Mastering Techniques - Using A Compressor - Part 2

This is Part 2 of a series of posts about using compression in mastering - the first post is here. For a more general introduction to using compression, click here.

Choosing the right compressor settings

Now your ears are focused on what the settings are doing, where should you set them ? Easy:

Adjust the controls until it sounds right

OK, that's next to useless, I know. "What sounds right" varies on the instrument or mix, the genre and taste. There is also a big difference between compressing an instrument in a mix and compressing a whole mix.

Compressing an instrument

For example, compressors are commonly used to even out very dynamic (spiky) intruments in a mix, like a snare drum, say. A snare note has a big spike at the beginning (the transient) created when the stick bashes the skin, followed by a longer sustained sound as the body of the drum resonates. If you push this uncompressed signal too high in the mix, the initial transient may start to sound annoyingly loud and percussive or even distort, if you push it into the red. As a result you can't hear enough of the "thump" of the drum to give it weight and impact.

To minimise this effect, you can compress the signal, using:

• a fast attack time - as soon as the signal starts to get loud, the compressor quickly starts to take effect
• a high ratio to have quite a dramatic effect reducing the spiky transient
• a fast release time to allow allow the compressor to "relax" quickly once the spike has been controlled
• a fairly high threshold so the ring of the drum is not compressed as well as the transient

With a little tweaking, the result should be a much fuller, punchier snare sound - the compressor jumps in quickly (fast attack) and holds back some of the big spiky transient (high ratio high threshold) but because the spike is very short, quickly releases again to allow the main note of the drum to sound. Overdoing it will have undesirable results though - for example if the attack time is too fast, it will remove all the attack from the note, making it sound dull and lifeless. But a bit of experimentation will soon sort this out, and as a result, the snare can be lifted higher in the mix without becoming annoying or distorting - the compression makes it louder. It also sound fuller and punchier, as a by-product.

Compressing a mix

Using settings like the ones above almost certainly won't work well on a mix, though. As a rule in mastering, the aim is for the compression to be as unobtrusive as possible. Usually we aim for a natural-sounding result, where the compression isn't easily noticed. Most final mixes have already had suitable compression used on the individual instruments, so the settings described above will probably result in something that sounds squashed and lifeless, or even pumping and distorted.

Remember in what follows that it's very common to use both a compressor and a limiter when mastering - the limiter catches the very fast transients, allowing the level to be lifted even further, but I'll discuss limiters in the next post. As far as compression goes, for a whole mix I often:

• Avoid very short attack times - and very long ones
• Use shorter release times
• Use low ratios
• Avoid large amounts of gain reduction

The medium attack time allows percussive elements of the mix to punch through, but still controls the overall signal level. If this gets too long, you'll hear the "thump-suck" effect I mentioned in the last post. Shorter release times also avoid obvious pumping, but you need to be careful. If they get too short, there won't be enough control and you may hear the compressor "bouncing" - rapidly triggering and releasing over and over, sounding crunchy or distorted as a result. Low ratios and moderate gain reduction avoid the compression becoming too ear-catching. Time for another rule of thumb:

If you compare the compressed signal with the level-matched original, it should sound better

Meaning not squashed or pumping (unless that's what you want !). You should always level-match the "before" and "after" levels before making your comparison, of course. When level-matching for comparison purposes, it's often worth using the vocals to judge the loudness. Our ears tend to latch onto a voice as the most important element in a mix, so get these as close as possible before comparing. Then listen and ask yourself questions like:

• Which do I prefer ?
• Am I sure one of them isn't louder ?
• Have I achieved what I wanted ? (ie. made it punchier, fuller, with more impact & excitement)
• Does the compressed version still sound lively and exciting, or is it too squashed ?
• Does it sound closer to similar tracks I'm trying to emulate ?
• Can I hear more of the quiet details in the mix, or is it getting "mushy" and confused ?
• Does it still sound natural ?

If you can answer these questions with a positive, you're doing well. If not, try varying some the settings and comparing again. If you still can't get a result you like, try a different compressor - or maybe it doesn't need compression at all.

All of this is very hard to describe in words of course, but hopefully by now you'll have a better idea of how compressors are used, what the controls actually do and what you should be listening for.

If you found this post useful you might also like to watch my free webinar on the more advanced technique of multi-band compression - for more information, click here.

 

Mastering Techniques - Using A Compressor - Part 1

There are still more things I want to cover in the area of general DIY Mastering, but I've decided it's time to start posting some specific information up here to help people get to grips with common mastering techniques. So this is the first in a new series of posts headed "Mastering Techniques", which will run alongside the DIY Mastering posts. I'm going to cover things like level-matching, EQ, stereo width adjustment and so on, but I think the subject most people struggle with is compression. Often people have been told that the answer to any number of problems is compression, but don't know what they're trying to achieve, or how to get there.

This post started to get much too long, so I've decided to split it into several sections, this is Part 1...

Know What You Want

In my opinion, the most important thing when using compression is to have a clear goal. You'll see lots of explanations of compression saying things like "a compressor reduces the dynamic range of it's input", but I don't find these very intuitive, and prefer the explanation I was given as a trainee:

Compression is used to make things louder

There can be all kinds of positive side-effects of this process, like making things sound fuller, richer, more controlled or punchier, but at the end of the day, especially in mastering, it's all about loudness. NOT excessive loudness, but something musically beneficial.

Mastering engineers almost always use a limiter somewhere to boost level, but the problem with this is that mixes very quickly sound crushed or distorted using only limiting. Using some gentle compression first means the limiter doesn't have to be hit so hard, giving a more natural sound. More on limiters in a later post.

The ideas here apply equally to compressing individual instruments or voices in a mix, incidentally, but you'll need different settings, usually. First we need to recognise a harsh fact of life:

Not all compressors are created equal

The best analogue compressors cost thousands of pounds. However there are some really good software plugins these days which cost far less. For example if you're looking for emulation of a traditional analogue compressor in a mix, complete with "musical" pumping, I really like Sonic Timeworks Compressor X. Would I use it for mastering ? Probably not. However there are high-quality compressors available on a DIY budget which are suitable for mastering - the TC Electronics System 6000 is a mastering "industry standard", and many of it's algorithms are available as plugins for their Powercore system, for example. Many people also use plugins by Waves or Izotope, and discussions rage about whether it's really possible to master with something so cheap, and which is better, on the Sound On Sound mastering forums and elsewhere.

Regardless of which compressor you decide to use though, they all share similar controls and concepts, and at the end of the day it comes down to whether you're happy with the result you get. With that in mind, lets dive in:

Exploring Compressor Controls

Try this experiment - we're going to overdo everything to begin with, so you get a feeling of what the different parameters do:

• Choose an instrument to compress and solo it (We'll move on to compressing a whole mix later)
• Patch a compressor across the stereo master output channel of your system
• Start with a ratio of 2:1.
• Set attack and release times of 100ms, if possible. You may have to disable "automatic" options first.
• Gradually reduce the threshold until the meters show you 4 or 5 dB of gain reduction

One of two things will have happened. Either:

• The sound will have got quieter, because the compressor is holding back the louder peaks. In this case you need to add some make-up gain, sometimes called output gain. Adjust it until the sound is a similar volume to when you hit the bypass or disable switch

Or:

• The sound will already be at a similar level, in which case your compressor automatically boost the output (make-up) gain

Now toggle bypass on and off and listen to the difference. Depending on your material, you will hear more or less difference. If you can't pick anything out initially, increase the ratio to 4:1 or reduce the threshold until more gain reduction is happening. (If you can't hear 6dB gain reduction with a ratio of 4:1 you should probably get your mastering - and probably mixing too - done by someone else !)

If your compressor has automatic attack and release times, it will probably sound OK (but a bit squashed). Even with manual controls, it shouldn't sound too bad. Once you can clearly hear the difference between the bypassed and compressed signals, you can try and figure out what it is you're hearing.

All the controls interact to give different effects, but before we get to that lets look at each in turn. First, ratio and threshold.

Ratio

Try increasing and decreasing the ratio. Higher values (4:1 etc) cause a more exaggerated effect - the compression "hits harder". Lower values are more subtle. If your compressor doesn't have auto make-up gain, you'll need to adjust it to match the bypassed version for a clear comparison. As a rule of thumb, use a high threshold and high ratio for a hard-hitting sound, but watch out for unnatural results. Lower ratios give a softer, "warming" or "thickening" effect. I rarely use higher than a 2:1 ratio.

Threshold

The threshold control determines when the compressor starts working. Lower values will give more compression, higher values give less. A low threshold with a high ratio will give lots of hard compression and probably sound very squashed and lifeless, whereas a higher threshold and low ratio will be a much more subtle. Whatever values you decide on, there is one rule of thumb worth remembering:

If the gain reduction meter doesn't return to zero several times a bar, you're almost certainly using too much compression

- because this means that the signal is being compressed all the time, and will probably sound flattened as a result. Try a higher threshold, and then higher ratio if it's not doing enough.

Attack & Release Time

Since I specified long attack and release times, you will probably hear the sound being "snatched" away right after a new note comes in, and perhaps "pumping" back up afterwards. Sustained notes are a good way to hear this. If you increase the attack and release times you'll certainly hear these effects. (For a deliberate example of this classic "thump-suck-relax" pumping listen to "One More Time" by Daft Punk). Automatic or "intelligent" settings reduce this effect, but increase the risk of using too much compression without realising it.

Now try reducing the attack time to it's minimum value. The sound will probably duck very quickly away to begin with - perhaps too fast to hear it happen, followed by the longer release back up to full volume. The chances are it will have lost any impact it used to have, especially with a higher ratio, and sound quite unnatural. Reducing the release time decreases the "pumping" effect, but as you get too short will start to sound "crunchy" or distorted.

Now your ears are focused on what the settings are actually doing, where should you set them ?

Answers in the next post.