Poly Grid - FM&PM Synthesis

Let’s leave additive synthesis behind for now and dive into another form of synthesis: Frequency Modulation (FM synthesis). Alongside that - and because it’s so closely related-we’ll also touch on Phase Modulation (PM), since the two are very similar in practice.

What is FM/PM Synthesis? #

Before we jump into building a synth together, I want to take a moment to explain the differences between FM and PM, and why you might choose one over the other. Understanding these distinctions will help you apply them more effectively when patching or designing sounds.

What is Frequency Modulation (FM)? #

Frequency Modulation means changing the pitch (frequency) of one signal (the Carrier) with another signal (the Modulator). When the modulator moves slowly, you hear a vibrato. But when it moves quickly, you create completely new sounds and tones. That is the concept. There are two types of Frequency Modulation:

• Linear Frequency Modulation: This changes the frequency by adding or subtracting a fixed amount, like ± 200 Hz, no matter what pitch you're playing. This type is musical and easier to control, making it good for melodic sounds.

• Exponential Frequency Modulation: Here, the frequency changes by multiplying or dividing the original frequency, for example by ± an octave. This can sound more dramatic and extreme but is often harder to control, especially when playing melodies. It is called exponential because the frequency changes exponentially, not linearly. The reason for this is that notes are not spaced evenly in frequency, but rather in pitch. So from C2 to C3, the frequency doubles from 130.81Hz to 261.63 (a difference of 130.81hz). But from C3 to C4, the frequency doubles again from 261.63 to 523.25 (a difference of 261.63hz). but the distance in pitch is the same (one octave). The pitch difference is the same (linear), but the frequency difference is not (exponential). This is why it’s called exponential FM.

What is Phase Modulation (PM)? #

Phase Modulation is similar, but instead of directly changing frequency, you're shifting the phase of the wave. The phase is like the starting point of a waveform. When you quickly shift the phase back and forth, you create new tones and sounds.

FM vs. PM: How Are They Connected? #

FM and PM often sound very similar, but technically they're different. Most synthesizers labeled as "FM" (like the Yamaha DX7) actually use Phase Modulation. Why?

Phase Modulation is easier to handle because:

• The pitch stays stable even when you change how intense the modulation is.
• Sounds are more predictable and easy to control.
• It is simpler to create musical and harmonic sounds.

Why is Phase Modulation Sometimes Called "Phase Distortion" (PD)? #

Phase Distortion is a special type of Phase Modulation, where the waveform is shaped or distorted in specific ways. The difference is usually small, so people often use "Phase Modulation" and "Phase Distortion" interchangeably, especially if a synthesizer uses its own special methods.

Simple Summary: #

• FM directly changes frequency.
• PM/PD indirectly changes frequency by shifting phase.
• Most FM synths use PM because it's easier to control.
• Linear-FM: easier to use, musical, and good for melodic sounds.
• Exponential-FM: more intense and experimental, better for effects.

Building a PM Synthesizer in Bitwig's Grid #

Alright, let’s build our first PM/FM synthesizer in Bitwig’s Grid step by step. Here’s what you need to do:

1. We need two sine oscillators, and since we already have three from the previous patch, we’ll delete the third one.
   Just select the last Sine Oscillator module and press the Delete key to remove it.

2. Now, connect the output of the second sine oscillator to the PM (Phase Modulation) input of the first sine oscillator.
   That’s the core of the modulation setup-your second oscillator becomes the modulator, and the first one is the carrier.

3. Since we don’t need to hear the modulator directly, go ahead and delete the mixer by selecting it and pressing Delete.

4. The modulation depth-how much the modulator affects the carrier-can be adjusted using the attenuator knob next to the PM input.

And just like that, you’ve built a basic Phase Modulation Synthesizer inside the Grid. It’s a super flexible starting point, and now you can experiment with modulator frequency, envelopes, and more to shape the sound even further!

Building a Exponential FM Synthesizer in Bitwig's Grid #

To turn our Phase Modulation Synthesizer into an exponential FM Synthesizer, all we need to do is move the cable from the PM (Phase Modulation) input of the carrier oscillator to its Pitch input instead. You can double-click + hold (the last click) the cable ending to drag it to the new input.

This changes the behavior significantly: instead of modulating the phase (which is a linear modulation), we’re now modulating the pitch (notes), which responds exponentially (in frequency / Hz). That means the modulator’s audio signal will cause the carrier’s frequency to shift up and down in pitch - creating those classic, metallic or bell-like FM tones. Great for percussions, bells, and other sounds that have a lot of overtones.

Check the Pitch Pre-cord on the Carrier
If it’s enabled, the oscillator will already follow the pitch from your MIDI keyboard or the piano roll - so your carrier oscillator plays the note you press.

Now, when you send another signal (like from your second oscillator) into the Pitch input, the two signals are combined - they’re basically added together.

This lets you:

• Set a main pitch via the keyboard (thanks to the pre-cord),
• And then modulate around it with your modulator oscillator.

The attenuation knob next to the Pitch input controls how much of that second oscillator’s signal is applied.
So if you keep it low, you’ll just get subtle modulation around your base pitch - which is great for more musical or harmonic FM.

So:

• PM input = linear modulation of phase
• Pitch input = exponential modulation of frequency

This small change gives you a completely different sound character and unlocks a new set of possibilities for FM-style synthesis in Bitwig’s Grid.

Building a linear thru-zero FM Synthesizer in Bitwig's Grid #

With this exponential frequency modulation setup, we now run into a problem. You very quickly leave the tonal domain and end up with metallic or dissonant sounds that are hard to play on a keyboard or to blend with other sounds. That’s why we’re now going to build a linear through-zero FM patch. The result will be similar to our first patch - that is, a PM or phase modulation patch. The outcomes are similar, but it's a bit trickier to build inside the Grid, as there are some issues to work around that are perhaps easier to solve in a Eurorack system or other modular synthesizer setups.

In Bitwig Grid, though, it’s not overly complicated, so let me explain how it works:

When we modulate the carrier frequency - that is, we use a second oscillator (the modulator) to modulate the frequency of our carrier oscillator - the frequency of the carrier goes up and down. That means we’re modulating the pitch up and down, and we're doing this at audio-rate speed.

Now, here comes the problem: let’s say we play a note on the carrier that’s close to 0 Hz - like a low bass note at 40 Hz. When we apply FM modulation, the frequency goes up and down. At some point, the modulation might push the frequency down to 0 Hz or even below 0 Hz, resulting in a negative frequency.

This doesn’t always happen, but depending on your modulation amount, it can easily occur. So what we need is an oscillator that can handle negative frequencies. Negative frequencies are technically the same as positive ones, except the waveform plays in reverse. For example, if 10 Hz produces a waveform that goes upward, -10 Hz would produce the same waveform but flipped - going downward. That’s why we need an oscillator capable of representing both positive and negative frequencies.

The usual audio oscillators inside Bitwig’s Grid can’t do this. But here’s a trick: instead of a standard oscillator, we use a LFO. A LFO (Low Frequency Oscillator) is, after all, just an oscillator. It’s typically used to modulate things slowly, but in the Grid, you can actually run LFOs at audio rate.

Let’s build it #

We can convert a LFO into a kind of through-zero oscillator by doing the following:

1. Drop in an LFO module in Bitwig Studio—in this case, a Wavetable LFO.
2. Switch its unit to kilohertz and set it to 0 kHz (Ctrl + click on the knob).
3. Instead of feeding it pitch or note signals like a regular oscillator, we need to feed it a kHz signal/value.
4. Add a Pitch In module to receive pitch from the MIDI keyboard or piano roll, and route it into a Pitch → Hz module. This converts pitch (notes) into a frequency signal.
5. In the Inspector, switch the Pitch → Hz module to kHz, since the LFO is using kHz as input.
6. Use an Add module to combine this signal with the audio signal of the second oscillator (the modulator).
7. Scale the audio signal using an Attenuator module.
8. You can now modulate the incoming pitch in the kHz domain with the modulator's audio signal.

Now that we're using a LFO as an oscillator, we can work with frequencies below 0 Hz, which helps us avoid ugly artifacts or unwanted changes to the main pitch. Sonically, you’ll get results that are quite similar to Phase Modulation (PM). However, PM is easier to patch and handle overall. In fact, many classic FM synthesizers actually use Phase Modulation under the hood.

Weirder FM/PM Synthesis #

We can also use FM synthesis with incoming audio signals as an audio effect. I’ll probably come back to that later in the course. Basically, all you need is a simple delay that’s being modulated at audio rate. But, look at this:

As you can see, we can also build a PM synth using a sampler by modulating the playback speed at audio rate. Both approaches stretch and compress the audio signal, which is very similar to stretching and compressing the phase of an oscillator!

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