Add an extra subtle harmonic to modulation for flavour

[anthonyalfimov]

Currently, the modulation is a simple sine wave with phase offset between channels. We could try breaking the simple sine pattern by blending in another sine wave with much lower amplitude. The phase shift of this additional harmonic can be inverted: offset the channel for which the main sine is not offset.

This extra sine might not work at high modulation rates, in which case in should fade out as modulation rate increases.

If the frequency of the extra harmonic is going to be faster than the modulation rate, we should consider extending the modulation range down.

The frequency of this extra harmonic should be the same for both channels and deterministic. This way, setting "Stereo Width" to 0 will always result in the same sound.

[anthonyalfimov]

Notes:

• This effect might not be desirable for flanger.
• When adding an extra harmonic like this, the amplitude of the main modulation sine should be adjusted to stay within specified limits.
• Instead of adding an extra sine, we can multiply by it instead (by a factor of (1 - a) + a * sin(1.33 * f + p)).

[anthonyalfimov]

Currently an extra harmonic is enabled for all FX types and for all modulation rates. To me it sounds more interesting (perhaps even usable?) with the added harmonic at high modulation rates.

[anthonyalfimov]

Using AM (multiplication) instead of an extra harmonic (addition) - not very impressed after a brief test. Probably should do direct comparison of the same preset on the same piece of audio.

[anthonyalfimov]

Sounds good enough for now.