Is it necessary to use mix bus compression if you've balanced the individual tracks and used volume automation as needed? It's all a matter of aesthetics. Art is subjective after all. When choosing to mix with bus compression it is recommended that it is added at the start of your mixing process so that all mixing is done through the mix bus compressor. In this article, we'll explain how to use master bus compression to improve your mixes and masters. This article is written by a professional audio engineer with over 7 years of experience, so buckle in!
What is A Master Bus Compressor?
Master bus compression is when you apply compression to a grouped mix bus or the master track of your DAW. The term ‘glue' is often used when describing what mix bus compression is. It glues together your mix, making it sound sharper and tidier, and is often used in a way that is separate from how normal music compression is applied.
There are four types of compressor and they each have their own unique strengths which they bring to the table: VCA, variable-mu, FET, and Opto (optical):
VCA Compression uses a Voltage Controlled Amplifier. Their behavior is dependent on the level of a control voltage.
This technology can act as a compressor by denoting the master track as the control voltage. The audio is compressed based on the signal from the master track, and how it behaves is determined by the attack and release.
They are renowned for their lack of coloration, or transparency, and also for having the quickest attack and release settings.
Variable-MU compression utilizes a vacuum tube for gain reduction. The more electric current hitting the tube, the greater the gain reduction. It's worth noting that MU essentially means gain.
As to be expected from a tube compressor, it offers pleasing harmonic distortions.
FET (Field Effect Transistor) compression recreates the sound of a valve using transistor circuits. Noted for their fast and bright sound, they're preferred for rock music, especially on drums. If you're looking for snappy transients then this is the one.
The last type of compression is the Optical Compressor. As the name suggests, it uses a photocell and a light bulb in order to control the gain reduction of a signal. The amplitude of the signal determines the strength of the light emitted from the bulb, which in turn attenuates the output signal.
The response time of the attack and decay are slowed by the delay between the light signal and the photocell. This mellow response lends itself well to processing vocals, mastering and anything that requires milder compression.
How To Use Compression On The Master To Glue Your Track
A compressor is generally used to control the dynamic range of a track. Downward compression will reduce any volume that exceeds a user-defined threshold.
The attack dictates how fast this volume decrease occurs, and the release time controls how long it holds the volume decrease before relinquishing its grip on the audio.
Upward compression on the other hand increases the volume of all audio below the threshold. Both achieve the same effect: the distance between the volume peaks and troughs is reduced.
Most people's understanding of the compression effect relates to two scenarios:
- How it is used on individual tracks, whether applied directly or via a send.
- Applied directly to the master channel, as used by the mastering engineer.
The settings applied will generally be far more subtle, leaving no noticeable compression artifacts. There should be no audible compression as the aim is simply to glue your mix together by adjusting the overall balance. If you can hear any noticeable compression, it's time to dial it back.
This process should merely enhance what is already audible in your mix, such as boosting the low-end, or taming some of the more high-end transients.
SSL G Master Bus Compressor
The SSL G compressor is a legend in the game, and rightly so. It is one of the few master compressors that sound great even with a 4:1 ratio on the master bus. Its versatility is one of the main reasons for its success.
It uses VCA to compress audio and the original hardware was built into SSL mixing desks. Its popularity and recognition led SSL to release it as a piece of hardware that can be incorporated into one's processing rig, and now thankfully it is available as a digital plugin.
To add glue to your mix it's recommended to add a relatively fast attack of around 10ms, with a slower release of around 1. Keep a low ratio, around 2, and the threshold at approximately 6dB. Check the output – if it's pumping at around 1-2dB then increase the make-up accordingly.
The results will be very subtle and therefore it's wise to take the time to bypass the compressor and listen for changes. You should notice the peaks of the vocals and high-end drums such as crashes and cymbals being reined in, while the low end should demonstrate greater clarity.
It can be tempting to increase the ratio and dial in a faster release, but this will simply lead to you having to boost the make-up dial and will make everything sound muddy. Mix bus compression at best is barely audible. Any noticeable compression should be immediately dialed back.
Remember, mix bus compressors are a lens through which your mix should be heard. You will still need to mix and add compression to individual tracks, not to mention send the final product to a mastering engineer for the mastering stage.
Let's take a look at digital transparent compression. Transparency in a compressor is when it doesn't color the mix it is being applied to. In other words, the audible level differences, such as peaking transients, are evened out without altering the sonic character of the mix.
So, how do we achieve this? Start by applying a compressor with a high ratio and a light threshold that's only catching the upper-most peaks. This will tame the most noticeable transients, so that should you add further compressors down the line they won't have to work as hard and you'll be left with fewer audible compression artifacts.
This is essentially transparency. Compression is taking place and tidying up your project while not overtly affecting the mix. We can then add additional compressors in order to create a chain where each plugin handles a particular task. This gives you greater control over your mix without having to rely on one plugin to do all the work.
Master Bus Chain
First things first, you do not need to add any effects to the master bus. As mentioned above, this is a subjective choice and is driven by aesthetic need as much as practicality.
That said, having a chain of effects on the master bus can glue all your tracks together and provide a desirable punch to the proceedings.
In addition to a compressor, you may also add parametric EQ, a saturator, and a harmonic enhancer. It's important to consider the order of your mix bus processing chain as each effect will be processing the previous one.
You need to consider whether you wish to EQ your signal first and then compress it, or the other way around. The order in which you do this can drastically affect your outcome.
A common technique is to add a parametric EQ first and roll off anything below 30Hz and above 19kHz.
Very little outside of this is audible to the human ear and rolling it off cleans up the mix's audio spectrum.
Adding a high shelf EQ to the sides will add depth and brightness without affecting the middle of the stereo field. In this case we are using Slate Digital's Infinity EQ.
Once the EQ is taken care of you can add your compressor and after that a saturator for analog warmth. If you're feeling extra indulgent why not slap a harmonic enhancer such as the Voxengo Warmifier on top.
There's a reason why FL Studio called theirs the ‘Sound Goodizer'. They make everything just sound a little easier on the ear without changing the overall character of the tracks.
Characterful Analog Compression
As we all know, there are no rules to music production. Some things might sound generally more pleasing to the ear, but much of it is subjective.
For instance, you may like the sound of a master bus chain that completely alters the character of your tracks, whereas others see it as an abomination.
An example would be creating a chain that begins with a compressor exhibiting audible analog artifacts (sometimes known as ‘deliberately crushed'), followed by an amp emulator that colors the whole thing in distorted harmonics or flat-out overdrive.
This is commonly applied in Vaporwave, a genre that seeks to transport the listener back in time by replicating the sound of vintage boomboxes and AM radio frequencies.
An added benefit of this approach is that it will sound louder, which our ears often perceive as better. This can sometimes be considered a low-hanging fruit approach to production, and as such many producers are wary of relying on this alone to create a unique sound.
Deciding whether characterful analog compression suits the overall aesthetic of the project should be at the forefront of the decision-making process.
Multi-band compression is the process of splitting compression by frequency band so that compression is only applied to certain frequencies. For instance, you might only want to compress the low end with a fast attack and fast release time in order to boost the kick.
Then applying a compressor to the mid-range with a slow attack time and fast release you can apply a transparent level of compression to a range of frequency that might otherwise sound muddled if the low end was included.
A great alternative to using multiple compressors is a dedicated multi-band compressor such as Slate Digital's MO-TT which divides compression across three frequency bands.
Blending Transparent, Characterful & Multi-band Compression
Now let's talk about blending all three on your master bus. This may seem like overkill to some, but it is not without its merits.
First off, by having a transparent compressor on your master bus you can glue together all the transients in your project so that nothing is out of place or sticking out where it shouldn't be. Coupled with some careful volume automation and you're already well on your way to a smooth mix.
Then by applying a characterful compressor you can add some analog warmth to the overall sound, adding depth and dynamics to a digital mix that might be suffering from a lack of personality.
Finally, by adding multiband compression you can hone in on each segment of the frequency spectrum and bring out the elements that you want to shine.
For instance: you can apply a generous level of upward compression to the high end in order to get the hats sounding nice and sparkly, while perhaps using downward compression on the middle frequencies to damp any vocal or guitar peaks that are coming too fast and frequent for meaningful volume automation to catch.
It's important to always ask yourself if it is necessary, as less is always better than more. Don't add a characterful compressor on your mix bus unless it serves the overall vibe of the production.
And likewise, multi-band master bus compression should only be used if you're trying to elevate and give character to certain frequencies.
How To Set Up Master Bus Compression
First, you must choose a mastering compressor. Each mix bus compressor out there brings different things to the table. If you want a nice, transparent compressor that will glue together any peaks and transients then the SSL G compressor is a great choice
If, on the other hand, you want an aggressive, fast compressor then the API 2500 is a solid selection. It will add character, energy, and movement to your mix. The ‘Mix' dial will also let you blend the wet and dry signals so you're always able to reign in any audible artefacts if you feel you've applied too much compression.
Once you're happy with your choice, open your compressor on the master bus and turn off auto-gain; set the threshold so that it's just capturing the peaks; then dial in your knee, attack, release, and ratio settings.
The knee dictates how fast or ‘smooth' the attack and ratio occur.
Lastly, and most importantly, add the gain that you have reduced via compression back to the output signal. If you're not sure how much gain reduction you've applied just take a look at the meter on your plugin:
In the above image, gain reduction is occurring at just under 1dB, and therefore you should then boost the output by the same amount to compensate, or else your mix will have its overall volume reduced.
Using Compression on The Mix Bus, or Grouped Instruments
Ok, so we have discussed the benefits of mix bus compression. Now let's talk about grouped instruments and the reasons why we would compress them.
It's worth noting that once you have added a compressor to the mix bus, any compression you add to, individual, or grouped tracks, doesn't need to work as hard.
You will obviously need to consider your CPU usage the more plugins you utilize, but thankfully compressors are on the low end of processing consumption.
Just the same way our mix bus compressor glues the whole project together, so will compression added to grouped instruments. Different instruments have different needs, and therefore in order to to get the most cohesive sound, you'll need to have different configurations to get your guitars, snare hits, and vocals to truly shine through.
Drums need a greater deal of dynamic control due to the variation in transients between the snare, cymbals, toms, and kick. You'll need compression with a fast attack and smoother release times.
Guitars should form a wall of noise and therefore require a high ratio of around 10:1, with a slower attack time and a fast release time.
Using a Limiter As a Compressor
Limiters and compressors share many similar qualities. In fact, a limiter is essentially a compressor with a really high ratio (above 10:1). If 10dB passes through the threshold they get reduced to 1dB. When the ratio is set to ‘infinite' it will remove anything that passes the threshold.
If you are looking to add color or harmonic distortion then a limiter is not for the job.
Limiters are used for ironing out instruments with inconsistent peaks, such as vocals or drums. They are quite transparent and often don't even have an attack knob. Anything passing the threshold is clamped down.
You can also use a limiter on the master bus to make the track louder. Start with a high threshold and dial back the output ceiling until you're happy with the results.
Boost your input gain and observe the gain reduction meter to see how much you're clamping down on volume.
Types of limiter
- Full band, limiters as the name suggests, will squash anything past the threshold regardless of frequency.
- Multiband limiters on the other hand let you limit certain frequency bands, much like a multiband compressor.
- True Peak Limiters will clip anything past a certain volume and are generally used in TV and radio where volume restrictions are dictated by industry bodies.
- Brickwall Limiters have their threshold set to infinite and will squash anything that passes the threshold.
What Compressor Should I Use For The Master Bus?
There is no one right answer to this question. Even the top mastering engineers will counter with: “well, what is it that you're after?”
Here are the best master bus compressors to start with during master bus compression:
- API 2500 – this is good for an aggressive sound and works great for rock, combined with other compressors
- SSL G Master Bus – this is the best master bus compressor for smooth glue that will bind your track together
- FabFilter Pro C2/Digital Transparent Compression – perfect for getting thickness and loudness without introducing colour
- Manley Variable Mu – will bring warmth and clarity to your mix. The slow attack and release time aren't great for vocals or drums with unpredictable transients, but this will glue things together with a nice analog colour.
- Fairchild 670 – possesses a lot of vintage character that will colour your mix without having to squash it. The attack time is quite slow and therefore might not be ideal for instrument grouping, but sits nicely on the master bus.
- Comp FET-76 – this FET compressor by Arturia provides some very subtle harmonic distortion that works wonders on warming up vocals and bass.
How Should I Use Master Bus Compression?
To summarise what we've gone through above, the effect of your master bus compressor should be barely audible and provide little to no coloration. It is the lens through which your project will be mixed and should only serve to tame transients and glue the overall project together.
Here are some suggested settings to get started with, which can be tweaked to taste:
- Ratio: 1.25:1 – 2:1. Take a 2:1 ratio – for every 2dB, 4dB, and 8dB that passes the threshold, the compressor will only allow 1dB, 2db and 4dB respectively. Anything beyond a 2:1 ratio therefore will start noticeably squashing the sound, leading to audible artifacts.
- Attack: a slow attack around 10ms will help preserve transients.
- Release: a quick release of around 0.8-1ms
- Threshold: 6-9db
- Make-up gain: Check the output – with these settings you should be balancing it out with around 1-2dB.
How Many Compressors on the Master is Too Much?
As long as the settings are in reason then you can't have too many compressors on the master bus. Each compressor is lightening the load of the next one. That said, is it necessary? Consider what compressors you'll be applying to individual tracks and instrument groups.
Anything from two to four compressors is standard on a mix bus chain, but you also need to consider as well that the track will also be sent off for mastering.
Where Does Compression Go In The Mastering Chain?
The most basic mastering chain looks like this:
In between, you might add a harmonic enhancer or additional limiters and compressors for specific tasks but that's about it. In some genres such as the aforementioned vaporwave, the mastering engineer might even place an amp simulator on the mastering chain to give it a washed-out sound, but that is for a very particular aesthetic purpose.
EQ goes first in the chain as it is a linear processor. That is, the way it alters the audio is not affected by the loudness of the signal. Then after that you apply your non linear-compressors – effects whose output is determined by the level of the input.
In this instance that would be a compressor, and finally, the limiter to attenuate or boost the overall volume of your end mix.