channel insert point comes before the EQ. In other consoles it comes after the EQ. In some consoles, this is switchable. Compressing before or after equalization are discussed in An Introduction to Compression: Advanced Compression.

Empirical Labs EL8X Distressor Compressor

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To use a hardware compressor in conjunction with a mixing console to compress the entire stereo mix, if the console has mix insert points. • Connect the mix insert point left channel send to the left input of the compressor. • Connect the mix insert point right channel send to the right input of the compressor. • Connect the left output of the compressor to the mix insert point left channel return. • Connect the right output of the compressor to the mix insert point right channel return. • Activate the stereo link function of the compressor (to be discussed later).

Samson SCOM4 4-Channel Expander Gate Compressor Limiter

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To use a hardware compressor in conjunction with a mixing console to compress the entire stereo mix, if the console does not have mix insert points. • · Connect the mix output left to the left input of the compressor. • · Connect the mix output right to the right input of the compressor. • · Connect the left output of the compressor to the left input of the stereo recorder. • · Connect the right output of the compressor to the right input of the stereo recorder. • Activate the stereo link function of the compressor (to be discussed later).

BBE MaxCom Dual Channel Compressor Limiter Gate

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To use a compressor plug-in with a software DAW to compress individual channels. Note that many software DAWs by default apply plug-ins to the monitoring and mixing processes. They do not record the output of plugins. This is a complex issue that is discussed in detail in An Introduction to Compression: Advanced Compression. • Insert a compressor plug-in into an insert point on the channel strip.

Chandler Compressor/Limiter Plug-in

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To use a compressor plug-in with a software DAW to compress the entire stereo mix. • If there is no mix (master) strip visible, make it visible or create one. • Insert a compressor plug-in into an insert point on the mix (master) strip.

Level Devil Compressor Plug-in

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The Basic Action of the Compressor Compressors, as normally found, work by reducing the level of loud sections of the signal. It would be possible to design a compressor that worked by increasing the level of the quiet sections of the signal, but that is very rare indeed. So the quiet sections of the signal are left unchanged. Only when the signal rises above a certain threshold does compression start. Let me explain that again. Sections of the signal that are lower in level than the threshold level that is set are not changed. Sections of the signal that are higher in level than the threshold have their level reduced. If you compare the uncompressed and compressed signals at this point, during quiet sections they will sound the same; during loud sections the compressed version will be quieter. In practice, it is not useful to leave the signal like this. It is better to raise the level of the entire signal so that the peak levels are the same as they were before compression. This is called make-up gain, or gain make-up. After gain make up, loud sections are as loud as they were before compression; quiet sections are louder than they were before compression. In some compressors, compression starts immediately when the threshold level is exceeded. In others, compression comes in gradually. This is called the knee of the compressor. A hard knee is where compression starts immediately. A soft knee is where it comes in gradually. Knee is explained in detail in An Introduction to Compression: Advanced Compression. An Introduction to Compression: Basic Compression

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Compression Ratio When the signal is above the threshold level, its level is reduced. The compression ratio sets the amount of reduction.

We would typically talk about a compression ratio of, say, 2:1. This means that when the input signal rises by 2 dB, the output signal rises by only 1 dB. If the input signal rises by 10 dB, then the output signal rises by only 5 dB. A compression ratio of 1:1 would mean no compression, nothing would be changed. 2:1 is actually a mild compression ratio, although useful in some contexts. 10:1 is quite a fierce compression ratio. Although it might be useful in some contexts, it will be clearly audible that compression is happening. If the compression ratio is set to 20:1, then if the input signal rises by 20 dB, the output signal rises by only 1 dB.

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When the compression ratio is 20:1 or greater, the effect is known as limiting. There are devices called limiters that have exactly this purpose. They allow the signal to rise up to a certain level, but then it can rise no further. Limiters are used in broadcasting to ensure that there is no distortion in the transmitter. They are used in live sound to protect the loudspeakers. They are also used in recording as part of the mastering process. They are generally not otherwise used in the recording or mixing processes. It is useful to have some idea of where to start with compression. A ratio of 4:1 is generally good to start off with as the effect of the compression will be clearly audible. If a stronger effect is required, then the ratio can be increased. If it is desired to make the compression less obvious, then the ratio can be decreased.

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Gain Reduction All good compressors have a gain reduction meter. It should really be called a ‘level reduction’ meter, but the phrase is now stuck. This text will refer to the gain reduction meter of the compressor, and the process of level reduction. The so-called gain reduction meter will show the amount of level reduction.

Although a compressor can be set purely by ear, it is useful to have visual confirmation of how much compression is taking place. The gain reduction meter will show from moment to moment how much the level of the signal is being reduced. When the signal is below the threshold level, the gain reduction meter will show 0 dB. When the signal rises above the threshold, the gain reduction meter will show by how many decibels it is being reduced, from moment to moment.

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Some compressors have an LED bargraph meter. Others have needle-type meters. The bargraph meter is likely to be more accurate and quicker to respond.

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Setting the Threshold Control [Please note that some compressors have a fixed threshold and therefore do not have a threshold control. Instead they have a gain control. This will be explained in the next section.]

As advised earlier, it is a good starting point to set the compression ratio to 4:1. Set the attack and release controls (to be explained later) close to their minimum values. Now apply a music signal that has both quiet sections and loud sections to the compressor. Set the threshold so that at in the very quietest sections the gain reduction meter shows 0 dB, which means no compression. Adjust the threshold so that when the signal goes any louder than this, compression starts to take place. From this point, setting the threshold and ratio controls is entirely up to your judgement. However, it is worth noting that there is rarely any merit in setting the threshold to be lower than the point you have arrived at by An Introduction to Compression: Basic Compression

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now. All that will happen is that after any period of silence in the signal, the compression will crash in suddenly, and unpleasantly audibly. Try the controls one at a time. • Raising the threshold will cause less compression to take place and the gain reduction meter will show less reduction in level. • Decreasing the compression ratio will cause less compression to take place and the gain reduction meter will show less reduction in level. • Increasing the compression ratio will cause more compression to take place and the gain reduction meter will show more reduction in level. It is incredibly useful to listen to what you achieve while watching the gain reduction meter. It is good to practise on as wide a range of signals as you can.

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Fixed-Threshold Compressors Some compressors have a fixed threshold and therefore do not have a threshold control. Instead there is a gain control, sometimes labeled level or input. When using such a compressor, you would turn up the gain until you could hear the amount of compression you want. The effect is exactly the same as a standard compressor, only the operational method is different. The gain reduction meter of such a compressor works in exactly the same way as that of a standard compressor.

The Empirical Labs Distressor features a fixed threshold and a gain control

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Setting the Gain Make-Up It is usual to set the gain make-up control so that the peak levels of the compressed signal are the same as the peak levels of the uncompressed signal.

It will now be possible to press the in/out button to compare the compressed and the uncompressed signals easily. It is worth bearing in mind that compression does not always improve a sound, so making a comparison like this will be a useful check. It may be apparent that there is some noise present during quiet sections of the signal. This is an inevitable result of the compression process. At this point, you should ignore any noise other than assessing whether it is excessive. If so, back off the ratio or threshold controls and apply less compression. This topic is discussed later.

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Setting the Release Control The release control is very much more important than it might appear. It is in fact the heart and soul of the compressor.

The release control sets the length of time it takes the compressor to respond when the signal changes from loud to quiet. That is important. Please read again. A compressor works by changing the level of a signal dynamically. When the signal is quiet it does nothing. When the signal is loud it brings it down in level. Imagine now a fader on a mixing console and you are listening to a signal passing through that fader. At first, set it to 0 dB, which will keep the level of the signal exactly the same as it was when it entered the fader. Listen to the signal for a while. Now reduce the level by 10 dB. In other words, set the fader to –10 dB. An Introduction to Compression: Basic Compression

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The signal is now quieter. The loud parts of the signal are quieter by 10 dB and the quiet parts of the signal are quieter by 10 dB too. THIS IS NOT COMPRESSION! Lowering the signal level using a fader is not compression because all the various levels of the signal have been reduced by the same amount. Compression only takes place when the loud sections of the signal are reduced in level more than the quiet sections. Let’s go back to the fader… Now, listen to the signal carefully. When it is loud, reduce the level. When it is quiet, bring the level back up again. Yes, this is compression. Manual compression. Now let’s turn to the compressor. Set all of the controls as previously instructed, so that you see a maximum reduction in level of around 10 dB on the gain reduction meter. Now set the release time to its maximum value. As the signal level changes, you will now notice that the gain reduction meter is showing a pretty constant 10 dB reduction in level. It hardly moves. This is exactly the same as lowering the fader by 10 dB and leaving it there. THIS IS NOT COMPRESSION! This point is vitally important and key to understanding how a compressor works. For actual compression to take place, the gain reduction meter has to An Introduction to Compression: Basic Compression

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move. If it is not moving, then there is no compression. Now experiment with the release control. As you lower the value of the release time, you will notice the gain reduction meter hopping about more and more quickly. You will also notice that the compression effect becomes more pronounced. This is why the release control is so important. Once all the other controls have been set properly, the release control governs the effective ‘amount’ of compression. The other controls have to be set properly of course. But once that is done it is the release control that wields the power.

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Setting the Attack Control (Basic) The attack control sets the length of time it takes the compressor to respond when the signal level changes from low to high.

In general, the attack control can be set to close to its shortest duration and left there. Often it is found that the very shortest settings produce a rather harsh sound, but you should judge that for your own individual compressor. There will be no reason to change this setting other than for special reasons and effects that are discussed in An Introduction to Compression: Advanced Compression.

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The Stereo Link Button In a two-channel compressor, the stereo link button connects the two channels in such a way that they apply the same amount of level reduction to the left and right sides of a stereo signal.

To put this simply, whenever you compress a stereo signal, you must press the stereo link button. The explanation of why is a little more complex. Let’s take as an example a recording of a piano and a kick drum. An unusual combination perhaps, but it will illustrate the point. The piano is recorded centrally in the stereo image, so it is at an equal level in both channels. The drum however is recorded only in the left channel. Both channels are compressed with the same settings, but with the stereo link function off. An Introduction to Compression: Basic Compression

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The threshold is set so that the piano is always below the threshold. The drum however is at a high level and easily exceeds the threshold. So while the piano plays and the drum is silent, everything is normal and the piano is heard as it should. But at the instant the drum places, suddenly the threshold in the left channel is breached. (Remember that the drum is only in the left channel). Level reduction is applied to that channel during the instant the drum places. But it is not only applied to the drum, it is applied to the piano as well, in the left channel. The piano in the right channel is unaltered. Now, since the level of the piano has suddenly gone quiet in the left channel, the right channel dominates, and the piano suddenly swings to the right of the stereo image. Then it swings back again as soon as the drum stops resonating. The effect of not switching in the stereo link when compressing a stereo signal is instability of the stereo image. There is never a useful purpose for this, so the stereo link function must always be activated when compressing a stereo signal.

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Noise and the Compressor All signals contain a certain amount of low-level background noise. It is an inevitable consequence of compression that this noise level will be increased. So if you can hear noise in a compressed signal, it is not the fault of the compressor (as long as it is a professional model), it is entirely due to the process of compression itself. There is no way of compressing a signal without increasing the noise, and as a consequence decreasing the signal to noise ratio of the signal. Noise can be controlled by using an additional noise gate, or an expander, which is discussed in detail in An Introduction to Compression: Advanced Compression.

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Breathing and Pumping One consequence of compression increasing the noise level of a signal is ‘breathing’. This is where the noise audibly rises and falls in level as the signal changes in level. Breathing corresponds very closely to the amount of reduction of level as shown on the gain reduction meter. When the gain reduction meter is low, the noise level will be high. When the gain reduction meter is high, the noise level will be low. If you reduce the release time so that the level reduction changes more quickly, then the breathing effect will be more rapid and more noticeable. Breathing is most strongly noticeable on speech where there is background noise, such as air conditioning noise. It is recommended that you record some speech with intentional background noise so that you can observe the breathing effect clearly. Once you have that characteristic sound in your mind, you will hear it when it is not so pronounced. In practice, for music signals breathing is normally not a problem. Pumping is closely related to breathing and applies more when the entire stereo mix is compressed than it does to individual tracks. Imagine a song in a slow tempo with a very strong back beat. The entire stereo mix is compressed. During the few tens of millisecond when the beat is actually present, the compressor brings down the level of the signal. In between, if the release An Introduction to Compression: Basic Compression

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control is set to a low value, the level will rise up again. So the level of the music continually and rhythmically swells up between the beats. Find a music track that has a strong beat and compress it with a short release time. This will show the pumping effect very clearly. Pumping is not always undesirable. Often it can add excitement to a mix. Some compression experts sometimes go to some trouble actually to increase the degree of pumping!

Avalon VT747SP Class A Tube Stereo Compressor/Equalizer

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Artistic Considerations At this point you have all the information necessary for successful compression to control dynamic range. Many types of signal will benefit from this, purely to decrease the difference between the quiet sections of playing and the loud. Compression to control dynamic range is useful for vocals, acoustic instruments, bass guitar and complete stereo mixes. Compression is generally not useful for heavily distorted electric guitar, as the distortion effect in itself reduces the dynamic range of the signal. Compression may or may not be useful for electronically and digitally synthesized sounds. It is up to the engineer to judge in individual cases. But there is another point to compression as well as controlling dynamic range – it simply makes things sound nicer! So for instance you could have a singer with a well-controlled voice who didn’t vary much in level, so there is no real need to use a compressor to control the dynamic range. But put his or her voice through a compressor and it will sound, almost magically, better. There are two reasons for this… One is that if you compress with a short release setting, the tiny details of the vocal are brought up in level. Subjectively, this seems to make an improvement in itself. Also however, compressors are often designed using techniques that produce distortion in the signal. Vacuum tube compressors, for example, add subjective warmth to the signal.

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And somehow a vacuum tube compressor provides a better quality of warmth than, say, a vacuum tube preamplifier. Judging the subjective sound quality of compressors is an important sound engineering skill that takes much experience to master.

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Conclusion Thank you for reading An Introduction to Compression: Basic Compression. However, there is something else you have to do now... You have to put your understanding into practice. And you have to listen. This e-Learning Module has introduced you to the foundation knowledge you need to master basic compression successfully. However, the only way you will truly understand is to use compression in real life recording situations. The more you work with compressors, and the more intensely you listen to the results you are achieving, the faster you will progress to being a master of basic compression. When you are ready to move on, An Introduction to Compression: Advanced Compression, from Audio Masterclass, will be your next step. Good luck!

David Mellor, Audio Masterclass

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