Re: about jitter... (digital does NOT mean perfect)

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On Fri, 30 Nov 2007, Paolo Saggese wrote:

>
> 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".

Your evidence for this is what?


>
> 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.

Your evidence is what?

>
> I guess not by chance audiophiles have long ago coined the term
> "digital sound"... not as a compliment. :-)

Audiophiles have also coined much other nonsense like the effect of
speaker cables on the sound. 
Just because someone who calls himself a audiophile prattles on about
something does not make it true.


>
> 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.

Fine. But that is not a problem. As I said, ppm which is not hard to obtain
is -120dB below the signal level-- ie totally and completely inaudible.

>
> (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...

No idea what this means.


>
>
>> 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.

Yes.

>
> 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-)

Exactly what hype is that? The hype appears to me to be coming from the
other end-- the analog junkies.


>
> OK, we ended up quite OT... sorry.
>

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