K404/K504 Assignment 2: FM Synthesis

Skim Audio-rate Frequency Modulation. You'll be responsible for some of this material on the midterm exam, so plan to revisit this web page later.

This week, you'll do these things:

continue working with Absynth, exploring all the features introduced last week; and
make a simple FM patch with Absynth.

FM synthesis with Absynth

Absynth lets you do classic Chowning FM synthesis with relative ease. In this simple form of FM, you modulate one oscillator (the carrier) with another (the modulator). The frequency input of the carrier, normally just a constant value (in Hz), becomes a sum of that constant value and the periodically changing output of the modulator. At sub-audio rates, this makes vibrato. At audio rates, this produces sidebands. As described in Audio-rate Frequency Modulation, the strength of the modulator output governs the brightness of the timbre produced by the carrier. In classic FM synthesis, this strength is expressed as a modulation index. The two most important parameters in FM synthesis are:

the modulation index, and
the carrier-to-modulator frequency ratio (C:M).

To set up a basic FM patch in Absynth, do the following.

Starting with a default patch (just a single oscillator with sine wave), change the oscillator mode to FM. This gives you two oscillators in an FM configuration: the "main" oscillator is the carrier, and the "mod" oscillator is the modulator.
Remember that each of the three oscillator modules actually contains two oscillators, as you already discovered with double mode.
Click the Mod button to switch from the carrier to the modulator.
Change the frequency mode of the modulator to Ratio, and set the ratio to 2. This gives you a C:M ratio of 1:2. Try other ratios. (To get irrational ratios, like 100:131, you'll have to put the carrier into Ratio mode also.)
Play a note and fiddle with the Index parameter to hear the effect of a changing modulation index.

Note that in classic FM, both oscillators use sine waves. This was also true in the Yamaha DX7 and related instruments. In later Yamaha instruments, you could select different waveforms, and certainly you should try doing that in Absynth. Both the main and mod oscillators have a waveform popup menu that lets you do this.

Solve this problem...

How can you make the modulation index vary over time? There's more than one answer to this question.

Your mission is...

Make a patch that incorporates one (or more) oscillator modules in FM mode, and do as many things as you can think of to make the sound change over time. Save the patch to a file.