Greetings everyone and welcome to my website Vettaville. This website is dedicated to providing unique information about some of Line 6's modern guitar/amp technology...particularly the Vetta as well the Variax. Also visit the Netherlands Vettaville and join us for equipment discussions at Andy Z's Institute of Noise Forums.

email: nathanshane@vettaville.com

 

 
 

Understanding Pre/Post Compression - by Nathan Shane

  
     
 

ATTENTION

This article is not finished yet, so please excuse if some parts seem to not make perfect sense.

 
 

 
 

Introduction

  
 

Working with compressors is not always easy, especially if you have no real idea of how a compressor works and is actually affecting the guitar's signal. But once you understand some of the basic functions, then it's just a matter of training your ear to hear how your guitar signal is being affected when you use one. So for those of you who are not really used to thinking like an audio engineer, or critically listening to every aspect of the guitar signal/performance, and picking it apart analytically...please read on.

  
 

 
 

Where - Before or After Amp Model?

  
  Where in the signal chain you decide to use compression will affect the final results.   
     
     
 

Compression - For Volume Control

  
 

The basic function of a compressor is to act like an automatic volume knob, to where signal peaks are controlled so that they're not as loud as they originally were, or so they do not exceed a certain threshold. Compression is used to help squeeze the dynamics of those subtle (but often noticeable) volume differences that occur when playing. It's a way to help average out a guitar signal to where it's not too loud or too soft and it maintains a more consistent sounding volume.

  
 

 
 

Volume differences from note to note are dynamic and occur naturally.

  
 

 
 

Also take notice of how the start of each note has a peak of volume.

This initial volume peak is called the attack transient.

  
 

 
 

Some guitar players are very good at controlling the dynamics of their strumming and picking. In other words, they've learned to hit each of the guitar strings with more dynamic consistency. The best way to think about this is like this, if you were to record yourself playing the guitar, doing arpeggios, strumming chords, and lead lines, then listened very critically to the playback, focusing on the volume of each and every note that you played, you'd start to think, I should have picked that note harder it sounds too low in volume compared to the other notes I picked. Or, it seems that every time I play that particular chord it comes out louder sounding than others. Once you've started to develop an ear towards your guitar signal this way, then you'll quickly realize how your guitar signal, your band, and you can benefit from using compression as a tone shaping tool.

  
 

 
 

Compression - For Limiting

  
 

So far we've learned that compressors can help to squeeze (or gently compress) the volume dynamics of a guitar signal into a more narrow range, but compressors can also be used as limiters. And although compressors and limiters perform similar tasks, there is one essential point that makes them different:

  
 

Limiters abruptly limit the signal above the threshold setting.

  
 

The term limiter typically describes a compressor whose compression ratio is 10:1 or greater. When such high compression ratios are used, the audio output remains at pretty much a fixed level. In other words, the output of the guitar signal is limited to a very narrow dynamic range above the threshold setting.

  
 

 
 

A very important point to remember is that high-gain amp models tend to heavily compress and limit the guitar's volume dynamics already. This has to do with the "physics" and inherent nature of distortion. Therefore, no additional compression or "subtle" compression settings will be used. But, rules were made to be broken, so whatever works for you - then go for it.

  
 

 
  The GAIN  
  Let's talk about the GAIN parameter before we move on to the other parameters. Keep in mind that when you squeeze/compress volume differences into a more narrow range, you are typically lowering the overall volume of the signal. The GAIN (makeup gain) parameter is what allows you to turn your newly squashed signal back up in volume, so that you can makeup any gain you lost when compressing the signal.  
     
     
     
     
 

The THRESHOLD

  
 

Compressors gently compress the signal above the threshold setting

  
 

So in order for a compressor to be setup so that it DOES NOT begin any compression until the input signal rises above a certain level, there is a threshold setting. This setting is defined as the point at which compression begins. Guitar signals which fall below the threshold setting pass through unaffected, those which rise above it will start to be compressed according to the compression ratio you've selected.
 
     
 

It may not always be necessary (or even sound good) to apply compression to a guitar signal all of the time. 

  
 

It might help to think of the threshold setting like this:

  
 

Look up at the ceiling in your house/apartment and pretend it's parallel with the floor (humor me if it's really not), think of that ceiling as a threshold and it's current position as its setting. Now look at the furniture, think of each piece as a different guitar note or chord that's been played. So right now, the ceiling is above everything else in the room, it's not touching any of the furniture below, therefore it has no affect upon anything...yet.

  
 

Once you start to lower the ceiling, or once you begin to lower the threshold setting, things then have the potential to be changed. So it should suddenly occur to you that if the ceilings position keeps on lowering, then the furniture below is going to start to be squashed. But wait, not everything will be squashed all at once, some of the pieces of furniture are taller (or louder in volume), and some of the pieces are shorter (or lower in volume). Therefore, the tallest pieces (loudest) will be squashed first.

  
 

Of course, in the real world of audio, guitar signals are typically dynamic in volume.

  
 

 
 

The RATIO

  
 

The RATIO setting is what determines the ratio of volume change that will occur in the output (when compared to the input). It works in combination with the threshold setting. When the signal exceeds the threshold setting, the output level is reduced above the threshold proportionally by a certain amount determined by the RATIO setting.

  
 

A ratio of 1:1 (one to one) means that what goes in is what comes out, no volume changes. A ratio of 2:1 means that if the input signal increases 2dB above the threshold setting, there will only be a 1dB increase in the output level.

  
 

 

  
 

Please keep in mind that the higher you set the compression ratio, the more that you run the risk of squashing too much of the volume dynamics out of your playing.

  
     
     
     
 

 
 

POST Comp: ATTACK & RELEASE

  
 

Too quick of an attack setting will often give your guitar a rather dull and "lifeless" sound because it cuts off the sharp musical attack (transient) that occurs when you first strike the strings (recall our first image above).

 

It's this initial attack transient that gives the guitar part of its sonic signature, so it's not always sonically good to compress the life of of this transient.

 

Too slow of an attack setting can allow the very volume peaks or transients you are trying to control to pass through the compressor without being compressed at all.
 
     
 

Remember: there is no specific attack or release setting that is optimum for all situations. 

But extreme settings can certainly hurt your guitar signal.

  
 

 
 

Remember, once your guitar signal has fallen below the threshold setting, the compressor wants to restore itself Release settings can often be much more forgiving in how they affect the overall sound of the guitar signal. But if the guitar signal is too heavily compressed and too short a release time is selected, than 

  
     
 

Comp Stompbox Sustain

  
 

The image below shows you exactly how a compressor can be used to change your incoming guitar signal for more sustain. The other comp stompboxes could be used as well, but for this example I've used the Vetta Juice stompbox. Vetta owner's have consistently commented on the Vetta Forum about how they often find themselves using the Vetta Juice much more than the other comp-stomps included in the Vetta. Here's why I think that is. There is one " essential feature" that sets the Vetta Juice apart from the other comp-stomps, it has a fixed threshold and variable compression ratio...whereas, the Boss CS-1/CS-1 Treble, MXR Dynacomp, and Vetta Comp all have a fixed compression ratio with a variable threshold.

  
 

When checking the owner's manual we can clearly read that the Vetta Juice Comp has a fixed threshold of -40dB with the SENS parameter varying the compression ratio from a gentle compression of 1.5:1 all the way to a limiting ratio of 20:1 (which is a lot of squash). 

I used a parameter setting of 12 o'clock for the SENS (compression ratio) and 12 o'clock for the LEVEL (gain). Look at the picture as you listen for the sustain difference. Since pickup strengths vary, you will need to dial in your settings by ear, trial and error. 

Click HERE to listen to this example.
 
 

This fixed -40dB setting works as a good "generic" threshold

  
 

 
 

Additional Thoughts

  
 

One of the more trickier things to set is the pre-comp stomp to add sustain to your incoming guitar signal, these settings are entirely dependent upon the signal level coming out of your guitar's pickup(s) into the amp...you'll need to experiment with the settings, and I'd suggest using a cleaner/crunch type of tone that will allow you to hear the differences when you're adjusting the parameters...if you are starting out using a saturated, over-driven, super distorted amp model settings to begin with, then you can almost forget about being able to distinguish how to set the parameters for the best results...remember, pre-comp stomp can be used for added sustain, a boost in signal, and to help keep your guitar signal be more consistent in it's volume since no one is capable of striking chords and notes with the exact same velocity every time to produce a consistent output volume.

  
 

 
 

More Additional Thoughts

  
 

When you use a stompbox comp "in front" of the amp model, you can affect two things:


1. help to re-shape the sustain of the in-coming guitar pickup signal.
2. adjust the actual volume of the in-coming guitar signal up/down in amplitude before it enters into the amp model.

Therefore, you can make your guitar signal to appear to be a "hotter" pickup before it hits the amp model...that is why you can use the pre-comp stomps to boost the amp model into CRUNCH or DISTORTION depending upon how much you turn up the comp's level.

**as an interesting "reverse" way of thinking, you can also take a really distorted amp model, and make it sound cleaner and less distorted, by using a pre-comp stomp which has it's LEVEL parameter turned down...in other words, you use the pre-comp stomp to turn down the in-coming volume of your pickup before it hits the distorted amp model, and since it's feeding a less hotter signal into the amp model, the distorted amp will have less distortion.**

Look at this like this...you can achieve the crunch or distortion in combination of many different ways, but here are the three basics:

1. turn up the DRIVE on the amp model
2. use a distortion stompbox before the amp model
3. use a comp stomp to increase the pickup volume going into the amp model.

If everyone were to go back and look at the Satriani Comp Lead that I created (downloadable from HERE on ION), you'll see that I used the Spinal Puppet Amp Model with the drive turned significantly down...down far enough to where it doesn't produce the distortion you hear in the patch...then you'll see that Stompbox1 is the Vetta Juice, which is first in the signal chain to make the incoming pickup signal "hotter" before it goes into Stompbox3 which is the Screamer, which adds it own flavor of distortion to the signal before it hits the amp model.

And something else to keep in mind, is that higher distortion amp models tend to produce their own natural compression, it's the nature of distortion, and therefore using a post-comp on the final ouput signal can often cloudy and make more mushy your final output signal...therefore, if you want sustain and/or consistency of signal...try getting it with a pre-comp stomp before the amp model first...then try it post if that doesn't work for you.