opaqueice;194637 Wrote: > > One way to see why that is to consider the time it takes an EM wave to > travel down the wire - a few picoseconds - versus the inverse frequency > of the signals being sent (typically 1000 ps). Imagine looking down at > the ocean through a magnifying glass. You only see a little bit of > water, so the height of the water doesn't vary across the part you can > see, but of course it varies with time as waves pass by. So to a good > approximation the part of the water you can see is flat, but moving up > and down with time.
This is not true, unless by "a few" you mean "a few thousand". A signal moves down a wire at about 5 _nano_ seconds per meter, depending on the dielectric and the construction of the cable, and the signaling rate of the s/pdif is a transition per 177 ns, way more than 1000ps - we know what these quantities are! I think the reason jitter is so poorly understood is that we intuitively can not grasp quantities like picoseconds (trillionths) of a second. We are inclined to believe that a wire changes voltage instantaneously because unless we work with ghz scopes all day long, we are not accustomed to seeing a transition moving along a wire. Personally it has taken me many years of working with these kinds of quantities to get any sort of "feel" for such tiny units of time. The notion that s/pdif moves so glacially as to be unaffected by reflections is just nonsense. What you are saying is equivalent to claiming that wire has no resistance because a lamp glows just as brightly with one foot of wire as with a hundred. Just because you don't have an ohm meter, or you can't perceive the difference in brightness, doesn't mean it is not there! With suitable equipment (like what telecom engineers use, not hifi magazines) we can easily measure jitter and we can see how an s/pdif signal is affected by these things. It is just silly to speculate about these quantities when they are so easily measured. We can see how slower rise times yield more jitter, or how an impedance mismatch affects the waveform. And with a high frequency test tone played through a DAC we can see how levels of jitter as small as ~50ps can be observed in the output of the DAC. How much of this is audible is up for debate, but it's all real and observable. -- seanadams ------------------------------------------------------------------------ seanadams's Profile: http://forums.slimdevices.com/member.php?userid=3 View this thread: http://forums.slimdevices.com/showthread.php?t=34406 _______________________________________________ audiophiles mailing list [EMAIL PROTECTED] http://lists.slimdevices.com/lists/listinfo/audiophiles
