Jitter and 75ohm cable length

"The Stereophile test showed it to be awful."

Not exactly. High jitter appears on analog outputs only. Digital is about 10x better (258ps vs 2400ps) and that's what Stereophile stated:

"The noise floor has dropped by 4–5dB, the word-clock jitter to a respectably low 258ps, which is actually better than the case with the standalone D/A processor driven directly by my PC's S/PDIF output (provided by an RME PCI card).

Considering that the AirPort Express's analog output is basically a freebie function added to a computer Wi-Fi hub, jitter aside, its measured performance is quite good. The beauty of this unassuming component, however, is its S/PDIF data output, which allows the AirPort Express to assume a respectable role in a true high-end audio system."

Kijanki, if one suspects they have a Jitter problem with their system, how might it manifest it's impact on the analog output. I had read an article indicating that high Jitter would make vocals sound overly warm, which actually may be desireable? Is there anything else you can add to the impact of sound reproduction? Thanks!

Rpg

Rpg,

Jitter is basically a noise in time domain. Applied to one frequency it creates sidebands at very low levels. In spite of low levels (less than -65dB) sidebands are audible being not harmonically related to root frequency. Now, take whole bunch of frequencies (music) and you'll get whole bunch of other frequencies at very low level - basically a noise. Amplitude of this noise is straight proportional to amplitude of music and without music (gap) is zero - therefore undetectable.

My first impression of Benchmark DAC1 that suppresses jitter was that sound was too clean (some people call it sterile or analytical). I had impression that some instruments had to be missing from the recording. I also understand that noisier (or distorted) signal sounds more lively the way that distorted guitar sounds more dynamic than clean jazz guitar at the same volume. Other than that sound is on neutral side - I would not attribute any warmth or lack of it to jitter. Imaging is more focused but perhaps a little narrower.

I don't see warmth as desirable quality. Benchmark technical director John Siau said that overly warm gear can negatively affect sound of instruments with complex harmonic structure like piano making it sound almost like out of tune. On the other hand cold sounding (expanded odd harmonics) gear is much worse. I had problem of brightness until I replaced speakers with aluminum dome tweeters. New soft dome Hyperion HPS-938 are wonderful - neutral and never bright on any CD. Sibilants are still very audible but always clean and natural.

I think that main difference between oversampling or upsampling DACs and NOS DACs is not the sampling itself but filtering. Traditional linear filtering adds pre-echo to impulse. Our hearing is very sensitive to it and getting rid of filter altogether (NOS) or using apodizing filter (extending post echo) might be a good thing. Here is some info on the subject:

http://mrapodizer.wordpress.com/2011/08/16/technical-analysis-of-the-meridian-apodizing-filter/

I don't care about any jitter on the analogue out.......It is is jitter on the digital signal which renders the 840c unusable. OR, is the 840c simply sensitive to even that moderate level of jitter....?

From the Stereophile test: This is analogue? not digitial?
"The AirPort Express stumbled when it came to its measured jitter performance—hardly surprising, considering it has to derive its 44.1kHz word clock from an asynchronous, probably encrypted datastream"

Now, with the above stated jitter at 258ps, I'm back to ground zero as to why my 840c doesn't like the dig out of the AE.

other questions....
Would the ATV be any better? (Apple TV). Would the DACMagic be prone to the same problem as the 840?

If one suspects they have a Jitter problem with their system, how might it manifest it's impact on the analog output.

I second Kijanki's excellent comments, and I would add the following thoughts, which I composed before seeing his response:

IMO it's very unpredictable, and the symptoms will vary widely depending on the spectral characteristics of the jitter, which can be expected to be a very complex mix of discrete frequency components and broadband noise-like components. Some of those components will be correlated with the values of the 1's and 0's defining the music data (which is different than being correlated with the music itself), and some will not be.

Perhaps essentially all that can be said is that there will be a loss of clarity, and an increase in distortion.

The following papers may be helpful. The second one, although highly technical, conveys a sense of how complex it all is, and by implication (as I see it) that the effects of digital cables and digital interfaces should not be thought of in the same kinds of ways that we use in describing the effects of audio frequency analog cables, e.g., "overly warm" (notwithstanding the fact that a given cable may create that perception in a given specific system):

http://www.positive-feedback.com/Issue43/jitter.htm

http://www.scalatech.co.uk/papers/aes93.pdf

Regards,
-- Al