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• Data compression, Source coding or Bit-‐rate reduc4on: kodiranje informacije tako da konačan oblik sadrži manje bita od originalnog.
• Dve vrste kodova: Losy i Lossless (sa gubicima i bez gubitaka)
Lossless data compression
• Lossless kodovi redukuju bite prepoznajući i otklanjajući tzv. sta4s4čku redundansu. (sta4s4cal redundancy)
• U teoriji informacija redundansa je razlika broja bita kojima je zapisana informacija i broja bita koji opisuju samu informaciju.
• Količina “nepotrebnih” bita u zapisu.
Run-‐length encoding • Većina realnih zapisa u sebi sadrži staGsGčku redundansu. • Primer: deo površine slike je konstantno zelene boje – umesto da
prenosimo informaciju: zeleni piksel, zeleni piksel, zeleni piksel, zeleni piksel... Dovoljno je zapisaG: 427 zelenih piksela!
• Primer: beli i crni pikseli u tekstu : WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW
• Primenom RLE algoritma ovaj niz se zapisuje kao: 12W1B12W3B24W1B14W
• Ovo je primer prostog, tzv. Run-‐length encoding (RLE) data compression algoritma.
• Veoma primenljiva tehnika kod zapisa u kojima ima dosta ponavljanja: prenos poruke faksom, veći deo informacije je beli papir... Nepovoljan je za kodovanje fotografija gde se paleta menja u konGnuitetu.
• KorisG se u Truevision TGA, PackBits, PCX i ILBM formaGma, kao i ispomoć u GIF i JPEG kompresionim tehnikama.
LZ77 i LZ78 • Abraham Lempel i Jakob Ziv, 1977. i 1978. godina. • Nalaze se u osnovi GIF i DEFLETE algoritama. Na DEFLETE algoritmu je baziran PNG kod.
• Pripadaju klasi tzv. DicGonary Coders tehnika (postojanje rečnika).
• Ulazna sekvenca se deli na fraze. Svaka fraza se kodira uz pozivanje na već postojeću u rečniku.
• Traži se najkraća fraza koje nema u rečniku, koja se zaGm koduje kao fraza iz rečnika + jedan karakter.
• Takva nova fraza se upisuje u rečnik i proces se ponavlja.
LZ77 i LZ78 • Primer: ulazna sekvenca:
AABABBBABAABABBBABBABB • Prva nova fraza je samo A –
(0,A). Druga nova fraza AB – (1,B). (pozivanje na prvu frazu iz rečnika + novi karakter) A|AB|ABBBABAABABBBABBAB
• Sledeća fraza je ABB -‐ (2,B) (pozivanje na drugu frazu iz rečnika + novi karakter) A|AB|ABB|B|ABA|ABAB|BB|ABBA|BB
• Reference Phrase Encoding • Referenca:1 Fraza: A Kod:
(0,A) • 2 AB (1,B) • 3 ABB (2,B) • 4 B (0,B) • 5 ABA (2,A) • 6 ABAB (5,B) • 7 BB (4,B) • 8 ABBA (3,A) • 9 BB (7,0)
Lossy data compression • Uklanja se deo informacije zarad uštede u veličini zapisa.
• Generalno, lossy algoritmi se zasnivaju na analizi percepcije ljudskih čula.
• Primer: oko je osetljivije na supGlne promene osvetljaja nego na supGlne promene u boji: JPEG se delom zasniva na “zaokruživanju” u tom smislu manje bitnih informacija.
• PsihoakusGčki kodovi koriste efekat maskiranja. • Govor ima posebne karakterisGke i za njegovu kompresiju se razvijaju posebni algoritmi (internet telefonija...)
• Procena je da bi bilo moguće, uz primenu već postojećih tehnika, 4.5 puta smanjiG memorijski prostor na kojem se danas čuvaju sve informacije.
• Procena iz 2007. godine: 1300 exabyte (10^18 byte) se može smanjiG na 295 exabyte memorijskog prostora.
• 60 CD-‐ROM ova po čoveku na planeG – oko 400 milijardi diskova...
• Tri glavne grupe audio formata: 1. Nekompresovani audio format dobijen putem PCM-‐a. Obično se zapisuje kao .waw format u Windows okruženju ili .aiff format za Mac OS. Na kompakt disku je to CD Audio format.
2. FormaG sa lossless kompresijom: FLAC, Monkey’s audio (.ape), WAVPACK (.WV), TTA, ATRAC, Apple Lossless (.m4a), MPEG-‐4 SLS, MPEG-‐4 ALS, MPEG-‐4 DST, Windows Media Audio Lossless (.wma lossless), SHORTEN (.shn)
3. FormaG sa Lossy kompresijom: MP3, VORBIS, MUSEPACK, AAC, ATRAC, Windows Media Audio Lossy
• 3gp -‐ mulGmedia container format can contain proprietary formats as AMR, AMR-‐WB or AMR-‐WB+, but also some open formats • act -‐ ACT is a lossy ADPCM 8 kbit/s compressed audio format recorded by most Chinese MP3 and MP4 players with a recording funcGon, and voice recorders • AIFF – standard audio file format used by Apple. It could be considered the Apple equivalent of wav. • aac – the Advanced Audio Coding format is based on the MPEG2 and MPEG4 standards. aac files are usually ADTS or ADIF containers. • ALAC -‐ Apple Lossless compression, a lossless compression format from Apple. • amr -‐ AMR-‐NB audio, used primarily for speech. • atrac (.wav) – the older style Sony ATRAC format. It always has a .wav file extension. To open these files, install the ATRAC3 drivers. • Au – the standard audio file format used by Sun, Unix and Java. The audio in au files can be PCM or compressed with the μ-‐law, a-‐law or G729 codecs. • awb -‐ AMR-‐WB audio, used primarily for speech, same as the ITU-‐T's G.722.2 specificaGon. • dct – A variable codec format designed for dictaGon. It has dictaGon header informaGon and can be encrypted (as may be required by medical confidenGality laws). A
proprietary format of NCH Souware. • dss – Digital Speech Standard files are an Olympus proprietary format. It is a fairly old and poor codec. Gsm or mp3 are generally preferred where the recorder allows.
It allows addiGonal data to be held in the file header. • dvf – a Sony proprietary format for compressed voice files; commonly used by Sony dictaGon recorders. • flac – File format for the Free Lossless Audio Codec, a lossless compression codec. • gsm – designed for telephony use in Europe, gsm is a very pracGcal format for telephone quality voice. It makes a good compromise between file size and quality.
Note that wav files can also be encoded with the gsm codec. • iklax – An iKlax Media proprietary format, the iKlax format is a mulG-‐track digital audio format allowing various acGons on musical data, for instance on mixing and
volumes arrangements. • IVS – A proprietary version with Digital Rights Management developed by 3D Solar UK Ltd for use in music downloaded from their Tronme Music Store and interacGve
music and video player. • m4a – A proprietary version of AAC in MP4 with Digital Rights Management developed by Apple for use in music downloaded from their iTunes Music Store. • mmf -‐ a Samsung audio format that is used in ringtones. • mp3 -‐ MPEG Layer III Audio. Is the most common sound file format used today. • mpc -‐ Musepack or MPC (formerly known as MPEGplus, MPEG+ or MP+) is an open source lossy audio codec, specifically opGmized for transparent compression of
stereo audio at bitrates of 160–180 kbit/s. • msv – a Sony proprietary format for Memory SGck compressed voice files. • mxp4 – a Musinaut proprietary format allowing play of different versions (or skins) of the same song. It allows various interacGvity scenarios between the arGst and
the end user. • ogg – a free, open source container format supporGng a variety of formats, the most popular of which is the audio format Vorbis. Vorbis offers compression similar to
MP3 but is less popular. • ra & rm – a Real Audio format designed for streaming audio over the Internet. The .ra format allows files to be stored in a self-‐contained fashion on a computer, with
all of the audio data contained inside the file itself. • ram – a text file that contains a link to the Internet address where the Real Audio file is stored. The .ram file contains no audio data itself. • raw – a raw file can contain audio in any format but is usually used with PCM audio data. It is rarely used except for technical tests. • TTA -‐ The True Audio, real-‐Gme lossless audio codec. • vox – the vox format most commonly uses the Dialogic ADPCM (AdapGve DifferenGal Pulse Code ModulaGon) codec. Similar to other ADPCM formats, it compresses
to 4-‐bits. Vox format files are similar to wave files except that the vox files contain no informaGon about the file itself so the codec sample rate and number of channels must first be specified in order to play a vox file.
• wav – standard audio file container format used mainly in Windows PCs. Commonly used for storing uncompressed (PCM), CD-‐quality sound files, which means that they can be large in size—around 10 MB per minute. Wave files can also contain data encoded with a variety of (lossy) codecs to reduce the file size (for example the GSM or MP3 formats). Wav files use a RIFF structure.
• wma – the popular Windows Media Audio format owned by Microsou. Designed with Digital Rights Management (DRM) abiliGes for copy protecGon.
• Lossless formaG mogu spusGG veličinu fajla na 50-‐60% od nekompresovane veličine.
• Lossy formaG, u zavisnosG od zahtevanog kvaliteta, mogu spusGG nekompresovan fajl na 5 do 20% originalne veličine.
• U zavisnosG od formata na CD može staG 1 sat nekompresovanog zapisa (PCM), oko 2 sata lossless koda ili oko 7 saG MP3 formata srednjeg kvaliteta.
CD istorija • The first compact disc was produced in a factory in Germany auer
years of development by Philips and Sony. • The compact disc project was launched following Philips' failure
with its video disc technology in 1978. • The video disc was one of the first commercial products to take
advantage of laser technology that could read informaGon from a disc without any physical contact.
• Research into the video disc began as far back as 1969, and itself was inspired by Italian Antonio Rubbiani, who had demonstrated a rudimentary video disc system 12 years earlier.
• In 1970 Philips began work on what was called the ALP (audio long play) -‐ an audio disc system to rival vinyl records, but using laser technology.
• Lou O{ens, technical director of the audio division at Philips, was the first to suggest that the ALP be made smaller than the dominant vinyl format and should aim for one hour of music.
CD istorija • The project iniGally flirted with the idea of quadraphonic sound but a disc
with one hour of music had to be 20cm in diameter and so the plan was abandoned.
• In 1977 Philips began to take the development of a new audio format much more seriously. A new name for the product was discussed and names considered included Mini Rack, MiniDisc, and Compact Rack.
• The team se{led on Compact Disc because it was felt it would remind people of the success of the Compact Casse{e.
• In March 1979 Philips conducted a press conference to show off the audio quality of its CD system in producGon and also to impress upon rivals how well it was progressing.
• A week later Philips travelled to Japan auer the Japanese Ministry of Industry and Technology (MITI) had decided to convene a conference to discuss how the industry could create a standard for the audio disc. The company leu Japan having agreed a deal with Sony.
• Philips' plan for a CD with a 11.5cm diameter had to be changed when Sony insisted that a disc must hold all of Beethoven's 9th Symphony. The longest recording of the symphony in record label Polygram's archive was 74 minutes and so the CD size was increased to 12cm diameter to accommodate the extra data.
CD istorija • In 1980 Philips and Sony produced their Red Book, which laid down all the
standards for compact discs. From that Gme on the companies worked separately on their own CD equipment but in the early days agreed to share components.
• In April 1982 Philips showed off a producGon CD player for the first Gme. "From now on, the convenGonal record player is obsolete," said Lou O{ens.
• The first commercial CDs pressed were The Visitors by Abba and a recording of Herbert von Karajan conducGng the Alpine Symphony by Richard Strauss.
• US record labels were iniGally very scepGcal about the CD. A year auer launch there were 1,000 different Gtles available.
• In 1985 Dire Straits' Brothers In Arms became the first CD to sell more than one million copies. It is sGll the world's most successful CD album.
• In 2000 global sales of CD albums peaked at 2.455 billion. In 2006 that figure was down to 1.755 billion.
CD Audio
• Red Book – dokument sa standardima. • Dvokanalni audio, 44.1 kHz, 16 bit. Red Book dozvoljava upis i 4 kanala (kvadrofonija), ali to nikada nije zaživelo. Mono zapis nije predviđen.
• 44.1 kHz je standard dobijen na osnovu činjenice da se digitalni audio zapis u to vreme čuvao na video trakama. 3 sempla po jednom kanalu su mogla biG zapisana u jednu video liniju. PAL standard: 294 linije, 50 polufrejmova u sekundi: 3x294x50 = 44.100 semplova u sekundi.
• Osnovni audio zapis je nasato klasičnim PCM kodovanjem. Na tu osnovnu informaciju dodaju se biG za zašGtno kodovanje i prilagođavanje zapisu na fizički medij.
• .waw Gp fajla je veoma sličan (klasičan PCM), samo je osnovna informacija prilagođena za upisivanje na računarske memorije.
• .cda nije audio format, već samo info fajl, koji obezbeđuje informaciju da se u računaru nalazi CD u audio formatu.
• Pohranjivanje audio zapisa direktno sa CD-‐a na hard disk nije moguće zbog razlike u strukturi podataka. Neophodna je souverska transformacija – ripovanje.
FLAC • Free Lossless Audio Codec • Codec = Koder + Dekoder • Fajl se na izvoru kodira i redukuje na 50 – 60% od originalne veličine.
• Na prijemu se vrši dekodiranje tako da konačni fajl nema gubitaka u odnosu na originalni.
• Besplatan dekoder, dostupan za javnu upotrebu.
• Omogućava zapis meta-‐data podataka, ime izvođača, naslov albuma, kao i brzo traženje.
FLAC • Razvoj dekodera započet 2000. godine (Josh Coalson)
• 2003. ga u svoje okrilje uzima Xiph.Org FoundaGon
• Xiph.org je fondacija koja se bavi razvojem besplatnih kodeka koji mogu da se takmiče sa komercijalnim formaGma.
• Ogg familija formata, među kojima je najuspešniji Vorbis, koji je pandan .mp3 formatu
• Danas najviše rade na razvoju formata koji treba da konkurišu .mpeg4
FLAC • It can handle any PCM bit resoluGon from 4 to 32 bits per sample,
any sampling rate from 1 Hz to 655,350 Hz in 1 Hz increments,[7] and any number of channels from 1 to 8.[8]Channels can be grouped in cases like stereo and 5.1 channel surround to take advantage of interchannel correlaGons to increase compression. FLAC uses CRCchecksums for idenGfying corrupted frames when used in a streaming protocol, and also has a complete MD5 hash of the raw PCM audio stored in its STREAMINFOmetadata header. FLAC allows for a Rice parameter between 0 and 16. FLAC supports ReplayGain.
• FLAC uses linear predicGon to convert the audio samples to a series of small, uncorrelated numbers (known as the residual), which are stored efficiently using Golomb-‐Rice coding. It also uses run-‐length encoding for blocks of idenGcal samples, such as silent passages.
• For tagging, FLAC uses the same system as Vorbis comments.[7]
MP3
• Audio format razvijen od strane MPEG (Moving Pictures Expert Group)
• MPEG 1 i 2, layer III = MP3 • 1992. godina (MPEG 1). • 1995. godina (MPEG 2) (dodatni bit rate protoci)
• Protok 128 kb/s omogućava smanjenja originalnog PCM zapisa na 10% veličine.
MP3
• Zasniva se na efektu maskiranja. • 1894. u literaturi se pojavio članak o uGcaju tonova nižih frekvencija i viših nivoa na gubitak čujnosG ostalih tonova.
• 1959. Richard Emer je detaljno proučio i opisao ovaj fenomen, definisavši set krivih na osnovu kojih je moguće predvideG efekte maskiranja.
• 1979. u SAD je prvi put predloženo korišćenje efekta maskiranja za potrebe razvoja kompresionih audio kodova.
• MP3 je direktan naslednik prethodno razvijenih kodova: OpGmum Coding in the Frequency Domain" (OCF), and Perceptual Transform Coding (PXFM)
• Oba koda su bila prihvaćena od strane MPEG-‐a, u to doba najkvalitetniji, ali zahtevni za postojeći hardver.
• Ništa bez Nemaca! Karlheinz Brandenburg je iskorisGo pesmu Suzan Vega “Tom’s Dinner” za procenu algoritma koji će postaG MP3. Pesma je skoro monofona, sa bogaGm spektrom, zgodna za analizu i poređenje.
• Suzan Vega – majka mp3 formata J • .mp3 ekstenzija zaživela 14. jula 1995. godine. Pre toga korišćena je .bit ekstenzija.
mp3 • U drugoj polovini 1994. mpr počinje da se korisG na internetu. • Pojavom Winamp plejera (1997. godina), popularnost formata
naglo raste. • Kodiranje mp3 formata nije do kraja precizirano. MPEG1
standard definiše načine i metode za otklanjanje nečujnih informacija.
• Razvijen je veliki broj različiGh mp3 kodera, od kojih zavisi kvalitet kodiranja.
• Koderi se razlikuju i na interentu su dostupne karakterisGke i uporedni testovi.
• Koder dobar za velike bitske protoke (npr. LAME), ne mora biG dobar i za male protoke.
mp3
• Dekodiranje je strogo specificirano. • Dekoderi se ne razlikuju po kvalitetu, već samo po memorijskom ili CPU angažmanu.
• Kvalitet konačnog fajla zavisi od kodera, bitskog protoka, ali i samog audio fajla koji se koduje.
• Zapisi sa puno brzih promena, tranzijenata i sl. Nisu pogodni za kodovanja sa niskim bitskim protocima – (ringing, pre-‐echo..)
• Dobar primer – aplauz publike...
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