Processing: Delay, Flange, and Chorus

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Processing Delay, Flange, and Chorus          
 

• Week 12
• IAT-204, Spring 2007SFU Surrey

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Todays topics

• Delay Lines
• Flange
• Chorus

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Learning Objectives

• Explain how to create echo effects
• Elaborates on how to create a variable comb
  filtering effect flanging
• Describe how to achieve the chorus effect

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Effects Achieved with Delayed Signals

• Delay a signal and mix it with the original
• Short delay (a few ms) ? subtle filtering effect
  but not a discrete echo
• Delay of about 100 ms ? slapback echo
• The delayed copy follows closely behind the
  original
• Longer delay time ? discrete events
• E.g. Wall or Mountain echoing

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Creating a Delay line with tapin and tapout

• argument ? the amount of signal it stores

• tapin a buffer continuously updated ? stores
  the most recently received signal

• tapout ? taps into the delayed signal at
  certain points

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Mixing Original Delayed Signals

• send the sound coming into the computer to two
  places directly to the output of the computer
  and to a tapin-tapout delay pair

• You can control how much signal you hear from
  each place for each of the channels ? mixing
  original and delayed signal in whatever
  proportion you want

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Mixing Original Delayed Signals (2)

• The hslider serves as a balance fader between a
  Dry (all direct output signal) and a Wet (fully
  processed output signal)

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Delay Emulates Reflection

• Combining a sound with a delayed version of
  itself ? emulating a sound wave reflecting off of
  a wall

• In the real world some of the sound energy is
  actually absorbed by the reflecting wall

• we can emulate that fact by reducing the
  amplitude of the delayed sound, as shown in the
  following example

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Delaying the Delayed Signal

• The reflections are in turn reflected off of
  other surfaces

• One simple way to model this reflection of
  reflections is to feed the delayed signal back
  into the delay line

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Stereo Delay with Feedback

• Stereo delay with feedback ? each channel of
  audio input is delayed, and fed back into the
  delay line

• Feeding audio signal back into a system has a
  potential for overloading the system ?
• scale the signal by some factor less than 1.0
  before feeding it back into the delay line

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Controlling Amplitude with normalize

• The normalize object is good for scaling
  unpredictable amplitude of summed signals

• Its often difficult to predict the amplitude of
  the summed signal that will go to the DAC

• specify a max amplitude ? normalize scales the
  peak amplitude so that it never exceeds it

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Variable delay time

• line object can be used to make a continuous
  transition between two delay times
• Its possible to provide a variable delay time to
  tapout using a continuous signal ? Doppler
  effect

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Doppler Effect

• Doppler effect occurs when a sound source is
  moving toward or away from the listener ?
  changing the perceived pitch
• Train Doppler

• Observers on this side receive more waves per
  sec ? perceive a frequency higher than the
  source's true frequency

• observers on this side receive fewer waves per
  sec ? perceive a frequency lower than the
  source's true frequency

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Doppler Effect (2)

• The moving sound source can outrun the wavefronts
  of the sound it is producing
• Doppler Animation

• Condensation cloud as an F/A-18 Hornet flys at
  or near the speed of sound

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Doppler Effect (3)

• Doppler effect can be emulated with delayed
  signal
• As the delay time decreases the (virtual)
  reflecting wall were moving toward you ?
• an increase in the received frequency of the
  sound
• As the delay time increases, the reverse is true
• The pitch variance resulted from continuously
  varying the delay time can be used to create some
  interesting effects ? e.g. Flanging

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Flanging

• Flanging - whooshing sound
• similar to the sound of a jet plane flying
  overhead
• E.g. dry drum loop with flanging applied
• How it Works
• Created by mixing a signal with a slightly
  delayed copy of itself, where the length of the
  delay is constantly changing

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Flanging

• Control how much of the delayed signal is added
  to the original ? depth control

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Flanging and Comb Filter

• When we listen to a flanged signal, we don't
  hear an echo because the delay is so short ? 1 to
  10 ms

• Points at which the frequency response goes to
  zero ? sounds of that frequency are eliminated ?
  comb filter

• Instead, the delay has a filtering effect on the
  signal ? create a series of notches in the
  frequency response

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Flanging and Comb Filter (2)

• If the amount of delay continuously changes ?
  varying comb filter flanging effect
• Sweeping action of the notches (flanger)
• As the delay increases, the notches slide further
  down into the lower frequencies, and vice versa

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Common Parameters of Flanging

• Delay specifies the minimum delay used on input
  signal
• Sweep Depth determines how wide the sweep is in
  terms of delay time

Max Delay Delay Sweep Depth
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Common Parameters of Flanging (2)

• LFO Waveform the waveform used to modulate the
  delay time
• Feedback taking a portion of the flanger's
  output and routing it to the input

• A large amount of feedback can create a very
  metallic and intense sound

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Flanging MSP Implementation

• Create sweep depth output of cycle is
  multiplied by 0.25 and then multiplied by 100 ms
  ? a signal with an amplitude 25

• Add to basic delay ? a signal that varies around
  basic delay of 100?25 ? used to express the delay
  time to tapout

• low-frequency sine wave is used to modulate the
  delay time

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The Chorus Effect

• Why does a chorus of singers sound different from
  a solo singer?
• Theyre not singing precisely the same pitch in
  impeccable unison ?
• The random, unpredictable phase cancellations
  that occur as a result of these slight pitch
  differences ? chorus effect
• How can chorus effect be emulated?

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Emulating Chorusing using rand

• Chorusing can be achieved by substituting a
  random fluctuation of the delay time for
  sinusoidal fluctuation for flanging
• Low-frequency noise rand

• rand chooses random numbers between -1 and 1,
  more gradually but still unpredictable ? suitable
  candidate for chorusing

• noise produces white noise ? not a good choice
  for modulating delay time ? sounds just like
  added noise

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Emulating Chorusing using rand (2)

• The cycle object for flanging is replaced by a
  rand for chorusing

• The setting for chorusing is substantially
  similar to flanging

• So a randomly delayed signal is used to express
  the delay time to tapout

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Multiple Delays for Improved Chorus Effect

• One way to do this ? feed the randomly delayed
  signal back into the delay line, where its
  combined with new incoming signal

• We can improve the chorus effect by increasing
  the number of slightly different signals to
  combine

• The output of tapout ? a combination of new
  variably delayed signal and the previously (but
  differently) delayed signal

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Summary of Todays Topics

• Create echoes by mixing original signal with
  delayed versions of it
• Create a variable comb filtering effect
  flanging
• Achieve the chorus effect by combining randomly
  delayed copies of a sound