Benefit of SuperClock?

[Corey Shay]

While it seems on the surface that having 256 times more clock pulses than word clock might be more accurate than word clock, now that I think about it I can't see the point really. Why would the other 255 pulses matter? All that really matters is when to sample, correct? What is the benefit of all the other pulses in between?

[JC925602]

Yes, other pulse matter.

Because every clock will drift a bit, so other pulse will allow the interface to readjust to the drift.

It's just like playing music, one note every four beat, with a click every beat.

JC

[Corey Shay]

So you are suggesting that an interface like an 888 needs a little time to get ready for that key sample pulse? It makes sense that a human would need it, but with a machine... I don't know. When it's told to sample, it samples, right? This suggests to me that the only things that would hurt regular word clock is a jittery source and/or cable issues. Are you saying that superclock can help the interface receiving the clock to detect and adjust for jitter?

[editor]

Exactly Corey,

Bob Katz has a lot to say about this at www.digido.com A very precise clock makes your entire system sound better and does reduce jitter.

Regards

e

[editor]

http://www.picosearch.com/cgi-bin/ts0.pl?index=50425

[Corey Shay]

Thanks for gracing the forum, editor. I have read Bob's site extensively and know him personally, and there is no doubt that a more precise clock makes for better sound, but how does simply having more clock pulses make it more precise? I would think that many clock pulses can be just as susceptible to jitter as fewer ones can. Really the question I have is why does the 888 care about all the other pulses in between if jitter is determined by the clock source?

[editor]

Ahh,... well, ahh, No problem Corey; and thank you for gracing us all with a well thought out, respectful reply. BTW: I started Bob out in the CD manufacturing end of this business. Your answer "is" on his pages, that is why I sent you there. May I suggest a re-read... or did you do a Vulcan Mind Meld when you mastered your album with him (got to know him so well) and acquire the sum total of his knowledge.

I know you are young, still in school and excited about all this stuff, (I have followed your posts since last year); and in fact called Bob to chat about your project, when you mastered it. Let me grace you with a life 101 general fact... You will glean more info from people if you don't insult them, as they are trying to help you.

Live long and prosper... and try not to be such a Barney.

[This message has been edited by editor (edited November 27, 2000).]

[Marc Edwards]

Editor, I have read this article before, and I've just browsed over it again (I searched for the word 'super' and 'clock' + had a quick flick through it). I can't find any reference to the Digi Super Clock, or anything similar. Personally, I think what Corey is asking is a perfectly logical question, and can't think of a reason why the extra pulses would be good for anything.
I'm sure Digi must have a technical reason for this. Digi????
BTW: I don't know Bob Katz personally, but i'm sure he's a nice guy!
Marc

[Ang1970]

I'm stumped too.
Speculations/Theories:
1. Digi wants all the 888's in the chain to be perfectly locked together while slaved to jittery source... so the whole system distorts together, in unison.
2. 256 is an impressive number.
3. Something about the multiplexing of divided time.
4. Oversampling?

Someone please clear this up.

[editor]

Corey,

OT: After receiving your email I was going to delete my post, as you explained that you didn't mean to be sarcastic, (Ok I will take you at your word here.) But I think if a guy digs his way in (me in this case), its un-cool to cheat your way out of it, by the delete key. I hate when people delete posts, that mess up a whole threads flow. In my own defense years ago there used to be a smart ass Professor in college, that had it in for me, (because I disproved one of his theories in front of a large class room) that would publicly annoy/humiliate me and always say "Well Mister editor, I'm glad you could grace us with your presence,"; whenever I was 2 seconds late to class, which was almost never (twice ever) and or usually after a harrowing near death experience, or on one of those days when you drop your tooth brush and after three miraculous, once in a million year bounces, it lands in the toilet. So the phrase, "Grace us with your presence" isn't all that big with me. I'm sorry to snap at you stoney bro.... Peace!


All,

Back to the topic; Clocking is all about density and PLL stability. BTW; Bob and I were at one time bitter "list" enemies (then at least friendly with each other), with a tat a tay' of verbal jousts on the Sonic Solutions user group more than a decade back, we beat this topic to death.

Its all about clocking and clocking seems to boil down to jitter, among OTHER FACTORS. (Selected Excerpt's from Uncle Bob's Jitter page) Jitter is time-base error. It is caused by varying time delays in the circuit paths from component to component in the signal path. The two most common causes of jitter are poorly-designed Phase Locked Loops (PLL's) and waveform distortion due to mismatched impedances and/or reflections in the signal path.

A typical D to A converter derives its system clock (the clock that controls the sample and hold circuit) from the incoming digital signal. If that clock is not stable, then the conversions from digital to analog will not occur at the correct moments in time. The audible effect of this jitter is a possible loss of low level resolution caused by added noise, spurious (phantom) tones, or distortion added to the signal.

A properly dithered 16-bit recording can have over 120 dB of dynamic range; a D to A converter with a jittery clock can deteriorate the audible dynamic range to 100 dB or less, depending on the severity of the jitter. I have performed listening experiments on purist, audiophile-quality musical source material recorded with a 20-bit accurate A/D converter (dithered to 16 bits within the A/D). The sonic results of passing this signal through processors that truncate the signal at -110, -105, or -96 dB are: increased "grain" in the image, instruments losing their sharp edges and focus; reduced soundstage width; apparent loss of level causing the listener to want to turn up the monitor level, even though high level signals are reproduced at unity gain. Contrary to intuition, you can hear these effects without having to turn up the listening volume beyond normal (illustrating that low-level ambience cues are very important to the quality of reproduction). Similar degradation has been observed when jitter is present. Nevertheless, the loss due to jitter is subtle, and primarily audible with the highest-grade audiophile D/A converters.

Jitter And the AES/EBU Interface

The AES/EBU (and S/PDIF) interface carries an embedded clock signal. The designers of the interface did not anticipate that it could cause a subtle amount of jitter due to the nature of the preamble in the AES/EBU signal. The result is a small amount of program-dependent jitter which often sounds like an intermodulation, a high-frequency edge added to the music. To minimize this effect in the listening, use a D/A converter with a high degree of internal jitter reduction. An external jitter reduction device that removes the subcode signal (containing time of day, start IDs, etc.) also helps.

The SDIF-2 (Sony Digital Interface-2) uses a separate cable for the clock signal, and thus is not susceptible to program-dependent jitter. However, the quality of the PLL used to detect an SDIF-2 wordclock is still important to low jitter. It is much easier to build a low-jitter PLL for a wordclock signal than for an AES/EBU signal.

Why are plug-in computer cards so jittery? Does this affect my work with the cards? Most computer-based digital audio cards have quite high jitter, which makes listening through them a variable experience. It is very difficult to design a computer-based card with a clean clock---due to ground and power contamination and the proximity of other clocks on the computer's motherboard. The listener may leap to a conclusion that a certain DSP-based processor reduces soundstage width and depth, low level resolution, and other symptoms, when in reality the problem is related to a jittery phase-locked loop in the processor input, not to the DSP process itself. Therefore, always make delicate sonic judgments of DSP processors under low jitter conditions, which means placing high-quality jitter reduction units throughout the signal chain, particularly in front of (and within) the D/A converter. Sonic Solutions's new USP system has very low jitter because its clocks are created in isolated and well-designed external I/O boxes.

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 is at removing incoming jitter, it can never be as effective as a well-designed crystal clock.

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.

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.

I took all this to mean that the more density the master clock has at all chain points, the less jitter or signal degradation at all points in the chain. A "Super Clock" in theory will hold PLL's more stable Vs. clocks that do not contain pulses of equal or greater density. Maybe I just read it wrong...


Regards

e

[This message has been edited by editor (edited November 30, 2000).]