Understanding FM8 Ratios and Pitch Offset Parameters

Whether you are a beginner or an experienced user, the FM8 ratios and pitch offset parameters can be a bit confusing. This quick lesson will help you get it all figured out!

Now there are actually a few things to cover in a conversation about FM8 ratios and pitch offset parameters and how they affect your sounds. This quick tutorial will break down the basics of each and we will dive deeper into the conversation in a follow up tutorial about ratios and how modulators and carrier waves figure into it all. But today, we will be sticking to the basics. I find it is much easier for most people to learn these things when broken down into two separate lessons.

First, you need to understand the difference between FM8 ratios and pitch offset parameters and how they work. It’s true that hey both affect pitch, but it’s how they do it that matters. A ratio affects pitch of an oscillator based on a ratio system. A value of 2.0000 represents a 2:1 ratio which will raise the pitch of that oscillator by an octave. If the ratio was set to 4.0000, it would increase the pitch of that oscillator by two octaves. So it is not quite as straightforward as ratio of 2 = 2 octaves, 3 = 3 octaves, etc. Here is a quick example of how this works out.

Using the root note of C4:

Note / Ratio

C3 = 0.5000
C4 = 1.0000
C5 = 2.0000
C6 = 4.0000
C7 = 8.0000
C8 = 16.0000
C9 = 32.0000
C10 = 64.0000

FM8 ratios are one thing, but the pitch offset parameter works differently. Pitch offset works by affecting the pitch value of an oscillator by a predefined amount of Hz. It is different to the ratio in that it stays constant regardless of pitch of whatever note is being played at any given time. For example an offset of 0.50 will detune the operator by 0.50 regardless of what note is played, whereas the ratio of an operator corresponds to whatever note is being played. This is used to create detuning and beating effects against other operators in play.

To find an octave above another note, you need to double the fundamental Hz value of that note. If you play an A directly above middle C, registering 440Hz with one oscillator and you add a second oscillator with the offset value set to 440Hz you have one oscillator an octave above the other. If you were to move to any other note from that A, however, it would be out of tune because the offset parameter deals in fixed values.

As you can see, this quickly becomes more complicated the deeper you dig. So we will pick back up in another tutorial. In the meantime, try to practice using the FM8 ratios and offset parameters to achieve basic whole octave changes. This will help you gain a better understanding of exactly how they work with each other and how they differ on a fundamental level.