Compression is a tool which compresses (reduces) the dynamic range of a signal. It developed as a tool in studios but quickly found its way onto our pedal boards. Unfortunately, amongst guitarists, the compressor has a reputation for having limited uses – usually used as a sustainer or to get a ‘country’ squashed attack. By looking at compressors from a recording engineers point-of-view we see that they are far more versatile than we might have first thought.
But first, some simple background info for those new to compression…
What Compression Does
Dynamic range compression is essentially an automatic volume control, where loud sounds (above a given threshold) are automatically reduced in level. This means that the dynamic range of the output signal is less than the input signal – hence the name compressor.
Its important to understand that compressors only pull down loud sounds – they are not designed to raise quiet sounds. As such the overall output of a compressor circuit will be lower in level than the input. To counter this loss of level, compressors are typically fitted with an output gain knob – commonly labeled ‘makeup’ gain since – to bring the overall signal back up to a desired level. Thus, the net effect of the compressor is that the low-level sounds are increased in volume. Bear in mind that noise is a low-level sound, so compressors will also raise the noise floor.
How Compression Is Used
Typical guitar pedal compressors are very limited in features compared to rack-mount studio compressors, so only some of the following knobs are likely to be found on any one guitar pedal:
- Output Level/Make-Up Gain
Of course, true ‘studio’ rack compressors have far more functions than those listed here, but these are the basic controls which you are most likely to encounter.
Only when the input level exceeds the threshold will the compressor begin to attenuate the level – signals below this threshold are normally not effected by the compressor circuit. So a lower threshold means the signal is being effected more often, whereas a very high threshold will only be affecting the very loudest parts of the signal.
Unfortunately, on all but the best guitar pedal compressors, the threshold is usually set at the factory, and there is no way for the end user to adjust it. The only pedals I’m aware of with a threshold control are the Carl Martin Compressor/Limiter,and the BYOC 5 Knob Compressor. The Joe Meek FloorQ uses a ‘Input Gain’ knob which serves essentially the same function as a threshold control.
The ratio controls the output level relative to the input level. For example, 3:1 would mean that an input signal 3dB above the threshold, would result in an output level only 1dB above the threshold. Again, for all but the best guitar compressors, the ratio is usually set at the factory. Both the Carl Martin and the BYOC have a ratio knob as does the Joe Meek though this pedal has the ratio control marked as ‘slope’.
Also EHX’s Black Finger and White Finger both have a switch marked as squash/norm which is similar to switching between a high ratio (squash) and a lower ratio (norm). Of course, since this is a binary switch not a knob, its either one or the other – you can’t have all variations in between.
Attack and Release
The attack setting is the time that it takes for the compressor to begin acting on the signal once it has passed the threshold. For instance setting a very short attack time will cause the attack of the note (the ‘picked’ part of the note) to be compressed. This is common for chicken pickin’ and funk guitar sounds. Conversely, for a more natural compression which preserves the natural attack of the guitar, a sufficiently long attack time will ensure that the attack passes through un-compressed and that the compressor only acts on the sustain of the note envelope.
The release is exactly the opposite to the attack control, meaning that it is the time taken for the compressor to reach zero gain reduction, once the input signal drops below the threshold. The release time should generally be set as short as possible provided that ‘pumping’ is not audible. ‘Pumping’ is where you can hear the compressor turning the level back up to zero gain reduction.
Most guitar compressors have the attack and release controls combined into a single knob, which is the case for the Carl Martin, Boss, Behringer, DOD, and Digitech products. Both the EHX Black Finger and White Finger have a switch for Lamp/LED which essentially switches between two different preset attack times. Vox’s “Snake Charmer”, the BYOC 5-knob and the Joe Meek “Floor Q” are the only guitar pedals that I’m aware of with independent attack and release controls at the time of writing.
The ‘Sustain’ Knob
Many stomp box compressors have a ‘sustain’ knob. In reality there is no standard function for a sustain knob because the sustain due to compression is actually a combination of the ratio, threshold, attack and release settings. As far as I’m aware, most of the pedals with a sustain knob (such as the BOSS, Behringer and Digitech) are designed so that full counter-clockwise is a limiting function, while full clockwise provides maximum sustain.
For the limiting function the ‘sustain’ knob would shorten the attack as much as possible, and raise both the ratio and threshold. But at full counter-clockwise both the ratio and threshold would be quite low and the attack time would be lengthened. Since these pedals also have a dedicated attack control, I can only assume that the attack control sets a range of possible attack times, from which the final attack time is determined from the setting of the ‘sustain’ control.
Chances are that the release times are automatic – i.e. program dependent – which is a sensible choice for a guitar stomp box compressor.
Where Should the Compression Go?
The typical answer is compression belongs at the start of the effects chain (or maybe after filter effects), but such a simplistic view can limit your tonal options. Compression is used to control and modify dynamics. And wherever dynamics need to be controlled – compression may be suitable.
In recording studios it can go right at the front of a recording chain – to ensure hot levels, without distortion – or it can be the final processing on a complete mix prior to CD replication. Compression has a use at every point along the signal chain.
Think back to Lead Guitar Tone Part 2 where I talked about the balancing act of complexity and interest, versus focus and clarity. Compression increases apparent loudness (focus) at the expense of expressive dynamics (interest). So what you may gain in thickness of tone, you could lose in expression and ‘feel’. Its always a balancing act, so use your ears to make the final judgment.
So, where should compression go? Anywhere dynamic range needs evening out.
Evening-out Distortion – Compressing at the Front of the Chain
Many pre-amps and OD/Dist pedals are designed to be dynamically responsive, where picking hard increases overdrive, but playing gently cleans up the tone. But depending on the song or player, this can be an unwanted feature.
Putting a compressor at the front of the effects chain controls the dynamics before the signal gets to the pre-amp/overdrive/distortion. This gives the player a solid and predictable amount of crunch. This is an easy way of cleaning up poor picking technique, and is a way of driving the pre-amp harder for longer, but without overly clipping the attacks of the note. In this case the compressor is used to produce a even predictable overdrive. This creates a more focused driven tone which is particularly useful in denser arrangements, or when the guitar part is not the center of attention, such as crunchy chordal backing (but really I can’t think of anytime that the guitar shouldn’t be the center of attention, right up front in the mix 🙂 ).
Evening-out Overall Dynamics – Compressing at the End of the Chain
In some contexts, particularly live band playing, dynamic players may have an unpredictable base level. This makes life hard for fellow band mates and for the front-of-house sound engineer. In these cases, compression is an effective way of decreasing your dynamic range from, say, 40dB to 15dB or even less. This way picking softly still seems like your playing softer, but in actuality your level doesn’t drop out of the mix. Conversely, picking hard will create the impression of playing harder/aggressively, without swamping out your band-mates.
Certainly, putting the compressor up front will even out the levels too, but for players who prefer a dynamically responsive, expressive distortion, it makes no sense to put a compressor before the overdrive. From a practical point of view, compression at the end of the chain can help you sit better in the mix, and ensures that no matter what amount of drive (high gain, crunch, or clean) you can always have a predictable output level.
Bear in mind though, if you go this route, you may want to have a boost pedal after your compression so that you can kick in a little extra level for solos – otherwise, without the boost, your lead tones and rhythm tones would have about the same level.
Make up your own mind about where to put compression. You shouldn’t necessarily put compression at the front of the chain just because everyone else does.
Although the attack control essentially exists to allow the attack to pass through unaffected, it is common for guitarists to use it for exactly the opposite. By setting the attack time to zero (or as close as possible), and the ratio and threshold fairly low, you can almost completely remove the attack from the note. This creates a percussive ‘thud’ rather than a ‘twang’ (highly technical terms 🙂 ) at the beginning of the note. This effect is common in funk music or country.
Unfortunately ‘thud’ and ‘twang’ probably aren’t helpful descriptive terms for someone who has never heard a squashed attack, so heres an audio clip which should make more sense:
mp3 coming soon.
The attack of single notes are different to the attacks of strummed chords. Similarly a guitar pick has a different attack to finger-picking. And lets not forget that different guitars, pickups and string gauges all have differing attack characteristics. But if you like to keep your attack fairly ‘transparent’ and uncompressed, it will be virtually impossible to find a single good attack setting to suit all of these different scenarios.
As I said at the beginning of this article most all compressors work by reducing the level of the louder sounds, which may include the attack of the note if the ‘attack’ knob is not set properly. Then, by boosting the entire signal we end up with the net affect of boosting low-level sounds. But if we want to have a nice, transparent attack, where one setting works equally well over a variety of input signals we need a compressor which works the other way around – a compressor that boosts quiet sounds without first reducing louder sounds such as the attack.
So, in this instance, an ideal compressor would be one which left the attack of the note alone completely and only worked on the sustain and tail portion of the note… enter parallel compression.
Parallel compression is where the original (uncompressed) signal is mixed back in with the compressed signal. This restores our attacks and transients to their original state, and by increasing or decreasing the mix level of the compressed signal we effectively have a single knob controlling how much ‘tail’ or low-level signal we want dialed in.
This means that if you need to have just one compressor set up for a variety of situations or guitars, and want only to thicken the tails while allowing the attacks to pass through virtually untouched, parallel compression may just be the ticket.
Unfortunately there are very few guitar compressors with a dedicated blend control for mixing in the uncompressed signal. The only two that I’m aware of are the BYOC 5 Knob Compressor and the Barber Tone Press. Alternatively you might like to use a regular guitar compressor and then add a blender so that you can mix in the uncompressed signal. Good blenders include the Xotic X-Blender and the Morely FX Blender
And that pretty much concludes everything about basic lead guitar tone, since we have already covered EQ, overdrive, and distortion voicing in previous posts. In the later installments I’ll look at time based effects such as delay and reverb, and also some of the more ‘genre specific’ effects such as filter effects, including wah and phaser.