[diviner]

mmilovan...prije svega dobrodosao

Sto se tice tvog pitanja upucenog direktno rumskom, pokusat cu ga zamjeniti (iako on ima drugaciju tezu) i iskazati svoje misljenje o pomenutoj temi koje cu (pokusati) debelo potkrijepiti kako tehnickim tako i licnim argumentima (ja sam ovu nedoumicu uspio sebi pojasniti vec poodavno pa cu samo pokusati prenijeti steceno (sa)znanje):

Argumentacija:

http://www.cdrfaq.org/faq04.html

Subject: [4-18-2] The audio data matches exactly, why do they sound different?
(2004/12/10)

Suppose you extract the audio track from the copy, and it's an exact binary match of the track you wrote from your hard drive, but the CDs don't sound quite the same. What then?

Most people don't notice any difference between originals and duplicates. Some people notice subtle differences, some people notice huge differences; on better CD players, the differences are harder to hear. Some say CD-R is better, some say worse. While it's true that "bits are bits", there *are* reasons why CD-Rs may sound different even when the data matches exactly.

An excellent paper on the subject is "The Numerically-Identical CD Mystery: A Study in Perception versus Measurement" by Ian Dennis, Julian Dunn, and Doug Carson, submitted to the Audio Engineering Society (Preprint 4339, 101st AES convention). It's available for download in PDF form at http://www.prismsound.com/downloads/cdinvest.pdf. The paper is primarily concerned with why pressed CDs created at different plants or with different methods sound different, but the observations are relevant to CD-R as well.

The conclusions in the paper suggest that low-frequency modulations in the disc affect the servo and motor electronics, causing distortion noticeable to a critical listener.


One prominent theory is jitter. This isn't the DAE "jitter" described in section (2-15), but rather a timebase error. A good overview can be found in the jitter article on http://www.digido.com/. A brief explanation follows.

The digital-to-analog ("D/A") conversion at the output of the CD player is driven by a clock in the CD player. The clock is tied into feedback mechanisms that keep the disc spinning at the proper speed. If the digital signal being read from the disc has irregular timing, small errors can be induced in the output clock. Even if the CD player gets all of the digital bits accurately, it will produce inferior results if the timing of the bits on the disc isn't precise. Put another way, something has to send a sample to the speakers 44100 times per second, and if it's speeding up and slowing down many times each second your ears are going to notice.

There is some question as to whether the clock driving the output will actually be affected by the input. If the output clock in the CD player is isolated and stable, jitter from the CD will not affect it.

If you play a CD digitally (e.g. by ripping it and then playing it through a sound card), the quality of the CD doesn't matter, because it's the timing of the clock in the sound card that drives the D/A conversion.

It has been asserted that the clocking of bits on a CD-R isn't as precise as on a pressed CD. Writing at different speeds on different types of media requires adjustments to the "write strategy" (section (3-31)) that can result in individual "marks" being sloppier than at other speeds. This could account for inferior -- or at least different -- sound.

Yamaha believes they have found a partial solution for jitter problems with their Audio Master Quality feature. See section (2-41).

There do not appear to be any carefully constructed (double-blind) tests published on the web that confirm that jitter is the cause of this phenomenon. The "Numerically-Identical CD Mystery" paper rejects jitter as a possible cause.


Some people have asserted that *any* two CDs, pressed or otherwise, will sound slightly different. Some claim to hear differences in identical CDs from different pressing plants. The former is silly, but the latter has a lot of anecdotal evidence to support it.

The manual for the CDD2000 reportedly states that the drive uses 4x oversampling when playing pressed CDs, but switches to 1x for CD-R. This affects the quality of the D/A conversion, and can make an audible difference.

http://www.mrichter.com/cdr/primer/losses.htm has some further thoughts, including a table showing signal level differences.

An extremely technical introduction to CD reading is available at http://www.tc.umn.edu/~erick205/Papers/paper.html. This may shed some light on why reading audio CDs is difficult, as well as explain concepts like aliasing and dither.

If you are finding your CD-Rs to be noticeably inferior, try different media, different write speeds, a different player, or perhaps a different recorder. There is some evidence that different brands of media and recorders may work better for audio, but in the end it's a highly subjective matter. Some people say CD-Rs sound worse, some people say they sound better (and some people think vinyl records are still the best).

Obratiti paznju na zakljucak!

Evo i direktan link vec spomenutogistrazivanja (Cambridge UK) na polju usporedbe presovanih diskova i audabilnih razlika (strogo kontrolisani blind testovi) izmedju istih proizvedenih u razlicitim fabrikama (naucni rad je pun tehnickih termina koje mnogi od vas nece razumjeti...potrebno je dosta vise ucenja i istrazivanja kako bi shvatili makar vecinu od onoga sto je ovdje navedeno...zato cu pokusati prevesti zakljucke rada):

www.prismsound.com/downloads/cdinvest.pdf

Zakljucak je, preveden na naski, da velika vecina koja je prijavljivala razliku izmedju diskova to prijavljivala autosugestivno (zapravo prijavljujuci nasumice, iz cega se kasnije nije mogla naci pravilnost).

Takodje, jos je potrebno shvatiti pojavu jittera...ko ne zeli citati artikle na

http://www.digido.com/
ili
http://www.rumski.com/forum/viewtopic.php?t=3020

moze procitati moj zakljucak (ako se meni ne vjeruje, pravac citanje i licno tumacenje onoga sto pise na postavljenim linkovima):

Da razjasnimo konacno i tu stvar !!!

Prvo argumenti :

http://www.digido.com/
articles -> Jitter -> Can Compact Discs contain jitter?

.... "CDRs recorded on different types of machines sound different to my ears. An AES-EBU (stand-alone) CD recorder produces inferior-sounding CDs compared to a SCSI-based (computer) CD recorder. This is understandable when you realize that a SCSI-based recorder uses a crystal oscillator master clock. Whenever its buffer gets low, this type of recorder requests data on the SCSI buss from the source computer and thus is not dependent on the stability of the computer's clock. In contrast, a stand-alone CD recorder works exactly like a DAT machine; it slaves its master clock to the jittery incoming clock imbedded in the AES/EBU signal. No matter how effective the recorder's PLL at removing incoming jitter, it can never be as effective as a well-designed crystal clock." ........

Objasnjenje:

Prilikom citanja i pisanja podataka vrlo je bitan proces sinhronizacije podataka koji pristizu sa audio medija, i to upravo zato (a sto je vec bilo spominjano) sto audio format nema crc (cyclic redudant checking), kao ni bilo koju drugu provjeru gresaka. U klasicnim kucnim plejerima su prisutne hardverske jedinice koje rade oversampling (uporedjivanje prethodnog i sledeceg dela muzickog materijala i zamjena greske, izgubljenog materijala adekvatnim, u istom nivou signala), a njihov rad se razlikuje od modela do modela!

Zaci, prilikom citanja podataka sa audio medija vrlo je bitno da noseci ili sync signal (44100 Hz ) bude sto ravnomjerniji , da na njega ne uticu drugi uredjaji (pojava interferencije, smetnje) kao i da bude sto precizniji (kvalitetniji oscilator). Ukoliko ovi uslovi nisu ispunjeni javlja se Jitter (pogledati dobro grafik na datoj strani gore) odnosno pomjeraj koji dovodi do misinterpretacije signala (nepravilnog citanja, greske) koji se obicno manifestuju u visoko frekventnom delu spektra, a zasto...pa vrlo jednostavno:

Ukoliko odmjeravate sinosuidu od 17 KHz (prodoran piskavi ton) sa 44.100 Hz signalom vise ne dobijate sinusoidu nego signal postaje zubasto kvadratast (dosta ostriji i prodorniji ton) i jasno je da ukoliko menjate pomjeraj u ovom dijelu spektra necete imati vise uopste pravilan periodican signal (jer je perioda odmjeravanog signala svega 2 puta veca od kontrolnog) . Ukoliko on nije periodican, znaci nije vise ni sine, ni square, ni triangle, vec pocinje da lici na noise i ovo bi trebalo da znaju svi koji su se barem jednom dotakli nekog softverskog synth-a u zivotu ! Odmah je jasno je da su srednji i duboki tonovi ovde manje podlozni upravo zbog svoje mnogo vece periode !

Sustina postavljenog pitanja (odnosno odgovora na njega) se nalazi upravo u jitter domenu, a ako se zna da SCSI rezaci koriste posebne klok oscilatore kako bi upravljali podacima sa rezaca, i da na njega skoro ne utice ostatak racunara , prilikom rezanja oni trebali da stizu u teoriji ravnomjernije i na taj nacin preciznije u timeline (vremenu), a samim tim jamice na narezanom cd-u bi trebalo da su dobro odmjerene i sto blize onima u teoriji !

E sad pazite:

Tehnologija je omogucila danas da se greske u citanju (grebovanju jer govorimo o d. audio formatu) smanje na apsolutnu nulu (kako i ne bi kad je unapredjivana skoro 10ak godina) ! To se radi najcesce uporedjivanjem i cheksumima prilikom samog citanja (hardverski)...ili cak ponovnim i visestrukim citanjem i provjere podataka iste deonice diska (softverski) ! Znaci uz pomoc C2 error report-a i rereading-om ! Ovu mogucnost imaju uglavnom racunarski citaci-rezaci !

Rezanje je proces koji je boljka svemu ovome jer ovde sad stupa na scenu sva prica odozgo (jer jednom kad se upisu podaci nema vise diranja ...nema rereading-a niti bilo kakve popravke). Znaci rezac primi podatke od kontrolera i ako kontroler vs mehanika vs pickup barem na jednom mjestu zakaze i produzi nama nam dragu jamicu ...ona ostaje duza no sto jeste ! Citac detektuje samo na prelazu pit -> land, recicete ......ali da li to zna vas cd- player kad je on za to vrijeme odbrojao za jednu binarnu cifru vise, poslao to u DA konverter a vi dobili pogresnu audio informaciju u uvetu (koju uzgred budi receno u ovom izolovanom slucaju nema sanse da cujete, samo povorku ovakvih gresakak, mozete ) !

Posto SCSI rezaci imaju napredniju kontrolu podataka zahvaljujuci SCSI kontroleru (i njegovom crystal-oscilatoru) , a sve ostalo isto (unutrasnjost , mehanika , pickup) tesko se moze govoriti da su superiorniji, posebno kada se zna da jitter najcesce nastaje u slucaju interferencije drugih signala, falinki i vibracija unutar samog rezaca kao i kvaliteta samog pickupa lasera, i posebno ako se zna jos i to da ranije nije bilo moguce za male pare napraviti mocan mikroprocesor (IDE kontroler) koji bi parirao SCSI po kvalitetu i ucinku, a zna se da tako nije vise danas !

Zakljucak: Razlika izmedju mozda postoji ...ali vi to svakako necete primjetiti pa makar imali kontrolnu sobu (studio) za test ! Ja sam pokusao da ovo objasnim sto plasticnije i opseznije i zato sam morao da davim Stvari je ipak najbolje razumjeti u potpunosti !!!

Nije lose da se procita cijeli clanak ili barem ova stranica gore pa da se onda postavljaju pitanja !

Ukoliko odradis ripovanje tehnicki korektno, ukoliko imas solidan cd rezac (ne treba ti nista posebno) i odlican/vrlo dobar cd medij, noviji model skupljih audio cd plejera (koji rade oversampling korektno) razliku u zvuku ne bi trebao primjetiti (presovani originalni audio cd vs kopirani audio cd).

Razlika u zvuku je najcesce proizvod autosugestije (u digitalnom domenu kopije jeste original ...i tacka), ponekad lose karike u lancu reprodukcije i replikacije (ako jedan elemenat/uslov medju onim navedenim na pocetku mog izlaganja, ne zadovoljava, imacete razliku u zvuku...da li ce on biti audiabilan, zavisi od mnogo faktora (jitter, lose ripovanje itd, vaseg sluha, stepena autosugestije, akustike prostorije, kvaliteta uredjaja u reprodukciji itd. itd.)...Svi ovi potencijalno navedeni uzroci i jesu razlozi tako oprecnih misljenja (jer se niko ne potrudi uraditi zaista jedan pravi i objektivan blind test...ako se i uradi objektivno, pitanje je koliko je isti tehnicki korektan i ispravan).


PS.
Sto se tice bugara, licno sam tokom 2001 godine poslao minimum 15-ak "mastera" koji su bili skinuti sa interneta (doduse, trudio sam se uvijek skidati 192 kbit/s mp3-e) i slati za bugarsku na izradu glass mastera. Ko ne vjeruje, i ne mora (njegov problem...ne i moj). Ostaje cinjenica da je mnogo bugara u pocetku bila replikacija originalnih diskova (ali samo u pocetku).