Chapter 3 Echo And Reverberation

Practical Audio Experiments using the TMS320C5505 USB Stick

“Echo and Reverberation" Texas Instruments University Programme Teaching Materials

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Echo and Reverberation

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Chapter 3 - Slide 2

Objectives

• To explain echo and reverberation • To implement them in C code • To test the models using the Texas Instruments TMS320C5505 USB Stick with a microphone and headphones/computer loudspeakers.

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Echo • Echo is audible because the speed of sound is relatively slow, about 400 meters per second • Click on icon to listen to echo.

Sound Source

Direct Sound Echo

Listener © 2010 Texas Instruments Inc

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Block Diagram of Echo • For the time being, we will consider only one echo path

• Output = Input + Delayed Input Simplified Echo Input

Output

+ + Delay

Gain Delayed Input

• Because of losses in the delayed path, Gain < 1. © 2010 Texas Instruments Inc

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Equation for Simplified Echo Simplified Echo x(n)

y(n)

+ + z-N

G

G.x(n)z -N

y (n)  x(n)  Gx(n) z  x(n)(1  Gz © 2010 Texas Instruments Inc

N

N

) Chapter 3 - Slide 6

Frequency Response • The frequency response of echo can be calculated using the following Matlab .m file.

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Frequency Response of Echo

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Multiple Echo Paths • A real room will have several echo paths. Multiple Echo Paths Input

+ Output

Delayed Input 1 Delay1

Gain1

+ + Delayed Input 2

Delay2

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Gain2

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Summary of Echo • The Echo output is derived solely from the input • Certain frequencies are attenuated • Because the poles lie inside the unit circle, echo is stable.

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Reverberation • Reverberation is similar to echo, but uses a slightly different configuration • Click on the icon to hear reverberation.

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Chapter 3 - Slide 11

Reverberation on Stage Sound reaches the microphone from both the performer and the the loudspeakers.

Loudspeaker

Loudspeaker Performer

Microphone

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Chapter 3 - Slide 12

Block Diagram of Reverberation

• Output = Input + Delayed Output

Simplified Reverberation Input

+

Output + Gain

Delay

Delayed Output

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Simplified Reverberation Equation Simplified Reverberation x(n)

+

y(n) + G

Gy(n)z-N

z-N

y (n)  x(n)  Gy(n) z  x(n) /(1  Gz © 2010 Texas Instruments Inc

N

N

) Chapter 3 - Slide 14

Matlab Model of Reverberation Using Matlab, the frequency response of reverberation can be calculated using the following .m file.

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Chapter 3 - Slide 15

Reverberation Frequency Response

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Summary of Reverberation • The output is derived from both the input and the previous output • At certain frequencies, the output will be amplified

• Because the poles lie on the unit circle, reverberation can become unstable.

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Chapter 3 - Slide 17

Real World Echo and Reverberation • A real room will have several echo paths

• When sound is reflected off a surface there will be “coloration”. Certain frequencies will be absorbed and there will be phase changes • Therefore, a commercial echo / reverberation unit will contain many different delay paths.

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Chapter 3 - Slide 18

C Code Implementation

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Chapter 3 - Slide 19

C Code Implementation

• This laboratory uses a sampling rate of 24000 Hz rather than 48000 Hz • This allows longer delay times to be implemented

• The echo and delay buffers are implemented as circular buffers.

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Chapter 3 - Slide 20

Introduction to Laboratory

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Chapter 3 - Slide 21

Practical Echo and Reverberation

• In the Laboratory you will build a C code implementation for echo and reverberation • You will be able to run different echo and reverberation effects on the TMS320C5505 USB Stick.

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Chapter 3 - Slide 22

USB Stick Setup TMS320C5505 USB to PC Microphone Headphones

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Installing the Application • Copy the code from Application 3 Echo and Reverberation to the workspace • Follow the steps previously given in Chapter 1 to set up the new project.

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Files In Project

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Console

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Changing Echo Buffer Length

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Adjusting the Echo • In the file echo.c, change the size of the delay buffer N • How does the echo effect change as the value of N is decreased and increased? • What is the maximum value of N supported by the hardware?

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Chapter 3 - Slide 28

Changing the Reverberation Buffer Length

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Chapter 3 - Slide 29

Adjusting the Reverberation • In the file reverberation.c, change the value of N to change the reverberation buffer length. The current value is 200ms • Change the value of DEPTH between 0 and 32767. Listen to the effect • Question: What happens when DEPTH is 32767, that is 100% is fed back?

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Chapter 3 - Slide 30

Programming Challenge • In the real world there are multiple echo and reverberation paths • Write your own code to combine both echo and reverberation.

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Chapter 3 - Slide 31

Questions • What are the differences between echo and reverberation?

• Which of echo and reverberation requires the least memory to write your program? • How would you ensure that reverberation remains stable?

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Chapter 3 - Slide 32

References • A Digital Signal Processing Primer by Ken Steiglitz. ISBN 0-8053-1684-1.

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Chapter 3 - Slide 33