IMP Homework Podcast Episode Six Transcript


Hello, I'm Wes Perdue from the East Bay area of San Francisco, California.

Welcome to the sixth and final episode of my podcast series that fulfills my weekly homework assignment for the online course from Coursera titled Introduction to Music Production.

This week we will be looking at the five main modules of synthesis. They are the oscillator, filter, amplifier, envelope, and LFO.

These five items are the building blocks for classical analog synthesis and for modern virtual analog synthesis as well. Because of their early prevalence, they became the foundation for almost all synthesizer architectures, whether analog-based or not.

I'll be using the synthesizer Diversion by Dmitry Sches for samples illustrating the various modules as I go along.


The oscillator, originally called the voltage-controlled oscillator, or VCA, is the module in analog synthesis that creates the audio signal. It can be as pure as a sine wave or as rich in harmonics as a saw wave. It could even be a noise generator. In some synthesizers it can be a user-generated waveform.

Here is a sample of a bare saw wave. [bare saw]


The filter, originally called the voltage-controlled filter or VCF in analog synthesizers, filters out part of the oscillator's signal. As you heard, a saw save's rich harmonics can be a bit harsh. Filters were first just low-pass filters, filtering out high frequencies above the cutoff. Now, they can be low pass, high pass, band pass, or band reject. More sophisticated and sometimes exotic filter types are included on some modern synthesizers. For instance, Zebra's default filter is LP Excite, and includes the options LP Vintage and LP AllRound.

Many filters include a resonance control that emphasizes frequencies around the filter's cutoff, which can add a great amount of character to the sound.

Here is that sine wave again, this time with a low pass filter and some resonance. [saw filter]

Much nicer, yes?

Here's a quick bonus example. When a synth has more than one oscillator, they can be stacked, and that can sound very nice. Here, there are two oscillators, with the second an octave down and detuned up four cents. [saw filter 2 osc]


The amplifier is often still called the voltage-controlled amplifier. It brings up the level of the signal coming out of the filter. Note that it is voltage-controlled. This is because the VCA is designed to be modulated.

Sidebar: modulation

Modulation is the changing of a setting over time. Note that all three previous modules, the oscillator (VCO), filter (VCF), and amplifier (VCA) are all voltage-controlled. A voltage that varies over time can be applied to the various parameters on these modules to create motion in the sound coming out of them.

On the oscillator, pitch is most commonly modulated. It is primarily modulated by key mapping: that is, the key pressed tells the oscillator to play the frequency corresponding to that key.

On the filter, cutoff and resonance are commonly modulated.

The amplifier's output level must be modulated; if it isn't, it'd never stop outputting a sound. Again, the synthesizer's keys play an integral role; when a key is pressed, we expect a sound, and when that key is released, we expect it to stop. The primary module that modulates the amplifier's output in relation to key presses is the envelope.


Every synth must have at least one envelope, and that first envelope is usually hard-mapped to the VCA.

The envelope is the most common modulation source on a synth, because as we established, one is needed on every key press. An envelope usually has four stages: attack, decay, sustain, and release. Attack comes first, and in this scenario is triggered by key press. Attack is the time the envelope goes from zero to max level. Decay is next; it is the time the envelope goes from max level to the sustain level. Sustain is the level the note is held at until it is released. The release stage is triggered when the key is released; it describes the time the signal goes from the sustain level to zero.

A fast attack and release can sound very harsh and abrupt. Here is an example with zero attack, decay, and release. [saw filter 0 release]

And here's an example with a long attack, decay, and release. I set the sustain level down from the max so you can hear each of the four stages clearly. [saw filter long release]


The final module we're visting today is the LFO, or low frequency oscillator. It's so named because it usually oscillates below the range of human hearing. It's the second most commonly used modulation source, and it is used to modulate any of the voltage-controlled parameters we've discussed, including oscillator frequency to create vibrato. It's also commonly used to modulate filter cutoff or resonance.

Here is an example of the LFO modulating oscillator frequency. [saw filter lfo]


It is not uncommon to use other modules, usually effects, and they can come from either inside or outside the synth. When I was building the synth patch for this example, I added a bit of saturation and delay with plugins inserted in Diversion's channel.

Here's the progression from bare oscillator [bare saw]

to the addition of the filter [saw filer]

and adding the lfo [saw filter lfo]

and adding the second oscillator [saw filter lfo 2 osc]

to adding the saturation and delay insert effects. [saw filter lfo 2 osc fx]

In summary, the oscillator creates the sound; the filter shapes the sound; the amplifier adjusts the level of the sound, and the LFO and envelope modulate the levels and other parameters of the other modules. That's synthesis in a nutshell.

I hope you've enjoyed this brief overview of the five modules that make up the synthesizer. If you'd like to submit feedback for this podcast, please use the contact form at

This concludes my podcast series on music production, as the class I'm attending is coming to an end. I hope you've enjoyed it. Thank you for listening.