What Is...Signal Processing

This article describes what a signal is, what processing one means and goes into some detail on how and why you might want to use the compressor.

Ultimate Guitar
People are always saying you should compress this, add reverb to that, try a little eq boost here. Signal Processing they say, its the way forward. What are these people talking about? Do they even know themselves? and... Can these things help me to make better sounding music? The answer of course... is yes and no! But first, let me explain a few things...

What Is A Signal?

In normal life, a signal is a means of communication. It's a way of one person letting another person know something. It could come in many forms, a wave of the hand, a shoutthe raising of a flag. In the context of audio and musical equipment, a signal is very similar. The sound of an instrument, or a voice must be conveyed from one piece of equipment to the next - in the form of an electrical message. For instance, from a microphone to a PA mixer and then from the mixer to the amp and speakers. The sound vibrations are converted into electrical messages and then back into sound again when they reach a speaker. They contain information such as amplitude (that's the loudness of a sound) and frequency (that is the pitch of the sound). There are many subtleties to the information in these signals that allow different sounds to be moved from one piece of equipment to another and still retain a representation of the original sound as heard by the unaided ear. The physics of sound is a fascinating subject in itself and just about every piece of audio equipment out there owes its functionality to a solid base of science. They say that Mozart was a brilliant mathematician, and that the parts of brain used for music and math are similar - and on many levels Audio Engineering is an excellent example of this idea. Technical and Artistic decisions often go hand in hand, so lets have a look at some technical stuff in action and how you can use it artistically.

What Is Signal Processing, What Is Compression, And Why Bother?

Compression is a type of Signal Processing. That is to say that a signal goes into a compressor, is altered (processed) and then leaves slightly or radically different. Signal Processing is the act of changing a signal in some way. That could be to make it louder, quieter, to adjust its pitch, its tone...the list is endless and its getting longer all the time. People never seem to get bored of making new ways to bend sound.

The Compressor

What a compressor does it to reduce the dynamic range of a signal. Dynamic Range refers to the difference between the loudness of different parts of a signal - the loud bits and the quiet bits. The highest peaks (the loudest parts) above a pre-set value (the threshold) of a sound are lowered in level by a pre-set amount (the compression ratio). A compressor listens for the loud parts of a signal and brings the level of these parts down so that the whole signal, both quiet and loud sections, sound more similar in level (loudness, volume).

Why Would You Want To Do This?

Well if you wanted to increase the volume of a whole recording that had a very loud sound in the middle, which caused distortion when you tried to turn it up, you could use compression to reduce the relative loudness of this sound. Then you could increase the gain of the whole recording by the same amount as the reduction you made to the peak levels. Overall, your recording would now be louder than before, and your problem section should not cause distortion. Compression is often used for vocals, as vocal performances can have a very wide dynamic range, from very quiet sounds to very loud ones! In the context of a mix, with a whole band playing in the background, the quieter sections may be completely lost - or the louder sounds may sound out of place. Compression can be part of the solution to this problem. A drumbeat for instance, usually has a few very loud sounds and some quieter ones. If I were to record a drum kit with a single microphone placed in front of the kit about 2 metres away, the chances are I would find the snare drum to be the loudest sound, followed by the cymbals and hi-hat, then the kick drum and the toms. That's not always the case, but in my experience this is quite regular. I might therefore be unhappy because the snare is punching right through my recording of a whole band and the rest of the drum kit is getting lost beneath the other instruments. If I try and turn up the drums to compensatethe snare drum pushes the meters into the red and begins to distort! ArgH! That's unacceptable! So, I could choose to compress the drum track. Using the two main settings of the compressor (which I will assume to be of a single band type, and I will discount attack and release times for the moment) the threshold setting and the compression ratio setting I can adjust the drum track so that it sits nicely in the mix and doesn't sound too flat either. Its important not to over compress the sound, as that can sound worse than the uncompressed version - so I have to experiment to find the right settings. The threshold setting allows me to select the level at which the compressor will start to work. By listening and watching the level meters I can adjust the threshold to a suitable level (lets say -6dB, or 6 dB below the Full Scale zero of the meter). This makes the compressor work when the snare is hit but not at any other time. A setting like this will target the loud snare hits and lower their level whilst allowing the rest of the drums to remain at or close to their original level. The compression ratio setting will determine by how much the level of the compressed material is reduced. If I choose a very low compression ratio, for example 1.5:1, then the compressed signal will be reduced by a very small amount. If I am more aggressive with the ratio - and set it to 6:1 - I reduce the compressed signal by a much greater amount. Over compressing will make the drum track sound flat and dull, as well as making the compression audible to even an untrained listener - most people can tell when something sounds unnatural and this is not the desired result in most cases! If you experiment with different settings, you will soon learn to listen for over compression and at the same time find the setting that achieves the result you are aiming for. There are many applications for compression, and it is one of the most important and widely used processing techniques at the disposal of an audio engineer. The crucial point to make here though, is that the art of knowing when to use compression, and how much of it to use is more important than the quality of your compressor or the fact that you have one at all.

So, In Review

01. Compression is used to reduce the dynamic range of a signal. 02. The results of compression can make your separate sounds sit better in a mix. 03. It can also be used to facilitate an overall gain increase without distortion. 04. It can be used to even out inconsistencies in a performance. 05. There are two main settings:
  • Threshold - that's the level at which the compressor starts to work, and
  • Compression Ratio - the amount by which the compressor will reduce the level of a signal. 06. Knowing when to use it is the key! Perhaps I will tackle EQ next, and my apologies if I have made any glaring omissions, gone too technical, not been technical enough or reproduced an article posted earlier by someone else. I had to do something while I download drivers to install my new PC's hard disks
  • 39 comments sorted by best / new / date

      nice article, can compression get rid of that annoying hum some notes seem to have when u play them back? or is that in eq..
      i dont really think all this is useful, we got by with out it before, why not now? Like do we need to know just how a light works, no, flik the switch. same idea (but its dark with signal processing). godd article tho
      Thanks. I know have an idea about compression. I have a feeling you explained it about as simply as it can be explained. I'm wondering - are modulated effects like chorus, flange, phase, tremelo, wah, different forms of compression?
      Very very nice article. Very informative and the stuff I didn't understand was explained with perynthesis. It taught me some interesting stuff that I didn't know before. 5/5
      this is the kind of stuff that would take hours of scanning owner's manuals and quarterlies, so thank you very much for the info.
      hey dont compresors also make youre sound longer cus when i use mine it lets note ring longer
      i must have yawned 5 times while i read this. Good article, just not appealing to somone who doesn't know electronics at all.
      Wow I thought this one got swallowed up in the UG masher...people actually read it. *fuzzy warm feeling* Well now its been a million months, I may even write that article on EQ
      cool... i was just looking for a device that did this kind of thing... compressor, that's what it's called... will invest in one...
      Great article. I would definitely like to hear your take on EQ cuz' i've been playin' guitar for more than year and I still have no idea what's goin' on there.
      I think that there are compressors pedals, but I have never tried one out.Does it worth?
      Great article, i learnt a lot about a subject that i didnt no existed. Im hopin u will write an article on EQ. 5 stars!!!
      Cool... have had this explained to me in one sentence before and didnt learn that much more from this, but it's good to know. Learnt a few words from it.
      gonstathis: I think that there are compressors pedals, but I have never tried one out.Does it worth?
      yeah, thyre definately useful for when youre using clean sounds
      Compression pedals are awesome, definately worthwhile. (to the dude somewhere above who asked about them)
      Good article, I hope you write more articles about related topics. 5 stars
      Was better than most stuff out there these days... It was detailed without boring going over the top, if you post more like this then you will be respected (which is hard to achieve) Ive done the physics behind most effects, as im doing Acoustics at uni, so if you need any info then get in touch. Oh by the way, you should hear how J Mascis (Dinosaur Jr) uses compression....
      excellent, good job, 5 stars i was wondering how i could fix things like your examples and figured it was just my recording equipment, im gonna go buy a compressor(well i guess it was because i didnt have a compressor) keep up the good work
      basically, send everything (guitars, drums vocals) into a mixer and then adjust the volume on each input individually until you're happy with it. then output it to recorder or program or whatever you're using.
      excellent article!!...very informative...taught me a hell lot of things..