Black CD's and low jitter

Could this be why hard drive lossless music and a good soundcard fares so very well as a music source?

http://www.genesisloudspeakers.com/newshmfeb03set.html

regards,
Rob.

 

a) If you don't believe in the green pen, you REALLY won't believe this.

b) What makes you think HDD sources are any better for jitter? Last night I tested a 40MB wav vs the original on CD, and the CD version was rather better, and both were being fed digitally to the same DAC. Jitter arises through clock accuracy (or inaccuracy), and a CD transport, with a dedicated, fixed-frequency clock will give a more consistent output (ie lower jitter) than a variable frequency clock as found on a computer soundcard. I haven't even had my CD transport clocked yet, but it still beats the computer (which has a decent quality Terratec soundcard that beats the **** out of any creative I've ever had).

 

Hi Isaac,
How did you get the wav file?

Did you ensure the kmixer "windows 48Khz resampling bug" was bypassed?

cheers,
Rob.

 

Copied off the article and read it on the train home (beats drafting patent applications). Interesting, but it sometimes seems that reality and Mr. Koh dramatically part company. (Of course, it may be reality and Tones parting company, but then we rarely see each other these days. What to do?).

For example, on p.4, is his definition of "jitter" correct? I thought that jitter was something encountered only on playback of a recorded CD, as a result of clock differences. Does it also figure in recording? He also seems deliberately to minimise the idea of buffering, as if it presents a problem to his idea.

Also on p.4, he says that pressed CDs are worse than burned CDs. Really?

On.p.7, do black CDs really have better reflectance than gold or silver ones? I would have thought that black was black and absorbed light, whether monochromatic or not.

The rest of it seems to me to be an exercise in applied wishful thinking - much subjectivity, not many hard facts. And of course the traditional "floored", jaw-dropping language that goes along with all these sorts of ideas. And of course the ultimate question - as a matter of fact, how do you change the sound when you record EXACTLY the same digital data?

I'm sure Mr. Koh is sincere. I hope he remembers that it's possible to be sincerely wrong. So, where have I misunderstood?

 

Tones - you have missed a few things.

Jitter is encountered on recording as much as playback. Unfortunately, once recorded there's not a lot that can be done about it, unless we could work out the error on recording each time, and compensate for it. This isn't possible however...

Pressed CDs are generally accepted as being "worse" than burned ones because of the ways they work. The edges of the "bumps" on pressed CDs aren't as clearly defined as the burned/not burned edges of dye in burned CDs.

If black CDs had no reflectance at all, how would they work? They may absorb at all visible wavelengths, but as I've pointed out to you before, CDs work at a wavelength we can not see, in the near infra-red.

 

Thanks for straightening me out, Isaac. As you're no doubt aware, I'm a polymer chemist by training and I have difficulty spelling the word "electronics". I do realise that black CDs will naturally have reflectance, otherwise they wouldn't work at all, but better reflectance, even in the near IR?

Anyway, since I don't believe in green pens either... However, I do wish I had the facilities to try it out, just for curiosity. Alas, I haven't.

 

The visible properties of a CD tells us nothing about how it will behave in the near IR. Better or worse than a normal, I do not know, but just because it is black visibly doesn't make a difference to what I know (ie nothing) about its behaviour in the near IR. FWIW, the one black CD I have encountered (a burn of some mp3s no less!) sounded uncommonly good through my old marantz CD63. Equally, the green pen being green doesn't tell us about its behaviour (absorbtion) in the near IR either. A black or blue pen with similar absorbtion in the near IR would work as well (and, of course, in your belief not at all).

I don't have access to the equipment to confirm or deny this either, but I haven't heard an appropriate demonstration either, so without such knowledge, I shall remain sitting firmly on the fence.

 

As Ted Moult once described it, a mugwump - mug on one side, wump on the other! (Is my vintage showing again?)

 

Hard drives ARE better in some respects. They are engineered to very high standards for high speed, error free transfer rates, and are vacuum sealed, unlike a CD which is at the mercy of your fingers / being scratched, vibration etc....

Soundcards naturally are a different matter, but mine makes a pretty pleasing sound to be fair to it.

 

From "Theoretical and Audible effects of Jitter on Digital Audio Quality", Benjamin and Gannon, 105th AES Convention 1998: 'for nearly all program material no audible degradation was heard for any amount of jitter added below the level at which the DIR (Digital Interface Receiver) lost lock'

The academic and engineering community would contend from their results that jitter isn't really an issue relevant to sound quality in anything like the way that the popular press makes it out to be. A recent addition to the Stereophile online archives by Rob Harley, with measurements and everything, also concluded that none of the digital tweaks he could find - green pen amongst them - had any effect on jitter. And BTW before anyone goes off on one (I'm really not trying to stir here), Rob Harley wholeheartedly believes in the value of digital tweakery and was trying to find out why they worked, not if they worked. So, in the absence of any effect of these tweaks on jitter, he suggests that another explanation besides jitter needs to be found as to how "digital can be different".

 

Jitter is not an issue in an entirely synchronous system like a computer, unless levels of jitter rise to over 1/2 of a given clock period. In fact, jitter may be deliberately introduced in order to lower RFI emissions (spread spectrum).

Jitter is only really an issue when a conversion to analogue takes place. Jitter and bit errors are NOT the same thing. An actual bit error (after error correction) is a very rare thing when reading a CD, and it usually upsets the transport rather more than for one 44100th of a second.

Jitter on a digital output from a computer will be determined by the accuracy of the sample rate clock on the sound card. As I already said, as these clocks have to be able to deal with a wide variety of possible frequencies they will often be more compromised than a dedicated clock in a CD transport. Furthermore, these clocks will be a lot more vulnerable to interference from the multitude of other clocks in a PC, motors, switch mode psu, etc....

I understand that people get pleasing sounds out of a PC, and I do too. However, by comparison, it is not as good as my CD transport; no two ways about it. It's a question of good or better.

 

All I get from mine is a deafening roar of fans Must do something about that one of these days...

 

Isaac, aren't the following two statements contradictory:

"Jitter is encountered on recording as much as playback"

"Jitter is only really an issue when a conversion to analogue takes place"

AFAIK jitter cannot be "recorded" if there has been no conversion to analog. When you copy a CD to a CD-R, even if you're doing it via TOSlink and a connection that's as jittery as hell - as long as there are no bit errors (highly unlikely) then what gets recorded is the same sequence of 1s and 0s as the original and any jitter along the way is irrelevant as jitter is time base errors, and a digital storage medium like a CD or CD-R has no timebase, and therefore no jitter.

Michael.

 

Should have said "conversion to or from analogue".

With a cd copy made via optical, no A-D or D-A conversion has taken place, and thus jitter is, as you said, not an issue.

With regard to digital interconnection and jitter, it can be reduced by reclocking/buffering incoming data (a-la DAC 64). However, many DACs don't (SuperDAC among others derives its internal clock signal from the incoming data), and this will make them more sensitive to transport clock accuracy, transport electronics (ref WM's thread showing the difference in digital output between the CDfix and the rotel 965BX), and digital cables.

 

Hmmm... just read the start of the report, and skimmed the details on drives, record speeds etc. Looks like the debate is on identical lines to the green pen. That different CD writers, different blanks etc. can result in burnt pits and lands of varying quality and position (time) accuracy makes sense, as does that this will then change the light signal reaching the photodetector. The issue then reduces to whether these changes affect the electrical digital signal generated from this, and how/if this affects upstream D to A conversion.

p.s. Just in case there's any confusion, jitter is certainly a problem when recording music in the first place i.e. when analogue mike feeds are mixed digitised. As Isaac said you'll be stuck with this from then on. The idea of jitter when making digital copies as descirbed here though is different...

 

In the link I gave above Rob Harley (who, again, 'believes' in the green pen) showed that the green pen makes no difference to jitter. So if the alleged phenomenon of these black CDs is jitter-related, it's not along the same lines as the green pen at all...

 

Well... I'd deliberately been more general than jitter (i.e. whether any change in the light signal, whilst retaining bit accuracy, could make a difference), but you're right the lack of measurable change in jitter for the Stoplight Pen you highlighted does make it distinct from the black CD tweak.

Thanks for sharing that article you linked to; I read it through over lunch and it has some interesting stuff in it. Well worth reading. There's lots of stuff about changes in CD production affecting sound that directly relates to the black CD issue here. I probably ought to have another more critical read through before I comment more here though...

Martin

 

Not saying this is the case but if it somehow did make the CD easier to read for the laser then this might well have sonic benefits as the servos have to work less hard to read the disc correctly and thus dont modulate the CD players power supply so much.