LFOs

As mentioned previously, LFO stands for Low-Frequency Oscillator which means that it’s like an ordinary oscillator but operates at sub-sonic ranges (typically below 20Hz, although some LFOs can run into the audio rate – that’s a topic for another day).

LFOs are free running, meaning that by default they are not re-triggered by gate information, although some synths do allow this option. It’s also noteworthy is that they contain both positive and negative information, so they oscillate above and below zero.

What does this mean? Well if, for example, we modulated our oscillator’s pitch by an LFO, the pitch would increase and decrease either side of the original pitch. So if our original pitch was C3 and we applied two octaves if modulation, the pitch would increase to C4 and down to C2.

The most common parameters LFOs are assigned to are pitch (vibrato at lower amounts, laser sounds at higher), pulse symmetry (pulse-width-modulation), filter (typical dubstep wobble) and amplitude (also known as tremolo).

There are two key controls on an LFO – the rate and amount. Here’s an example of an LFO modulating a sine wave’s pitch (we’re increasing the amount of modulation):

lfo_amount.gif

Here’s the same LFO but this time I am increasing the rate of the LFO:

lfo_speed.gif

Let’s modulate the symmetry of a pulse wave. Observe how the sound thins as the LFO cycles. At faster rates you can create the classic Reese bass sound:

Now, let’s add a low-pass filter and modulate the cut-off of that with our LFO. I’ve changed our oscillator to a sawtooth now and I’m adjusting the cutoff manually, too – watch how it affects the oscilloscope:

filtermod.gif

Finally, here’s the same saw, however this time we’ll modulate the amplitude:

ampmod.gif

Envelopes

The last thing we’re going to look at is envelopes. Envelopes have two key differences from LFOs. Firstly, they only contain positive data so if we we were to modulate the pitch of an oscillator with an envelope, the pitch would only rise past the start position (whereas an LFO would swing above and below it).

Secondly, envelopes are gate-triggered. They require a Note On signal to be triggered. Ordinary envelopes contain four stages – attack, decay, sustain and release – commonly referred to as an ADSR envelope.

Every synth will have at least one ADSR hardwired to the amplitude, and every time you press a key you engage this ADSR. This is just simple example but envelopes can vary in complexity, especially synths such as the Native Instruments’ FM8 – more on this another day.

The attack stage is the amount of time the envelope takes to reach the maximum value after note on information is received so, if the attack time is 1ms, then the envelope will engage instantly. If it’s 250ms, it will fade in over that time. Decay is the amount of time the envelope will take to reach the sustain stage after the maximum value is reached.

Sustain is different to attack, decay and release in that it isn’t a time duration but the value a note will sustain until a note off message is received. So if the sustain is zero, the sound will tail off to nothing after the decay time has elapsed. If the sustain is maximum, the decay stage will have no effect.

Release is amount of the time the envelope takes to reach zero after a note off message is received. No release means the sound will tail off very quickly whereas a longer release will make the sound fade out.

The simplest type of envelope is on/off, like a binary message (actually, some synths allow you to disengage envelopes all together and have the sound constantly sustain but nevermind about that for now).

An AD envelope is the next most simple type, there being virtually only three ADs you can have:

  1. No attack, some decay
  2. Some attack, some decay
  3. Some attack, no decay.

AR envelope.png

The x-axis is time in all three examples; attack and time are represented by blue and decay by yellow.

N.B sometimes, as in the case with the MS-20, you might see AR instead of AD – this has virtually the same application.

ADSR envelope are by far the most common type of envelope you will see. To visualise this, let’s look at the next diagram:

ADSR.png

Aside from amplitude, envelopes can be wired to control pitch and filter cutoff. Let’s look at some examples of each of these. Firstly, we’ll start with an amplitude envelope with no attack or decay, full sustain and a little release:

Screen Shot 2015-04-04 at 09.45.39.png

This sound has no real movement to it so let’s add some attack and decay and drop the sustain to half its value:

Screen Shot 2015-04-04 at 09.48.31.png

The sound fades in, drops quite slowly, sustains and then fades out abruptly. Let’s increase the attack further and add more decay and release:

Screen Shot 2015-04-04 at 09.51.43.png

Now, I’ve reset the amp ADSR to it’s starting point. Let’s have a look at a second envelope, the pitch ADSR, I’ve added some decay, no attack, sustain or release:

Screen Shot 2015-04-04 at 09.54.36.png

The sound now has a sharp blast of high-frequency energy at the beginning. This sort of effect is useful when synthesising leads, pads and even drums. I’m going to increase the attack to match the decay value:

Screen Shot 2015-04-04 at 09.56.46.png

Now, the sound’s pitch rises and falls before settling at the original pitch determined by the sustain. It’s important to understand that sustain is a value so if we add some, the pitch doesn’t not fall to the note we’re playing but a note or two above it (depending on how much sustain was added).

Now let’s reset the pitch ADSR to its starting values and look at our filter ADSR. I’m using a resonant low-pass filter for this example. Using a short decay, we can make the synth almost sound like a ‘pluck’:

Screen Shot 2015-04-04 at 10.03.01.png

Finally, adding a lengthy attack with some sustain:

Screen Shot 2015-04-04 at 10.05.38.png

And there you have it, those are the foundations of basically every synth. If you can get your head around LFOs, ADSRs, waveforms and filters there’s not really much more to it.

Of course, different types of synthesis employ other techniques but, to give two examples, the way wavetable synthesis morphs between waveforms will likely be just LFO modulation and the way frequency modulation works is by using simple sine waves and envelopes. So it really is worth having a good grounding in subtractive before moving on to more complex types of synthesis.

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