Digital Audio Signal Processing Topic-7: Active Noise Control & 3D Audio Marc Moonen Dept. E.E./ESAT, KU Leuven [email protected]

Digital Audio Signal Processing

Topic-7: Active Noise Control & 3D Audio

Marc MoonenDept. E.E./ESAT, KU Leuven

[email protected]

Digital Audio Signal Processing Version 2012-2013 Topic 7: ANC & 3D Audio p. 2

Lecture-6: Active Noise Control & 3D Audio

• Active Noise Control– General set-up– Feedforward ANC & Filtered-X LMS– Feedback ANC – Reference : S.J.Elliott & P.A.Nelson, Àctive Noise Control’, IEEE Signal

Processing Magazine, October 1993, pp 12-35

• 3D Audio – Head related transfer functions(HRTF)– Binaural synthesis– Cross-talk cancellation


Active Noise Control - Intro

• Passive noise control : sound absorbers, …,

works well for high frequencies (`centimeter-waves’)

• Active noise control : for low frequencies (e.g. 100 Hz>lambda=3,4m.)

– General set-up:

- ANC works on the principle of destructive interference between the sound field generated by the `primary’ (noise) source and the sound field due to secondary source(s), whose output can be controlled

aim: generate `quiet’

at error microphone



– Secondary source(s) : • mostly loudspeakers• sometimes mechanical `shakers’ (excitation of structural

components)

– Signal processing task : generation/control of electrical signal(s) to steer secondary source(s)

– Two approaches will be considered: • Feedforward ANC : solution based on `filtered-X LMS’• Feedback ANC : see also control courses

– PS: First ANC Patent in 1936 (!) (Paul Lueg)

`describes basic idea of measuring a sound field with a microphone, electrically manipulating the resulting signal and then feeding it to a secondary source…’



– Destructive interference relies on superposition & linearity :• Propagation of acoustic waves is approximately linear. • Non-linearity may be due to loudspeakers (secondary sources)

After destructive interference at main frequency, harmonics generated

by loudspeakers may become distinctly audible.

– Destructive interference at one point, may imply constructive interference at other points:

secondary source to be placed close to error microphone, so that only modest secondary signal is required, and hence points further away from secondary source are not affected. Produce `zone of quiet’ near the error microphone (e.g. 10dB reduction in zone approx (1/10).lamba)

secondary

`shut up…’[quiet]

`SHUT UP…’


Feedforward ANC (1)

Basic set-up:

– C(z) = secondary path = acoustic path from secondary source to error microphone, including loudspeaker and microphone characteristic. C(z) can be modeled/identified, based on training sequences, etc. (calibration)

– PS: feedback in filter coefficient adaptation path

C(z)

d

eW(z) y

secondary source

primarysource

x


Feedforward ANC (2)

Design problem:

– given (?) secondary path C(z), design W(z) that `minimizes’ E(z)

– ìdeal’ solution is W(z)=-H(z)/C(z) …H(z) generally unknown

C(z)

d

eW(z) y

secondary path

primarysource

x

H(z)

)().)(

)().(1()( zD

zH

zWzCzE


Filtered-X LMS (1)

– straightforward application of LMS :

…does not work here

(example C(z)=-1, then steepest ascent instead of steepest descent)

C(z)

d

eW(z) y

secondary path

primarysource

x

H(z)

kkkk e..1 xww


– This would have been a simpler problem (swap C and W)…

...allowing for straightforward application of LMS, with filtered x-signal

– Only time-invariant linear systems commute, hence will require slow adaptation of W(z) (see page 11)

kfkkk e..1 xww

C(z)

d

eW(z) y

H(z)x

fx

Filtered-X LMS (2)


– filtered-X LMS scheme : swapping of C and W in adaptation path (not in filtering path)

…with C’(z) an estimate of C(z)– PS: H(z) unknown and not needed for adaptation (like in AEC)

C(z)

d

eW(z) y

secondary path

primarysource

x

H(z)

C’(z)

xfx k

fkkk e..1 xww

Filtered-X LMS (3)


– Filtered-X LMS convergence (empirical result)

N=filter length W(Z)

L=filter length C’(z)

– Stability also affected by the accuracy of the filter C’(z) modeling the true secondary path C(z).

Found to be `surprisingly’ robust to errors in C’(z)... (details omitted)

}{).(

10 2f

kxELN

Filtered-X LMS (4)


Feedforward ANC (3)

Additional problem-1:

Feedback from secondary source (loudspeaker) into reference microphone.

This is an acoustic echo cancellation/feedback problem :– Fixed AFC based on model of F(z), obtained through calibration, is

easy – Adaptive AFC is problematic (combination of 2 adaptive systems)

C(z)

d

eW(z) y

secondary source

primarysource

x

F(z)


Feedforward ANC (4)

Additional problem-2:

Additive noise in error microphone (e.g. due to air flow over microphone, etc.)

Cancellation of primary source signal corrupted by noise, similar to near-end noise/speech in AEC

C(z)

d

eW(z) y

secondary source

primarysource

x

noise


Feedforward ANC (5)

Extensions: multiple reference signals/multiple secondary

sources/multiple error signals

– Applications: airplane/car cabin noise control, active vibration control,...– Needs generalization of Filtered-X algorithm, where coefficients of

control filters are adapted to minimize the sum of the mean square values of the error signals.


Feedforward ANC (6)

Multiple Error (filtered-X) LMS:

• K reference signals• M secondary sources• L error microphones• MxL different secondary paths between M secondary sources and L error

microphones• all K reference signals are filtered (cfr `filtered-X’) by all MxL secondary

path models, …• …to generate collection of KxMxL filtered reference signals, which are input

to the adaptive filter• etc..

K M L


Feedback ANC (1)

Basic set-up :

– C(z) = secondary path (see page 6) – 1 microphone instead of 2 microphones– Applications : active headsets, ear defenders

W(z)

C(z)

primary source

d

ey

secondary source


Feedback ANC (2)

Design problem :

• given C(z) design W(z) (=feedback control) such that E(z) is `minimized’

• For `flat’ C(z)=Cnt : W(z)=-A for large A (like in an opamp) • For general C(z) : see control courses

W(z)

C(z)

d

ey

+

)(.)().(1

1)( zD

zWzCzE


Feedback ANC (3)

An interesting feedback controller is formed as follows :

…with C’(z) is an estimate of C(z) and W’(z) yet to be defined.

Note that if C’(z)=C(z), then W’(z) is fed by d (!), i.e. …

d

e

+

W’(z)

C(z)

y

-C’(z)

+ )(').('1

)(')(

zWzC

zWzW

)(.)(')}.()('{1

)(')('1)( zD

zWzCzC

zWzCzE


Feedback ANC (4)

Note that if C’(z)=C(z), then W’(z) is fed by d (!), i.e. …

…which means the feedback system has been transformed into a feedforward system, similar to page 12..

d

e+

W’(z)

C(z)

yd

)(.1

)(')(1)( zD

zWzCzE


Feedback ANC (5)

In the set-up of page 12, this is …

– with H(z) =1, and for C(z) containing pure delay, this means W’(z) must act as a predictor for d.

– Adaptation of W’(z) based on filtered-X algorithm

C(z)

d

eW’(z)

y

secondary path

primarysource

x

1


Feedback ANC (6)

Application : active headsets / ear defenders : • 10-15dB reduction can be achieved for frequencies 30-500Hz• Problem: variability of secondary path (headsets worn by different

people, or worn in different positions by the same person, etc.)• Headset can also be used to reproduce a useful signal ù’

(communications signal, music, ..) : electrically subtract u from error microphone signal

W(z)

C(z)

d

e

y

+

+-u

Prove it !


3D Audio

Virtual acoustic displays = systems that can render sound images positioned

arbitrarily around a listener. Two approaches…

– Acoustic soundfield synthesis : reproduce original soundfield èverywhere’, with large number of transducers. Suitable for multiple listeners.

– Binaural audio : reproduce original soundfield at (2) eardrums, with headphones or -at least stereo- loudspeakers Suitable for single listener

virtual source location


Head Related Transfer Function (HRTF)

HRTF is acoustic transfer function from a specific sound location to the eardrum, and describes diffraction of sound by the torso, head and external ear– HRTFs differ significantly across subjects (especially for high

frequencies (>6kHz))– àverage’ HRTFs measured on mannequins

– Applications use HRTF data base (HRTF for each position)

source location p

),( pH


Binaural Synthesis

For source X(z) to be virtually placed at position p, signals to be delivered at left/right eardrums are

– multiple sources

– referred to as `binaural’ signal, because it would be suitable for headphone listening. Head-phone reproduction (with non-individualized HRTFs) often suffers from in-head localization, front-back reversals, ...

– TFs may include desired room acoustics (e.g. concert hall, …)

)(.),(

),(

)(

)(zX

H

H

zX

zX

R

L

R

L

Η

p

p

ii

iR

iL

R

L zXH

H

zX

zX)(.

),(

),(

)(

)(

p

p


Cross-talk Cancellation

To correctly deliver the binaural signal to the listener, the signals must be equalized, to compensate for transmission paths from loudspeakers to eardrums. Transmission path inversion is referred to as `cross-talk cancellation’, as it involves cancellation of unwanted cross-talk from each speaker to the opposite ear.

A_LL is HRTF from left speaker to left eardrum,

should also include actual room acoustics….

PS: Channel inversion, see Topic-6 (easier with e.g. 3 loudspeakers for 2 ears) PS: Equalization zone (`sweet spot’) typically small: translation<10cm, rotation<10degrees

)(.),(

),(.

)(

)(

1

1

zXH

H

AA

AA

zY

zY

R

L

RRLR

RLLL

R

L

HC

p

p


Compare to feedforward ANC...

(see page 6))(.

),(

),(.

)(

)(

1

1

zXH

H

AA

AA

zY

zY

R

L

RRLR

RLLL

R

L

HC

p

p

d

Ce

W y

secondary path

primarysource

x

H

HCW .1


Compare to feedforward ANC...

Adaptive ?

– head movement tracking (e.g. video-based) + compensation, provides

• larger equalization zone

• dynamic localization cues (by maintaining stationary virtual sources during head motion)

– error signal only àvailable’ during calibration, hence difficult to compensate for variations in acoustic channels

C

eW y

secondary path

primarysource

x

H


Sound Field Synthesis

Huygens’ principle: Synthesize sound field in a listening area, based on secondary sources

(loudspeakers) on an enclosure of listening area, playing back recorded

(with microphones on the same enclosure) sequences

virtual

sound source

…

…


Sound Field Synthesis

Huygens’ principle: This may be realized as a multichannel ANC system which then allows for

an equalization of the actual listening room, as well as a reproduction of a virtual listening room

virtual

sound source

…

…

= Multi-channel extension of p. 25-26:• H(z) contains L (virtual) acoustic TFs from virtual sound source to mics

• C(z) contains MxL (real) acoustic TFs from loudspeakers to mics

M loudspeakers

L microphones


Conclusions

• Active Noise Control :

- Feedforward systems (with implicit feedback)

- Feedback systems (turned into feedforward)

• 3D Audio :

- Binaural synthesis & cross-talk cancellation.

- Soundfield synthesis

Documents

Digital Audio Signal Processing Topic-7: Active Noise Control & 3D Audio Marc Moonen Dept. E.E./ESAT, KU Leuven [email protected]