Implement "robotic" effect about scratch-audio

[ericrosenbaum]

We now have a robotic effect based on the Tone.js feedback comb filter: here. (A vocoder effect would be even cooler, but after some experiments I think it is too processor intensive).

The robot effect creates a low buzzing, with the effect setting controlling the pitch of the buzz, scaled to 10 steps per semitone (same scaling as the pitch effect). Setting the effect to 0 mutes it, and all other values scale the pitch (including negative values, which as they get extreme turn into more of an echo or delay effect).

This effect is pretty cool and robotic sounding (fairly similar to the classic Dalek voice from Doctor Who, especially if you combine it with the fuzz effect), but open questions include:

• Is the performance good enough? We may be able to replace this with something simpler.

• Should we add a wet/dry scaling, so that low values of the effect (e.g. 10 or 20) make barely audible changes to the sound, but increasing it increases the 'wet' proportion of effect passed through?

• Should we clamp the effect value, to prevent it become too extreme in the positive or negative direction?

• Is the 'tail' of the effect a problem (i.e. the echo effect that lasts after the sound is over, more so for large negative values)?