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Author Topic: Proper word clock implementation  (Read 132872 times)

bananahill

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Re: Proper word clock implementation
« Reply #195 on: March 24, 2005, 08:04:13 am »

Dan,

I have to agree with Logichead.  You are out of line here.
You have every right to express your opinions and findings and argue these points.
The conflict is your role as moderator.  From the tone of many of the posts, no one is moderating this thread.  You are a very involved participant.

I think you should remove yourself as moderator and continue to post.
If you want to remain moderator, you need to do this in an objective and unbiased manner.
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Stephen Krause

Logichead

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Re: Proper word clock implementation
« Reply #196 on: March 24, 2005, 08:16:27 am »

Nika Aldrich wrote on Thu, 24 March 2005 10:43

Is this thread really about semantics?  Cables don't directly induce jitter - the jitter comes from EMI, RFI, other types of noise.  Good cables merely reject that jitter better.  In this capacity, bad cables are said to "induce" jitter because it is accepted that some degree of noise is a part of the environment, so cables that allow this noise to affect the signal are said to "induce" that jitter.  This seems like pretty basic stuff to me?

As far as the filtering of cables, I assume it has been discussed that cables act as filters and can thus cause modulation of the square wave pulses and can therefore create jitter that was otherwise not present.  Lowering the cutoff frequency of the filter can increase the modulation.  The less square the waveform the greater likelihood that the cable's inherent filtering properties exacerbate the jitter present from noise sources as well.  For these reasons as well, good quality cabling helps reduce jitter manifested through the process.

I think that the filtering properties of a cable do indeed "induce" jitter, but even if we reject this for some reason and say that cables don't "induce" jitter, but they can be said to "reject" jitter, and if you accept that that jitter from noise is an inherent part of the environment one could certainly decide to describe the use of poor cables as "inducing the manifestation of this problem."  It seems petty to try to pick apart these semantics.

As for Apogee's "low jitter cable" (or "ULTRA low jitter cable" or whatever) I think the implication is that it has high jitter rejection, has better inherent filtering properties, and thus exudes lower amounts of jitter?  

Something seems petty about debating the semantics at play here, especially when we all know what was meant, and when AES whitepapers have been written on the significance of using good quality cabling so as to provide lower jitter to digital devices.

Nika


No doubt there are semantics involved here, as is often the case on a forum. That's not what I find important.

First, if you read that white paper (the pdf is included a few posts up) it appears that low bandwidth cable actually adds jitter to jitter-free digital audio. I am only interpreting what I read, but I'm pretty sure I have it right. This was news to me, as I suspect it is to you.

Next, Dan has been using this forum inappropriately, just read his posts if you are uncertain. He has directly singled out Apogee, his former employer and a current competitor for his most severe criticism. Fine, criticise away, but not as the moderator.

Dan, again - you are damaging your reputation as well as Pro Sound Web's. Imagine your response if it were someone at Apogee moderating this thread and saying these things about you.

IMO Apogee should contact the operators of the forum and complain. If only to save Dan from himself, this entire thread should be deleted.

Best regards....H

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Nika Aldrich

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Re: Proper word clock implementation
« Reply #197 on: March 24, 2005, 09:10:26 am »

Logichead wrote on Thu, 24 March 2005 08:16

First, if you read that white paper (the pdf is included a few posts up) it appears that low bandwidth cable actually adds jitter to jitter-free digital audio. I am only interpreting what I read, but I'm pretty sure I have it right. This was news to me, as I suspect it is to you.


If you're talking about the ability for the filtering characteristic of cables to provide a modulation effect on the square wave data run through the cable this isn't exactly new stuff - it's pretty basic waveform theory.  The question has to do with the practical results of that - what type of bandwidth is necessary in order to "induce" this jitter on a bad cable, especially when dealing with 44.1kS/s pulses?  I suppose I could run a test in Matlab and see.

So I concede Dan's point in advance of actually knowing - that even in this pathological case, jitter isn't induced for some reason.  Still, there can be little denying that some cables reject jitter better than others.  

Nika
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bobkatz

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Re: Proper word clock implementation
« Reply #198 on: March 24, 2005, 09:36:20 am »

danlavry wrote on Wed, 23 March 2005 21:36



Cables do not make jitter.




It's also the language that is being used in the promotion of the product. The fact is that a cable must have the right impedance, and ideally maintain that impedance when flexed or bent in order to reduce reflections in the circuit and have a small eye-pattern. As I understand it, these reflections can confuse a receiver and cause time-based problems. Whether that is being a "low jitter cable" is a matter of how you interpret the language. I'd certainly want to buy a cable with the right impedance, with the stabilizer wires inside to maintain that impedance. Those are important characteristics. But it makes for dull marketing language!

BK
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danlavry

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Re: Proper word clock implementation
« Reply #199 on: March 24, 2005, 09:37:30 am »

I thought more about what I said last night, and I just want to be more specific than I was.

I find the term “cable induced jitter” to be a poor choice of words. Julian did the math well, and he performed his experiments and collected data. I guess he was focused on the math derivation, and on getting the massage across that AES signals are impacted by a cable.

I would say that the jitter in an AES link is impacted by the cable, impacted by the transformer or capacitor. Impacted, not induced. The word induced suggests that the cable does something actively. But the cable does not act. It reacts. There is no capacitor induced jitter either for the same reason – the capacitor is passive. As a passive device, a cable reacts differently to different input signals and the reaction depends on interaction with other components.

The original discoverer of the effect of jitter on a digital audio link was Hawksford. He explained the mechanics very well. The signal has droop due to low frequency coupling. The droop is related to the time duration of each logic state. With data changing on an AES link, the droop time duration changes also which causes a “wobble”. The waveform voltage goes up and down tracking the digital logic states. Given that the signal transitions between states has a finite rise time (it is not vertical) the time where a comparator reference (threshold) is crossed by the wobbly signal also wobbles. That is the jitter we are taking about. Data link jitter.

I find the term cable induced jitter to be off the mark. The cable does not induce jitter. That is not the way I heard Hawksford explanation to be, nor is it my understanding of how the mechanism is. The cable does not do anything by itself. It does not induce anything it is passive. It “just sits there”. Just like other passive parts (resistors, caps, diodes, empty circuit boards, metal chassis and more). Is a metal chassis a low jitter chassis?  Supose it that help shield a circuit from interference that cause jitter in one application, and it increases it in another case. The chassis has attributes (length, width, material data…)  but you can not say it is an low jitter chassis. It is just a passive piece of metal. It is passive material and it does not “make waves” by itself. Just like a cable.

I assume there was no harm intended by Julian, just a poor choice of a word. But someone is using that for commercial promotion of a product. And we are not taking rocket science here, it is just a piece of passive copper with some insulation. Why does someone home in on the one “off phrase” in an article? Is it lack of depth and technical competence? Readiness to just say anything? A misinterpretation of a poor choice of words.

Does someone that says that cables make jitter and sell a low jitter cable be required to explain how cables make jitter? Just pointing at “so and so said this or that” does not make it so. Where is the mechanism for cables to induce anything? It is not there. If you believe it and sell it, measure it. Guess what- whatever jitter numbers you measure in an AES data link will change with different transformers and even with sample rate. Cables are reactive. They are put into a structure and become a part of an interaction.

A piece of cable on the ground, connected to nothing does not “make signals”, and jitter without a signal makes no sense. A cable has characteristics. There are mechanical (weight length…) and electrical (capacitance, resistance…) so you can say “low capacitance cable” or 10 foot cable. Those characteristics go with the cable wherever it is in all applications.

It is improper to take a passive device into a specific application with specific conditions and assign to it attributes that belong to that specific case. The number 2 can mean 2 billion dollars or 2 cents. Shell we call the number 2 very significant? It is just there passively. It is not inducing money. Like a cable, the number 2 is “just there” until you give it some use, some context. It is passive, like a cable. The word is passive.  

And suppose that I am dead wrong (I am not). Then the cable induced jitter explanation is a reason to stay away from external clocking in favor of internal clocking. External clock box always requires cables. If Apogee thought that a cable makes jitter they should encourage the use of internal clock, not an external Big Ben.  

Regards
Dan Lavry
www.lavryengineering.com
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Logichead

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Re: Proper word clock implementation
« Reply #200 on: March 24, 2005, 09:52:37 am »

Nika Aldrich wrote on Thu, 24 March 2005 14:10

Logichead wrote on Thu, 24 March 2005 08:16

First, if you read that white paper (the pdf is included a few posts up) it appears that low bandwidth cable actually adds jitter to jitter-free digital audio. I am only interpreting what I read, but I'm pretty sure I have it right. This was news to me, as I suspect it is to you.


If you're talking about the ability for the filtering characteristic of cables to provide a modulation effect on the square wave data run through the cable this isn't exactly new stuff - it's pretty basic waveform theory.  The question has to do with the practical results of that - what type of bandwidth is necessary in order to "induce" this jitter on a bad cable, especially when dealing with 44.1kS/s pulses?  I suppose I could run a test in Matlab and see.

So I concede Dan's point in advance of actually knowing - that even in this pathological case, jitter isn't induced for some reason.  Still, there can be little denying that some cables reject jitter better than others.  

Nika


Thanks for your reply Nikka. I also appreciate the reasoned tone of your comments. The practical implications of cable induced jitter appear to be small. There are indeed limits on what the numbers tell us! While you appear to say there is essentially no such thing (in practical terms) as cable induce jitter, the white paper seems to say otherwise. It is an infinitismal point we are dealing with here, no need for histrionics. I can see both conclusions.

As you say, "there can be little denying that some cables reject jitter better than others." That is all Apogee appears to claim, at least on their website.

I don't want to read too much into it, but your silence on Dan's actions as participant and moderator leaves me wondering how you feel about a manufacturer using a forum as Dan has.

Best regards....H






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Nika Aldrich

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Re: Proper word clock implementation
« Reply #201 on: March 24, 2005, 10:01:40 am »

Eh gad, that was the longest, most vindictive and most nuanced apology I've seen in a long time.

Nika
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Logichead

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Re: Proper word clock implementation
« Reply #202 on: March 24, 2005, 10:37:00 am »

Nika Aldrich wrote on Thu, 24 March 2005 15:01

Eh gad, that was the longest, most vindictive and most nuanced apology I've seen in a long time.

Nika


What he said.

Best regards....H
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danlavry

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Re: Proper word clock implementation
« Reply #203 on: March 24, 2005, 11:13:14 am »

Logichead wrote on Thu, 24 March 2005 14:52

Nika Aldrich wrote on Thu, 24 March 2005 14:10

Logichead wrote on Thu, 24 March 2005 08:16

First, if you read that white paper....


...necessary in order to "induce" this jitter...

Nika


While you appear to say there is essentially no such thing (in practical terms) as cable induce jitter, the white paper seems to say otherwise.
Best regards....H




You obviously did not read a word I said. The white paper is misusing a single word "induced" instead of "react" and Max the salesman from Apogee thinks it is sufficient reason to sell a cable. A separate and valid point: The white paper does not apply to word clocks, thus word clock low jitter cable is out. Third, if Max were correct (he is not) then it means that external clocking is even worse then I pointed out because of the added jitter (it calls for a cable)...

You argue without registering the content of what is being said!!!

You are not staying with it. You are not participating in a technical dialog. You are DIVERTING THE ISSUE to Dan as a moderator.

You asked Nika to "rate me" as a moderator.

I am asking all the participants and viewers to rate you on your level of understanding and your style of participation here.  

Dan Lavry
www.lavryengineering.com
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Logichead

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Re: Proper word clock implementation
« Reply #204 on: March 24, 2005, 11:36:30 am »

What our levels of technical understanding are is not the issue - unless we're going to be taking entrance exams before posting. However much this may have begun as a technical discussion, your actions as moderator have taken matters elsewhere.

Do the right thing, Dan.

Best regards....H
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danlavry

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Re: Proper word clock implementation
« Reply #205 on: March 24, 2005, 11:43:21 am »

I am trying to stop another run away train. That "low jitter cable buzz" is starting to spread. It is just one more of those faulty "audio concepts" that in my opinion should be stopped. I present technical input and ask questions. There should be no need for insults but I will not be walked over. Cables don't induce signals and the audio world should learn this.

I saw cable  advertisement  by Gepco, where they saying: “They (the cables) feature minimal attenuation, low jitter and”....

Meanwhile, the misconceptions about external clocking have become wide spread, and now the ultra low jitter cable notion is starting to spread as well.  

I do not view Apogee as a competitor. My opinion of their gear is that it is consumer quality judging from the signals it generates.  Period.  I do view Apogee as having a lot of influence and marketing power, much of it from the quality of the electronics that I designed while there in the past and I question their claims for technical leadership. Avoidance and diversion is not the behavior of a technical leader.

Apogee refuses to answer the posted technical questions. They do not answer others and they do not answer me.  I am not the problem. It appears that I am the only chance to get things straight.  Apogee should not send a salesman here to argue with a high powered engineer. That is the reason the conversation turns personal and off subject.
 
What is coming up next? 384fs? I will not design gear above 96KHz regardless of the monetary consequences to my company. My stand is right and honorable.

We have here 3-4 Apogee supporters - “regulars” that instead of insisting that Apogee  engage in a technical dialog and answer questions, choose to go after me for doing a poor job of moderating. One guy seems to think that quoting a poor use of language (Cable induced jitter) is sufficient to make the principles of physics (Maxwell's equations - the foundation of electricity and magnetism) go away.

Cables do not generate signals!!! Cables don’t induce anything!

Where is a single technical comment from Apogee explaining what “temperature controlled cable” means?   One cannot put a passive device in a circuit and have it “wake up” and start doing active things like inducing anything. Cables do not manufacture signals or jitter.


One more time: Apogee engineers. What is a temperature controlled cable? Is any passive device capable of inducing a signal? Does an explanation of a cable interaction with other components under AES (with data) have anything to do with Word clock cable? If so what?

Regards
Dan Lavry

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trevord

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Re: Proper word clock implementation
« Reply #206 on: March 24, 2005, 01:56:22 pm »

I am a long time lurker here
but i have to add my 2 cents on this one

first let me say i agree with dan lavry.

let me add a different take on his explanation
1) there are many jitters in engineering. clock jitter can be defined as the varying position of the edges of the clock
for example a 10ns clock would be
0 - rise edge
5 - fall edge
10 - rise
15 - fall

a jittery clock would be
0 - rise edge
5 - fall
11 - rise
14 - fall

this results in all the bad effects of jitter in the word clock for digital audio

this is a function of the clock source - if a jittery clock were transmitted thru a perfect medium it would still be a jittery clock

now for cables
assuming a perfect clock - a low jitter clock SOURCE
what happens at the end of the cable as far as clock jitter is concerned has more to do with
the DRIVER of the signal
the TERMINATION logic at the end
and
the EMI of the space around the cable
THAN the cable itself.

if your cable has some horrendous characteristic it is in relation to some standard it is expected to be used in

for example if your cable has horrible capacitance specs - it can be taken care by an accommodating design at the driving and terminating ends of the cable

saying a cable is low-jitter is laughable without mentioning what is expected at either end of the cable
the most perfect word-clock co-ax would produce the most noisy signal at the end with the wrong termination

a claim can be made that these cables would have the least effect if a perfect word-clock is perfectly driven and perfectly terminated
but this is dan's point - all the "perfection" concerns things that are NOT the cable!!!

it is also a circular marketing argument
it can be  said once "standard termination and driver specs" are met, these cables are better than the average
but if "standard termination and driver specs" are met then an average cable is adequate - by the definition of the standard.

i am surprised at the opposition to dan's observation in a professional engineering forum

trevor

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Eliott James

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Re: Proper word clock implementation
« Reply #207 on: March 24, 2005, 06:05:02 pm »

Oil and water. "Ear" people and "Tech" people. The two seldom meet in understanding, although both are equally right and wrong on a consistant basis about the same topics.
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danlavry

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Re: Proper word clock implementation
« Reply #208 on: March 24, 2005, 06:17:05 pm »

It seems to me that many people here do not appreciate the important distinction between the AES mechanism that introduces jitter and the Word clock transmission that does not have that mechanism. So I will explain (credit for the discovery of the mechanism belongs to the very brilliant professor Hawksford):

The plot below is a “scope picture” of 4 waves. The top one (in red) shows a sequence of “highs” and “lows”, but the duration of each state is not equal. The wave stays high for “extra long time” between about 250 and 575, it stays low for “extra long time” between 700 and 850. The point is: it is not a periodic wave!

index.php/fa/885/0/

The blue plot represent the same wave but with slower rise and fall times. The slowing down of rise and fall represent the combined limitations of the upper frequency capability of the transmission link. That limitation is due to components such as the signal driver, driver output transformer, cable and more.

The black trace below shows what happens to the blue trace (rise time limited signal) when passing it through a transformer or capacitive coupling. Transformers are common for AES. Capacitive coupling is more common for SPDIF. Note that the AC coupled signal droops further down when the input has a longer high state, and drift further up when the input has a longer low state. The outcome is a “wobbly” signal which does depend on the duration of the low and high state as is the case with AES signal.

The last trace, the purple one, is a receiver comparator threshold. Note that the transition times are when the black wave crosses the purple wave. Those points can be early or late because of the time dependent droop (due to variation of the low high states). For example the negative transition around 780 is very early. The signal (black trace) barely moved and the comparator reacts. That variable data dependent timing interacts with BOTH rise time a droop to generate that data link jitter.      

See next post for the word clock case

Regards
Dan Lavry
www.lavryengineering.com
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danlavry

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Re: Proper word clock implementation
« Reply #209 on: March 24, 2005, 06:28:03 pm »

The presentation for word clock is very similar to the one I posted in the last thread. But word clock is simpler because the state alternate between low and high at a periodic manner (low, high, low, high....)

index.php/fa/886/0/

The red trace is the input.
The blue trace represents the rise time limitations (driver, cable...)
The black trace represents the low frequency limitation (transformer or capacitive coupling.
The purple is the comparator threshold.

Due to the periodic nature of the word clock signal, all cycles are identical, and there is no signal wobble therefor the mechanism that applies to jitter generation with AES signals DOES NOT APPLY HERE.

So quoting Julian's paper here (with or without the mis- statement about cable induce jitter) is OUT OF PLACE.

Regards
Dan Lavry
www.lavryengineering.com
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