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An Introduction to Self-oscillation and FM Synthesis - Part 2

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This post is part of a series called An Introduction to Self-oscillation and FM Synthesis.
An Introduction to Self-oscillation and FM Synthesis - Part 1

This pair of quick tips will give you an introduction to FM synthesis. In this second part I am going to be looking at FM synthesis. I will be using the relatively simple Subtractor synth in Reason to give you an introduction to how this kind of synthesis works, and how you can begin to use it in practice.

So... Frequency Modulation?

The FM stands for frequency modulation, which is quite literally what is going on with this kind of synthesis. In the simplest terms, this kind of synthesis involves modulating the frequency (pitch) of our main oscillator. In this example I am modulating the pitch with a simple triangle shaped LFO.

If you increase the speed of the LFO the pitch wobbles begin to blur into one, and after a certain point you hear strange overtones and a change from one changing note to what appears to be a new, steady tone.

If you followed my previous tut on self oscillation and you're sensing a link here you're on the money. The same thing is happening here with the speed of the LFO as was occurring before with our loop speed - once it reaches about 30 Hz (30 cycles per second) we are able to hear the rate of the LFO as an audiable sound in its own right, hence the new note we perceive.

The 'Bumble Bee' Analogy

When explaining this to my students at college I developed an analogy using a bumble bee. Imagine there's a bee sitting on the wall in front of you. It's lazily flapping its wings. This slow flapping is causing an vibration in the air, but you can't hear it - it's too slow - it's too low a frequency for our ears to detect.

Now if the bee decides to take off it begins flapping its wings much, much faster - when the frequency of its wings flapping reaches the range our ears can detect (about 30-20,000 Hz) we begin to hear the sound.

In this audio example I am using an LFO to modulate the volume on the note - listen to how this affects the sound at higher frequencies:

This 'amplitude modulation' is the principle behind AM synthesis, but for now let's focus on FM.

Modulating With a Normal Oscillator Instead of an LFO

Although at higher frequencies the LFO we were using did create a FM effect, it would be better if we could use something that is able to operate at a higher range of frequencies. Fortunately standard oscillators are specifically designed to operate at audible frequencies, and the Subtractor allows you to modulate oscillator 1 with oscillator 2.

After making sure ocillator 2 is switched on, I have set the mix control all the way over to oscillator 1 - I don't want to hear the tone generated by oscillator 2, I just want to use it to modulate the pitch of oscillator 1, just like I did with the LFO.

Listen to what happens as I slowly turn up the FM knob and add some modulation:

By changing the pitch and waveform of oscillator 2 you can get some really interesting tones. Things often get really crazy if you adjust the pitch by semitones or cents, as only changing the octave control means that the frequency of Osc2 will always be a multiple of the frequency of Osc1.

Making Use of These Tones

You will notice that the pitch of your note often goes way off when you begin to apply the FM. At this stage you can choose to manually retune Osc1 to meet concert pitch, though this can often change the tone of the sound. Another way is to export an example of the tone, and load it into a sampler to be retuned in exactly the same way as we did in the previous tut.

Tuning Your Note Once Within a Sampler

Now we need to manually adjust the 'root' and 'tune' dials so that our note is tuned to the rest of our instruments. Using another instrument play C3, and adjust the samplers controls until when you play C3 on the sampler the 2 notes sound the same. This can be a little tricky if you're not used to tuning other instruments such as guitars - you could use a tuner or tuning plugin (not pitch correction plugin) to help you. Once you're done you can play your sample like any other instrument.

In this example all the sounds apart from the drums are made using FM techniques on the Subtractor.

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