How to test your compressor plugins. Which one is really best?

I am just passing this information on. It is in this latest issue of Audio Technology written by Mike Stavrou a well known engineer here in Australia.

We all have a collection of digital plugin compressors. And why is it that some might impress us and others not so much. This is a test that will find out which compressors are best for use in our productions.

Music compression is all about the shape of its movement. This can be difficult to hear. There is the dynamaic movement of the compressor and artifacts that are also created. This test exposes the inaccuracies and strange anomalies in bad compressor designs. While the music is present and it is constantly moving up and down it can be hard to hear the volume changes that are attributed purely to your compressor. To eliminate any confusion the idea is to eliminate the music leaving behind only the volume shaping the compressor is performing.

What to do:

Find the right source. A single instrument or a punchy complex dynamic uncompressed mix is also good.

Insert this on a track. And copy it to an second track. Invert the phase of the second track and adjust volumes for a complete null. Faders should be at unity.

Insert test compressor on second track. Perhaps first set the compressor so it is not doing anything. (eg high threshold low ratio etc but still inserted and operating) Check for null. A small delay in the plugin will prevent a complete null. If this is the case then try advancing the second track a few ms or so and see if that improves it.

What you will hear:

Each time the compressor grabs hold of the sound and reduces its gain the source will jump into view. You should not hear and clicks that are not part of the music. Any clicks farts or noises generated by this particular unit are not a byproduct of the compression itself. These strange noises, thumps or distortion are being added by the workings of this compressor and will appear in your mix.

If you use this same compressor plugin on 6 individual tracks than these things multiply as well. Some compressors are better than others. Instead of smooth or quick ramping levels you might hear the swell made of jittery steps. If it jumps all over the place it is bad. You can smooth this by changing release time (200 ms or more)

It is a great test to do on demo versions before you invest in some expensive compressor plugin. Test all the ones you have. You can use this test to also setup a compressor in mastering as you normally would then do the null test to see how it effecting your audio. Then fine tune for better results in the null state.

What to Aim for:

You play with the controls to find the most elastic setting where the sound rushes towards you and away with a spring in its step. Try to get the movement to compliment the rhythm of the source. That is when compression feels the best. When it bounces along with the energy and the vibe of the track. Then the compressor becomes the finishing touch to your production instead of the first stage of accidental destruction.

Take turns inserting each of your compressors and limiters into the second track and hear with clarity the vast differences between them. Expensive analog compressors have a smooth rubber band like action. But does not mean we can't get our best digital plugins to do the same.

Sounds like an interesting exercise.  

Listening to a difference signal and trying to draw any subjective conclusions is never ever a good approach.

Null tests are a very good way to establish whether 2 signals are different, and to what degree. And you can potentially analyze the difference signal to, for instance, see if it's random noise or restricted to a certain frequency range or something like that, or by comparing it in microscopic detail to the original signal, or by using test signals.

But you can't really get anything out of just listening to the difference signal on its own because you don't know whether what you're hearing was added to or taken away from the original signal. 

And you can't tell whether what you're hearing is even audible in the context of the original signal, particularly if the difference signal is amplified (as it usually is). Some people may want to argue this, but, IMO, it's a terrible argument - if the differences are so easily audible, why bother with listening to the difference signal at all?

I setup in both Sonar and Studio One for these tests. You do have to tweak the volume of the track with the plugin inserted for finer null tuning for each plugin. You don't have to alter track delay or advance settings very often either. When I do this I do it in Studio One as it is easy there.

I found that certain compressors performed well. The standard Sonitus compressor did well. The Sonitus multiband not so well. The LP64 Multiband EQ also did very nicely. Could get to bounce in and out very well. A few other compressors and limiters did not do well, especially the limiters. Very jerky response. Won't use them again after hearing them in this situation.

It is interesting when you do get something like the LP64 working nicely it is amazing how much the attack parameter effects things in terms of that bounce feeling. Slowing it down means the compressor is not doing so much and the result is mostly quiet. Speed up attack and you start to hear it more often. It is very interesting playing with all of the (compressor) parameters without the music being present most of the time.

Other things show up some interesting results like tape simulation. Some of them added some not so nice distortion and artifcats. It exposes what your plugins are adding.

EQ's are interesting. When you effect a band (boost or cut) you only hear the frequencies in that band. You really get to hear what parts of the spectrum you are effecting with eq.

I have found with digital plug ins that the attack and release is always a clouded affair. Mild attack and release seem to work fine but most break down after even a 4:1 ratio and around -6 gain reduction. They just sound smeared past that point. One notable exception is the Precision Buss Compressor from UAD which can be pushed farther than most. I have not found even one that will emulate an 1176 in all button mode. Most get really smeared when trying to duplicate this sound. There is not the tight grab and slow clear release that hardware provides.

Another challenge is the depth & weight of the sound that transformers impart. A lot of plug ins don't map to the real world counterparts in the low mids and bottom end. That said I agree that the Sonitus is a good digital compressor. Neutral and effective but unfortunately not enhancing which is sometimes a desire. Overall in my experimentation, digital compressors, when applied throughout a mix as you would real compressors, give a haze and cloudiness to the sound which at first may be appealing but when compared to commercial music makes it obvious that transparent compression in hardware is a better approach.

Jeff Evans

A few other compressors and limiters did not do well, especially the limiters. Very jerky response. Won't use them again after hearing them in this situation. 

Jeff, this is exactly what I would predict, and illustrates the problem with this approach.

Want "jerky" response? Here's the recipe:

1. High compression ratio
2. Hard knee
3. Peak (vs. RMS) detection
4. Fast attack
5. Fairly high threshold

Sounds like a typical peak limiter to me. It's "worse" only because it's doing something you've artificially defined to be "bad". Even though it's doing exactly what it's designed to do. Even though it might sound fine when you listen to the actual audio.


If you want to use a difference signal to find which areas to listen to more closely in the actual audio, that might be a reasonable approach. But it's not a "real" signal you're listening to here, it's a difference signal, that's all. 

Making any subjective judgments on an artificially constructed, out of context difference signal doesn't make any sense when you can just listen to the actual audio. 

I live by one rule when it comes to compression - Compressors are meant to be seen and not heard.

I feel the same about reverb and EQ. But that's considered "old skool" these days. Nowadays the compressor is a rhythm instrument.

Compression reduces the dynamic range of a sound (by attenuating the highs).  This gives you a more consistant sound.

This can be particuarly important for real sounds (like vocals ) so you can control how they sit in the mix.

Many compressors also tend to "glue" multiple sounds together so they all sound like they are part of the same song.   I'm not sure exactly why they have this effect (maybe someone that knows more about audio theory can answer).

So for example with drums, you might put gentle compression over the whole drum mix to glue them together.  Same with multiple vocal taxes etc.   And of course most people do some gentle compression over the whole mix.   

I'm not a hard core guy either way, but I do find it quicker for whatever reason to find a nice sound with an outboard comp. Some of it is that they often have quite distinct personalities and if you know what those are you use them for the things that that would be good for. That probably starts you off a certain way there. Some of it probably is that you can easily grab knobs and adjust physically. And they do seem more forgiving sometimes.

BTW, the thing about compression working best with it doing X, Y, or Z is another one of those things that immediately runs up against the 'whatever sounds good' rule. It works best when it sounds best in the context. Sometimes, flatting something out heavily is exactly what you want. It's often done with drum room mics for instance. And sometimes you want to make it not jump out at you, and give more emphasis to the after ring or air or whatever that follows the initial attack.

And, sometimes you very much want it heard and want to abuse it. This is not at all a new phenomenon, and it goes back to the 70s at least. Even a band like CSN&Y would sometimes use hugely compressed acoustics. Lady Madonna from The Beatles has a hugely compressed piano.

I think that what really is the goal is to achieve a variety of textures that layer over each other in a nice way to create depth and space. That means that some things may get smashed and others punched up and others with tempo based emphasis, some bright, some dark, some mid-rangy, some scooped out, some dry and some ambient, depending on how you are layering things.

This is one thread that in my opinion more people should perhaps look into and try. This is not my idea but rather Stav's but again I used this technique recently to determine how a certain compressopr plugin was behaving.

It is much harder to hear how a compressor is working and what it is doing to your individual track or mix when the music is present! By removing it, you only hear then how a compressor is behaving and very clearly too and only when it is engaging. A jerky peformance can be readjusted to become a smooth movement toward and away from you thus when you put the music back in it all seems to sound much better.

Many people have asked how to set up a compressor properly and this is certainly one way to do it. It is not too difficult to set up the track and routing etc to create the null.

"Rise from your grave..."
 
I just came across this thread, and I find it interesting. However, I am not finding the particular faults, as described by the OP, in any of my compressor plugins.  Maybe I am, but I need a good, bad example to know what to listen for.

I tried the Sonitus Multiband compressor, but the only thing I really heard was that it really didn't do much.   On the other hand, I tried the single-band Sonitus Comp, and it sounded very good and clean with no defects.  Martin Eastwood CompressivePro was about as USELESS (capitals for a reason) as it can get! There was no real change in sound or defects, but the controls didn't do anything! FXpansion DCAMFreeComp...just a "tinny" sound that was always present, probably caused by the plugin's delay and a dry signal (i.e. some parallel compression) always present in the mix. Variety of Sound's DensitymkIII performed well, with relatively light compression and a decent attack--the sound seemed to ebb and flow well. This one is actually regarded as a good, freebie compressor plugin.
 
Fast release times and a high ratio on the PC76 introduced "sputtering noise." This however, could be a characteristic of the real-deal Holyfield, so perhaps it is OK in terms of the quality being true to form. The PC4K, on the other hand, sounded clear and clean, but not very aggressive.
 
Melda's MCompressor was very "sputtery" when set to fast attacks and moderate release times. It only sounded good when the knee was set to super soft.
 
Fish Filet's Blockfish put a lot of bass into the difference mix. It also added some "swirling" type noise into the mix that could be heard if low cut was enabled. It also wasn't very tweakable, as the only setting that sounded decent was to have the compression knob remain at the halfway mark.

A demo of Fabfilter's Pro-C was very clean and all controls worked as they should, yielding very smooth and flowing results. Obviously, this is a high-end plugin with much praise. Overall, it was very tweakable and predictable.

This test was also very useful to understand how the ProChannel Console Emulators work. I could definitely hear the saturation/distortion if the Drive knob is pushed way up. I guess distortion is easy to hear, but because it's subtle, it may not be obvious on the channel or in the mix.
 
So maybe I did the test correctly and heard the right things. What do you guys think?

I would think that they're all different, and can be used for different applications? I like Fab... But I also use Alloy and it's compressor(s) and I like PSP Vintage Warmer for acoustic guitar? I'm sure there are others that are really sweet, these are my favs. 
 
I did enjoy this,
"If I understand the use of compression right, is it mean to be used to shorten the width of the sound in the stereo spectrum or sound of a song? I dunno, For some reason I can not hear any difference using compression. To me it's more or less been a glorified volume increasing utility."
 
That was pretty funny.

My best is Logic's own compressor. I did the above test and it confirmed that it behaves adequately and doesn't add anything nasty to the mix.
 
At this point, it's practically the only compressor I use, because it does it all - VariMU, Opto, VCA, FET, etc. and it does it extremely well.
 
Others may offer instant gratification and nicer graphics, but the truth is, the more I use this one, the better I know it and the closer I get to instant gratification. Even on the stereo bus - I prefer it to every bus compressor I've tried except maybe the Duende SSL. 
 
Good enough for me.

It is not so much about which plug-ins are bad and which are good. It is about setting up your compressor to give you the right result. Something many don't know how to do and this test goes a long way towards getting it right.
 
A really good plug-in can be set badly and it will perform badly in this test just as an another compressor (more average or less expensive) can be set to work very smoothly under this test. (eg Rain's Logic compressor which I agree with him is very good in fact)
 
If however you cannot get your plug-in (good or average) to perform well under this test then it is a matter of perhaps realising it is a not so good plug-in after all and you may be better off looking for something different.

Interesting how Tom picked some plug-ins that did not perform well.  That mixmkr is exactly what I mean.  If the attack part created some artifacts, adjusting the attack will usually fix the problem.  The fact they could not means there may be something seriously wrong in how these plug-ins are handling the attack part of the sound.  And these artifacts do get added into your signal source and are adding unwanted sounds.  You just don't notice them so much while the music is present but when you take it away suddenly it becomes quite clear.
 
BTW Tom keep persevering with various parameters until the attack nasties go away. Other factors can influence things such as ratio (were you using a very high ratio) and threshold as well. You might find these plugs ins will suddenly come good when you start backing some of these other parameters off a bit.
 
If you however you cannot do anything to reduce the attack artifacts then simply keep that in mind before using them.