The big flaw with this particular measurement (other than the horizontal sweep has been blown up to greatly exaggerate the "problem")
What makes you think that?
Because I can read a scope legend. The scope has been blown up in the horizontal direction to exaggerate the "jitter" You're seeing only a tiny fraction of a bit time.
What information on that image tells you that? It seems to me you're not understanding what you're seeing.
Not only that, but it's showing us the raw signal jitter, before the bit clock has been reconstructed by the receiver's PLL and divided down to the sample rate, which decreases the jitter accordingly. And furthermore, there's no indication of the exposure time, so we don't know anything about the frequency spectrum of the jitter. That's important too.
It's the same for both cases. Level playing field, anybody?
can't do this perfectly.
Not true. The jitter is clearly the result of constraining the bandwidth of the cable so that intersymbol interference is introduced. With a good cable, there wouldn't be any intersymbol interference.
But you don't believe in all this audiophile "mumbo jumbo" about good cables and bad cables, do you?
And even if what you said is true, so what? It doesn't *have* to do a perfect job. Nothing can. It only has to do it well enough so that you can't hear a degradation.
You're general POV seems to be that if *you* can't hear a difference then no-one else should either.
Triode posted a link to a paper published by the AES that discussed the problem of jitter concludes:
"It can be shown that, compared to low oversampling multi-bit designs, pulse density modulation converters are much more sensitive to jitter when producing low frequency audio signals. This phenomena may explain certain subjective characteristics of PDM DACs which cannot otherwise be rationalised. A simple model of jitter error audibility has shown that
white jitter noise of up to 180 ps can be tolerated in a DAC, but that even lower levels of sinusoidal jitter may be audible."
I'm not up on *all* the various DAC topologies, but I still can't understand why this should be such a problem. If anything, I'd expect a given amount of jitter to have *less* effect on a low frequency signal than on a high frequency one. That's because jitter is really just undesired phase modulation by a noise process, and a variation by a given amount of time represents a much smaller number of degrees in a cycle of a low frequency waveform than a high frequency waveform. This is analogous to the fact that phase modulation and frequency modulation are closely related, but with a 6 dB/octave pre-emphasis or de-emphasis because of the fact that frequency is the derivative of phase, and phase is the integral of frequency.
Have you ever heard the effect of phase modulation on an audio signal? The sort of improvements audio enthusiasts wax lyrical about can often be attributed to phase. Soundstage, depth, clarity, all that sort of stuff. Even a small phase error can radically smear the sound.
Note that the paper analyzed the effect of jitter on full amplitude 20 KHz sine waves. That's about as worst-case as you can possibly get. Can you still even hear 20 KHz? I know I can't.
I don't hear discrete sounds or tones, no, but I do hear relationships between sounds and positional information that is encoded in the HF band. If you lose that you lose clarity and detail in the sound.
The discussion was originally about the possibility for improving the performance of the SB by adding a more accurate clock/crystal oscillator. As you rightly say, with no external DAC there is no SPDIF link but there is still the possiblity that the internal clock can be improved upon.
Yet according to that same AES paper, internal clock phase noise is utterly negligible compared to the jitter source he was investigating, namely data-dependent jitter introduced by intersymbol interference. This is exactly what I suspected given what I know about crystal oscillators -- they have the lowest phase noise of any kind of RF oscillator, and suffer only from frequency inaccuracy and slow drift.
And what happens if the crystal is inaccurate?
So it seems to me that if you're really worried about jitter, just use the analog outputs on the Squeezebox and don't even connect anything to the S/P-DIF outputs!
That is of course not feasible if you want to use an external DAC, in which case you need to minimise jitter, which brings back to where we started.
One could ask why you even need an external DAC in the first place, as the Squeezebox already provides exactly that function. At least it gets the DAC out of my computer where 100A ground currents circulate!
Because I can hear that my DAC sounds better than the DAC in the Squeezebox.
I know some people think that something costing only $200 can't *possibly* be any good, and that at least $1K of fancy, gold-plated high-end accessories is needed to even be worthy of their consideration. God bless em', they do keep the audiophile industry in business.
It's not about spending money for the sake of it. I want to get the best possible sound with the least possible outlay. I don't go chasing "snake oil" gadgets. I have a Squeezebox, which is stock (£190). I have an external DAC, which cost me £70 plus £10 for a UK power supply (I bought it from the US); I've tweaked my (inexpensive) amplifer (cost new £120, 15 years ago. £20 on tweaks) and added a power amplifier for biamping (£90). The sound I get from my setup now is at a level comparable to systems costing well into four figures for an outlay of only £190
You seem to be very closed to the possibility that others can hear things that you obviously can't. You're not wrong if you can't hear it, but you are wrong to insist that others can't either.
Please don't dismiss as impossible everything you can't explain or don't understand in engineering terms.
R. -- http://robinbowes.com
_______________________________________________ Discuss mailing list Discuss@lists.slimdevices.com http://lists.slimdevices.com/lists/listinfo/discuss
