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