Sound Synthesis

1. Sound Synthesis Part V: Effects

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

3. Chorus

4. 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.

5. 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).

6. 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).

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

8. Treble & bass

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

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

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

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

13. DISTORTION

14. Saturation

15. Distortion as waveshaping result Transfer function

16. Distortion: diagram SOUND IN SATURATED AMP Transfer function SOUND OUT

17. [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

18. 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)

19. Autotune

20. 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

21. 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

22. 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.

23. [flashback] spectrogram

24. 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.

25. “Believe”, Cher (1998)

26. 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.