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

+

fcA

fm*I

A fc

+DELAY

fdd

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

EqualisationSOUND IN

LOW PASS BAND PASS HIGH PASS

AMP AMP 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 threecomponents

First fourcomponents

Saturation

• 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. 1493-1509, 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.