sound synthesis part v: effects. plan overview of effects chorus effect treble & bass...
TRANSCRIPT
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.