chapter 3 echo and reverberation

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© 2010 Texas Instruments Inc

Practical Audio Experiments using the TMS320C5505 USB Stick

© 2010 Texas Instruments Inc

“Echo and Reverberation"

Texas Instruments University Programme

Teaching Materials

Chapter 3 - Slide Chapter 3 - Slide 22© © 2010 Texas Instruments Inc 2010 Texas Instruments Inc © © 2010 Texas Instruments Inc 2010 Texas Instruments Inc

Echo and Reverberation


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.


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

Listener

DirectSound

Echo


Block Diagram of Echo

• For the time being, we will consider only one echo path

• Output = Input + Delayed Input

• Because of losses in the delayed path, Gain < 1.

Delay

Input +

+

Simplified Echo

GainDelayed Input

Output


Equation for Simplified Echo

)1)((

)( )()(N

N

Gznx

znGxnxny

z-N

x(n) y(n)+

+

Simplified Echo

G.x(n)z -NG


Frequency Response

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


Frequency Response of Echo


Multiple Echo Paths

• A real room will have several echo paths.

Delay1

Input

+

+

Multiple Echo Paths

Gain1De layed Input 1 Output

Delay2 Gain2De layed Input 2

+


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.


Reverberation

• Reverberation is similar to echo, but uses a slightly different configuration

• Click on the icon to hear reverberation.


Reverberation on Stage

Microphone

Performer

LoudspeakerLoudspeaker

Sound reaches the microphone from both the performer and the the loudspeakers.


Block Diagram of Reverberation

• Output = Input + Delayed Output

Delay

Input Output+

+

Delayed Output

Simplified Reverberation

Gain


Simplified Reverberation Equation

)1/()(

)( )()(N

N

Gznx

znGynxny

z-N

x(n) y(n)+

+

Gy(n)z-N

Simplified Reverberation

G


Matlab Model of Reverberation

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


Reverberation Frequency Response


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.


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.


C Code Implementation


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.


Introduction to Laboratory


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.


USB Stick Setup TMS320C5505

USB to PC

Headphones

Microphone


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.


Files In Project


Console


Changing Echo Buffer Length


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?


Changing the Reverberation Buffer Length


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?


Programming Challenge

• In the real world there are multiple echo and reverberation paths

• Write your own code to combine both echo and reverberation.


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?


References

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

chapter 3 echo and reverberation

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