t

Whena conversation takesplaceinsidea room,theacoustic speech signal islinearlydistorted bywallreflections. The room'seffecton thissignalcanbe characterized by a roomimpulseresponse. If the impulseresponse happens to be minimumphase,it caneasilybe inverted.Synthetic roomimpulseresponses weregenerated usinga pointimagemethodto solvefor wall reflections. A Nyquistplot was usedto determinewhethera givenimpulseresponse wasminimumphase.Certainsyntheticroomimpulseresponses werefoundto be minimumphasewhentheinitialdelaywasremoved.For thesecases a mimimumphaseinversefilter wassucessfullyusedto removetheeffectof a roomimpulseresponse ona speech signal. PACS numbers: 43.55.Br, 43.45.Bk

INTRODUCTION

When a conversation

takes place inside a room,

With this. notation H(co) can be factored into a minimum' phase and an allpass part.

the

speech signals are distorted by the presence of nearby

reflectingwalls. Soundstravel not only the direct p{th from source to receiver,

but also reach the receiver

after bouncingoff one or more walls.

The total "room

effect"

in the time

can be viewed

as a convolution

and is often

undesirable.

One

scheme

for

removing the room effect is to pass the distorted speech through a second filter which exactly inverts the effect of the room. The purpose of this paper is to characterize a room impulse response in terms of parameters relevant to its invertibility. An impulse response, like any other finite energy time function, can be characterized by the magnitude and

phase of its Fourier transform. If H(co) is the Fourier transform of h(t) and 4>(00)is the phase of H(oo), then

H(co) = [H(co) I exp[i½ (co)]. For

a certain

class

of functions

(1) known

(3)

do-

main of the speech signal with a room impulse response. This room effect is perceived as echo and reverberation

H(co) = M(co)A(co), where

as minimum

and

A(0•)= exp[iCa(•o)].

Notethat

(5)

- x for any½•(co).WhenH(co)is

minimumphaseq•,(•o)=0, whichimpliesthat A(o0)=i. Minimum phase impulse responses are of particular interest because their inverses are guaranteed to be minimum phase and casual. (The truth of this statement can be seen by considering the s plane of the Laplace transform where the inverse filter replaces poles with zeros and vice versa. If the original impulse has poles and zeros only in the left half-plane, then its inverse must have poles and zeros only the left halfplane.) If a room impulse response is minimum phase, then an inverse filter will exist capable of completely

phase functions, q•(•o)can be uniquely determined from

removingthe room's effect from a speechsignal. Fur-

IH(•o)[. A function is saidto beminimum phaseif its

thermore,

Laplace transform contains no poles or zeroes in the right half-plane. When a function is minimum phase, the log magnitude and phase of the Fourier transform

tionship depends upon the fact that the log of the Laplace transform is analytic in the right half-plane for minimum phase functions.

can be determined

from

If A(•o) is not identically equal to one, then it will be (as a consequenceof its definition) nonminimum

phase. Thus,H(•o)will notbe minimumphaseandits inverse may be either unstable or acasual.

If, how-

ever, A(•o) represents a "pure delay" it will introduce

We expect

h(Dto be a stable,casual,but, in general,nonminimum phase impulse response.

filter

estimated from the signal power spectra.

are related throughthe Hilbert transform. • This rela-

Let h(t) be an arbitrary room impulse.

this inverse

knowledge of only the magnitude of the room's frequency response (i.e., unknown phase), which can be

The phase of the Four-

no perceptual distortion in the speech signal. In terms of an impulse response, "pure delay" is defined as an

ier transform q•(•o)can be expressed as the sum of a

allpassfunctionwith a groupdelay re•(•o)xvhichis con-

minimum phase component•m(•o) (as determined from

stant for all frequencies,

I

1)anda component whichrepresent thedeviation

from minimum'phase

½

+

(2)

i.e.,

T,a(CO) = - [dCPa(Co)/dco ] - constant.

(6)

If a room impulse response were minimum phase with pure delay, then an inverse filter would only need to remove the minimum phase component.

The worst case, for inverse filters,

a)Presentaddress:ComputerSystemsLaborat9ry,724South Euclid Avenue, St. Louis, MO 63110.

165

J. Acoust. Soc.Am.66(1), July1979

is a room im-

pulse for which A(•o) has a group delay which is not independentof frequency. Perceptually effective inverse filters

may exist for this case, but will not be

0001-4966/79/070165-05500.80

(D1979Acoustical Societyof America

Downloaded 21 Jan 2012 to 129.16.87.99. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp

165

considered

in this paper.

component.

To investigate the usefulness of the minimum phase inverse filter, synthetic room impulse responses were generated by computer and separated into their minimum phase and allpass components. It was found that certain room impulses are truly minimum phase with pure delay. For these cases a minimum phase inverse

A(t•)= H(I•)/M(t•).

Because of the special significance of a pure delay, the group delay of the allpass component was of particular interest. Group delay was computed by the formula

(})J' r,a (/z) __ im[jJ

filter was effective in removing the "room effect" from a distorted speech signal.

(13)

where A'(k)is

(14)

the frequency derivative of A(k),

Im(-) denotes the imaginary part,

I. METHODS

N-1

A'(/z) =- i DFT[na(n)]--i •, ha(n)exp[-i(2•r/N)trn],

Synthetic impulse responses were generated by an

n=0

existing computerprogram2 on a Data GeneralS200 Eclipse computer.

This synthesis program

(15)

accepts

and

values for room size (length, width, and height) source location, and reflectivity for each of the six walls. The synthetic impulse responses are found by using a point image method to solve for wall reflections. Duration

of the computed responses was 2048 samples (or 204.8 ms assuming a 10 kHz sampling rate). The minimum phase component of the impulse response was determined by zeroing the cepstrum for

negativequefrencies.a For a finite sequenceh(n) (such

i N-1

(16) a(n) =DFT'•[A(k)] =• • A(k) exp[i(2•/N)kn]. •-o

The motivation for determining the minimum phase component of the impulse response was the relative ease of computing a minimum phase inverse filter. The impulse response of the minimum phase inverse filter

g(n) was computedby taking the inverse DFT of the reciprocal of the minimum phase spectrum M(•z):

as the truncated room impulse responses) a real, per-

G(k)=i/M(k),

iodic approximationto the cepstrumc•(n), is definedby the following equations-

g(n) =DFT'•[G(k)] - • G(kiexp[i(2•/N)kn]. (18)

N-1

•o

H(/z) - DFT[h(n)] - Z h(n)exp[-i(2•r/N)kn], (7) r•--0

C(k)= log[H(/z) [,

(8) 1

To test the effectiveness of the inverse filter perceptually, a selected speech sample was filtered with the

room impulse response and inverse-filtered with the

c•(n) =DFT'I[C(/z)] =• • C(/z) exp[i(2•r/N)trn], (9) minimum •-0

where

DFT

indicates

a discrete

Fourier

transform.

cy responseH(k) is computed f•romc•(n) in the following manner. Sincec•(n) is a periodic functionof n, the cepstrum may be effectively

zeroed for negative

quefrenciesby settingthe secondhalf of c•(n) to zero. is transformed

back to the fre-

quency domain to obtain M(/z). This procedure is described by the following equations. If

ct,(n), n = O,N/2,

The resultant

speech was

III.

Finally, a necessary and sufficient condition was desired for determining whether or not a given impulse response was, indeed, minimum phase. For this purpose the Nyquist criterion was used to detect the presence of nonminimum phase zeros. The Nyquist criterion is based on a mapping theorem of Cauchy. If a complex variable z in the z plane describes a contour

C• in a positive sense, then F(z), a function of the com-

•n(n)= 2c,,(n),1-