Fm Synthesis in Reason 5 part 1

Reason 6 Tutorials

FM synthesis is a method of generating sounds using 2 or more signals. One modulates the other and by definition it would return a different sound. FM Synthesis is probably also one of the difficult methods to explain in just a few words. Just as a reference to a complete math book about it: standford documentation on FM Synthesis

To back up my own thoughts on the whole FM Synthesis, I understand most of this while programming the FM-Pac (or FM-Pack, or OPLL) made by Yamaha in 1986. The chipset was based on one of its keyboards (DX-2 keyboards at the time) and I got around understanding most of it what FM-Synthesis did by the time. Even while I have dropped the whole FM generation from around 1997 and moved on to wavetables and such, I still have some common knowledge about what makes FM tick.

Personally I find the explination about FM-Synthesis made by propellerheads themselves (An introduction to FM Synthesis - part 1, and part 2) and interesting read, however I feel there is one minor issue with this story: it only focusses on the FM Oscillator to sell copies of Reason 5. For some reason it makes you feel like FM synthesis was introduced when Reason 5 came, however this is far from the truth. It was already there when the subtractor was introduced.

To demystify FM synthesis, I will take a different look then just looking at the FM Oscillator. Because there is one thing I am missing inside the picture of FM-Synthesis: Waveform!

When you take any of the oldschool generation game councils (like the Sega Genesis for instance), it uses a sound chip that is based of FM Synthesis. The synthesis is defined by 2 different parameters: Carrier wave and the modulator. This is also refered to the 2 operator FM. Like the FM-Pack, it had 2 operators and both parameters have a series of definitions:

Waveform, Attack, Decay, Sustain, Modulation Amount, LFO and so on. This was originally designed for both the carrier and the modulator.

When we look at the 2 devices that originally have FM, we can see 2 of them that have this: the Subtractor and Thor. Most common thing that people will 'talk' about is the Thor FM Oscillator. Yet there is something much more advanced inside thor to really emulate 100% fm-synthesis. This is the part where it usually goes beyond understanding because FM is hard as hell to understand in just a few minutes.

Ok, enough for the intro, lets get started.

FM and the Subtractor

To understand the subtractor FM it isn't really hard to get things going. The only thing you need to know right now is that FM uses 2 oscillators (the carrier and the modulator). So, in a way, we need both oscillators to make FM effective.

fm no fm

In this setup I am using a single sawtooth wave form and then use that to make FM with it. However, in this setup there isn't much happening with the FM knob, since it needs a modulator. Which isn't here yet.

RNS Download: No FM
MP3 Download: No FM

The only way that frequency modulation works inside the subtractor is to have a second waveform which is different then the original. In this case one could use a Sawtooth, but lower the octave (or increase the octave). In a way, the waveform becomes different (smaller or larger in size), thus it modulates the frequency.

RNS Download: Frequency Modulation Subtractor
MP3 Download: Frequency Modulation Subtractor

Modulation on the sine form

The biggest problem with FM-synthesis is that it can be radical in changes. Because we are chaning the frequency of the first wave form, depending on what the second wave form is. In most cases waveforms suchs as Saws and Square forms will often make a sound "harsh", roughed edged and totally out of sync with the kbd (eg: they don't play along with the notes that nicely). In most cases the safe spot using FM is below the setting of 32. Everything above it will make it go 'off note' at some point. This is an avarage take, since it depends on the waveform.

The sine form is most likely the most secure one put FM on it, since it is clean and mostly harmonic (no gradual changes since it slightly goes up and down).

RNS Download: Frequency Modulation Subtractor
MP3 Download: Frequency Modulation Subtractor

To fully understand what makes FM tick, one could use a long note instead. Notice the changes of the tone while the waveform goes up per octave:

RNS Download: Frequency Modulation Subtractor
MP3 Download: Frequency Modulation Subtractor

One thing I hope will be noticing in this setup is that as soon as the modulator waveform gets closer to the original waveform, the fm-effect becomes lesser noticable. This is because the modulation goes faster and thus it becomes more stable. Another setup you could compare it too is using and LFO and control the pitch while you are at it.

RNS Download: Pitch changing Subtractor
MP3 Download: Pitch changing Subtractor

See the similarties over here? One might think there is a misconception here. It might seems like we are controlling the pitch of the waveform. However, we are not. We are modulating the frequency of the waveform (the speed of the waveform), so in a way, the faster the waveform goes... it will lead into a difference in pitch. This is confusing... I know.

Lets try a more simple aproach. The square wave form is something that just goes on-off-on-off. You normally wouldn't recognize it, because it sounds like a harmonic tone. However, when you go really low, you will hear what a square wave form does.

fm suare

RNS Download: Square wave Subtractor
MP3 Download: Square wave Subtractor

Just a simple low square form... like I said, it just goes on and off. Almost like a ticking sound. Ok, what if we add a sine form to that and throw that agains the Frequency modulation while we are at it. This will make the Square form change accordingly to the sine form.

RNS Download: Square wave Subtractor FM
MP3 Download: Square wave Subtractor FM

There, now you will definitly notice whats happening over here. The square wave form changes shape accordingly to the second oscillator (which is in this case a sine wave). The FM knob will tell how much the frequency is going to change. So, if you would do it really drastic it will more or less become an LFO set on sine form and connect that to the pitch at the same time.

The difference is: if you would an LFO to control the pitch, the LFO rate will remain the same and thus the pitch effect remains the same, yet with FM the rate changes as soon as you going to trigger different notes. Because different notes also means that the oscillators changes at the same time. Therefor: FM is hard as hell to understand in just a few minutes.

Anyways, I will just leave at this for now. Maybe it may give you some ideas to play around with. Before I will close this topic down, here is a different aproach which I will be discussing in the next part(s):

RNS Download: Square wave Subtractor FM
MP3 Download: Square wave Subtractor FM