Cc thuc tnhCc bn nhc s thng cthu mtrong cc studio vi kh nhiu thit b phc tp. Nhng v bn cht, thu m l qu trnh chuyn i sng m thanh thnh tn hiu s. Nh bit, m thanh thc l cc sng c hc, c dng hnh sin tun hon lin tc (analog) trong khi m thanh s ch l nhng xung in t ri rc (digital). Do , bng nhng mu ri rc, m thanh s ch c th m phng mt cch gn ging nht vi m thanh thc t m thi. Vic m phng c c trng bi cc thng s sau:- Sample:L thnh phn nh nht ca bn nhc s. c cc xung in t ri rc, cn phi tin hnh rt nhiu ln ly mu. Mi mu gi l mt sample l gi tr bin ca tn s sng m ti thi im ly mu. Cng ly nhiu mu, tn hiu s thu c cng chnh xc hn.- Sample Rate:(Sampling Rate, Sampling Frequency): L s ln ly mu trn mt giy, c n v Hz. Mt bn nhc c sample rate l 44100 Hz th mi giy nhc s c ly mu 44100 ln.- BitDepth: lu li di dng s, mi mu c biu din bng mt lng bit d liu nht nh, gi l BitDepth. Cc bn nhc hin nay thng c BitDepth l 16 bits, 24 bitsBitDepth cng lnm thanhcng sc nt, trung thc nn n cn c gi l Resolution ( nt).- Channel:Bng cc thut ton, tn hiu s s c tch ra thnh nhiu knh (Channel) sao cho khi nghe bng h thng loa thch hp s c cm gic nh khi ang nghe nhc trong khng gian thc t.T 4 thng s c bn trn, ta khng nhng bit c cht lng m cn c th tnh c dung lng ca bn nhc. V d mt pht ca bn nhc c : Sample rate = 44100 Hz, BitDepth = 16 bits = 2 bytes, Channel = 2 knh s c dung lng: 44100 t ly mu x 2 bytes x 60 giy x 2 knh = 10.584.000 bytes, tc khong 10.1 MB.

- BitRate:L thng s thu gn, i din c bn cho cc thuc tnh trn. Bitrate c n v Kbps (Kilobits per second) dung lng (tnh theo bit) ca m thanh s trn mt giy. Vi Bitrate, ta c th xc nh nhanh chng dung lng cng nh phn no cht lng ca bn nhc. Mt pht nhc 128 kbps c dung lng khong 1 MB v bn nhc 320 kbps th chc chn s hay hn bn nhc 128 kbps.Cc nh dngSau qu trnhthu m, ta c mt file nhc wav c cht lng nguyn gc nhng dung lng rt ln, khong 10 MB cho mi pht nhc. Bi vy, tin vic lu tr hay chia s, ngi ta phi nn cc bn nhc li di cc nh dng. Mi nh dng ng vi mt thut ton nn nht nh v t l nn cng nh cht lng sau khi nn ca bn nhc cng khc nhau. C hai cch nn chnh l: nn c mt (lossy compression) to ra cc file nhc mp3, wma, oggv nn khng mt (lossless compression) to cc file nhc flac, apeKhi nn c mt (rip hoc convert nhc), chng trnh nn s ct bt i nhng di tn s m thanh nht nh (thng l di tn trn 20 Khz, theo c im v kh nng nghe ca tai ngi), t gim c dung lng bn nhc. Nhng ci phi tr gi l cht lng m thanh s gim i. Do vy, cng gim t th nh dng nhc hay thut ton m ha cng tt.Theo nhiu nh gi th nh dng ogg, wma cho cht lng tt hn mp3 vi cng mt dung lng. Bn cng nn nh rng vic chuyn i qua li cc nh dng cng lm gim cht lng bn nhc.C nhiu cch nn vi loi bitrate khc nhau nh: CBR/ABR/VBR:- CBR: Constant bitrate:Ngha l bitrate ca stream l mt hng s v khng thay i ti bt k im no ca stream.S dng bitrate c nh m ho ton b file. y l thit lp mc nh ca hu ht cc my nghe nhc- ABR: Average bitrate:Ngha l stream c th s dng bitrate thay i cho mi frame,nhng bitrate trung bnh ca ton b stream l c nh.gn ging VBR ngoi tr kch thc file bit trc (vi VBR, kch thc file c khi ln, khi nh tu vo phc tp ca m thanh) nh iu chnh mc thay i quanh gi tr bitrate trung bnh n nh trc.- VBR: Variable bitrate:Ngha l stream c th s dng bitrate thay i cho mi frame v ty bin t c bitrate cn thit cho mi frame,v vy bitrate trung bnh khng th xc nh trc khi encode hay tnh ton c th....S dng bitrate thay i tu theo yu cu ca tng on m thanh. Thng th VBR cho cht lng tt hn vi kch thc file nh hn CBR nh cp pht bitrate "thng minh" hn. Tt c nh dng "lossless" u c m ho theo VBR, nhiu nh dng "lossy" mi (nh WMA phin bn 9) cng h tr VBR.Lu l vic convert li tng bitrate khng th lm tng cht lng bn nhc. Mt bn nhc 320 kbps c to ra t bn nhc gc c bitrate 64 kbps ging nh bn ly 90% nguyn liu ca mt chic bnh nh lm mt chic bnh ln hn.Khc vi nn c mt, vic nn nhc khng mt ging nh ta nn dng zip, rar. T l nn ca nh dng ny khng cao (ti a l nn cn bng 1/3 dung lng bn nhc gc) nhng b li, cht lng m thanh li tng ng vi bn nhc gc (bi thc cht ta khng ct xn bn nhc gc, khi chi nhc, cc phn mm ch tin hnh gii nn m thi). Khi lu tr cc a nhc, mun tit kim dung lng m cht lng m thanh vn khng i, ta hy rip thnh cc bn nhc nn khng mt.m thanh c ghi trn a CD v nh dng file m thanh WAV c s dng chun nh dng pulse-code modulation (PCM) (tm dch l iu bin nhp, ngha l trong analog ta thy 1 tn s sine din t m thanh, nhng trong k thut s ta khng th c sng sine m ngi ta s dng nhng "nhp p" cao thp khc nhau 1 cch lin tc din t cn ng nht hnh dng sng sine") . y l nhng tn hiu m thanh gc v hon ton khng c nn.Theo chun PCM, mi giy m thanh c ly mu vi tn s ly mu 44.1KHz, v mi mu c din t bi 16 bit d liu. C ngha l trong 1 pht nhc/m thanh ta c:44100 t ly mu X 2 knh tri phi X 2 bytes (16 bit = 2 bytes) X 60 giy = 10.584.000 bytes = 10.1 MbNh ta bit, 1 CD thng c dung lng l 750Mb, hoc lu c 74 pht nhc, v th nu bn nhn con s 10Mb ca mi pht nhc cho 74 bn s thy r ti sao CD n li nh vyNh vy tm li, 1 giy ca m thanh gc s c bitrate l 1411kbpsMP3, WMA, ... - Lossy compression (nn mt d liuVi s pht trin ca PC v internet, nhu cu chia s thng tin v nhc cng ngy cng c i hi cao. Nhng ngi ta khng th no gi c album nhc n 700Mb qua internet vi tc o ut 56kps thi y c. Do cc nhm nghin cu, cc t chc, v nhiu cng ty khc nhau c gng tm ra nhng nh dng m thanh mi s dng nhng thut ton ring nhm gim bit dung lng d liu cn din t m thanh gc cng lc c gng gi cho m thanh gn vi m thanh gc nht.C rt nhiu nh dng khc nhau ra i nh mp3, wma, aac, ogg, mpc, atrac, ... Chng hot ng gn ging nhau nhng mi nh dng c 1 thut ton khc nhau xc nh xem gi li mum thanhno, b mu m thanh no, hoc iu chnh mum thanhth no. Th th ti sao li c th b, hoc gi? V theo l thuyt tai con ngi s rt kh nhn ra s hin din ca 1 tn s m thanh nht nh no (c th l qu 20Khz). Vic b i 1 phn d liu m thanh ny gip cho cc nh dngm thanhmt d liu nh Mp3 c th gim d liu cn thit din t 1 ln ly mu (s t hn rt nhiu so vi 16bit cho 44100 ln 1 giy nh ca m thanh gc).Ngoi ra cc nh dng m thanh ny cn to ra nhng m thanh gi nhm p vo nhng phn n loi b, iu ny l thc s khng th chp nhn c, n to ra nhng m thanh ta hay gi l "o o" hoc vang hoc mo hn so vi m chun, /v nhng file c nn vi bitrate cng thp th hin tng ny xy ra cng nhiu (v d in hnh nht: bn hy nghe th 1 on khn gi v tay ca 1 file mp3 v 1 track trong CD gc hoc 1 file nn khng mt d liu (lossless) s ngay lp tc nhn ra. V sao ting v tay li gy ra nhiu vn nh vy ? Bi v ting v tay l 1 m thanh hn hp ngu nhin, nu trong m thanh chun gc n s c din ta y , th nhng vi m thanh nn, nh dng nn buc phi "p" bitrate ca mnh vo khong cho php do n to ra nhng m thanh v tay u u nhau rt t s khc bit hoc b hiu ng vang).Chng ta thng thy rng MP3 hay c nn vi bitrate l 128, hoc 192, hoc 320 kilobit 1 giy (kbps) . Bn c th nhn thy rng n ch bng 1/10 so vi biterate ca WAV (1411kbps) l l do ti sao 1 pht nhc MP3 128kbps ch tn khong 1Mb.ng l trong 1 s trng hp nht nh, hoc 1 dng m thanh/nhc no , s rt kh phn bit s khc nhau gia m thanh gc v MP3. Bn cnh cc thut ton nn ca cc nh nhc mt d liu c ci thin rt nhiu. Th nhng khng c g hon ho, v chc chn ci g mt i th s lm cho n hng i. c bit l m thanh. i vi nhng album nhc nh vocal, nhc c, hay c bit l c in th y l 1 tai ha, v vi nhng album nhc ny, thng nhng nhc c c s dng hoc ging ht c tn s m thanh rt cao hay rt trm do rt nhiu d liu b loi b hoc iu chnh khc i so vi thc t.MP3, m thanh nn, nhiu ngi cho rng ch thch hp vi nhc pop hoc cc dng nhc bnh thng khc.Lossless Compression (Nn khng mt d liu)Trong cng vic hng ngy vi my tnh, hn khng t ln bn nn 1 file ti liu gi cho ng nghip. C th bn s dng Zip hoc Rar lm nh dng nn.File ti liu c bn nn sau khi qua Zip hoc Rar s tr nn nh hn rt nhiu nhng khi ngi nhn nhn c file, h s gii nn v c c file ti liu gc m bn to. Vy Zip v Rar lm g ? Ni n gin, l nhng thut ton nhm tm ra nhng quy lut lp ca d liu t tm 1 cch hin th khc ti u hn, tn t d liu hn. (v d ta c chui: aaaaa bbbbbbb aaa 11111 , bn thy rng cch din gii tt hn nhiu m tn t ch hn l ax5 bx7 ax3 1x5). y l 1 v d rt n gin bn hiu, cn th n phc tp hn rt nhiu .Nh vy khi ngi nhn nhn file v gii nn, Zip v Rar ng nhim v s dng nhng chui d liu nn y tp hp v to li file gc ban u. cng l mc ch ca nh dng m thanh nn khng mt d liu (lossess). Vi cu trc trn ca zip hoc rar th bn c th thy r rng i vi lossless audio, n ly u vo l m thanh gc ca CD, c gng tm ra nhng quy lut m thanh v nn n li. Vic nn li ny l khng cao v d liu m thanh rt a dng v s dng nhiu d liu. Hin ti mc nn cao nht c th ca k thut nn khng mt d liu l bng khong 1/3 dung lng gc ca m thanh gc. Do mi album lossless s c dung lng khong 200 n 300 Mb.Khi gii nn hoc khi nghe lossless iu chc chn ta t c chnh l tn hiu gc ca m thanh CD (44.1Khz, 16bit, 1411Kbps) . iu ny l cu nhn cho mi ngi yu m nhc lun i hi m thanh trung thc nhng khng c iu kin c CD gc hoc mun s dng my tnh lm ni lu tr albums.Hot ng ca vic ghi CD nhc:Nh cp, nh dng m thanh ca CD l PCM 1411kbps. V u vo ca n cng phi nh dng PCM 1411kbps. Do khi ta ghi 1 CD nhc vic u tin ca 1 trnh ghi a l n phi convert (chuyn) bt k nh dng cho vo ra WAV, bt k n l mp3 hay ape, lossy hay lossless. l l do v sao m ngoi mp3 thng c h tr sn, i vi cc nh dng m thanh khc ta phi cn plugin cho trnh ghi a mi c th ghi c.Nh th bt k nh dng nhp vo l g trc khi ghi ra a ta s c 1 d liu m thanh nh dng WAV, m WAV th lun l PCM 1411kbps. Cho nn d d liu vo "xu" hay "p" n cng s c cho mc 1 ci o c dt bi 1411 k si ghi ra CD. Ti sao cng 1 album, ta c 2 nh dng mp3 v ape , mp3 ch 50Mb, ape n 200Mb m ghi ra a vn y, vn cng ngn y pht nhc ? c cu tr li ti sao.Hot ng ca vic nn CD nhc:Nh vy sau khi ghi ra CD 1 r d liu "xu" y, nu bn s dng n c trong my s vn thy rng bitrate ca n l 1411kbps . Tip theo nu bn s dng software rip CD ny v xc nh bitrate l 320 hay cao hn i na th n s vn thc hin cng vic nn 1411kbps d liu "xu" y tr thnh 320. Nhng cng phi ni thm rng d nn 320kbps nhng liu "xu" ca bn s cng tr nn xu hn v chnh trong lc nn 320kbps, n s tip tc b mt tip d liu . xu li cng xu .Vy theo l thuyt bi trc, gi nguyn "xu" gc bn ch c cch nn nh dng lossless khng mt d liu ... "xu".Phn ln, hay ko mun ni l tt c nhng a nhc copy (c nhc Vit ln nhc ngoi) m ta thy ngoi tim u l ghi ra a vi ngun l MP3 trong my tnh. Bn c rip vi bt k nh dng no th cht lng vn l hng ph phm, khng ni g cht lng CD, m cht lng m thanh khng th no bng a gc.Vy vi lossless n s th no ? Cng vn th, nhng khi APE c trnh ghi a gii nn ra WAV ta s c li d liu p ban u 1411kbps, to ra 1 a CD chun 1411kbps, ri ta li rip lossless, ri li ghi ra. Cho d bao nhiu ln i na th d liu vn (c th) c gi nguyn. ti ni c th l v n cn ph thuc nhiu vo cht lng CD, cht lng u c, 2 th y c m bo c cho s an ton, hon chnh ca d liu khi ghi v c hay khng. V th m ngi ta lun ni l vi CD th phi l TDK, a th phi l Plextor, hn na khi ghi hay c th ch tc 1x , vng 1x , nh th mi gim thiu ti a s li c ghi.Cng ngh ghi a v loi a c s dng l rt quan trng do a hiu mi t nh vy. Ngoi ra cn c loi a dnh cho dn audiophile nh a vng, a thy tinh. Cng ngh th c XRCD, DCC, Chesky, MFSL ,... rt rt nhiu. S khc nhau ca h l cch thc x l tn hiu gc t n hon chnh, sau s dng cng ngh my mc c ft trin ring ghi ln a c hiu, my ghi a lun m bo rng khng c li xy ra, d liu khng bi nhiu, v khi ghi ln b mt a t c hiu qu ti u.Cc bn khi c Bin i F th lu - Chui F (FS) dng cho tn hiu tng t, tun hon- Bin i F thi gian - lin tc (CTFT) dng cho tn hiu tng t, khng tun hon- Bin i F thi gian - ri rc (DTFT) dng cho tn hiu ri rc, DTFT l hm lin tc- Bin i F ri rc (DFT) dng cho tn hiu ri rc, DFT l ri rc (thc cht DFT l DTFT c ly mu trn min tn s)- Bin i F nhanh (FFT) dng cho tn hiu ri rc, l cch tnh DFT nhanhTrong phn ny chng ta s tm hiu cc dng m thanh gc v nn (trong nn mt d liu v khng mt d liu)Trc tin, ta tm hiu v m thanh gc ci :CD Audio ( nh dng WAV ) c s dng chun nh dng PCM (Pulse-Code Modulation)Trong tin hiu, Dng sng ca tn hiu Analog l dng SIN, tn hiu Digital l nhng nhng bc cao thp khc nhau 1 cch lin tc (kiu nh dng bc thang)n din t gn ng dng song SIN.m thanh dng PCM l cha b nn .PCM c cc chun :+ Tn s ly mu l 44.1KHz+ S bt m ho ca mi mu : 16biti khi c ngi t hi, ti sao lu MP3 vo 1 ci CD th c hn hn trm bi, m Burn thnh a nhc ch c c trn 1 chc bi . Cu hi ny cng u km phn th v phi khng ? D tha n l th ny :By gi ta th tnh dung lng 1 file nhc dng WAV trong 1 giy l bao nhiu nh (Theo cng thc in hnh : Dung lng = Tn s ly mu X S bt m ho ca mi mu) .Ta phi : 1 bi nhc c 2 knh tri v phi .Nn dung lng phi nhn 2 (bi v 2 knh c lp m) v 16 bit = 2 bytesTa c : 44100 Hz X 2 knh X 2 bytes X 1 giy = 176400 bytes = 172.27KbCh : 1 Kb = 1024 byte ; 1 Mb = 1024 Kb ; 1Gb = 1024 MbSuy ra : 1 pht nhc WAV c dung lung = 172.27 X 60s = 10336.2 Kb = 10.1Mbn y chc cc bn hiu ti sao 1 a CD ch lu c khong 70 -80 pht khng ?Bi v : 1 CD c dung lng khong 700-800Mb , mi pht nhc mt khong 10Mb, v chi ly 10Mb nhn cho 70 pht c phi bng 700Mb ng l dung lng cha ca ci a CD khng . Hehe, hay nh . th ra l th !!Qua cch tnh trn ta cng c th suy ra c tc chun ca WAV (file gc) s l : 44100Hz X 2knh X 16bit = 1411Kbps (phi l th ny nha cc bc)Lossy Compression(nn mt d liu)Tiu biu :MP3,WMA,Trong thi i Internet hin nay th nhu cu chia s thng tin, Multimedia ngy cng ln. Nhng ngi ta khng th no gi c album nhc n 700Mb qua internet vi tc con ra 56kps ca Dial-Up c, hay tc ca ADSL cng khng kh thi.

Ngi ta lun c gng sang to ra cc nh dng m thanh khc nhau, s dng phng php khc nhau nhm mc ch gim thiu dung lng ca file nhc nhng vn phi gi c c trng vn c ca file nhc gc . Cc bn c th thy ngy nay c rt nhiu nh dng file m nhc khc nhau nh :MP3, WMA, AAC, MPC, AMR Mi nh dng m thanh li s dng mt thut ton khc nhau ly mu, m ho v c cch ring gi li c trng ca file nhc gc .Nh cc bn bit, tai con ngi c kh nng cm nhm nhng m thnh c tn s nm trong khong 16Hz -20KHz, m thanhnm ngoi di coi nh v ngha i vi tai ngi . Chnh v th m ngi ta tm cch loi b thnh phn tn s nm ngoi di i, chnh v vy gip cho cc nh dng m thanh nn kiu mt d liu nh MP3,WMA tit kim c s ln ly mu cng nh tit kim c rt nhiu s bit m ho cc mu , dn n tit kim c dung lng .Song song vi qu trnh tit kim trn, cc nh dng ny cng phi lm sao b p vo nhng phn m n lc b i, n to ra nhng m thanh mo so vi m thanh chun gc, vi m thanh nn vi tc cao th khng roc rt lm, nhng nu nn vi m thanh c tc thp th hin tng ny rt r rt to cho ta cm gic mt f . Bn hy so snh ting n nga phi t 1 CD gc v t 1 file MP3 m xem, ting t CD gc c din t chi tit, y nn ta c th nghe r tng bc tng bc chy ca n nga, nhng vi 1 file MP3 th 1 s thnh phn m thanh b gin lc i nn ta nghe c cm gic ting nga n t tng tng, u u nhau, chng r ting chn nga phi .Chc mi ngi cng bit l chng ta hay download nhc MP3 c bit rate nh 128kbps, 192kbps, 256kbps, 320kbps . Nh vy nhiu anh em cho l bit rate qu cao ri, nhng thc ra so vi m thanh CD chun (1411Kbps) th n ch bng khong 1/9 m thi, th nn dung lng file MP3 so vi WAV cng ch bng khoang 1/9 -1/10 m thi .Cng c 1 s trng hp nn nhc tt khin cho khng cm nhn c s khc bit gia file nn v file gc . Cc bn cng nn lu rng, vic s dng file nhc dng nn nh MP3,WMA ch l gii php tnh th m thi, bi v n c th p dng tt cho dng nhc tn thi nh POP, ROCK nhng khi p dng n vi trng hp nhc ho tu, nhc c in th coi nh b i, l do v sao , n gin l nh sau :Cc dong nhc ny thng c nhng nhc c pht ra loi m thanh rt trm (tn s cc thp) hoc cc bng (tn s cc cao), do khi nn chng cc m thanh trong di tn s cc thp hoc cc cao ny s b loi b th nn coi nh 1 phn ca dn nhc b g nn ho gt ra kh danh sch, he he he . Vy th cn u l trn vn na phi khng cc bc .m thanh dng nn mt d liu n c u v nhc im nh th y, vy cn m thanh nn khng mt d liu th sao ?Lossless Compression(Nn khng mt d liu)Chc mi ngi chng xa li g vi 2 thng WINZIP v WINRAR. 2 thng ny gip ta gim thiu dung lng file ng k, nht l file dng Office . Nn vo ri bung ra m d liu vn nguyn si, hay nh !Vy n lm cch no, thut ton ca n l g ?Thut ton can tht ra cng chng cao siu g, an hem ta c khi cng ngh ra, hihi . l n tm ra nhng on d liu trng lp sau n tm cch hin th sao cho cng tit kim c s liu bao nhiu cng ttV dlien tng thc t 1 cht, cn trong my tnh n l cc chui s 1,0 c: Trch t on th ti tn gi c ni dung nh sau :Anh yu em, anh cn em, anh mun emon ny c c thy 3 t anh, mi t c 3 ch, v chi mt 9 chTng t c 3 t em, mi t c 2 ch, v chi mt 6 chnu gi thay v vit ch anh ta ch cn thay th bng ch X, ch em thay bng ch Y , t gi ta ngm hiu c ch X l anh, ch Y l emSuy ra : X yu Y, X cn Y, X mun YNh vy ta tit kim c tng cng 9 ch ci y .Qa trnh gii nn ra th lm ngc li : X -> anh ; Y -> emNn file m nhc bng WINZIP, WINRAR cng lm theo kiu nh th, n tm ra quy lut ca m thanh ri nn li . Nhng cc bn bit y, file m thanh u c nhiu on lp li, m thanh n rt a dng nn kh nng nn l rt thp, do dung lng sau khi nn chng thp hn ban u l bao, khng tin cc bn th nn 1 file WAV m xem .Thut ton nn khng mt d liu lun dm bo cht lng ban u sau khi gii nn v n khng lc b i bt k ci g, n ch tm cch hin th d liu khc m thi, nhng khi ti to li th y nh ban u.Cn vi kiu nn mt d liu th khng th ti to li cht lng ban u, v 1 phn ca file gc b lc b i khi nn .iu ny cng tng t vi truyn hnh, video. a s th chng ta chp nhn s dng cc nh dng nn mt d liu, nht l video, truyn hnh (v dung lng ln hn m thanh), v so snh vi cht lng gn nh khng i my (mt thng kh nhn ra), nh dng nn mt d liu cho dung lng nh gp hng chc ln.No related posts.Bin i tn hiu Analog & Digital

Tn hiu tng t v tn hiu s

Trong thc t ta gp rt nhiu trng hp chuyn qua li gia tn hiu tng t (Analog) v tn hiu s (Digital)Trong phn ny ti s gii thiu qu trnh chuyn i gia 2 lo tn hiu ny di dng l thuyt n ginPhn ny cng s gip cc bn hiu c cc bit rate c to ra nh th no v n nh hng nh th no n cht lng tn hiu thu c .

Phn trn l tn hiu Analog (c dng sng hnh sin), l 1 min lin tc, c rt rt nhiu gi tr (trc thng ng l bin tn hiu, v l lin tc nn c v s gi tr - cc bc hc vi phn trong lp 12 chc bit ri ch )

Phn di l tn hiu Digital (c dng cc xung vung) ch c 2 gi tr : 0 v 1

Mc 1 khng nht thit phi in p dng hoc ln hn mc 0 . in sao cng c, min l c 2 mc khc nhau.V d: 15V l mc 0 cn -5 l mc 1, ty thuc vo h thng. C th t mc theo khong in p, nh t 0V->3V l mc 0 ; t 3V->6V l mc 1.Trn hnh, c ch in p l 4V nhng vn l mc 1. Nhiu ch c th tng/gim bin tn hiu nhng trong h thng s, tn hiu ch c 2 mc nn 4V vn l mc 1. R rng, tn hiu s c kh nng hn ch nhiu.

Nu tn hiu l 3V th sao nh ? Lc , n khng x l -> treo my , hihihi. Ni chung, ngi ta lun chn khong cch gia mc 1 v mc 0 sao cho h thng vn hot ng tt vi mc nh hng ln nht ca nhiu. Nhiu thng b lm ! C mili vn thi !

Nhiu nh nh vy, c mili vn hoc micr vn m nghe ct-xt th vn nghe ting x ? Bi v tn hiu c c bi u t th c vi mili vn thi! M nhiu th ch no cng nh hng, cng xm nhp c, tc l u t cng b nhiu. Do vy, lc , ch vi mili vn nhng nh hng rt gh! Mi c t bng ln ch c 10mV m nhiu ti 3mV ri! Suy ngh 1 cht, ta thy nh gi mc nh hng ca nhiu th khng th da vo in p nhiu. Tn hiu 5V cn nhiu vi 5mV th khng sao nhng tn hiu 200mV m nhiu c vi mV l gh ! nh gi iu ny, ngi ta s dng thng s S/N ( signal/noise ) , tc l t s tn hiu/nhiu

V d 1 : S/N = 5V / 5mV = 1000V d 2 : S/N= 200mV/ 5mV = 40R rng, v d 1 cho cht lng tt hn v mc tn hiu gp 1000 ln nhiu (nhiu qu nh), v d 2 tn hiu gp c 40 ln nhiu (nhiu tng i l ln)

Cc bn hay chi Ampli, ct-xt, Equalizer chc cng hay nhn thy ci ch dB, vy n l ci g?Ngoi cch nh gi mc tn hiu so vi nhiu nh trn, cn c i lng PSNRPSNR = 20lg S/N (peak signal to noise ratio )V d 1 : PSNR = 20log 5V/5mV = 20log 1000 = 20x3 = 60dBTi sao phi ly log ? V tai ngi, mt ngi thng khng tuyn tnh, tc khng tng theo hm bc nht .V d : bn nghe 1 m thanh 2W v 8W th cha chc g c cm gic ln gp 4 ln . Ga tr dB c th m v s dng log m, v d lg (0/1000) = -2 . Khi th nghe nh hn .Hm lg l php ton ngc vi php ly tha102 = 100 =>lg 100= 2

Chc nhiu ngi ang thc mc, vy qu trnh ghi v c a CD n din ra nh th no ? Xin tha l i khi nh sau :

Trong qu trnh ghi a CD, lung tn hiu dng s c a vo u vo ca tia lazer (chui cc bt 1 v 0, , v d nh l 100110111010 .),gp bt 0 th tia lazer bn 1 l vo b mt ca a, cn bt 1 th khng bn.Khi c a CD th qu trnh ngc li, mt thn (len) ca u c a s pht ra tia lazer chiu vo mt a CD. Khi gp l trn mt CD, tia lazer khng b phn x li, u c hiu l bt 0. Ch no cn nguyn (khng b l), tia lazer b phn x ngc li, mt thn nhn c tia phn x , u c a hiu l bt 1.R rng, vi h thng tng t, cc my mc phi c thit k phn bit rt nhiu mc tn hiu. Trong khi , vi h thng s, my mc ch cn phn bit 2 mc. Vi 2 mc l rt d ch to. V d nh, cng tc=ng hoc m, n=sng hoc tt, dy in=dn hoc ngng, t in= phng in hoc np inTm li, vi a CD th c v t sao chp, min a tt th khng bao gi x x do sao chp nhiu.Chnh v th nn nu sao chp cc file nhc (khng c convert) t a ny sang a kia, t PC sang USB. Th khng baio gi mt cht lng .

Chng ta bt u tm hiu cch bin 1 tn hiu tng t sang tn hiu s v ngc li .

Ly mu: (SAMPLING)

C 1 ng bi ht ca Tun V, ly ngu nhin 1 bi ra nghe ri kt lun : i ca ht hay qu => Ci l gi l ly mu y.

T1, T2... l cc ln ly mu. T1 l ln ly mu th nht, T2 l ln th 2 .t1, t2... l khong thi gian tip tc ly mu tip theo, theo hnh trn ln ly mu th nht T1 cch ln ly mu th hai T2 mt khong thi gian l t2.Ly mu l giai on u ca qu trnh chuyn tn hiu tng t thnh tn hiu s (tc in p micro thnh in p s lu v a CD). C sau 1 khong thi gian c nh, ngi ta o mc tn hiu tng t - tt nhin l thit b in t ch c cha no ngi o c. T t1 n t2, c v s gi tr nhng ta ch ly 9V lm gi tr i din. Tng t cho cc thi im khc. V sao khng ly ht cc gi tr m ch ly thi im T1 ? D hiu thi ! V sao khng kim tra nguyn bao go m ch ly 1 nhm ra coi ri kt lun go ngon hay d? V nhiu qu ! Nu ta ly mu ht th r rng ta cn nhiu a lu tr. Bi vy m t t1->t2, ta ly 1 gi tr thi !S ln ly mu trong 1 giy gi l tn s ly mu (Sampling rate)V d : Sampling rate f= 8KHz => T=1/f = 1/8000Hz = 0,125msNgha l : c 0,125 giy th ngi ta li o ly mu mt ln , tng ng 1 giy ly mu 8000 ln . Kinh khng .

Lng t ha (Quantize)

Ngi ta chia in p o c thnh cc mc Mt mc i din cho 1 khong gi tr.V d 16 mc0 - 1V : mc 01V - 2V: mc 1........15V - 16V : mc 15

gi s in p o c l :0.5V ---> mc 0 ,1V ---> mc 1 ,1.2V ---> mc 1 ,3V ---> mc 3 ,5.4V ---> mc 5 ,

Sau bc lng t ha, min gi tr ca cc in p c c l rt rng s tr thnh 1 s gi-tr hu hn nht nh. Nh ta thy, 1V v 1.2V u l mc 1 . Vy bc ny, 1 ln na ta li lm s lc hn tn hiu.-----S ln ly mu l v hn --> trong 1 khong thi gian th ch ly 1 s mu nht nh----- gi tr ca cc mu ly c l v hn ----> chia thnh cc mc v 1 khong gi tr s c i din bi 1 mcBc ny, ni cho ton hc 1 cht l " Ta ri rc ha tn hiu". ang l 1 ng lin tc th bin thnh 101010

M HO

Bc ny n gin l ta i h 10 thnh nh phn. Sau , d nhin l truyn i hoc l lu v a CD, a cng (nu ang thu m... )Vd: ly v d trn, gi s dng 4 bit m ho cc mc .0.5V ---> mc 0 (0000)1V ---> mc 1 (0001)1.2V ---> mc 1 (0001)3V ---> mc 3 (0011)5.4V ---> mc 5 (0101)Lung tn hiu thu c s l 00000001000100110101Thit b thu s c ln lt, da vo s bt dng m ho tn hiu thng nht t trc, n tch tng nhm 4 bit trong dy s trn ra, v d 0000 n hiu l 0, 0001 l mc 1

S cc mc gi l rng ca mu (Resolution, ), v d trn rng l 16 mc => dng 4 bit nh phn lu tr.Dng 1 s nh phn c 4 bit th c tt c 16 s nh phn : 0001, 0010,....... 1111 . Bi v : 2 m 4 =16 . Mun lu tr 256 s th s bit cn dng m ho 8 (v 2 m 8 = 256 )

Tc ly mu = (tn s ly mu) x ( rng mu) . Ting Anh gi l sampling ratevd: 16khz x 8 bit = 16000 x 8 = 128000b/s = 128kb/s

He he, n y chc mi ngi bit bit rate bi ht c hnh thnh v tnh ton ra sao ri ch .

trn l qu trnh to tn hiu PCM. Khi bn thu m bng micro vo PC, CD th thc s ta ang thc hin qu trnh trn. By gi nhn vo thng s nh "PCM 44khz 16bits " th chc l bn hiu c ngha ca chng ri chKhng bit c hiu tht khng, ti c gii thch li 1 ln na cho chc, l+ Tn s ly mu (sampling rate) = 44KHz+ S bt dng m ho cc mc tn hiu = 16bt=> Tc (bit rate) = 44Khz x 16 bit = 44000Hz x 16 bit = 704000b/s = 704kb/s

l qu trnh bin i t Analog sang DigitalVy c c file dng Digital ri, c c n ta phi lm saoQa trnh i ngc li so vi qu trinhg trn m thi .

Gii m :

Cc bn thy sau khi gii m th ta thu c 1 ng gp khc trong khi ng gc l 1 ng cong. Chuyn ny d hiu ng khng ?

Cc bn c thy hnh trn ng gp khc nhn r lm khng ?Cng d thy, nu ta cng tng tn s ly mu, tng rng mu th ng gii m c s cng mn (trn hnh : cc ng ni gia cc ln ly mu s nh li, cng nh ta cng thy n mn hn ng khng) v cng ging ng gc. iu ny gii thch ti sao bitrate cng cao th nghe cng hay, cng ging thc, v kt qu ti to li s chnh xc hn.Mt ng tn l Nyquyst nghin cu v ni l tn s ly mu ln hn 2 ln bng thng th kt qu thu c l gn trung thc .

V d: tai ngi nghe 0Hz->20Khz,suy ra bng thng = 20khz-0hz = 20khz, vy th tn s ly mu t nht phi l 2x 20khz =40khz. Thi nay, vi a DVD, ngi ta dng ti 96kHzD tn s ly mu c tng ln th r rng ta cng mt thng tin. Ta khng lu tr tt c. Ci g m chuyn sang Digital th ct thnh tng khc ht ri ! Khi ch c mt i ch khng th thm ra. Bi vy ng dng phn mm convert tng bitrate hay tn s ly mu nh cc bc. nh s cng tng t. Khi phng to nh trn my vi tnh th qu gh ! Cng phng cng gh ! Qu trnh scan hnh cng nh trn ! Ngi ta chia tm hnh thnh nhng nh, gi l pixel. Trong 1 nh vy chc c l c nhiu im vi mu khc nhau nhng ngi ta ch ly 1 mu lm i din. D thy, nu s pixel cao v su mu cao th nh s p v sc nt. Vi my nh s th mn hnh bao gm nhng cm bin. Mi cm bin l 1 pixel, mu ca 1 cm bin l mu ca 1 im nh. Cm bin l t ch thit b cm nhn c s bin i. Ni chung, ci no m bin nh sng, nhit ,.. thnh in th l cm bin (sensor). nh sng, nhit ,.. thay i th in cng thay i theo l c. V u c gi l thit b mc d n nh xu

Ti y, c th mi ngi thc mc l " Ti sao tn hiu mt i m khng cm thy nh vy" . l v nhanh qu nn mnh kh nhn ra y!, tai ngi m ch c phi tai . u , hihihihi .

Kt lun:

- Bitrate, tn s ly mu cng cao th nghe cng hay- Convert li tng bit rate :nu dng 1 file MP3 64kbs Convert ln MP3 128kbs hon ton khng hay thm t no.Nu dng 1 file MP3 64kbs 32kHz Convert ln MP3 128kbs 44kHz hon ton khng hay thm t no.- Hn ch s c mt ca thit b Analog trong qu trnh x l/bin tp m thanh/hnh nh.

Cc file WMA, MP3 , AAC, u l dng lossy(mt thng tin). Cc encoder Mp3,... s ct bt nhng phn m tai ngi rt t nhn ra, v d nh khong tn s cao t 16khz-20khz, hoc l trong lc c rt nhiu m to, nh th nhng m nh s c b i .

Ni chuyn vui cht, cc bn chc cng mua a ngoi ch tri ri ng khng ?Cc bn 1 iu : a VCD mua ch thng l ca nhc lng thm ngi mu ThaiLand hoc phim m h h. l do h lng vo bn cho chy. Ngun ca my ng lm a l trn mng m thi ! Trn mng th l mp3, wma,.. u l nhng dng lossy . Sau , my ng phi chuyn thnh MPEG ghi ra a VCD. M MPEG li l 1 dng lossy . Hai ln lossy nn cht lng km l phi. Hehe, ng nh .

Chc n y cc bn c cht t hiu bit thm v lnh vc in t s, c bit l lin qua trc tip n cc file MP3 Tun V ti sn v