Why two numerally identical CDs could sound different?

I have two Cds one is the original pre-master and the other is a pressed copy from a very known plant. I have tested both for DATA discrepancies and have found the two are identical DATA wise. Why these two CDs sound different regardless the player or converter. I have been auditioning both CDs through a Bow CD Player with a external dCS 954 converter on Dynaudio Confidence C4 monitors, the sound of the pressed CD is much more congested, less transparent and "airy" than the original pre-master. I am new to this forum and I do not know if this kind of question is under the forum's scope, but I know that there are some people here involved with audio production so I decided to post this intriguing question.

 

Welcome to the forum, there are a lot of fun and knowledgable people that hang out here in this forum, (unfortunately I am not one of those kind). But to your question, you will most assuredly get some great responses because this exact topic has been covered a few times before, as a matter of fact there is another thread that deals with this very topic. I believe the consensus was that identical data should provide identical results, but there are other variables, such as dither and quantization errors. If I can find the other thread, I will post a link here later.

 

Welcome, Homero. Jitter problems? Check out this thread:

http://www.stevehoffman.tv/forums/showthread.php?s=&threadid=18118

 

Additionally to the jitter, the shape of the pits on the CD is most likely different on a self-burned CD than on a pressed one from a plant. This alone could be a reason why they sound differently (different pit shape = different 0/1 (on/off) changes). I always thought there are so many variables when it comes to CD, even two samples from the same plant, but from different stampers, may sound differently... a VERY complex field...

 

It would be interesting if you put a "jitter reducer" between the player and and dCS 954 converter and listen again. The differences may be smaller or non-existant if the "jitter reducer" is good enough. An Infinity Digital Time Lens would probably do the trick.

 

I wish I could give you a definite technical reason, but I can't. I know Steve has commented that this does happen and is one of the reasons why DCC was so successful because of very strict quality control against this problem.

You'll find here at Steve's basically 2 different camps on this subject. Those, like you and I who do hear differences, and those that don't. Those that do, accept this, and those who don't, swear over and over again that "they must be the same because the data is the same exactly and bits are bits."

Welcome Homero. BTW have you heard the differences between the MFSL Ultradisc (Made in Japan) and the Ultradisc II (Made in the USA) of the same "exact" albums?

 

I'll admit it. I used to be in the "bits are bits so they must sound the same" camp but recently I've switched over to the other side. I'd almost bet the bank that the so-called identical MoFi UDI and UDII discs that have discussed here before, for example, would sound identical when extracted using Exact Audio Copy on a computer and played back through a high-end sound card or D/A converter. But I'd also bet that they would sound different from each other when played back on a standard CD player. EAC, or similar software, will read each individual bit over and over until it gets it "right" but during normal playback data is, for the most part, just streamed off the disc leaving the audio open to all kinds of errors starting with variations in the physical composition of the CD itself. Ah, wouldn't it be great if everyone had Steve's attention to detail?

Or maybe none of this applies to Homero's problem. Either way, I'm done rambling.

 

Not exactly -- the data are interleaved and checksummed in a complex way so that errors of up to 4000 (I think) consecutive bits can be fully corrected with no interpolation. With one exception, you simply can't chalk up audible differences (apart from gross drop-outs, skips, etc.) to any physical properties of the disc. Unless the disc is obviously damaged, you'll get the same bits every time.

The exception, of course, is jitter, which is real but 100% dependent on the behavior of your particular components. Some D/As handle jitter well, some don't. (Ironically, fancy two-box converter/transport setups are *more* prone to jitter because of the S/PDIF interface and its wonky clock.) Jitter has nothing to do with reflections or stray light or any of that other stuff -- it's slightly malformed pits on the actual pressing master that slightly "distort" the timing that the D/A converter depends on.

The upshot of which is, CD-R copies of two "digitally identical but sound different" CDs made on the same equipment ought to sound identical. Some here have reported otherwise, and I have no explanation for that.

In any case, though, I think the effect of jitter is something of an all-purpose bogeyman that may or may not explain what many people hear.

Ryan

 

I should've said "in comparison to EAC, data is just streamed off the disc." EAC can take several minutes to determine the value of a few bits. I was speaking in relative terms. With mentioning the physical properties of the disc I was just pointing out the obvious starting point of the chain of possible problems (which wouldn't exist with a meticulously extracted audio file "frozen" in time on a hard drive, if you know what I mean.) Sorry for the lack of clarity.

 

While I agree that EAC definitely works harder than most any other process to retrieve the data, my point was more that the act of reading bits off a CD isn't anywhere near as fragile as many think. The disc has to be pretty beat up before serious problems crop up.

It is true, however, that CD-ROM has higher data redundancy than CD-Audio (since even 4000 consecutive bits isn't good enough when one misplaced bit invalidates the entire file). So the chances of bit errors *are* greater on CD-Audio than on CD-ROM (or, as you point out, on a hermetically sealed hard drive). I just don't think error correction via interpolation (or "guessing") happens nearly as often as people think it does. (Or as often as tweak manufacturers want us to think it does, at any rate.)

Ryan

 

Thanks all for the replies. I am just thinking how interface or converter "jitter" could be involved... If I play the reference disc on a system and after wards I play the pressed disk on the same system, "jitter" should affect both disks at the same extent. I have read a paper from Prism entitled "Sonic differences between numerically identical CDs" - http://www.prismsound.com/psdownload, it is a very interesting research about this subject but the test results are still open to discussion. I will search the forum's archives for more information I have missed.

 

That paper is a very interesting and recommended read.

If I understand jitter right, the idea is that different "pressings" of the same digital material will sound different on the same system due to variances in the pits on the CD, which are too small to affect the actual data but are large enough to affect the timing of the data being fed into the D/A converter.

Ryan

 

Sure, some physical characteristic of the media should interfere with the player performance, probably in the time domain.

 

Re: Re: Why two numerally identical CDs could sound different?

No, unfortunately I don't know these records

 

Re: Re: Re: Why two numerally identical CDs could sound different?

They are Gold CD's made by Mobile Fidelity Sound Labs. You can see them here. Scroll down and see the links.

 

a

 

I forgot to say that this CD was pressed in Brazil but it's a SACD production, I should be receiving the Hybrid SACD pressed in Germany next week so I will have a very good way to compare the 44.1 layer from both plants. I will comment the audition and tests results soon.

Thanks all

 

I couldn't get that link to work. Here's where I found the paper:

http://www.prismsound.com/downloads/cdinvest.pdf

Thanks for the reference, Homero. Looks like an interesting article!

 

I have continued investigating the subject, its very difficult indeed to establish a method that compares objectively some measurements against the listening expierence. A found it from Phillips:

Audio Losses - pressed vs. CD-R

At first glance, there should be no reason for loss of fidelity between an original disc and a CD-R copy. In practice, there are often reports of such degradation. For example, Philips reports at their WWW site in cddd3610e.pdf that you can expect the following differences at the analogue line output, with similar values for headphone.
Line Out On pressed CD On recordable CD
Amplitude Linearity 1.5 dB (20 Hz - 20 kHz) 2.5 dB (20 Hz - 16 kHz)
S/N-ratio 81 dB (84 dB A-wtg) 80 dB (82 dB A-wtg)
Total Harmonic Distortion + Noise 65 dB (1kHz) 55 dB (1kHz)
Channel separation min. 70 dB (20 kHz) min. 65 dB (16 kHz)

They also state - emphatically

THE SOMEWHAT REDUCED AUDIO QUALITY WHEN PLAYING BACK AUDIO TRACKS ON CD-R DISCS HAS NO RELATION TO THE DIGITAL QUALITY OF THE AUDIO TRACKS AS THEY HAVE BEEN RECORDED ONTO THE CD-R DISC.

And this from Quantized.com web site, about jitter:

Jitter is a statistical measurement of the variation in pit or land length around the mean value for each run length. For each pit and land length a large number of lengths are measured and the standard deviation is reported.

If the standard deviation is high (greater than 35nS) this can cause instability in the clock and data decoding circuitry on target players and hence reduce the readability of the disc.

I have checked how much would cost the test equipment to perform intensive media check, the prices go over US$ 50.000.

So for now I have to admit that there is no easy way to go further on this research.

 

I've never been able to detect any differences between a pressed CD and a properly ripped and burned CD-R copy, but common sense implies that it stands to reason that playback issues could make an audible difference, i.e. lasers tuned to read pressed discs better than dye-based recordable blanks, etc. (I have a Pioneer Elite PD65 that plays absolutely everything except for black CD-Rs. It just doesn't like these things, although it is widely claimed that the black CD-Rs have higher compatibility than their "regular" counterparts. Go figure.)

Let's face it, CD is pretty much an analog medium. Yes. Just like a vinyl record or magnetic tape, it is a physical carrier read linearly (note that I mean standard CD audio discs and NOT data CD-ROM) and prone to physical damage that may interfere with the signal being read correctly (at least past a certain threshold of redbook standard error correction).

This would actually be a good argument in favor of computer-based audio - getting rid of the physical carrier and its attendant problems altogether. As for the jitter bugaboo, contemporary computer interface standards eliminate the problem completely. Traditional SPDIF is quite an inefficient manner of moving data, as the DAC clock must always be synched precisely with the frequency of the transport. If it isn't ... hello jitter. Even lowly USB 1.1 is a much better method, as the USB standard is bidirectional - automatic error correction and buffering on both sides, no special tricks or magic cables or additional software needed, so long as the protocols between the USB device and the computer are properly set. Hell, with a little bit of knowledge and a $10 Ethercard one can easily build a box that even the most expensive SPDIF setup couldn't dream of touching.

Of course USB 1.1 is limited in bandwidth (not very useful for going beyond 16/48kHz), but Firewire and Firewire 2 are much more robust and capable anyway. If you replaced all the outdated methods (like SPDIF) with current standards in the computer world, jitter wouldn't even have to be a topic of discussion.

 

I read your entire post, and I still don't see how you figure that CD is an analog based storage medium. Maybe it's just me, but I think that CDs are a digital medium.

 

This is actually from the users manual from a 1997-vintage 2x cd-r drive, and not any sort of technical paper on sound quality of CD-R vs. pressed CDs. In addition, the page you quote from above is just some guy's theories based on the old user manual, not anything from Phillips.

(BTW, the correct filename is cdd3610e.pdf -- there's a typo on the site you quoted from.)

The more likely explanation is that on such an early drive, Phillips was still struggling to deal with different brands of discs, and may have set up their analog output circuitry differently for reading them. In any event, the restricted specs above are only valid for playback on that particular Phillips drive, and not any other drive or CD player.

The only errors that would affect sound quality would be C2 (uncorrectable) errors, and any halfway-functioning CD writer should be able to make a disc without those.

Ryan

 

:

Thanks for your clarification about that paper.

I don't have any problem to record a CD without C2 errors from my workstation, Sonic Solutions and Plextor drives are professional standards worldwide, what intrigues me is why the CDs pressed at the plant in Brazil did not sound as good as the original master CD we have sent. Frankly I can easily detect the sonic differences between the two CDs when playing both on the same player with the same converter. This Master CD was sent to Germany Sonopress to a Hybrid-SACD pressing so briefly I will have another sample to compare.

Regards

Homero Lotito

 

What might be interesting would be to check the master CD vs. the pressed disc on a variety of different players/converters to see if the differences vary. If it's jitter (which makes the most sense), it may well differ in severity depending on the hardware used.

Ryan

 

"Not exactly -- the data are interleaved and checksummed in a complex way so that errors of up to 4000 (I think) consecutive bits can be fully corrected with no interpolation. With one exception, you simply can't chalk up audible differences (apart from gross drop-outs, skips, etc.) to any physical properties of the disc. "

I'm sorry Ryan but this is wrong. There is such a thing called "pressing jitter" which impacts the glass master process. Production jitter mistakes occur as we found at Chesky Records and as Telarc has found.

New glass master processes from Sony and JVC address this and it is also a part of the XRCD process.

The error correction of ones and zeros is simply not the offending issue.