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  • This module is included inLens: Digital Signal Processing with NI LabVIEW and the National Instruments Platform
    By: Sam ShearmanAs a part of collection:"Musical Signal Processing with LabVIEW (All Modules)"

    Comments:

    "This online course covers signal processing concepts using music and audio to keep the subject relevant and interesting. Written by Prof. Ed Doering from the Rose-Hulman Institute of Technology, […]"

    Click the "NI Signal Processing" link to see all content selected in this lens.

    NI Signal Processing
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Tremolo Effect

Module by: Ed Doering

Summary: Tremolo is a type of low-frequency amplitude modulation. Learn about the vibraphone, a mallet-type percussion instrument that can create tremolo, experiment with the tremolo effect using an interactive LabVIEW VI, and learn how to model the tremolo effect mathematically.

LabVIEWq.png This module refers to LabVIEW, a software development environment that features a graphical programming language. Please see the LabVIEW QuickStart Guide module for tutorials and documentation that will help you:
• Apply LabVIEW to Audio Signal Processing
• Get started with LabVIEW
• Obtain a fully-functional evaluation edition of LabVIEW

Overview

Tremolo is a type of low-frequency amplitude modulation. After learning about the vibraphone, a mallet-type percussion instrument that creates tremolo, experiment with the tremolo effect using an interactive LabVIEW VI and learn how to model the tremolo effect mathematically.

Physical Tremolo: Vibraphone

The vibraphone is a mallet-type percussion instrument similar to the xylophone and marimba. The percussionist in the right foreground of Figure 1 is playing a vibraphone.
232084492_4d55954909_b.jpg
Figure 1: Vibraphone (right foreground); click for larger version. Photographer: Charles Dietlein (http://www.flickr.com/photos/dietlein/232084492/). Copyright holder has granted permission to display this image under the Creative Commons Attribution-NonCommercial-NoDerivs license.
Following are the vibraphone's key characteristics:
  • Range of three octaves, beginning on F3 (the F below middle C)
  • Playing surface is covered by metal bars; the pitch of each bar increases as the length decreases; bars are typically struck by soft cord- or yarn-covered mallets
  • Sound intensity is increased by placing a series of resonating tubes (resonators) directly under each bar
  • Sustain pedal controls whether or not a damper is applied to the metal bars, giving the vibraphonist similar expressive control as a piano
  • Motor-driven disks rotate between the metal bars and resonators cause sound intensity to fluctuate (tremolo effect)
The name "vibraphone" was originally derived from the term "vibrato," since the undulating sound of a vibraphone resembles that of a vocalist singing a long note with vibrato. However, vibrato refers to a low-frequency fluctuation in frequency, an altogether different effect (see Vibrato Effect for details).

Tremolo Demonstration

LabVIEW.png Download and run the LabVIEW VI tremolo_demo.vi, which demonstrates the tremolo effect applied to a sinusoidal oscillator. Tremolo normally requires two controls: rate determines how quickly the amplitude should fluctuate, and depth establishes the amount of amplitude fluctuation. The third control adjusts the pitch of the sinusoidal oscillator.

Modeling the Tremolo Effect

Tremolo is a type of low-frequency amplitude modulation. The screencast video of Figure 2 develops the mathematical equations needed to model the tremolo effect. After watching the video, try the exercises below to ensure that you understand the main concepts.
lfmod_tremolo-eqn.html
Figure 2: [video] Develop the mathematical equations needed to model the tremolo effect
Problem 1
What is the name of the term f R f R ?
[ Click for Solution 1 ]
Solution 1
Rate
[ Hide Solution 1 ]
Problem 2
Which ratio is the basis of depth when expressed in decibels (dB)?
[ Click for Solution 2 ]
Solution 2
Ratio of maximum to minimum loudness
[ Hide Solution 2 ]
Problem 3
True/False: Tremolo rate is typically above 20 Hz.
[ Click for Solution 3 ]
Solution 3
False; tremolo rate is typically between 3 and 10 Hz
[ Hide Solution 3 ]
Problem 4
Which modification to the basic envelope equation is required to avoid clipping?
[ Click for Solution 4 ]
Solution 4
Subtract the depth value D
[ Hide Solution 4 ]
Now that you have been introduced to the main concepts of the tremolo effect, return to the interactive VI of the previous section. Experiment with the depth and rate controls, and confirm that the typical values mentioned in the screencast video in Figure 2 seem reasonable.

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