Digital Audio Myths - Listening on a PC

oliverstoned wrote: The extraction of audio involved the mechanic issue, isn'it? So read again what "What hifi" said: "Our recent reviews of various CD-RW and MiniDisc recorders have attracted a flurry of correspondence, much of it via e-mail, denouncing the results we reported. 'How can different CD-RW discs/MiniDisc blanks/CD-RW recorders sound different?' they howl, adding, 'Surely the machine/disc combination either records the ones and zeroes or it doesn't. After all, different floppy discs don't make word-processor documents read better or worse, do they?' It's hard not to argue with that last bit of logic, even if sometimes the reviewing staff on the magazine would love to be able to blame their floppies (!). But experience has taught us that, just as different CD players impose their own sound on a recording, so the various digital recorders on the market, and even the various brands and types of blank media available for them, can make a difference. So what's going on? Digits are either there or not, right? The answer to that is 'kind of...' since all digital systems rely on error correction to get the sound from the disc to the analogue outputs in a recognisable form. The less hard the correction systems are having to work to reconstitute the original sound, the better the reproduction becomes. What's being corrected is faults in the data, caused by anything from scratches on discs to mistracking of the laser pickup, from fluctuations in disc speed to wobbles in the spinning disc, and from low reflectivity causing misreading to vibrations caused by someone walking across the room. And that's before you get into electronic failings such as jitter... Trouble is, a CD or MD player looks dead simple: you bung in a disc and it just plays music, just like these words are about to be saved to a hard disc on a computer and when we want to read them they'll come back on the screen exactly as they were typed. Hopefully. But the fact of the matter is that CD players, and digital recorders, are all about high-precision engineering operating in a fairly hostile environment. For example, the laser pickup system in a CD player, or the write head in an optical or magneto-optical recorder, needs to move in three dimensions, alter its power and focus, and deal with a disc spinning at a constantly-changing speed, and do all that on a microscopic scale. Thus anything that can make this task easier, be it discs with greater reflectivity, more even spirals of pits, or even a more consistent optical layer, is likely to give a better sound. No, all digital equipment doesn't sound the same, however much logic might suggest otherwise - in fact, it's a miracle most of it is so consistent... " WhatHiFiSound+Vision "

The highlighted bit is wrong. More precisely, it draws the wrong conclusion. In the digital world, any bit of information can either be read wrong or right. If it is 1 (or 0) and is read as 0 (or 1) it is wrong, if it is 1 (or 0) and is read as 1 (or 0) then it is right.

Now: As long as the bits are read correctly (that is in my case: the bits are saved on disk like they are on the CD), better mechanics don't change the sound at all. Not in the least. As soon as the mechanics are so poor that the bits can not be read properly, we have problems. Standalone CD players then start skipping, or you hear dropouts (missing frames). When ripping the CD with CDex, you get an error message indicating that some parts of the tracks contain dropouts. Then indeed you might have to buy a better drive. But NEVER will a bad drive on the computer result in just a "poor" sound. Either the file has errors - then ANYONE who has ears will hear the problems. Or there weren't any errors - then NOBODY will be able to hear a difference, because there is none.

But any good drive in the $20-$30 range (Plextor, preferably) will read CDs without any errors or even slowdowns.

cobb wrote: Correction- after looking at more reviews on this card, I would definitely be buying it. If I wasn't so attached to my trusty old DSP I would be ordering it myself. Very impressive card and not overly priced. 51 million transistors on board. Unheard of for a soundcard.

Actually, their sound enhancement feature called "Crystallizer" doesn't seem to be as good as the musical CD player algorithms. But the card is not yet available, and maybe further tests can clarify that.

BTW: I'll most surely buy this card ... even if that feature doesn't live up to the expectations, it still boost gaming performance and allows 128 simultaneous sounds instead of 64, and it simulates the audio filters in hardware instead of using the main cpu. I can't wait to play Battlefield 2 with this card ...

cobb wrote: Just looked up the specs the X-Fi. It looks perfect for the job.

Agreed. And even if its D/A converter isn't good or seems too "cheap", one can always use the digital interface to connect it to a top-notch amp, and have the amp do the conversion.

I still think that the amp and speakers are the most important link in the chain.

cobb wrote: I believe this is what Mike is already doing. I think he posted earlier that his music is coming from hard drive- not cd [edit] this is in relation to goose's post 2 back

Exactly. I'm talking about ripping the music to the hard disk using lossless formats (WAV, WMA Lossless, Apple Lossless, FLAC) and then either playing it using either a digital connection to the amp, or a really good PC sound card like the Creative X-Fi or sound cards used for hard disk recording (studio equipment). 

Here you go ... I higlighted Bob's statements which support my theory in blue, and added my own comments in red.

Jitter on CD

From: Paul R [email protected]

My comments are:
Thank you for the best article on jitter I've ever seen anywhere.
Bob Katz's comprehensive treatment in enlighening.
He loses me on a few points, though, and I'd love it if someone could clarify.

First, I believe he states that Jitter is introduced in the conversion process, but is eliminated in the digital storage (hard disk, etc.). But then he speaks of jittery CDs. How is a CD different from other storage media? Why is jitter recorded on a CD?

Hello, Paul... Thanks. your comments are cogent. Apologies for the "work in progress". If we knew all the answers, we'd be geniuses! I will say that a large group of mastering engineers and critical listeners agree that CDs cut in different ways tend to sound different. The CD differs from other storage media in many ways, but the critical point is that the timing of the output clock and the speed of the spinning disc are related. The output of the CD player is a clocked interface, and the data are clocked off the CD disc in a "linear" fashion, one block of data after another. A buffer is used, which theoretically cleans up the timing to make it regular again. And for the most part, it does.

A lot of this is theory... no one has proved it as fact. And there may be more than one mechanism causing jitter taking place.

To obtain jitter in the low picosecond region requires extremely accurate timing. Any leakage current (interference) between the servo mechanism controlling the speed of the spinning disc and the crystal oscillator controlling the output of the buffer may unstabilize the crystal oscillator enough to add jitter to the clock signal. This does not change the data, by the way. If the servo is working harder to deal with a disc that has irregularly spaced pits or pits that are not clean, perhaps leakage from the servo power affects the crystal oscillator. It doesn't take much interference to alter a clock by a tiny amount.

This jitter is "ephemeral", though, because you can copy this data (irrelevant to the clock), and then play it back again from a more steady medium... and make it sound "good" again. This is not a permanent problem.

See? Ripping the CD on a computer means exactly that - copying the data (irrelevant to the clock).

What makes the CD different from a hard disc, is the HD uses an asynchronous interface (SCSI or IDE which is what computer CD-ROMs use for extracting the data). The disc is always spinning at the same high speed and the heads land on the spot you need when the data is requested. The data coming out is completely unclocked (it comes out in bursts) and has to pass through the SCSI barrier into a buffer located in a different chassis than the hard disc (the computer)... thus, there is great distance between the varying currents of the spinning disc motor and the oscillator driving the output of the buffer in the computer chassis. Since the computer chassis power supply only has to power the output oscillator, the result can be much more stable. Depends on how good the designer did his/her homework. Same for a CD Player... there are audiophile CD players where great attention has been made to power supply design, and these players exhibit much less jitter and better sound.

It is also possible to build a CD player based on a SCSI mechanism... possibly such a player would be more stable in playback than a standard CD player. You would have a computer in its own "cleaner" environment buffering the data Thanks again Bob for clarifying that computer based audio files can be clean. The Alesis Masterlink is such a player, and in another "chapter" of my work in progress I will have something to say about its audible performance.

I'd like to tackle a 200 page booklet to put all the pieces together someday, but haven't the time. I think in our FAQ there are some explanatory letters which help to cover the rough spots.

He also states that a 99th generation copy of CD is apparently identical to the original. But then talks about the degradation of making CDRs at 4x speed vs. 2x speed. Please help me reconcile this.

The data is identical... It's important to separate the message (the data) from the messenger (the clock). That's what I'm talking about - identical copies sound the same, no hearing necessary to determine this.

It's all in the playback of the last disc in the chain, Paul! The "old" clock is NEVER transferred on each copy, only the data. No matter what speed you write at, there is a new writing master clock in the CD recorder that determines the spacing of the pits on the newly written CD.

This is also true when extracting data from a CD and storing it on the hard disk. The clock is not transferred.

But each time you copy, that clock is not transferred through the SCSI barrier of the next CD Recorder. I will have to write about this in more detail and diagram it for my readers, hopefully soon...

And each playback is anew... if the clock of the final playback is irregular, you will have jitter on the final playback of the last generation.

This also applies to the computer. Jitter is ONLY introduced at the point where the digital signal is converted to analog. BTW: The Creative X-Fi has a jitter rate of approximately 110 picoseconds in its D/A converters. That's well in the range of most musical CD players. 

But you can clean that up yet again and start the whole cycle all over again.

I'm hoping the answers to these questions are within my grasp.

I think they will be, if I can just get the hang of explaining it properly!

Take care,

Bob