What Is a Morph Synth? Playing the Sound Between Instruments

A morph synth lets you play the sound between instruments. Put four sounds at the corners of a square, move a point inside it, and you hear one sound that changes continuously as you move. The interesting part is how the in-between sound is made.

The simple way to blend two sounds is to play both and fade one down as the other comes up. In the middle you hear two instruments at once, a bass and an organ stacked on top of each other. That's a crossfade.

A morph does something else. It describes each sound, how loud it is, how its harmonics are spread, how much noise it has, and blends the descriptions. Then it plays the blended description as one sound. The middle of a bass and an organ is a single new tone, not two.

Contour is an ambient morph synth for iPhone and iPad.

• Four neural decoders. Each corner holds a small model trained on one real instrument from the NSynth dataset. They respond to pitch and loudness 250 times a second.
• One blend. Your finger's position becomes four blend weights. Harmonic energy, filtered noise and loudness move together.
• Real-time resynthesis. A bank of 60 harmonics and 65 noise bands plays the blended description, then the envelope and space shape it.

The square is drawn as a survey sheet, and the contour lines are real. They come from what the four corners actually produce, measured so that equal distances on screen are roughly equal changes in sound. Cross a line and the timbre has changed by a known amount. That's what lets your finger learn the pad.

1. Hold a note and drag slowly from one corner to the opposite one. Listen for the point where it stops sounding like either.
2. Draw a small circle near the middle. That's where the most new sounds are.
3. Switch sheets. Each sheet is a different set of four corners and a different landscape. Up to eleven come with Contour Pro.

Contour also runs as an AUv3 in your DAW and takes a MIDI keyboard. With Pro it records what you play. The how it works chapter of the manual goes deeper into the terrain.