How to Build a Crossfading Synth

Our latest sound package of presets and note clips, Crossfading Synths, is based around a simple method for making your sound design more complex. Crossfading allows you to manually or automatically blend between different sound sources and morph through timbres and pitches.

In this article, we'll break down three methods for creating a crossfading synth in Bitwig Studio's modular environment, The Grid. A lot of these synthesis methods were covered in more depth in our Modular Concepts video “Synthesis World Tour.” These ideas will help you get started making your own crossfading synths.

Crossfading Synth 1: A two-oscillator synth with an automated Blend knob

1. Start with the initial preset patch for Poly Grid: a Triangle oscillator whose amplitude is controlled by an AR envelope, then passed to the audio buss via Audio Out module.

2. Now add a second oscillator. Since we’re making a synthesizer that fades between two different voices, a different waveshape would be nice so let’s go with a Sine oscillator. And then we can use the Ratio controls to tune the oscillators to different harmonics.

3. Use a Blend module to route both oscillators into the envelope’s in port. This module allows us to crossfade between the two sound sources, and it’s located in the Mix category along with other signal-routing tools. Connect them by dragging from each oscillator’s out port to one of the Blend in ports, which will create patch cords.

4. Go to the Level category and drag a Velo Mult between the AR envelope and the Audio Out. Velo Mult module is aware of the velocity coming in from your keyboard and uses that value to scale each note’s level, making the patch velocity sensitive to notes you play or program.

5. So far we have one envelope controlling the output volume. Now let’s add a second envelope with a longer attack, for modulating the balance of our two oscillators. We can select an ADSR (from Envelope) for this, and bring the Sustain level down for a little extra movement.

6. The blend knob can be controlled in the same fashion as nearly every other parameter in Bitwig: via modulator. The top-right corner of the ADSR module has a little blue-ish icon, suggesting a circular port with a cable coming out of it, awaiting connection. Clicking this modulation routing button switches us into a mode where every potential target parameter is lightly shaded. So now we can click and drag on the Blend knob to set the range of modulation from the ADSR.

Even without cables connected, the ADSR module’s envelope signal is now controlling the balance of the two oscillators at sample rate. And pressing your computer’s [ESC] key will exit mode.

You now have a simple “crossfading synth” that blends from the first to the second oscillator with each note played. But who says we have to use this crossfading pair of oscillators for audio?

Crossfading Synth 2: An FM synth with morphing modulators

1. Let’s add another Sine oscillator between Blend and the AR envelope/amplifier module. Make sure to connect Blend’s output to the purple phase in port of Sine. And finally, for this connection to have effect, the attenuator knob beside the in port must be opened some amount, reflecting the amount (AKA the “index”) of modulation taking place. Holding a note while you adjust this maximum value will help you find a sound you like.

2. While we are at it, let’s add another Velo Mult, this time between the Blend module and the new Sine oscillator. By scaling the signal with each note’s velocity, we will get a slightly different modulation amount for each note.

Since we are acting on a control signal in this case, we can switch the Velo Mult to Linear mode. You can do this by selecting the Velo Mult module and then going to the Inspector Panel on the left side.

3. We have already maxed out Blend, so let’s upgrade. Right-click the Blend module and select Replace With > Merge from the dropdown menu. You can also delete the ADSR since she isn’t controlling anyone anymore.

4. To recreate what Blend was doing, we can add a Value (Level) module and route it into the Merge module's blue control input. Adjusting the Value knob now crossfades between the two oscillators going into Merge.

5. Now select Merge and go to the Inspector Panel. The number of in ports is adjustable so let’s go from 2 to 4. The additional inputs will appear.

6. Add a new oscillator and route it into the third in port. (In the picture below, we are just duplicating the Sine oscillator tweaking the Skew and Detune values for a bit of contrast. But feel free to add something completely different.)

7. Add a Noise (Random) generator, and connect it into the fourth input of Merge. Now setting the Value between zero and one crossfades across all four modulators. Do play some notes and listen!

8. Aside from blending between up to eight sources, Merge is also an upgrade as it can be controlled directly by any signal. Let’s add an LFO module and connect its output to Merge's blue Select In port. And now the blend position is moving on its own at whatever rate/speed LFO is set to, with whichever waveshape is selected.

9. And one more benefit of signals in The Grid is that they are stereo — including control signals. LFO is one of several modules that make working with stereo easy, so try adjusting the Phase Offset (Right) numeric near the bottom right corner of LFO. Going from +0 º (completely aligned with the left channel) to something like +72 º (as in the picture below) will have a clear effect on both the waveform visual and the sound you hear.

Now you have a unique FM synthesizer, with a sine wave carrier that is being modulated by various crossfaded sources over time.

Crossfading Synth 3: A wavetable oscillator as crossfading synth

1. Now right-click the Sine carrier and swap it with a Wavetable (by selecting Wavetable from the Replace With section of the context menu). The device will default to a sine wave, so you won't hear any difference… until you move the Index knob.

2. Click the wavetable visual in the center of Wavetable and then try different shapes in the pop-up window (you can keep moving Index as well). We picked Bata Seq, which starts with a nearly flat wave. So as the wavetable cycles through the different indexes/shapes, the audio has the effect of “dropping out” whenever it crosses the straight-line waveform.

3. Replace the LFO with a Steps (Data) module. Unlike an LFO, this step sequencer defaults to modulating all played notes with the same, transport-synced position.

4. BUT our Steps can easily have some stereo fun as well. Try adding a Ø Reverse (Phase) module between Steps and Merge. Then with Ø Reverse selected, go to the Inspector Panel. By default, Ø Reverse reverses both channels, changing a zero to one ramp into a ramp from one to zero. The Stereo-ness parameter, however, lets us pick only one of the channels to be reversed.

5. Finally, let’s add a control source to move the index position of Wavetable. We’ll use an LFO to control the position of the wavetable. And again, open the attenuator beside the in port to get a result. Then adjust the LFO parameters to your liking, including stereo options or anything else.

You now have an FM synth with four morphing modulator sources and whose carrier waveshape is being faded and changed. Come to think of it, a wavetable oscillator alone is a crossfading synth. But with such a wide variety of single controls that can crossfade and alter the sound, the options here are quite rich already.

For more crossfading synths patches and presets, check out our Crossfading Synths sound package, which includes a preset called “Genesis” that uses ideas from this tutorial.. And check out our other Modular Concepts and Let's Build A... tutorials here.

October 14, 2021

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