LinPlug Octopus - Manual (Page 57)

6. Alternatively, a second Oscillator can be used to modulate Osc 1.
Switch on Osc 2, set its Amplitude to 1.0 but don't send its output to
the Mix. Instead, set it up to modulate Osc 1 by increasing the value
in second row of the first column of the Oscillator Matrix (as shown
below). The setting of this Matrix node determines the amount of
signal that is sent from Osc 2 to Osc 1, and hence the amount that
Osc 1 is modulated by Osc 2. This produces an effect known as
Frequency Modulation resulting in rich, complex timbres.
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