Effect: Tremolo / Vibrato
Vibrato: Oscillate the pitch of a signal. Resample as an effect (later), or just mess with the instrument
Tremolo: Oscillate the volume of a signal. Easy effect
But…: "The tremolo arm on your favorite guitar, for example, is actually a vibrato arm." (source) So these terms are used interchangeably.
Classic frequency-domain effect
Idea: lowpass filter with variable passband
Human vocal tract is subtractive via low-pass filters: "ooh" is low-pass, "aah" is less low-pass (note inverse connection to speaker size)
My sample implementation uses FIR filters
More typical implementation is with an IIR filter cascade (need to keep number of coefficients small to be able to update the filter in reasonable time). Biquad sections are a popular choice, as here.
See above for fancy demos
Effect: Flanger / Phaser / Chorus
Phaser: Simulation of flanger using cascade of biquad "all-pass filters" to get multiple phase cancellations. Originally an analog flanger substitute, but different enough to survive (Audacity effect)
These terms are used pretty interchangeably; no standard vocab
Not normally an "effect": just a thing. We've talked about it in detail.
As an effect: "sped up" (Chipmunks) or "slowed down" (sleepy) version of the sound
Effect: Frequency Stretching
Frequency domain equivalent of resampling
Idea: lengthen or shorten a signal while keeping its harmonic content the same
Time domain: Try to find the period of the signal, interpolate whole periods; useful period may be impractically long
Frequency domain: Take DFT, scale it up or down (interpolate / decimate in frequency domain), take inverse DFT; Fourier uncertainty is a problem, multiscale is hard
The difficulty of this plan is why video is time-scaled rather than audio
AKA pitch-shifting, because pitch is log-frequency, so multiplication becomes addition