Adaptive Active Noise Control for Headphones Using the TMS320C30 DSP Application Report 1997 Digital Signal Processing Solutions Printed in U.S.A., January 1997 SPRA160 Adaptive Active Noise Control for Headphones Using the TMS320C30 DSP Angela Kuo Wang and Benedict Tse Advising Professor: Wei Ren University of California at Berkeley Department of Electrical Engineering and Computer Sciences Berkeley, California 94720 This application report consists of one of the entries in a design contest, The 1995 TI DSP Solutions Challenge. The report was designed, prepared, and tested by university students who are not employees of, or otherwise associated with, Texas Instruments. The user is solely responsible for verifying this application prior to implementation or use in products or systems. SPRA160 January 1997 IMPORTANT NOTICE #OPYRIGHTÀ À4EXASÀ)NSTRUMENTSÀ)NCORPORATED Contents ABSTRACT............................................................................................................................ 1 INTRODUCTION.................................................................................................................. 2 OUTLINE .............................................................................................................................. 2 COMMERCIALLY AVAILABLE ANC HEADPHONES....................................................... 3 THEORY AND STRUCTURES FOR ACTIVE NOISE CONTROL...................................... 3 THE FILTERED-X LMS ALGORITHM................................................................................ 6 TMS320C30 IMPLEMENTATION OF AN ADAPTIVE ACTIVE NOISE CONTROL SYSTEM FOR HEADPHONES ........................................................................................ 9 System Components........................................................................................................... 9 System Identification Algorithm...................................................................................... 10 Control Algorithm ............................................................................................................ 11 Implementation of an Adaptive ANC System for Headphones ....................................... 11 RESULTS ............................................................................................................................ 11 SUMMARY AND CONCLUSION ....................................................................................... 16 REFERENCES .................................................................................................................... 17 ACKNOWLEDGMENTS..................................................................................................... 18 Adaptive Active Noise Control for Headphones Page iii List of Illustrations À&)'52%ÀÀ!#4)6%À!#/534)#À./)3%À#/.42/,À&%%$&/27!2$À3#(%-% À&)'52%ÀÀ!$!04)6%À&%%$&/27!2$À#/.42/,À3425#452% À&)'52%ÀÀ!$!04)6%À&%%$"!#+À#/.42/,À3425#452% À&)'52%ÀÀ3934%-À$)!'2!-À/&À4(%À&),4%2%$ 8À,-3À!,'/2)4(-ÀÀÈ&%%$"!#+À#/.&)'52!4)/.É − À&)'52%ÀÀ#/-0/.%.43À).À4(%À0,!.4À 0QÈÉ À&)'52%ÀÀ.!22/7 "!.$À./)3%À#/-0!2)3/.À/&À2%$5#4)/.ÀÀÈ$"À63À&2%15%.#9À).À(:É À&)'52%ÀÀ.!22/7 "!.$À./)3%À!À (:À3).53/)$À7)4(/54À!$!04)6%À!#4)6%À./)3%À#/.42/, À&)'52%ÀÀ.!22/7 "!.$À./)3%À!À (:À3).53/)$À7)4(À!$!04)6%À!#4)6%À./)3%À#/.42/, À&)'52%ÀÀ7)$% "!.$À./)3%À!$!04)6%À!.#À0%2&/2-!.#% À&)'52%ÀÀ7)$% "!.$À./)3%À#/--%2#)!,À(%!$0(/.%3À0%2&/2-!.#% Page iv Adaptive Active Noise Control for Headphones ABSTRACT Commercially available active noise control headphones rely on fixed analog controllers to drive "anti-noise" loudspeakers. Our design uses an adaptive controller to optimally cancel unwanted acoustic noise. This headphone would be particularly useful for workers who operate or work near heavy machinery and engines because the noise is selectively eliminated. Desired sounds, such as speech and warning signals, are left to be heard clearly. The adaptive control algorithm is implemented on a Texas Instruments (TI™)1 TMS320C30GEL digital signal processor (DSP), which drives a Sony CD550 headphone/microphone system. Our experiments indicate that adaptive noise control results in a dramatic improvement in performance over fixed noise control. This improvement is due to the availability of high- performance programmable DSPs and the self-optimizing and tracking capabilities of the adaptive controller in response to the surrounding noise. ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 1 TI is a trademark of Texas Instruments Incorporated. Adaptive Active Noise Control for Headphones Page 1 INTRODUCTION This work addresses the problem of acoustic noise. Since prolonged exposure to excessive levels of acoustic noise can cause permanent hearing loss, safety problems, and lower worker productivity, reduction of noise is an important goal. Machinery and engines are a major source of noise, and much of this noise occurs in the low frequency range. Passive methods used to reduce noise, such as earmuffs, are not especially effective at low frequencies due to the relatively long wavelength of the sound [1]. However, active noise control (ANC) has proven to be effective at these low frequencies [2]. The underlying principle of ANC is to generate a secondary sound wave to destructively interfere with the unwanted noise, thereby reducing the net sound pressure. Active noise control technology can be used in a wide variety of situations, including car or airplane headrests, automobile exhaust mufflers, and refrigerator fans. Headphones that employ ANC would be particularly useful for airport ramp workers, ambulance drivers, and many others who operate on or near heavy machinery. Aside from low-frequency noise control, another benefit is that the offending noise, in some cases, can be selectively eliminated, leaving desired sounds such as speech and warning signals to be heard clearly. For the ANC technique, there are two approaches to designing the controller that drives the "anti-noise" loudspeakers: fixed and adaptive. Fixed (analog) controllers, which are easier to implement, are used in commercial ANC headphones. This paper describes the implementation of adaptive control. The self-optimizing and tracking capabilities of the adaptive controller in response to the surrounding noise suggest that improved performance over fixed controllers is achievable. Furthermore, the availability of high- performance programmable DSPs makes the implementation of computationally-intensive adaptive algorithms feasible. OUTLINE This paper includes: • A brief overview of ANC headphones available on the market • An explanation of ANC theory and structures • One family of adaptive algorithms used for ANC: the filtered-X least- mean-square algorithm (FXLMS) Page 2 Adaptive Active Noise Control for Headphones • A description of the C-language implementation of the FXLMS algorithm, including the associated system identification • A complete TI TMS320C30 implementation of an adaptive ANC system for headphones • A performance comparison between the ’C30-based adaptive ANC system and a commercially available ANC system for headphones COMMERCIALLY AVAILABLE ANC HEADPHONES There are currently three companies that sell ANC headphones: ANVT and NCT Inc., which are active noise control technology companies, and Koss Corporation, a headphone manufacturer. The ANC headphones produced by these companies use fixed analog controllers and claim to achieve a reduction of about 18 dB within the 30 Hz to 1400 Hz frequency bandwidth. However, testing of one pair of headphones shows that an average of 8---10 dB reduction is achieved for both narrow-band and wide-band noise. Because of the limitations of the fixed controller, the commercial headphones are not able to maximize the noise reduction and maintain stability for various types of noises. THEORY AND STRUCTURES FOR ACTIVE NOISE CONTROL Active noise control uses the principle of superposition: two sound waves combine additively. Therefore, if one can deliberately generate a sound wave to oppose the pressure fluctuations of an unwanted sound wave, then the net result is lowered sound pressure fluctuation. As an example of an active noise control problem, consider the situation of a helicopter pilot. A headphone set mounted with actively controlled speakers, which selectively cancel the engine-induced noise, would be quite desirable. The environment would be made safer since the selectivity property means that speech and warning sounds would still be heard clearly. Moreover, such a device could be much less expensive to build than a cockpit that uses passive means to reduce noise [10]. The simplified representation of such a feedforward active noise control system is given in Figure 1, where all the signals shown are sampled signals, − and Q is the unit delay operator. The goal of adapting the feedforward − controller, #QÈ É K , is to cancel the noise at location (c), the pilot's ears. This noise originates from a source, the engine, located at point (a). A detection microphone is placed at location (a), and the signal that is picked up is Adaptive Active Noise Control for Headphones Page 3 − filtered through #QÈ É K and fed to a loudspeaker mounted in the headphone set. The loudspeaker then emits an "anti-noise" (also referred to as secondary noise) to cancel the primary noise at location (c). The uncancelled noise is picked up by an error microphone, which is also mounted in the headphone set, and that error signal, EKÈÉ, is used to tune the filter driving the loudspeaker. Figure 1.