DIGITAL AUDIO WATERMARKING: SURVEY

Transcription

1 DIGITAL AUDIO WATERMARKING: SURVEY MIKDAM A. T. ALSALAMI * MARWAN M. AL-AKAIDI ** * Computer Scece Dept. Zara Prvate Uversty / Jorda ** School of Egeerg ad Techology - De Motfort Uversty / UK Abstract: Dgtal audo watermarg s a techque for embeddg addtoal data alog wth audo sgal. Embedded data s used for copyrght ower detfcato. A umber of audo watermarg techques s proposed. These techques explot dfferet ways order to embed a robust watermar ad to mata the orgal audo sgal fdelty. Ths paper maes a tutoral geeral dgtal watermarg prcples ad focus o descrbg dgtal audo watermarg techques. These techques are classfed accordg to the doma where the watermar s embedded. Coclusos are reported ad a geeral wor frame s explaed. Keywords: Dgtal watermarg, audo, copyrght protecto.. INTRODUCTION As dgtal multmeda wors (vdeo, audo ad mages) become avalable for retrasmsso, reproducto, ad publshg over the Iteret, a real eed for protecto agast uauthorzed copy ad dstrbuto s creased. These cocers motvate researchers to fd ways to forbd copyrght volato. The most promsg soluto for ths challegg problem seems to le formato hdg techques. Iformato hdg s the process of embeddg a message to dgtal meda. The embedded message should be mperceptble; addto to that the fdelty of dgtal meda must be mataed. Iformato hdg ule cryptography. I cryptographc techques sgfcat formato s ecrypted so that oly the ey holder has access to that formato. But, oce the formato s decrypted the securty s lost. I formato hdg, message s embedded to dgtal meda, whch ca be dstrbuted ad used ormally. Iformato hdg does t lmt the use of dgtal data. Iformato hdg ca be classfed to two d of techques: Stegaography ad Watermarg. The ma purpose of stegaography s to hde the fact of commucato. The seder embeds a secret message to dgtal meda (e.g. mage) where oly the recever ca extract ths message. The warde of commucato chael wll otce the trasmtted meda, but he/she wll ever perceve the bured secret message sde ths meda. Fgure. llustrates a smple stegaographc system. I ths system the message m s embedded to the Cover-object C (could be mage, audo or vdeo) to produce the Stego-object S that should has the same fdelty of C. The Cover-Object s oly used for the Stegoobject geerato ad s the dscarded. The embeddg operato s parameterzed by the ey that s ow for both eds of commucato: seder ad recever. O recever sde the bured message s extracted from Stego-object detecto process. Embeddg message should be perceptually ad statstcally udetectable for the warde. A deal stegaographc system would embed a large amout of formato perfectly securely wth o vsble degradato to the coverobject. Watermarg s very smlar to stegaography that both see to hde formato the Coverobject. However stegaography s related to secret pot-to-pot commucato betwee two partes. Thus, stegaography techques are usually havg a lmted robustess ad protect for the embedded formato agast modfcatos that may occur durg trasmsso, le format coverso, compresso or A/D coverso. O the other had, Message m Cover-Object C Embeddg Stego-Object S Detecto Embedded Message Key Warde Key Fgure. Stegaographc System

2 watermarg rather tha stegaography prcples s used wheever the meda s avalable to partes who ow the exstece of the embedded formato ad may have terest removg t. Thus, watermarg adds addtoal requremets of robustess. A deal watermarg system would embed formato that could ot be removed or altered wthout mag sgfcat perceptual dstorto to the meda. A popular applcato of watermarg s to gve a proof of owershp of dgtal data by embeddg copyrght statemets. Ths paper s orgazed as follows. Secto 2 descrbes the modules of watermarg systems ad the fucto of each module. Secto 3 ad 4 are to expla the applcatos ad requremets of dgtal watermarg. Secto 5 covers dgtal audo watermarg techques through four subsectos. Fally, coclusos ad geeral wor frame for audo sgal are preseted. 2. WATERMARKING SYSTEM MODULES A watermarg system cossts of three modules that are watermar sgal geerato module, watermar embeddg module ad watermar detecto module. Watermar sgal s geerated by usg a o-vertble fucto that taes, as a put, a watermar ey. I some systems the host sgal (cover-object) s tae to accout whe watermar s geerated. Ths wll help watermar geerator producg a mperceptble sgal-depedet watermar. Watermar embeddg s performed tme doma or trasform doma (DFT, DCT, DWT, etc) usg a sutable embeddg rule (e.g. addto or multplcato). Fally, watermar s detecto s performed by some sort of correlato detector or statstcal hypothess testg, wth or wthout resortg to the orgal sgal. 3. DIGITAL WATERMARKING APPLICATIONS The requremets that watermarg system has to comply wth are always based o the applcato. Thus, before we revew the requremets ad desg cosderatos, we wll preset the applcatos of watermarg [Cox et al, 2002; Katzebesser ad Pettcolas, 2000]: 3. Copyrght protecto Copyrght protecto s the most mportat applcato of watermarg. The objectve s to embed formato detfes the copyrght ower of the dgtal meda, order to prevet other partes from clamg the copyrght. Ths applcato requres a hgh level of robustess to esure that embedded watermar caot be removed wthout causg a sgfcat dstorto dgtal meda. Addtoal requremets besde the robustess have to be cosdered. For example, the watermar must be uambguous ad stll resolve rghtful owershp f other partes embed addtoal watermars. 3.2 Fgerprtg The objectve of ths applcato s to covey formato about the legal recpet rather tha the source of dgtal meda, order to detfy sgle dstrbuted copes of dgtal wor. It s very smlar to the seral umber of software product. I ths applcato a dfferet watermar embedded to each dstrbuted copy. I cotrast the frst applcato where oly a sgle watermar s embedded to all copes of dgtal meda. As well as copyrght protecto applcato of watermarg, fgerprtg requres hgh robustess. 3.3 Cotet Authetcato The objectve of ths applcato s to detect modfcato of data. Ths ca be acheved wth socalled fragle watermar that have a low robustess to certa modfcato (e.g. Compresso). 3.4 Copy Protecto Ths applcato tres to fd a mechasm to dsallow uauthorzed copy of dgtal meda. Copy protecto s very dffcult ope systems; closed system, however, t s feasble. I such systems t s possble to use watermars to dcate the copy status of the dgtal meda (e.g. copy oce or ever copy). O the other sde, copy software or devce must be able to detect the watermar ad allow or dsallow the requested operato accordg to the copy status of the dgtal meda beg coped. 3.5 Broadcast Motorg Producers of advertsemets or audo ad vdeo wors wat to mae sure that ther wors are broadcasted o the tme they purchase from broadcasters. The low-tech method of broadcast motorg s to have huma observers watch the broadcastg chaels ad record what they see or hear. Ths method s costly ad error proe. The soluto s to replace the huma motorg wth automated motorg. Oe method of automated broadcast motorg s to use the watermarg techques. Wth watermarg we ca embed a detfcato code the wor beg broadcasted. A computer-base motorg system ca detect the embedded watermar, to esure that they receve all of the artme they purchase from the broadcasters. 4. PROPERTIES OF DIGITAL WATERMARKING Watermarg systems ca be characterzed by a umber of propertes [Cox et al, 2002; Katzebesser ad Pettcolas, 2000]. The relatve mportace of each property depeds o the requremets of the system applcato.

3 The propertes beg dscussed ths secto are assocated wth watermar embedder, watermar detector, or both. 4. Embeddg Effectveess The effectveess of a watermarg system s the probablty that the output of the embedder wll be watermared. The cover wor s sad to be watermared whe put to a detector result postve detecto. The effectveess of a watermarg system may be determed aalytcally or emprcally by embeddg a watermar a large umber of cover wors ad detect the watermar. The percetage of cover wors that result postve detecto wll be the probablty of effectveess. 4.2 Fdelty I geeral, the fdelty of a watermar system refers o the perceptual smlarty betwee the orgal ad the watermared verso of the cover wor. However, watermared wor may be degraded the trasmsso process pror to ts beg perceved by a perso, a dfferet defto of fdelty may be more approprate. We may defe watermarg system fdelty as a perceptual smlarty betwee the uwatermared ad watermared wors at the pot at whch they are preseted to a vewer. 4.3 Data Payload Data payload refers to the umber of bts a watermar embeds a ut of tme or wors. For audo, data payload refers to the umber of embedded bts per secod that are trasmtted. Dfferet applcatos requre dfferet data payload. For example, Copy cotrol applcatos may requre a few bts embedded cover wors. 4.4 Bld or Iformed Detector We refer to the detector that requres the orgal, uwatermared wor as a formed detector. Iformed detectors may requre formato derved from the orgal wor rather tha orgal wor tself. Coversely, detectors that do ot requre the orgal wor are referred to as bld detectors. Iformed detector has a good performace watermar extracto. However, ths wll result a huge umber of orgal wors have to be stored. 4.5 False Postve Rate A false postve s the detecto of a watermar a cover wor that does ot actually cota oe. Whe we tal of a false postve rate, we refer to the umber of false postves we expect to occur a gve umber of rus of the detector. 4.6 Robustess Robustess refers to the ablty to detect the watermar after commo sgal processg operatos. Audo watermarg eeds to be robust to temporal flterg, A/D coverso, tme scalg,..etc. ot all applcatos of watermarg requre all the forms of robustess. Ths depeds o the ature of applcato of watermarg system. 4.7 Securty The securty of a watermar refers to ts ablty to resst hostle attacs. Hostle attac s the process specfcally teded to thwart the watermar s purpose. The types of attacs ca fall three categores: uauthorzed removal, uauthorzed embeddg, ad uauthorzed detecto. 4.8 Cost Cost of watermarg system refers to the speed wth whch embeddg ad detecto must be performed ad the umber of embedders ad detectors that must be deployed. Other ssues clude the whether the detector ad embedder are to be mplemeted as hardware devce or as software applcato or plug-s. 5. DIGITAL AUDIO WATERMARKING Watermarg dgtal meda has receved a great terest the lterature ad research commuty. Most watermarg schemes focus o mage ad vdeo watermarg. A few audo watermarg techques have bee reported. Dgtal audo watermarg s the process of embeddg a watermar sgal to audo sgal. Audo watermarg s a dffcult job because of the sestvty of Huma Audtory System (HAS). The requremets metoed earler are commo to both mage ad audo watermarg techques. Despte ther smlartes, audo ad stll mage watermarg systems exhbt sgfcat dffereces. Frst of all, the fact that mages are two-dmesoal sgals provdes attacers wth more ways of troducg dstortos that mght affects watermar tegrty. For example, scalg, rotato or removal of rows/colums. Audo watermarg methods eed ot to deal wth such attacs, as audo s a oe-dmesoal sgal. Due to the dfferece betwee Huma Audtory System (HAS) ad Huma Vsual System (HVS), dfferet masg prcples should tae to accout each case. Dgtal audo watermarg techques ca be classfed accordg to the doma where the watermarg taes place. Followg sectos wll dscuss audo watermarg techques ad classfy them to four categores. 5. Frequecy Doma Audo Watermarg Audo watermarg techques, that wor frequecy doma, tae the advatage of audo masg characterstcs of HAS to embed a audble watermar sgal dgtal audo. Trasformg audo sgal from tme doma to frequecy doma eables watermarg system to embed the watermar to perceptually sgfcat

4 Iput Sgal Frequecy Trasform Watermar Embeddg Iverse Frequecy Trasform Watermared Sgal compoets. Ths wll provde the system wth a hgh level of robustess [Cox et al, 997], because of that ay attempt to remove the watermar wll result troducg a serous dstorto orgal audo sgal fdelty. Fgure 5. shows a dagram of frequecy based watermarg system. The put sgal s frst trasformed to frequecy doma where the watermar s embedded, the resultg sgal the goes through verse frequecy trasform to get the watermared sgal as output. Watermar ca be embedded to frequecy doma compoets by mea of dfferet methods, Cox ad et al [Cox et al, 997] propose usg the spread spectrum techque frequecy doma. I spread spectrum commucato, oe trasmts a arrowbad sgal over a much larger badwdth such that the sgal eergy preset ay sgle frequecy s mperceptble. Smlarly the watermar s spread over very may frequecy compoets so that the eergy of ay compoet s very small ad certaly udetectable. I ths method the frequecy doma of cover sgal s vewed as a commucato chael ad the watermar s vewed as a sgal that s trasmtted through t. Attacs ad utetoal sgal dstortos are thus treated as ose that the trasmtted sgal must be mmue to. They clam that order for the watermar to be robust, watermar must be placed perceptually sgfcat regos of the cover sgal despte the rs of potetal fdelty dstorto. Coversely f the watermar s placed perceptually sgfcat regos, t s easly removed, ether tetoally or utetoally by, for example, sgal compresso techques that mplctly recogze that perceptually wea compoets of a sgal eed ot be represeted. Suppose that the watermar W cossts of a sequece of real umbers, W w, w 2,, w. I order for W to be embedded to a cover sgal, S, a sequece of values, V v, v 2,, v, s extracted from frequecy spectrum of S, the watermar W wll be embedded to V to obta V v, v 2,, v. V s the serted bac to S place of V to obta a watermared sgal S. Oly copyrght ower ows the locatos of V sequece values frequecy spectrum of S. Ths wll esure the securty of the watermar. S maybe altered, by tetoal or utetoal attacs, to produce S *. Gve S ad S *, a possbly corrupted watermar W * s extracted ad compared to W. W * s extracted by frst extractg V * from S * ad the geeratg W *. Watermar Sgal Fgure 5. Watermarg Frequecy Doma Fgure 5.2 depcts watermar embeddg ad extracto. There are three atural formulae for computg V : v v + αw v v (+ αw ) v v (e α?w ) α s scalg parameter (cotrols robustess ad fdelty). There are a umber of ways that oe ca use to evaluate the smlarty betwee two watermars. A tradtoal correlato measure ca be used, for example. Smlarty of W ad W * ca be measured by: sm( S Value Extracto V Combg V Value Iserto S * W,W ) Where X.Y * W. W * W. W W * x. y Value Extracto Dfferecg Aother audo watermarg techque uses statstcal algorthm wors Fourer doma [Arold, 2000; Arold, 200]. Ths method s based o the patchwor algorthm [Beder et al, 996] ad does t eed the orgal audo detecto process. Audo sgal s broe to frames; each frame s used to embed oe bt. Each frame s trasformed to frequecy doma usg DFT. Assume that the trasformed frame cotas 2N values, the the embeddg process wors as follows:. Map a secret ey ad the watermar to the seed of radom-umber geerator. Start the geerator to pseudoradomly select two termxed subsets A {a },,M ad B{b },,,M S V * Postprocessg W * Fgure 5.2 Watermar Embeddg ad Extracto S * V

5 of equal sze M N from the orgal set of audo sgal frequecy spectrum. 2. Alter the selected elemets a A ad b B,,,M accordg to the followg embeddg fucto: a a +?a b b -?b?a ad?b are two patters geerated by the secret ey. There are two patters for 0 ad aother two for. We have to select the correct patters accordg to the value of the bt beg embeddg. The alteratos of frequecy doma coeffcets have to be performed a way that acheves audblty. Therefore,?a ad?b are drve from psychoacoustcs model. Thus,?a ad?b are reshaped for each dvdual frame. For more formato about psychoacoustcs model see [Pater ad Spaas, 2000]. I watermar detecto process, hypothess testg s used. We formulate test hypothess, H 0, ad alteratve hypothess, H, the approprate test statstc z wll be a fucto of the sets A ad B wth probablty dstrbuto fucto PDF (z) the uwatermared case ad m (z) watermared case. H 0 : the watermar s ot embedded; z follows PDF (z). H : the watermar s embedded; z follows PDF m (z). Two d of error are corporated hypothess testg: I : + φ (z)dz P II : T T φ m(z)dz I P II (Type I error) (Type II error) Hypothess testg s used the detecto to decde whether the watermar bt s embedded or ot. The threshold T s used the detecto step. Detecto procedure s as follows:. Map the secret ey ad the watermar to the seed of radom-umber geerator to geerate the suset C ad D. C A ad D B f a correct ey s used. 2. Decde the probablty of correct rejecto PI accordg to the applcato ad calculato the threshold T from error type I equato. 3. Calculate the sample mea E(z) E(f(C,D)) ad choose betwee two mutually exclusve propostos: H 0 : E(z) T the watermar bt s embedded. H : E(z)>T embedded. the watermar s ot Hypothess testg depeds o approprate test statstc. Two test statstcs ca be used watermar detecto:. The frst test statstc uses the fucto to measure the dfferece betwee populato meas of A ad B: z a b f ( A, B) σ a b Therefore the two mutually exclusve propostos become: H0: (z) N(0,) H: m (z) N (zm, ), z m ( a + b) σˆ a b Where N (µ, σ 2 ) s the ormal dstrbuto wth the mea µ ad stadard devato σ, ad 2 2 ( z P + z ) ε I P II 2. The secod test statstc uses aother fucto: z f ( A, B) a b σ 2 a b a + b σ a + b a b 2 a + b The threshold T must be computed ad compared wth the mea value calculated by oe of the above statstcs fuctos. It s clear that the detecto process does t requre the orgal audo sgal whle t wors to detect the statstcs chages the meda to determe whether t s watermared or ot. Further research has bee acheved to mprove the performace of above watermarg system, for more formato see [Hog et al, 2002; Yeo ad Km, 200] 5.2 Tme Doma Audo Watermarg I tme doma watermarg techques, watermar s drectly embedded to audo sgal. No doma trasform s requred ths process. Watermar sgal s shaped before embeddg operato to esure ts audblty (Fgure 5.3). The avalable tme doma watermarg techques sert the watermar to audo sgal by smply addg the watermar to the sgal.

6 Watermar Watermar Shapg Modulated Watermar Watermar Embeddg Watermared Audo Sgal Iput Audo Sgal Fgure 5.3 Tme Doma Watermarg Embeddg a watermar to tme doma volves challeges related to fdelty ad robustess. Shapg the watermar before embeddg eables the system to mata the orgal audo sgal fdelty ad reders the watermar audble. As for robustess, tme doma watermarg systems use dfferet techques to mprove the robustess of the watermar. Worg frequecy doma eables watermarg system to embed a robust watermar, whle t s possble to detfy the most sgfcat compoets of the cover sgal. Also, masg characterstcs of audo sgal ca be exploted, order to reduce the dstorto of embedded watermar. I ths secto, two methods for audo watermarg tme doma are show. The frst oe preseted [Bassa ad Ptas, 998; Bassa et al, 200] ad whch the watermar sgal s modulated usg the orgal audo sgal ad fltered by lowpass flter to reduce the dstorto that mght be result from embeddg the watermar. The orgal audo sgal s dvded to segmets ad the each segmet s watermared separately by embeddg the same watermar. Watermar sgal, w {, -}, 0,,,- s geerated by thresholdg a chaotc map a way smlar to the oe descrbed [Bassa et al, 200]. The seed (start pot) of the chaotc sequece geerator s the watermar ey. Usg the chaotc sequece geerator s to esure the securty of the watermarg system.e. the sequece geerato mechasm caot be reversed egeered. Suppose that we have a segmet of audo sgal S s, s 2,,s the the watermarg process beg by modulatg the watermar sgal w by usg audo sgal S, w α s w 0,,, - Where deotes a superposto law whch ca be multplcato, power law, etc, ad α s a costat cotrols the ampltude of the watermar sgal. The maxmum allowable watermar ampltude s the lmted by the maxmum perceved sgal dstorto. I ext stage, w s shaped usg a lowpass Hammg flter of legth (order) L: w L l 0 b w l l Where b l are the flter coeffcets. Ths process results audble watermar sgal. Fgure 5.4 [Bassa et al, 200] shows the power spectral desty (psd) of two watermar sgals, oe s shaped ad the other s ot. It s clear that the ushaped watermar sgal s audble whle t has a psd exceeds the power of the orgal sgal certa frequeces. The psd of the shaped watermar sgal les udereath the orgal audo sgal the etre frequecy rage. Fally the shaped watermar sgal s embedded to audo sgal: y + w s 0,,..., - It s obvous that the calculato of watermared sample y s based o the eghbors of the sample s Fgure 5.4 Power spectral desty of two watermar ad orgal sgals

7 ad the chaotc sgal (watermar) w. I detecto stage, the receved sgal, Y, broe the same way that orgal sgal s broe. Let us cosder the followg sum: C 0 y ( + ) mod C s the correlato of W wth Y, evaluated for all possble crcular shft of Y. By substtuto ad rearragg the above equato we get: C ( s ( ) mod + w + w( + ) mod 0 0 w w ) The expected value of the frst sum s zero f ether the watermar mea value m w or the sgal mea value m S s equal to zero. I case m w s ot zero (the umber of ad s ot the same), the quatty?w w 0, must be tae to accout. Let us deote by B a set of N B?w dex values for whch the correspodg w values are equal the or wth the most occurreces. It s easy to show that: B w w Let us deote by A the set of all dex values that do ot belog to B. obvously, the cardalty of A s N A -?w ad the followg equato holds A w 0 So, C ca be expressed as follows: C ( s ) modw + s( + A B Let us defe the followg terms: T, ( s( + )modw ) A T 2, ( s( + ) mod w ) ( ) mod + w + w 0 ( + ) mod T B 3, ( w 0 ( + ) mod w ) It ca be easly show that E(T, ) 0, where E() deotes the expected value operator. For the term T 2, t s easy to show that: T2, sg( w) ( Therefore B s ( + ) mod w w ) N B B s ( + ) mod w ) E w ( T2, ) m S If o watermar has bee embedded the sgal, T 3 0ad thus: C T 2, w m S O the other had, f the sgal s watermared C T w 2, m S w 0 ( + ) mod + ( w ) For watermar detecto we costruct the rato r : r C T T 3, 2, The orgal sgal S s requred for evaluato of T 2, ad T 3,, but t ca be replaced by Y wthout sgfcat error. The value of r s computed for every 0,,,-, for all segmets. We compute the detecto value of the audo segmet j as R r, the fal detecto value s j R 0 N s R j 0 j segmets sgal., where N s s the umber of The decso about the exstece of the watermar s made depedg o a threshold value compared wth R. It s clear that ths watermarg system s mmue agast tme-shftg ad croppg. The fact that C s computed for all possble crcular shft of Y, esures sychrozato betwee Y ad W wll occur for certa value of 0,,,-. Aother watermarg system uses the HAS masg effects to shape the watermar sgal [Boey et al, 996; Swaso et al, 998]. Shapg operato s performed frequecy doma, but the shaped watermar s embedded to audo sgal tme doma. Watermar s a ose-le sequece geerated by usg two eys x ad x 2. The frst ey x s author depedet. The secod ey x 2 s computed from audo sgal that the author wat to watermar. It s computed from the sgal usg a oe-way hash fucto. The two eys are mapped to pseudoradom umber geerator to geerate a ose-le sequece, watermar. Orgal audo sgal s requred detecto process to compute the secod ey x 2, ad to extract the embedded watermar.

8 Temporal Masg s FFT Frequecy Masg IFFT FFT y s Fgure 5.5 Audo Segmet Watermarg Procedure The watermarg process begs wth dvdg the audo sgal to segmets, ad the each segmet s watermared separately. Suppose that you have a geerated watermar y, the the algorthm of watermarg a dvdual segmet, s, wors as follows:. Compute the power spectrum S of audo sgal segmet s as follows: N 2 S 0 log 0 sh( ) exp( j2π 0 N N Where h() s a Ha wdow: 8 3 h( ) cos 2π 2 N N s the umber of samples oe segmet ad j s 2. Compute the frequecy masg threshold M of the power spectrum S. 3. Use the mas M to weght the ose-le watermar, P M * Y, where P s the weghted watermar ad Y s the power spectrum of the watermar sgal y. 4. Compute the verse of FFT of the shaped watermar p IFFT(P ). 5. Compute the temporal masg t of s. 6. Use the temporal masg t to further shape the frequecy shaped watermar to create the fal watermar w t *p of the audo segmet. 7. Create the watermared segmet s s + w. Fgure 5.5 shows a dagram of watermar shapg ad embeddg. 2 the watermar sgal ca be recostructed. Also the embedded possble dstorted watermar ca be extracted. Assume that r 0,,,N s a recovered pece of audo sgal, the we ca compute x r - s. If r has a watermar the x w +, where s ose (tetoally or utetoally added to the watermared sgal). Otherwse, x. Smlarty betwee extracted watermar, x, ad the recostructed oe ca be measured by correlato as follows: sm(x,w) N 0 N 0 x w w w The the value ca be compared wth a threshold T. The recovered sgal r s possble shfted. Ths leads to lose the sychrozato betwee the extracted watermar ad the recostructed oe. I such case we ca assume that r s +τ + x, where x as metoed before. τ s a uow delay, thus, a geeralzed lelhood rato test must be performed to determe whether the audo sgal s watermared or ot. max τ exp( max τ N ( 0 exp( r ( s + w + τ + τ N 2 ( r ( 0 + ) ) s τ The, ths rato s compared to a threshold. )) 2 I detecto process, the orgal audo sgal s ow. Thus, secod ey ca be computed ad the

9 PCM Audo Samples Mappg Flter Ba Bt Allocato Quatzer Codg Frame Pacg Ecoded Bt Stream MPEG Psychoacoustcs Model Acllary Data 5.3 Compressed Doma Audo Watermarg A umber of techques are proposed to embed a watermar sgal to MPEG audo bt stream, rather tha gog through decodg/ecodg process order to apply watermarg scheme ucompressed doma [Qao ad Nahrstedt, 999; Neubauer ad Herre, 2000a; Neubauer ad Herre, 2000b; Neubauer ad Herre, 998]. Such systems are sutable for pay audo scearo, where the provder stores audo cotets compressed format. Durg dowload of musc, the customer detfes hmself wth hs uque customer ID, whch therefore s ow to the provder durg delvery. I order to embed the customer ID to the audo data usg a watermarg techque, a scheme s eeded that s capable of watermarg compressed audo o the fly durg dowload. MPEG audo compresso s a lossy algorthm ad uses the specal ature of the huma audtory system (HAS). It removes the perceptually rrelevat parts of the audo ad maes the audo sgal dstorto audble to t huma ear. For more formato about MPEG audo Compresso see [Pa, 995]. MPEG ecodg process has the followg steps:. Iput audo samples pass through a mappg flter ba to dvde the audo data to subbads (subsamples) of frequecy. 2. At the same tme, the put audo samples pass through MPEG psychoacoustcs model, whch creates a masg threshold of audo sgal. Masg threshold s used by quatzato ad codg step to determe how to allocate bts to mmze the quatzato ose audblty. 3. Fally, the quatzed subbad samples are paced to frames (coded stream). Fgure 5.6 shows the basc structure of a MPEG audo ecoder. Flter ba dvdes the put audo sgal to 32 equal-wdth subbads, the the umber of bts used Fgure 5.6 Structure of MPEG Audo Ecoder Header CRC Bt Allocato Scale Factors Ecoded Samples Acllary Data Fgure 5.7 Frame Format of MPEG Audo quatzato s determe upo masg threshold to mmze the audblty of possble dstorto maybe troduced by quatzato. The MPEG audo stream cossts of frames. Frame s the smallest ut whch ca be decoded dvdually. Each frame cotas audo data, header, CRC (Cyclc Redudacy Code), ad acllary data. I frame, each subbad has three groups of samples wth 2 samples per group. The ecoder ca use a dfferet scale factor for each group. Scale factor s determe upo masg threshold ad used recostructo of audo sgal. The decoder multples the quatzer output to recostruct the quatzed subbad sample. Fgure 5.7 depcts the geeral format of MPEG frame. MPEG audo decodg process s smple a reverse of the ecodg process. The decodg taes the ecoded bt stream as a put, upacs the frames, recostructs the frequecy samples (subbads samples) usg scale factors, ad the verses the mappg to re-create the audo sgal samples. Fgure 5.8 descrbes ths process. Oe audo watermarg techques [Qao ad Nahrstedt, 999] embeds the watermar to scale factors of MPEG audo frames. I ths techque, DES ecrypto algorthm s used geeratg o-vertble watermar. Orgal data s appled to ecrypto algorthm to get the watermar as follows: Frst, a ey KEY s selected ad for each MPEG audo frame a j j,,n ( umber of audo frames), we apply DES wth KEY to t to get a radom byte sequece RBS: RBS DES KEY (oe audo frame a j ) Secod, let RBS be -th byte of radom byte sequece ad w be the -th bt of the watermar bt stream, the the watermar ca be created by: f RBS eve umber w otherwse Each scale factor taes 6 bts; therefore, we have as may as 63 levels of scale factors (dexed

10 Scale Factors Frame Upacg Decodg the Quatzed Samples Estmatg Masg Threshold Decoder Part Quatze ad Ecodg Frame Pacg Spread Spectrum Modulato Watermar Geerator Mappg Flter Ba Shapg the Watermar Shapg ad Embeddg Ecoder Part Watermar MPEG Audo Bt Stream Fgure 5.9 MPEG Audo Bt Stream Watermarg Watermared MPEG Audo Bt Stream from 0 to 62, 63 s ot used by the stadard). The level chage of scale factor has a audtory effect that the soud becomes stroger whe the scale factor level creases, ad becomes weaer whe the scale factor decreases. Icreasg or decreasg scale factor by oe level ormally caot be detected by lsteers. Let ScaleFactor (dex) be the -th scale factor wth the level dcated by dex ad SW be the -th watermared oe. The watermarg procedure wors as follows: ScaleFactor ( dex) f dex + w or 63 SW ScaleFactor ( edex + w ) otherwse Ths scheme has drawbacs. The frst oe s that the scheme does t has much data to watermar due to the few umber of scale factors audo frame. Also, the watermar scheme s ot robust eough agast attacer who s tryg lower scale factors by 2 or 3 levels. O the other sde, multple watermars caot be appled. The reaso s that whe multple watermars are appled, certa scale factors would be creased by multple levels ad perceptble ose would be troduced. Aother watermarg scheme embeds the watermar to the ecoded data. However, chagg the all ecoded samples shows a perceptble dstorto. Spacg Parameter sp s troduced to solve ths problem. sp s used way le that every sp samples, we radomly select or 2 samples to be watermared. The watermar geerato procedure wll be modfed to corporate spacg parameter: w 0 f RBS 0 (mod sp) f RBS (mod sp) otherwse Let Sample be the -th sample audo frame ad SW be the -th watermared sample. The watermarg wll be: Sample SW Sample f every + w bt of (Sample otherwse + w ) s Both watermarg schemes descrbed above use the cocept of spread spectrum watermarg, but through compressed doma. The orgal MPEG audo s requred detecto process ad the watermar ca smply extracted ad verfed. Aother techque [Neubauer ad Herre, 2000a; Neubauer ad Herre, 2000b; Neubauer ad Herre, 998] MPEG audo stream watermarg s to partly decode the put bt stream, embed a perceptually hdde watermar the frequecy doma ad fally quatze ad code the sgal aga. Fgure 5.9 llustrates a geeral structure of bt stream watermarg system. Ths watermarg system cossts of four parts. Each part has a specfc fucto. We ca see that ths watermarg system has assembled parts of MPEG ecoder ad decoder, addto to parts of frequecy doma audo watermarg systems (watermar geerato ad watermar embeddg). These parts have bee modfed order to eable the system to embed the watermar subbads samples. The frst part, decoder part, taes MPEG audo bt stream as a put ad gves frequecy subbads samples as output. Ths part supples the other parts wth scale factors that are ecessary masg threshold estmato ad ecoder process. The secod part, watermar geerator, s used to covert the watermar to subbad represetato order to be ready for embeddg. The watermar ca be ay data provded by copyrght ower. The

11 geerated watermar s fed to watermar shapg ad embeddg part, whch tur, taes the decoded subbads samples ad scale factors to estmate the masg threshold of the audo sgal ad use t shapg the watermar. The last two parts have much smlarty to the techque proposed [Swaso et al, 998]. The last part, ecoder part, taes the watermared subbads samples ad scale factors. It decodes the samples usg the orgal scale factors ad the pacs the resultg decoded samples. I order to avod the possble dstorto of requatzato, the orgal scale factor s used ad o eed to recomputed ew scale factors. The embedded watermar ca be detected ucompressed doma as well as compressed doma. Orgal audo data s requred to extract the watermar ad the measure the smlarty betwee the extracted watermar ad the orgal oe. The watermar detecto ucompressed doma ca be acheved, exactly le the way preseted [Swaso et al, 998], by usg correlato measuremet. 5.4 Wavelet Doma Audo Watermarg Wavelet trasform ca be used to decompose a sgal to two parts, hgh frequeces ad low frequeces. The low frequeces part s decomposed aga to two parts of hgh ad low frequeces. The umber of decompostos ths process s usually determed by applcato ad legth of orgal sgal. The data obtaed from the above decomposto are called the DWT coeffcets. Moreover, the orgal sgal ca be recostructed form these coeffcets. Ths recostructo s called the verse DWT. The process of decomposto s depcted Fgure 5.0. For more formato o Wavelet trasform, see [Daubeches, 992; Daubeches, 988]. robustess. Also a umber of bts are added frot of watermars bts to locate the pot where the watermar bt s embedded watermared sgal. These bts are called sychrozato bts. For example, wth local redudacy rate 3 ad sychrozato bts 000, we chage the orgal watermar as: w 0 w w 2 000w 0 w 0 w 0 w w w w 2 w 2 w 2. Suppose that B s a bloc of audo sgal beg watermared, we use DWT to have D 0,D, D 2,,D, C, for some teger. the after patchwor algorthm s used to embed the watermar by artfcally modfyg a patch value P N as P N? D j J Where I ad J are two subset of dexes radomly geerated. Proposed algorthm modfes P N a way that the modfed P N s devato away from expected. To be specfc, we modfy some wavelet coeffcets D as D D D D For I ad j J, w s a watermar bt beg embedded ad δ s a real umber. Dfferet two subsets of dexes I ad J are used to embed the sychrozato bts for securty purpose. D + δ, D D δ f w j j δ, D D + δ f w 0 j j j S C 0 D 0 C D C C - C 2 D 2 D Fgure 5.0 Wavelet Decomposto A method of audo sgal watermarg wavelet doma uses patchwor algorthm [Km et al, 2002]. I ths method, a bary watermar w s embedded oe bt oe data bloc. Watermar bts are locally repeated for the purpose of Fgure 5. t N for watermared Audo Sgal I detecto process, t N P N t+ PN t, are computed, where P N t+ ad PN t are two patch values of bloc B t+ ad B t, respectvely. Fgure 5.0 [Km et al, 2002] shows t N for watermared audo sgal [Km et al, 2002]. The peas show ths fgure refers to the watermar bts locatos audo sgal. The detecto s made accordg the followg crtera, for β > 0:

12 If t N > βnδ ad Nt+ <0 the s detected bloc B t. If t N < -βnδ ad Nt+ > 0 the 0 s detected bloc B t. If prevous two codtos are ot satsfed, the o watermar bt s detected ths bloc. Sychrozato bts must be foud frst to determe the locato of watermar bts. Ths watermarg system shows a hgh performace sychrozato ad resstg tme shftg attac [Km et al, 2002]. 7. CONCLUSIONS All watermarg systems are desged to acheve oe goal that s embeddg a hdde robust watermar to dgtal meda. These systems have to satsfy two coflctg requremets. Frst, watermar must be mmue agast tetoal ad utetoal removal. Secod, watermared sgal should mata a good fdelty,.e. watermar must be perceptually udetectable. To accomplsh ths tas, varety of techques have bee exploted, ad dfferet domas are volved to ehace a certa applcato of watermarg ad/or mprove fdelty ad robustess of watermared sgal. However, watermarg systems have a umber of dffereces. These dffereces ca be cosdered evaluatg performace of watermarg systems ad sutablty of these systems for a specfc applcato. These dffereces ca be explaed as follows:. Some audo watermarg systems requre the orgal audo sgal, or ay formato derved from t, to be preseted detecto process. Ths wll leads to a large umber of orgal wors have to be stored ad searched durg detecto. Systems that requre the orgal audo sgal are ot sutable for some type of applcatos, case that detecto process has o access to the orgal wor or t s ot acceptable to dsclose t. O the other had, presetg the orgal sgal yelds effcet watermar extracto, cosequetly, effcet detecto. Audo watermarg systems that are based o patchwor algorthm use a statstcal detecto process (hypothess testg) ad do t eed the orgal audo for detecto purpose. But most techques that are base o correlato measuremet of smlarty requre that sgal except method preseted [Bassa ad Ptas, 998; Bassa et al, 200]. I spte of that a umber of audo watermarg techques requre oly the watermared sgal detecto, watermar ey s eeded both embeddg ad detecto. 2. I order to mata the watermar securty, watermar would be embedded to selected regos of some doma trasform of audo sgal. These regos are selected radomly by geeratg a sequece of dexes. Sequece geerato s paramerzed by a ey called watermarg ey. Ths ey s requred both embeddg ad detecto. I some watermarg systems, watermarg ey s used to geerate the watermar tself. I ths case, the watermar would be a radom sequece of bts or dgts geerated by some sort of algorthms esure o-vertblty of watermar order to mata the securty of watermarg ey. Watermarg ey could be provded by the copyrght ower or a combato of formato provded by hm/her ad formato derved from orgal sgal. I such case, orgal sgal wll be requred detecto process for ey geerato purpose. I all scearos, the ey s used as a seed for radom umber geerator. Sometmes, dsclosg the watermarg ey or havg a access to t becomes mpossble. Thus, usg the same ey detecto ad embeddg wll ot be acceptable. A soluto to such problem could be foud usg two eys, oe for embeddg ad aother for detecto [Hog et al, 2002] (.e. publc-ey or asymmetrc watermarg system). 3. Durg embeddg process, orgal audo sgal s dvded to frames. The after, each frame s watermared separately. Some watermarg systems embed the same watermar to a umber of frames to ehace watermar robustess. But, other systems each frame s watermared wth dfferet watermar. 4. Because of sestvty of HAS, watermar sgal must be shaped to ret t audble. Masg characterstcs of audo sgal ca be used for ths purpose. Psychoacoustcs MPEG model s commoly used to calculate masg threshold that s used weghtg the watermar. I some other audo watermarg systems, dfferet techques are used. These techques use the orgal audo sgal modulatg the watermar. Therefore; the ampltude of watermar sgal s cotrolled by ampltude of audo sgal. Watermar shapg process may effect the exstece of the watermar cover wor, cosequetly, false egatve rate wll be creased.

13 A geeral wor frame for dgtal audo watermarg systems ca be stated as follows:. Watermarg system should be able to embed ay set of data to audo sgal, ad the detector should be able to retreve the embedded data (.e. ot just report that watermar s preseted or ot) 2. Watermar embedded (detecto) module should be depedet of mode of operatg. (e.g. the same watermar s embedded to multple frames of audo sgal or dfferet watermar s embedded to each frame). 3. Watermarg ey geerato should be depedet of watermar embeddg ad detecto (e.g. embeddg ad detecto wll ot be effected whether orgal sgal s volved ey geerato or ot). The above pots eables audo watermarg system to be sutable for varety of applcato ad mae t possble to put stadards (e.g. [SDMI, 2000] ) ad evaluato bechmars. 7. REFERENCES. Arold M. 2000, Audo Watermarg: Features, Applcatos ad Algorthms. Multmeda ad Expo. IEEE teratoal Cof., Vol. 2, pp Arold M. 200, Audo Watermarg, Dr. Dobb s Joural, Vol. 26, Issue, pp Bassa P. ad Ptas I. 998, Robust Audo Watermarg the Tme Doma. Sgal Processg IX, theores ad applcatos: proceedg of Euspco-98, Nth Europea Sgal Processg Cof., Greece, pp Bassa P., Ptas I., ad Nolads 200, Robust Audo Watermarg Tme Doma, IEEE Tras. O Multmeda, Vol. 3, pp Beder W., Gruhl D., Mormoto N. ad Lu A. 996, Techques for Data Hdg, IBM Systems Joural, Vol. 35, No. 3&4, pp Boey L. Tewf A. H. ad Hamdy K. N. 996, Dgtal Watermarg for Audo Sgal. I Proc. of EUSIPCO 96, Sep., Vol. III, pp Cox I. J., Kla J. Leghto F. T. ad Shamoo T. 997, Secure Spread Spectrum Watermarg for Multmeda. IEEE Tras. O Image Processg, Vol. 6, No. 2, pp Cox I. J., Mller, M. L. ad Bloom J. A. 2002, Dgtal Watermarg. Morga Kaufma Publshers, USA. 9. Daubeches I. 988, Orthoormal Bases of Compactly Supported Wavelets. Comm. Puse ad App. Math., Vol. 4, pp Daubeches I. 992, Te Lectures o Wavelets, SIAM, Phladelpha.. Hog D. G., Par S. H. ad Sh J. 2002, A Publc Key Audo Watermarg Usg Patchwor Algorthm. Proceedgs of ITC- CSCC 2002,pp Katzebesser S. ad Pettcolas F. A. P. 2000, Iformato Hdg Techques for Stegaography ad Dgtal Watermarg. Artech House, UK. 3. Km H. O., Lee B. K. ad Lee N. -Y. 2002, Wavelet-Based Audo Watermarg Techques: Robustess ad Fast Sychrozato. I 4. Neubauer C. ad Herre J. 998, Dgtal Watermarg ad ts Ifluece o Audo Qualty. 05 th AES Coveto, Audo Egeerg Socety preprt 4823, Sa Fracsco. 5. Neubauer C. ad Herre J. 2000a, Audo Watermarg MPEG-2 AAC Btstream, 08 th AES Coveto, Audo Egeerg Socety Preprt 50, Pars. 6. Neubauer C. ad Herre J. 2000b, Advaced Audo Watermarg ad Applcatos. 09 th AES Coveto, Audo Egeerg Socety Preprt 576, Los Ageles. 7. Pater T. ad Spaas A. 2000, Perceptual Codg of Dgtal Audo. Proc. of IEEE, Vol. 88, No. 4, pp Pa D. 995, A Tutoral o MPEG / Audo Compresso. IEEE Multmeda, pp Qao L. ad Nahrstedt K. 999, No-vertble Watermarg Methods for MPEG Ecoded Audo, Cof. o Securty ad Watermarg of Multmeda Cotets, pp SDMI, 2000, Call for proposal, 2. Swaso M. D., Zhu B., Tewf A. H. ad L. Boey L. 998, Robust Audo Watermarg Usg Perceptual Masg, Elsever Sgal Processg, Sp. Issue o Copyrghts Protecto ad Access Cotrol, Vol. 66, No. 3, pp Voyatzs G. ad Ptas I. 998, Chaotc Watermars for Embeddg Spatal Dgtal Image Doma. I Proc. ICIP98, Chcago, Vol. II, pp Yeo I.-K. ad Km H. J. 200, Modfed Patchwor Algorthm: A Novel Audo

14 Watermarg Scheme. Proc. of the Iteratoal Cof. O Iformato Techology: Codg ad Computg, pp