I'm quoting others not myself.
A top engineer who runs one of the best recording schools and mastering house told me he hears a difference between the X1 and the rest. True or false I never checked it out.
Also here is a interesting posted by a colleague.
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Can Compact Discs contain jitter?
When I started in this business, I was skeptical that there could be sonic differences between CDs that demonstrably contained the same data. But over time, I have learned to hear the subtle (but important) sonic differences between jittery (and less jittery) CDs. What started me on this quest was that CD pressings often sounded deteriorated (soundstage width, depth, resolution, purity of tone, other symptoms) compared to the CDR master from which they were made. Clients were coming to me, musicians with systems ranging from $1000 to $50,000, complaining about sonic differences that by traditional scientific theory should not exist. But the closer you look at the phenomenon of jitter, the more you realize that even minute amounts of jitter are audible, even through the FIFO (First in, First Out) buffer built into every CD player.
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.
I've also observed that a 4X-speed SCSI-based CDR copy sounds inferior to a double-speed copy and yet again inferior to a 1X speed copy.
Does a CD copy made from a jittery source sound inferior to one made from a clean source? I don't think so; I think the quality of the copy is solely dependent on clocking and mechanics involved during the transfer. Further research should be done on this question.
David Smith (of Sony Music) was the first to point out to me that power supply design is very important to jitter in a CD player, a CD recorder, or a glass mastering machine. Although the FIFO is supposed to eliminate all the jitter coming in, it doesn't seem to be doing an adequate job. One theory put forth by David is that the crystal oscillator at the output of the FIFO is powered by the same power supply that powers the input of the FIFO. Thus, the variations in loading at the input to the FIFO are microcosmically transmitted to the output of the FIFO through the power supply. Considering the minute amounts of jitter that are detectable by the ear, it is very difficult to design a power supply/grounding system that effectively blocks jitter from critical components. Crystal oscillators and phase locked loops should be powered from independent supplies, perhaps even battery supplies. A lot of research is left to be done; one of the difficulties is finding measurement instruments capable of quantifying very low amounts of jitter. Until we are able to correlate jitter measurements against audibility, the ear remains the final judge. Yet another obstacle to good "anti-jitter" engineering design is engineers who don't (or won't) listen. The proof is there before your ears!
David Smith also discovered that inserting a reclocking device during glass mastering definitely improves the sound of the CD pressing. Correlary question: If you use a good reclocking device on the final transfer to Glass Master, does this cancel out any jitter of previous source or source(s) that were used in the pre-production of the 1630? Answer: We're not sure yet!
Listening tests: I have participated in a number of blind (and double-blind) listening tests that clearly indicate that a CD which is pressed from a "jittery" source sounds worse than one made from a less jittery source? In one test, a CD plant pressed a number of test CDs, simply marked "A" or "B". No one outside of the plant knew which was "A" and which "B". All listeners preferred the pressing marked "A", as closer to the master, and sonically superior to "B". Not to prolong the suspense, disc "A" was glass mastered from PCM-1630, disc "B" from a CDR.
Attention CD Plants---a New Solution to the Jitter Problem from Sony: In response to pressure from its musical clients, and recognizing that jitter really is a problem, Sony Corporation has decided to improve on the quality of glass mastering. The result is a new system called (appropriately) The Ultimate Cutter. The system can be retrofitted to any CD plant's Glass Mastering system for approximately $100,000. The Ultimate Cutter contains 2 gigabytes of flash RAM, and a very stable clock. It is designed to eliminate the multiple interfering clocks and mechanical irregularities of traditional systems using 1630, Exabyte, or CD ROM sources. First the data is transferred to the cutter's RAM from the CD Master; then all interfering sources may be shut down, and a glass master cut with the stable clock directly from RAM. This system is currently under test, and I look forward to hearing the sonic results.
Can Jitter in a Chain be Erased or Reduced?
The answer, thankfully, is "yes". Several of the advanced D to A converters now available to consumers contain jitter reduction circuits. Some of them use a frequency-controlled crystal oscillator to average the moment to moment variations in the source. In essence, the clock driving the D/A becomes a stable crystal, immune to the pico- or nano-second time-base variations of jittery sources. This is especially important to professionals, who have to evaluate the digital audio during recording, perhaps at the end of a chain of several Phase Locked Loops. Someday all D to A converters will incorporate very effective jitter-reduction circuits.
Good Jitter vs. Bad Jitter
The amount of jitter is defined by how far the time is drifting. Original estimates of acceptable jitter in A/D and D/A converters were around 100 to 200 picoseconds (pS). However, research into oversampling converters revealed that jitter below 10 pS is highly desirable. For D/A converters, the amount of jitter is actually less important than the type of jitter, for some types of jitter are audibly more benign than others (I repeat: jitter does not affect D-D dubs, it only affects the D to A converter in the listening chain).
There are three different "types" of jitter:
1. The variations in the time base which are defined as jitter are regular and periodic (possibly sinusoidal)
2. The variations are random (incoherent, white noise)
3. The variations are related to the digital audio signal
Jitter can also be a combination of the above three.
Periodic fluctuations in the time base (#1 above) can cause spurious tones to appear at low levels, blocking our ability to hear critical ambient decay and thus truncating the dynamic range of the reproduction. Often this type of jitter is caused by clock leakage. It is analogous to scrape flutter in analog recorders.
On the other hand, Gaussian, or random jitter (#2 above, usually caused by a well-behaved Phase Locked Loop wandering randomly around the nominal clock frequency) is the least audible type. In addition to adding some additional noise at high frequencies, gaussian jitter adds a small perfume of hiss at the lowest levels, which may or may not be audible, and may or may not mask low level musical material. Sometimes, this type of jitter puts a "veil" on the sound. This veiling is not permanent (unlike the effects of dither, which are generally permanent), and will go away with a proper reclocking circuit into the D/A converter.
Finally, timing variations related to the digital audio signal (#3 above) add a kind of intermodulation distortion that can sound quite ugly.
