GCT535- Sound Technology for Multimedia Delay-based Effects - - PowerPoint PPT Presentation

GCT535- Sound Technology for Multimedia Delay-based Effects

Graduate School of Culture Technology KAIST Juhan Nam

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Delay-based Audio Effects

§ Types of delay-based audio effect

– Delay – Chorus – Flanger – Reverberation (this will be handled in the topic of spatial audio effect )

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– Implemented by circular buffer: move read and write pointers instead of shift all samples in the delayline

Comb Filter

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𝑧 𝑜 = 𝑦 𝑜 + 𝑕 ' 𝑦 𝑜 − 𝑁

𝑨+,

𝑦 𝑜 𝑧 𝑜

+

FIR Comb Filter 𝑧 𝑜 = 𝑦 𝑜 + 𝑕 ' 𝑧(𝑜 − 𝑂)

𝑨+0

𝑦 𝑜 𝑧 𝑜

+

𝑕

IIR Comb FIlters

𝑕

Comb Filter: Frequency Response

§ ”Combs” become shaper in the feedback type

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𝑧 𝑜 = 𝑦 𝑜 + 𝑦(𝑜 − 8) 𝑧 𝑜 = 𝑦 𝑜 + 0.9 ' 𝑧(𝑜 − 8)

Perception of Time Delay

§ The 30 Hz transition

– Given a repeated click sound (e.g. impulse train):

• If the rate is less than 30Hz, they are perceived as discrete events.
• As the rate is above 30 Hz, they are perceive as a tone

– Demo: http://auditoryneuroscience.com/?q=pitch/click_train

§ Feedback comb filter: 𝑧 𝑜 = 𝑦 𝑜 + 𝑏 ' 𝑧(𝑜 − 𝑂)

– If N <

8

9

:; (𝐺 >: sampling rate): change tone of the input sound

• If N is large under this condition, it can generate a pitched tone as it models

sound propagation and reflection on a string (e.g. Karplus-Strong model) – If N >

8

9

:; (𝐺 >: sampling rate): repeat discrete events with gain loss

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Fractional Delay

§ Necessary when the length of delay continuously changes

– Chorus, flanger and other modulations

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𝑦 𝑜

𝑁 − 1 𝑁 𝑁 + 1

. . . fractional delay (=𝑒)

𝑦 𝑜 − 𝑁 − 𝑒

Linear Interpolation

§ The output is delayed approximately by 𝑒

– The phase delay in the low frequency range is close to the fractional delay 𝑒 – The output is attenuated in the high frequency range – Useful in “random access mode” (no recursion)

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𝑨+B

𝑦 𝑜

𝑨+,

+

𝑒 1 − 𝑒

𝑧 C 𝑜 − 𝑒 𝑧 𝑜

𝑧 C 𝑜 − 𝑒 = (1 − 𝑒) ' 𝑧 𝑜 + 𝑒 ' 𝑧(𝑜 − 1)

Linear Interpolation

8 https://ccrma.stanford.edu/~jos/pasp/Fractional_Delay_Filtering_Linear.html

§ Frequency Response

First-Order Allpass Interpolation

§ The output is delayed approximately by 𝑒

– The phase delay in the low frequency range is close to the fractional delay 𝑒 – The output is unity at all frequency range – Be careful in “random access mode”

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𝑨+B

𝑦 𝑜

𝑨+,

+

−𝑏 𝑏

𝑧 𝑜

𝑧 C 𝑜 − 𝑒 = 𝑏 ' 𝑥 𝑜 + 𝑥(𝑜 − 1)

𝑧 C 𝑜 − 𝑒 + 𝑥 𝑜

𝑥 𝑜 = 𝑧 𝑜 − 𝑏 ' 𝑥(𝑜 − 1)

Allpass Filter

𝑒 = 1 − 𝑏 1 + 𝑏

First-Order Allpass Interpolation

§ Frequency Response

10 https://ccrma.stanford.edu/~jos/pasp/First_Order_Allpass_Interpolation.html

Delay Effect

§ Generate repetitive loop delay

– Parameters

• Feedback gain
• Delay length

– Ping-pong delay: cross feedback between left and right channels in stereo – The delay length is often synchronized with music tempo

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+

𝑦 𝑜

feedback

𝑧 𝑜

Dry

+

Wet

Delay Line

Chorus Effect

§ Gives the illusion of multiple voices playing in unison

– By summing detuned copies of the input – Low frequency oscillators (LFOs) are used to modulate the position of output tops

• This causes pitch-shift

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LFOs

x(n) y(n)

Dry

+ +

Wet

Delay Line

Flanger Effect

§ Emulated by summing one static tap and variable tap in the delay line

– “Rocket sound” – Feed-forward comb filter where harmonic notches vary over frequency. – LFO is often synchronized with music tempo

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x(n)

+

LFOs Static tap Variable tap

y(n)

+

Wet Dry

Delay Line