THE MLS ANALYSIS TECHNIQUE AND CLIO CONTENTS Itroductio... MLS Sequece... Te cro-correlatio...3 Example... Bibliograpy...5 Rev.0-9 Dec 997 Itroductio Te MLS aalyi aim to caracterize te beaviour of te ytem uder tet bot i time ad frequecy domai; te meauremet itelf acieve a precie time meauremet of te baic parameter of a ytem, it Impule Repoe IR, tu allowig everal pot-proceig to be made; amog tee we recall: - requecy ad pae repoe requecy - Acoutical aecoic aalyi Time ad frequecy - Acoutical room repoe Time ad frequecy - Reverberatio time RT60 Time - Eergy time curve ETC Time - Cumulative pectral decay Waterfall Time ad frequecy D.U.T. MLS DUT RESPONSE CROSS- CORRELATION T TRANSER UNCTION IMPULSE RESPONSE IGURE - Te MLS aalyi tecique igure ow te flow diagram of a MLS meauremet. Te aalyi tecique i baed o te matematical teory beid particular igal kow a Maximum Legt Sequece MLS; tee igal are digitally
yteized biary equece wic, amog oter propertie, are te followig: we fed to te ytem iput, teir cro-correlatio wit te ytem output give exactly te ytem impule repoe; te crocorrelatio ca be carried out, i te time domai, wit a efficiet digital algoritm tat miimize te uer waitig time. Regardig te pae repoe it a to be uderlied tat it i ufficiet oly a igle meauremet cael, due to te complete temporal cotrol of te geerated igal. I fact, te aalyzer take te geerated igal itat by itat ad compare it to te acquired oe, o it i capable of performig real pae meauremet. It i alo poible to ull te igal group delay caued, for example, by te propagatio time from te loudpeaker to te micropoe or by a equalier etwork, terefore obtaiig better quality preetatio of pae meauremet. Pay attetio to te fact tat oly a aalyzer, like CLIO, tat perform te digital MLS algoritm decribed i ti article ca acieve te performace offered by te MLS tecique, ad ca terefore be called a MLS aalyzer; tere are ome itrumet tat ue MLS equece oly a timuli ad te directly perform T: te reult i very poor. Two fudametal quatitie like te impule ad pae repoe, tat everal aalyzer oly calculate, are itead meaured oe; ti let te uer ave te bet picture poible of te ytem. MLS Sequece MLS O 5 SAMPLES N N L - Pxx +x+... 0000000... + + - - + -/L igure ow a example of Maximum Legt Sequece; ti i te imple cae of a equece of oly 5 ample ad we ay tat it order N i ad it legt L i 5; i te figure we ca ee alo te o called primitive poliomial Px ad te relative feedback ift regiter tat are eeded to defie ad geerate te equece; i practice you obtai a bipolar igal aigig a ig level to zero tate ad a low level to oe tate uig itegrated ift regiter ad D/A coverter. Sow are alo te equece of biary value zero ad oe, te aociated equece of electrical tate + ad - ad te value of te auto-correlatio cro-correlatio of te equece wit itelf. rom a electrical poit of view te MLS igal a oter iteretig feature; it pectral cotet cloely reemble a wite oie o, we you ear it, you feel like a oie burt i geerated i fact it i played for o loger ta fractio of a ecod; but, o te oter ad, it i determiitic well kow at ay itat ad periodic wit a certai temporal legt alo referred a it order: ti i wy it i called a peudo-radom oie. Beig a oie igal it a a low cret factor, trafer a lot of eergy to te ytem compared wit te claical impule excitatio ad acieve a very good igal-to-oie ratio. IGURE
3 Te cro-correlatio 6 5 ] [ ] [ 3 0 0 y y b L L a MLS y y»» < < - d D.U.T. y Ad ow a cloer look at te cro-correlatio. I figure 3 ad i te aociated equatio we may decribe matematically wat appe i a MLS meauremet. We upply to our DUT a MLS igal equatio ad meaure it repoe yt. Te value of a MLS auto-correlatio i give i equatio ad we may well tate tat it i almot equal to a perfect impule eq. 3; ad ti approximatio i more ad more valid a te legt of te equece icreae wit practical legt tartig from 095 poit. rom teory we kow tat te output of our ytem i equal to te iput igal covoluted wit te ytem impule repoe eq.. We may ow ee te expreio of te cro-correlatio of te iput MLS equece wit te meaured output igal; wit very imple tep decribed i equatio 5 ad ubtitutig te value of te auto-correlatio a i eq.3 we obtai te deired reult: te ytem impule repoe equal te cro-correlatio of te MLS iput wit te meaured output. IGURE 3
Example We will ow decribe te claical applicatio of te MLS aalyi for recoverig te aecoic frequecy repoe of a loudpeaker, i.e. te frequecy repoe a if te loudpeaker were poitioed i a aecoic IGURE camber but meaured i a ormal room. igure ow a impule repoe tat we may obtai a te reult of a MLS meauremet; te importat tig to uderlie i tat, beig ti te product of our meauremet, we ca tik of it a te tart poit from wic we ca obtai te deired fial reult after we ave made ome particularly importat coice; te impule repoe ow i of a loudpeaker meaured i a ormal room; a we ca ee te impule i ometig like. m after time zero wic idetifie te begiig of te MLS timulu ad wit ome eay calculatio ivolvig te peed of oud aroud 3 m/ we are able to meaure exactly te ditace of te meaurig micropoe from te acoutic ceter of te loudpeaker i ti cae 7 cm; we ca alo ee tat after ome time 9 m tat te impule i decayed to egligible value more eergy arrive, it i due to te firt reflectio of oud i te room; it i ti te time to cooe if we are itereted to te total frequecy repoe of te loudpeaker plu te room or we wat to obtai te aecoic beaviour of te former. I te firt cae we ave to traform te complete impule repoe meaured ad we will obtai te IGURE 5
repoe a i figure 5. I te ecod cae we ave to traform oly te part of te impule repoe tat i due to te loudpeaker; i oter word we ave to elect ti part by mea of a time widow alo ee i figure ; we will obtai te repoe a i figure 6. Tee two figure clearly ow te cotributio due to te eviromet tat add to te proper frequecy repoe of te loudpeaker; oter meaurig trategie may lead u to ivetigate differet part of te ytem impule repoe; for example we could iolate oe wall reflectio ad tudy te aborbig propertie of te wall material i fuctio of frequecy. Bibliograpy IGURE 6 [] D.D. Rife ad J. Vaderkooy, Trafer uctio Meauremet wit Maximum-Legt Sequece, J. Audio Eg. Soc., Vol. 37, 989 Jue. [] W.D.T. Davie, Geeratio ad propertie of maximum legt equece, Cotrol, 966 Jue/July/ Augut. 5