On Friday 30 November 2007 13:15, Sergei Steshenko wrote:
> I am sorry, but all the above said can be summarized in very few sentences:
>
> 1) any phase/frequency modulation produces a signal with infinite
> spectrum;
[...]
well, sure... as far as one knows what all that means. I tried
to explain jitter effects in terms (hopefully) understandable by
anyone, not just by those who have some EE background. 8:-)
Apart from that, from a purely "technical" point of view of course
you're right. Analogue W&F and other forms of "analogue jitter" (if
we like to call it such) does indeed introduce phase and frequency
modulations of the source signal, which in turn of course cause the
appearance of infinite terms in the spectrum. That's about the same
that digital jitter does when just looking at a spectrum analyzer
screen (though typical levels of "analogue artifacts" may look even
worse from that point of view).
Nevertheless, from a "perceptual" point of view, reasonable amounts
of "analogue artifacts" (modulation) seems to be much more "benign"
to our ears (brain, actually) than most currently common levels of
"digital artifacts".
That is, it looks like we can easily tolerate some relatively large
amounts of "analogue artifacts", while even small amount of "digital
artifacts" may result in a quite "unnatural", unpleasant sound.
I guess not by chance audiophiles have long ago coined the term
"digital sound"... not as a compliment. :-)
I've also tried to give some hypothesis about why it is so...
though of course they were just that, no more than educated
guesses.
> 4) any high end digital receiver should have a PLL-based receiver that
> resamples (with the same frequency) input signal end eliminates jitter
> - rather, the jitter becomes the one of the PLL;
this would be another long story... way out of this list scope, I'm
afraid.
BTW, to try making a long story short, IMO/IME PLL-based resampling
techniques does not completely "replace" the "incoming" jitter with
the local PLL one as you seem to imply.
Resampling does indeed (usually...) reduces the overall amount of
jitter but, unfortunately, the resulting "output" jitter is still
somehow correlated to the "input" one (and, usually, to the signal
too). That is, resampling _may_ (or may not) help mitigating the
problem, but surely it can't really solve it altogether.
A better approach is (IMHO) to try to "avoid" jitter in the first
place, by using the "cleanest" possible clock for the DAC (that is
a free, local XTAL oscillator placed next to the DAC itself), and
then to somehow slave the source data stream to that clock.
(that's in fact the idea that made me start this whole thread. :-)
BTW, compare the above with the "standard" approach used in consumer
devices, that rather do the other way around. That is place the clock
at the source, embed it in the data stream (S/PDIF) and then, in the
long run, eventually (and hopelessly) try to fight the large amounts
of nasty, correlated jitter resulting from so doing afterwards...
> 5) from my experience the decisive factor nowadays is analog circuitry
> in the power amplifier and speakers, not digital jitter.
well, yes and no.
Analogue circuitry and speakers are indeed still the most "sensible"
parts which limits the performance of an audio systems.
Yet this does not mean that nowadays digital sources are "perfect"
or anyway so good that their problems may be neglected altogether.
That _may_ be true for low to average quality audio systems, but it
is by no means true once you move toward truly high quality ones.
If you have a really good audio system, what I have (improperly)
called "digital artifacts" (of which jitter is one of the possible
causes - though not the only one) results in clearly audible and
quite annoying defects.
Of course, again that's IMO/IME.
To conclude, in case I was misunderstood, what I meant to point out
was not that digital audio is in itself a "step back" with respect to
traditional analogue techniques. Of course it is not. Rather, I just
meant what I said: digital audio is no magic, it does have it's own
problems and it is by no means "intrinsically perfect".
This may seem (and should be) an obvious statement... was it not for
the massive marketing hype trying (and often succeeding) to convince
everyone of the contrary. 8-)
OK, we ended up quite OT... sorry.
Ciao,
Paolo.
--
Skype: Paolo.Saggese
http://borex.lngs.infn.it/saggese
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