Robin Bowes wrote:
What information on that image tells you that? It seems to me you're not understanding what you're seeing.
The legend says the time trace is 5 ns/div. A pair of measuring lines implies that the jitter on the uncorrected is somewhat less than that, about 3 ns.
As I recall (I can't find good, recent references), each subframe in S/P-DIF carries a (usually) 16-bit PCM sample in a 32-bit subframe, so the clock rate of the composite stereo signal would be 64 times the 44.1 KHz CD sampling rate, or 2.8224 MHz. At this rate, a bit time is 354.31 ns, so the <5 ns jitter is a tiny fraction of the bit time. That's why you only see one bit transition on the scope -- because its horizontal sweep rate has been blown way up to make the small jitter visible.
Even if that jitter were directly imposed on the local VCO, which it is not because of loop filtering, it would still be reduced by a factor of 64 as the VCO clock is divided by 64 to produce the DAC sample clock. 5ns of jitter would become 78 ps. Even tinier when you consider that a cycle of 20 KHz (the highest frequency the CD can reproduce) takes 50 *microseconds* to complete. What fraction of an audio cycle is that? What's the FM modulation index? What is the resulting spectrum of sidebands around the 20 KHz signal? What about lower frequency audio? I'll leave the precise numbers to the reader, but it should already be obvious that they're already tiny for the 20 KHz signal, and even smaller at the lower frequencies.
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?
Except that, in both cases, it's just too small to matter!
But you don't believe in all this audiophile "mumbo jumbo" about good cables and bad cables, do you?
Don't put words in my mouth. You don't need cables that can pass 2 MHz if you're carrying baseband analog. If you're carrying S/P-DIF, which has its spectral peak at 2.8 MHz, then you do.
It would be hard to find a coaxial cable that couldn't do an adequate job of passing the spectral components of a 2.8 MHz S/P-DIF signal over a few feet in a home stereo system. We regularly use even smaller coaxes to carry far higher frequencies from cell phones to external antennas.
I *can* see how there might be a problem with ordinary shielded, twisted-pair microphone cables such as the kind long used in professional work to run 600 ohm analog signals over significant distances. Here you'd probably want a redesigned cable better suited for megabit digital signals. Something like Cat-5, for example, which is really cheap and goes up to 100 MHz. It doesn't have to be expensive or gold plated to be good.
You're general POV seems to be that if *you* can't hear a difference then no-one else should either.
Not at all! For one thing, I'm 48 and my hearing is not what it was at 18. But if I can't hear a difference, and some calculations cast strong doubt on *anyone* hearing the difference, then I think it reasonable to ask those who claim to hear a difference if they have conducted any proper blinded listening tests. If not, then I question their assertion. Audiophiles have a very long history of "hearing" all sorts of amazing differences that seem to disappear as soon as proper controls are introduced. That's a fact, and it would be foolish to ignore it here.
Have you ever heard the effect of phase modulation on an audio signal?
Sure I have. Remember I said I help design modems for a living. Phase modulation (e.g., PSK) is one of the modem designer's standard methods. I'm well aware of what large amounts of phase modulation sound like; I've spent many hours listening to these things while developing and using modems on satellite links. But that doesn't mean very tiny amounts of phase modulation sound at all alike.
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.
Yeah, but can you do it in a properly controlled test? How do you know you're not just fooling yourself?
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.
Again, so you say. People can claim to hear anything. Prove it with a properly controlled test. That's all I ask.
And what happens if the crystal is inaccurate?
If it's in a wall clock, the clock runs a little fast or slow. If it's a local oscillator in a radio receiver, then its dial frequency calibration is a little off. If it's the oscillator in a PLL recovering the clock from a digital signal, then what happens depends on how far off frequency it is, and the bandwidth of the loop filter. If it's sufficiently far off and/or the loop bandwidth is too narrow, then the loop will never lock, or it might take a very long time to lock. (BTW, this was the big fear about the recent Huygens probe to Titan. They were so concerned about a design flaw in the receiver PLL causing loss of lock and all the data to be lost that they completely redesigned the mission.) But if it's not so far off, then the oscillator will still lock to the exact frequency of the incoming signal. There will be a DC "stress" in the error control voltage to account for this offset, but that won't affect the loops' ability to track the signal.
Again, crystals -- even cheap ones -- have the cleanest phase noise spectra of just about any oscillator out there. Even atomic clocks. Such devices actually generate their output with crystals, and they use the exotic rubidium or caesium stuff to slowly "steer" the crystal's frequency to compensate for frequency offset and slow drift.
Because I can hear that my DAC sounds better than the DAC in the Squeezebox.
Once again: how do you *know* this?
It's not about spending money for the sake of it. I want to get the best possible sound with the least possible outlay.
Quite frankly, you could have fooled me.
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.
Not at all. I am still open to the possibility that others can hear it, but if that goes strongly against what is known in the scientific literature, then I insist on proper scientific proof, not a mere anecdote. I think I'm quite right to insist on that.
Please don't dismiss as impossible everything you can't explain or don't understand in engineering terms.
Not at all. It's just that before we can try to understand something in engineering terms, we have to first find out if it's even real. Science gives us the tools we need to determine that, even when it's something as subjective as audio quality.
--Phil
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