Sound Synthesis Part V: Effects

Plan Overview of effects Chorus effect Treble & bass amplification Saturation Pitch vocoder Summary

CHORUS

Chorus effect “A chorus effect […] occurs when individual sounds with roughly the same timbre and nearly (but never exactly) the same pitch converge and are perceived as one.” (Wikipedia) This is found naturally in pianos, where each hammer hits multiple strings tuned to (nearly) the same pitch.

Chorus effect (cont’d) In practice: –Add the signal to delayed and frequency modulated versions of itself. –FM modulation needs to be low frequency and low amplitude (ie, vibrato).

Chorus effect (cont’d) In practice: –Add the signal to delayed and frequency modulated versions of itself. –FM modulation needs to be low frequency and low amplitude (ie, vibrato).

Chorus effect (cont’d) fm + fc A fm*I A fc + DELAY fd d original sound chorus effect (N times)

TREBLE & BASS

Treble & bass Vocabulary: –Treble: high frequency sounds –Bass: low frequency sounds –medium –Equalizer: tool that allows selective amplification of frequency bands.

Filters: low & high pass Low-pass filter: Bass selection High-pass filter: Treble selection

Equalisation SOUND IN LOW PASS BAND PASS HIGH PASS AMP + SOUND OUT

Equalisation Allows to –reduce unwanted sounds –make instruments or voice more prominent –adjust timbre of instruments –at home, adapt to individual taste and hardware...

DISTORTION

Saturation

Distortion as waveshaping Transfer function result

Distortion: diagram SOUND IN SATURATED AMP SOUND OUT Transfer function

[Flashback] Fourier Transform Idea: “All functions can be decomposed in a (possibly infinite) sum of sinusoidal functions of varying frequencies.” Transforms a function from time domain to frequency domain. Eg, right, for a square wave. First component First two components First three components First four components

Saturation –clips signal at high amplitudes hard soft –Adds more overtones Some equivalent vocabulary: –to overdrive the amplifier (physical) –to distort the signal (waveshaping) –to saturate the signal (result)

AUTOTUNE

Autotune “Autotune is a proprietary tool by Antares Audio Technologies, for analysing and correcting pitch in vocal and instrumental performances” (Wikipedia) Used (post-hoc) to hide pitch inaccuracies in vocal track and make them sound perfect. Proprietary – but basically built on a phase vocoder

Pitch correction: Resampling? Simplest way to change pitch of a sound: resample. –played faster, all frequencies will increase, leading to higher pitch; –played slower, all frequencies will decrease, leading to lower pitch. so-called Chipmunk effect –record sound at half the normal speed –played back at normal speed  one octave higher

Phase Vocoder Originally proposed by Flanagan & Golden (1966) Steps: –Decompose the signal using Short Term Fourier Transform (STFT) –  yields a spectrogram –correct individual components –resynthesize the sound by inverting the STFT. J.L.Flanagan & R.M. Golden “Phase Vocoder” Bell Systems Technical Journal, vol. 45, pp , 1966.

[flashback] spectrogram

Autotune Apply a phase vocoder Bends the pitch to the nearest semi-tone –correct for inaccurate voice pitch “"Photoshop for the human voice” Josh Tyrangiel, Time. The rest is part of the engineer skill and “secret sauce” strong settings lead to robotic voice –eg, popularized by Cher’s “Believe” song.

“Believe”, Cher (1998)

Summary Sound synthesis techniques can be used to apply a number of effects on recorded sounds They have been used extensively –artistic purpose (distortion, chorus) –professional purpose (autotune) They now form an essential part of recording and production practices.