Servo Feedback Control of an Audio Subwoofer

by

Dalimar Vélez

Advisor: Shawn Hunt

Overview

• Objectives• Background and Concepts• Problem• Implementation Design• Procedure• Results• Conclusion

Objectives

• Designing and building a Digital Servo-Feedback Control of an Audio Subwoofer.

Background and Concepts

• A subwoofer is a speaker who reproduces low frequencies (20 to 500 Hz).

• An accelerometer measures the acceleration of a system. (ex. ADXL150)

• Harmonics are integral multiple of the fundamental frequency.

nffh

Background and Concepts

• Filters:– Low pass

– Adaptive filter structure for system identification

– Adaptive filter structure for noise cancellation

Pass Band

Wc

Stop Band

W

H|(jw)|

Unknown System

X(n)

d(n)

y(n)+

- e(n)Adaptive Filter

n‘(n)

d(n)

y(n)+

- e(n)Adaptive Filter

s(n) + n(n)

Problem

• The subwoofer creates most of the harmonic distortion in a speaker.

• To improve this:– Measure subwoofer motion cone– Compare the output with the input– Correct the difference

This is called a Servo Feedback Control

Implementation Design

• In order to reduce the noise in the subwoofer output is essential to know the distortion added by the system to the input signal.

• A system identification adaptive filter was developed to obtain the response of the subwoofer.

• This response is going to be used to modify the input of the subwoofer.

• These modifications will cancel the distortions, making the output of the subwoofer clearer.

Procedure

• Music is filtered by a low pass.

DSP Board

ADC LPF AdaptiveFilter DAC

Weights

Music

Acc

AdaptiveFilter

Procedure• The accelerometer is connected directly to the

cone, without signal amplification, to an Analog to Digital Converter (ADC) in the DSP board.

DSP Board

ADC LPF AdaptiveFilter DAC

Weights

Music

Acc

AdaptiveFilter

Procedure

• Develop algorithm for feedback loop.

DSP Board

ADC LPF AdaptiveFilter DAC

Weights

Music

Acc

AdaptiveFilter

Procedure

• Develop algorithm for frequency response and motion deviations correction.

DSP Board

ADC LPF AdaptiveFilter DAC

Weights

Music

Acc

AdaptiveFilter

Photos

Photos

Photos

• Distortion produced by the cone:

Results

• The green graph represents the input of the system and the violet graph shows the output distortion created by the subwoofer

Results

Results

• The adaptive filter identified the response of the subwoofer system.

• The algorithm reduced the harmonic distortion.

• The 180 Hz harmonic for this case was attenuated from -46.5dB to -65 dB.

Conclusion

• The distortion was caused by the physical properties of the subwoofer driver.

• The reduction of harmonics at the output of the subwoofer was accomplished thanks to the usage of the coefficients found by the adaptive filter.

• The hardware can be implemented in any other application independently of the driver used.

References

• [1] ±50g Single Axis Accelerometer with Analog Output - http://www.analog.com/

• [2] Harmonic – http://whatis.techtarget.com

• [3]Chassaing, Rulph. DSP Applications Using C and the TMS320C6x DSK. John Wiley & Sons, nc, NY 2002

• [4]Antoniou, Andreas. Digital filters, analysis, design and applications, 2th ed. Mc Graw Hill, Inc ,1993.

• [5]Doucet, Dougla., Fasani Rob. Hasenkamp, Pete r. and Senesky Matthew. Using an Accelerometer to Make an Active Subwoofer.

www.webee.com/Schematics/Active%20Subwoofer/SubWoof.htm#con

Questions?

For more info:http://ece.uprm.edu/~s998705/investigacion.htm