How Synthesizers Make Noise

While playing keyboard, you make lots of cool sounds if you tweak some of the knobs while you are playing. In fact, this is what makes a keyboard player different from a piano player who is playing on a keyboard. To tweak the knobs, it helps to know what it is you are tweaking. Today, I'll describe oscillators, filters, envelope generators, and how they work together.

Most of you have seen an audio waveform like this before:
...and most of you intuitively know what this is and what it's doing. If you don't, leave a comment and I'll go back and explain it. If you zoom in on a small section, you'll see this:
Notice how it's an oscillating signal. It's our goal to create a slightly complex waveform like this. This one is a little jagged because it is the combination of drums, guitars, bass, and several synths. So, since our goal is to create an oscillating signal, the typical way a synthesizer works is to start with an easy-to-get oscillating signal, and then season it to taste. Here's logically how most amplifiers process it:

This is what each part is and does:

• OSCILLATOR: This is the fundamental audio signal. It can be a sine wave, a square wave, a saw tooth wave, a recording of something (sample), noise, or anything else you could dream up.
• FILTER: This is like a tone control on your guitar amp or on a radio. Another way to think of it is that it's like an equalizer. It mutates the incoming signal to reduce (or enhance) certain frequencies.
• AMPLIFIER: This makes the signal larger (lounder) or smaller (quieter). This part is pretty boring except for.....

ENVELOPE GENERATORS

Have you ever turned down the volume knob on your guitar, pluck the note, and then raise the volume? Kind of give a glassy or violin sound, doesn't it? Or how about played with a wah pedal? You pluck the note and move the pedal — which gives is a new dimension while you play it. Well, the volume knob swell is like an envelope generator for the amplifier. The wah pedal is like an envelope generator for a filter. An envelope generator changes the settings of your oscillator, filter, and/or amplifier in a time-dependent way. The most common approach to an envelope generator is the ADSR (Attack, Decay, Sustain, and Release) envelope. Here's an image that explains the ADSR envelope applied to the amplifier (the bottom axis is time):

• ATTACK: After the signal to start playing a note (key pressed), this determines how fast and how loud the note should be (initially). To get a really harsh attack (like a trumpet), the attack time is short and the amplitude is high. To get a really soft attack (like spacey music that goes bwwwwaaaaaAAAAAHHH), the attack time is set much longer.
• DECAY: After finishing the attack phase, this sets how the amplitude should change. For an organ or violin, there often is no decay at all. However, for a plucked instrument (guitar) or brass instrument, the decay will usually drop back down substantially.
• SUSTAIN: After finishing the decay phase, if your finger is still pushing the keyboard for the note, this is how loud it will be while you hold the note. It is not time dependent.
• RELEASE: This determines how fast the note should go away after the key is released. For an organ, the signal will immediately drop, for percussion this usually has no meaning, but for something like a piano it takes time for the instrument to stop vibrating.
  

Most folks only play with the ADSR on the amplifier/amplitude. However, you can do some other cool things by using the EG's (envelope generators) for the other parts as well.

HANDS ON

This is all pretty boring unless you have something to play with. If you already have a synthesizer... get a hold of its manual and find out how to change all these settings on one of your tones. (Pick a simple tone like a flute, or start one from scratch.) Or perhaps you could visit a local music store and find a MicroKorg or a Moog to play with. (I did that yesterday, and it was pretty phun.) You can also find a softsynth and play around with its settings. Here's some softsynth suggestions:

• Windows/Mac: Check Out KeyToSound's Remedy
  
• Linux: ZynAddSubFx
  

And don't forget: have fun!

Notes and further reading:

• To see what an analog oscillator circuit looks like, check out this sine wave oscillation circuit.
• Sine wave scope image courtesy of , and permission is granted on that site to use its content for non-commercial use.
• The ADSR envelope image courtesy of tge Wikipedia article ADSR Envelope and is licensed under the GNU Free Documentation License. That article also has a lot more info on the ADSR Envelope concept.