The Music Espionage

Compression


When it comes to understanding compression it can be a little bit tricky knowing not only how to use it, but when. Basically, you could think of compression as a very helpful friend that is willing to give all his time to sit and listen to just one audio track. This new friend, some lonely so-and-so who’s only girlfriend was made from odds and ends from his Dad’s shed, will carefully listen to the specified track and when a signal goes above a certain point he will very kindly bring down the level and by a certain amount.

From the Floor up!

To deal with compression you will need to know about dynamic range. First, we have the noise floor. This is the lowest level, where tape hiss and electrical hum lives. Next is the normal level, best for recording incoming signals in order to minimize distortion and overcome the noise floor. The distance between the noise floor and the normal level is called the signal-to-noise ratio. Next is the maximum level, which is where distortion occurs at when your incoming level reaches it. This is the highest level in the total dynamic range. The difference between the normal level and the maximum level is referred to as your headroom. This is your safety zone, and this is needed to account for some stray peaks here and there without hitting the maximum level. So from the noise floor to the maximum is called the dynamic range, see it’s easy!

The ideal level would be mainly within this normal level area. If you have a level meter on your recording desk or software, you normally want something that is at the top end of the Green just going into the yellow. Never the red!

Because no performer, drummer, guitarist, whatever is perfect there will be times when you have sudden jumps in the dynamics. This could be from a heavy hit on a snare or just a strongly plucked guitar string. Compression is here to help. It is through compression that you will be able to lower the higher, more energetic transients in an audio recording, but also bring up the lower ones. This will make the whole recording smooth and more uniform, so there are not big jumps in the dynamic range.

The Nuts and Bolts:

So lets get down to the nitty-gritty. Keeping in mind the idea that the compression is a helpful guy that is listening closely to your audio signal. We need to tell him a few things to listen out for so he knows when to reduce or expand the signal level, by how much, how quickly to work and what to send to the main output so we get a nice smooth signal.

Threshold

This is the parameter that sets the level for the signal, if the amplitude exceeds a certain threshold is will be compressed. It is generally given in dB, where a lower threshold (e.g. -60 dB) means a larger portion of the signal will be treated (compared to a higher threshold of −5 dB). I like to think as lowering a line down onto a signal, everything after this point will be compressed. Shown below.

Ratio

Once you have set the level for compression to happen (Threshold) you now need to state by how much you want to compress the signal. This ratio relates to the amount of signal that has jumped over the threshold. So for 4:1, for every 4dB that goes over only 1dB will be allowed to pass, so it has been compressed by 3dB.

Gain

The final principal element of compression is gain. You have set the level when to start compression, stated by how much you want it compressing so now just the output to deal with. The gain will increase the whole signal level, meaning you achieve a more rounded and uniform signal, without out the sudden peaks and jumps in level. The mechanism of compression means that loud sounds are reduced in level, but compressors have an output level control that allows any gain lost by compression to be restored or made up for. If you carefully apply enough gain to bring the signal peak levels back to where they were before compression, the quieter signals will be louder than before, so you can think of compression as both a way to make loud sounds quieter and to make quiet sounds louder.

Attack, Release and Auto

In additional to these main three, the Compressor will also have a number of additional elements that allows you to refine how the signal is compressed and how the compressor works.

Firstly is the time it takes for the compression to work. Attack and Release controls are provided to determine how quickly the gain is pulled down once the threshold is reached, and how long the gain takes to rise back to normal once the signal falls back below the threshold. Creating a deliberate overshoot by setting an attack time of several milliseconds is an effective way of emphasising the percussive nature of drums. Too short a release time can result in level ‘pumping’, while if the release time is too long quieter sounds following a loud beat may be reduced in level even further.

Setting the best attack and release values for a given type of material can take a certain amount of skill, and if the programme material is constantly changing in dynamics, no one setting is going to be quite right. Auto function continually adapts the attack and release characteristics to the material being processed, by monitoring not only the input level but also the rise and fall times of signal peaks. Such systems can be very effective, especially on complex mixes or vocals.

A Good Knees-Up

A compressor has no effect on signals that are below the threshold, but as soon as they reach the threshold, gain reduction is applied. This is known as hard-knee or hard-ratio compression because the onset of compression is sudden and occurs as soon as the threshold level is reached.

Because compression can sometimes sound a little abrupt or heavy-handed, the soft-knee compressor was developed. With this type of compression, gain reduction starts a few dBs below the threshold, but at a very low ratio. As the signal gets close to the threshold, the ratio increases, until at the threshold the ratio is that set by the user. This can often sound smoother and more musical.

Soft-knee compression is often used when the compression needs to be ‘invisible’, such as when you’re keeping a mix level under control, whereas hard-knee compression is used in situations where it does not matter if you can hear the compressor working.

Peak and RMS

The compressor works through a very smart circuit that is measuring the incoming signal and determining when it needs to compress, based upon the threshold. This in turn controls the gain circuit that normally comes in a number of different types, Voltage-controlled Amplifier (VCA), a Field Effect Transistor (FET) or even a valve. Just like the TL-Audio Dual Valve compressor, shown here.

The compressor will behave differently, depending on whether it responds to the average signal levels (RMS) or to absolute signal peaks (Peak).

An RMS (Root Mean Square) level detector works rather like the human ear, which pays less attention to short-duration loud sounds and through fancy magic and mathematics is a means of determining average signal levels. The pros of compression often find that the implications of using RMS control are that the compression will sound natural, but short duration, high amplitude sounds may pass through at a higher level than expected. This can be a problem when making digital recordings, as clipping is to be avoided at all costs.

The difference between Peak and RMS sensing tends to show up most on music that contains percussive sounds, where the Peak type of compressor will more accurately track the peak levels of the individual drum beats for example. In Peak mode, the gain control responds more accurately to brief signal peaks than in the RMS ‘averaging’ mode. This ensures peaks, sudden audio transients like drums, are more accurately controlled.

Side Chaining

So with all that bagged and out of the way. You are now a pro at compression! We can now start to use dynamic effects, such as compression in more creative ways. To begin this creative use some compressors will have what is called a Side Chain function. Essentially, this allows you to take one sound and use it to manipulate another sound. So in the most basic form, you might have a compressor unit on one channel compressing the signal for say a Bass guitar, however the compressor is not reacting to the Bass’ signal but to that of something else, for example maybe a kick drum or snare. This has now opened up a whole Pandora’s box of tricks and gimmicks of how to use the compressor.

In terms of the Bass side-chained to a Kick for example this is a great way to give the bass extra punch through the rhythm of the kick drum. Also, because the two share a very similar band of frequencies, it can give the two more clarity.

Here is an example of a kick drum pattern and the bas guitar playing at the same time.

[audio:http://www.themusicespionage.co.uk/wp-content/uploads/2012/06/9-Bass-and-Kick-Together.mp3|titles=9 – Bass and Kick Together]

This is the Bass Guitar without the side-chained compression:

[audio:http://www.themusicespionage.co.uk/wp-content/uploads/2012/06/10-Bass-NO-Compression.mp3|titles=10 – Bass NO Compression]

Now here is the same section, now with a compressor on the Bass track, this side-chained to the kick. A Ratio of 6:1 will give it a good thump as the Kick jumps in, with a quick Attack to catch the rapid transients.

Here is the Bass with Compression:

[audio:http://www.themusicespionage.co.uk/wp-content/uploads/2012/06/12-Bass-WITH-Side-Chained-Compression.mp3|titles=12 – Bass WITH Side-Chained Compression]

De-essing with Compression

One of the first uses od Side Chaining is ‘de-essing’ vocal recordings, these being the positive ‘S’ and ‘T’ sounds while singing. This is often called Dynamic-EQ, as you are using an EQ to trigger the compressor. To achieve this is real easy, copy a vocal recording onto a separate track. Now on this second track place a band-pass EQ filter boosting the problem areas with sibilance, normally between 2kHz and 5kHz. On the original track have a normally run of the mill compressor. You now go to the Side Chaining menu and select the second vocal track, the one you just copied. This will now be triggering the unit, so the compressor only work on the strong sibilance sounds.

Click here to read about this process in greater detail on our how to mix vocals page. Or jump straight to the related video, on YouTube – Dynamic- EQ on Vocals.

Ducking

Another extremely common use of being able to make a compressor of one track listen to the signal of another is Ducking. You may think this is like Dogging but maybe on a Canal Boat, but it’s not! This method is used very frequently in radio broadcasts. A DJ will have a music track playing; on this channel will be a compressor that is listening (side-chained) to the signal from the DJ’s microphone. Whenever they talk the compressor will drop the volume of the music so the audience can hear what the DJ has to say, normally some rubbish, this making it much clearer and easier to understand.

Parallel Compression and the New York Mix
The final major use of compression is through Parallel compression. This is NOT a form of side-chained compression. It is however a tried and tested mixing trick most often used for achieving loud, punchy drums, but can be used on anything. It gets the name from electronic circuits. Where in Series it would go through the unit before the output, in Parallel it goes straight to output and also through a unit, shown below:

The idea is simple: blend a heavily compressed version of your drum mix with the uncompressed original. The compressed version is slowly blended in behind the original, adding excitement and more drive to the recording. Your asking, ‘Why the New York mix’?, well simply because this method became popular with New York producers and got the name New York Compression. Because you have a heavily compressed sub-mix of say the drums, this would greatly reduce the louder peaks and through the gain you are able to bring up the smaller, softer parts. This giving rise to the name ‘Up wards Compression’, basically three names for the same thing! Producers, trying to confuse everyone!