Jitter and 75ohm cable length
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 performancehardly 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 |
Let me add to Al's great post. As he stated, typical transport has transition times in order of 25ns. Threshold resides most likely at the half of that - 12.5ns while impedance boundary, that causes reflection will reside on the other end of the cable. Sharpest slew rate change, causing reflection, is usually at the very beginning (knee). From that point signal travels forth and back (reflection) over distance of 2x1.5m=3m with speed of about 60-70% of light speed - let say 0.2m/ns. Reflection will return in 15ns missing time-wise threshold point. I would use 1.5m-2m length or less than a foot where transmission line effect is non-existent yet. Rule of thumb says that we're dealing with transmission line when transition time is less than 8 times propagation delay (one way). It would imply that typical 25ns transport digital cable becomes transmission line when propagation is longer than about 3ns being equal to about 0.6m. It sounds strange but good cable should be very short or 1.5m-2m. When transitions are slow we don't have much of reflection induced jitter problem but rather noise induced jitter (noise affecting threshold point). When transport has fast transitions noise induced jitter is reduced but reflection induced jitter is dominant requiring very good cable. Long cables in addition add to noise pickup so whole thing becomes system dependent. Same cable might sound great with one system but no so great with the other. Many people report better results with Toslink, in spite of slow transitions, perhaps because of noisy environment or ground loops that coax might create. Magfan, I commented only on Stereophile findings. AE sounds very clean in my system but I use it with Benchmark DAC1 that is jitter suppressing. It is possible that your AE is bad but it is also possible that one sent for evaluation to Stereophile was extremely good (selected?). It is even possible that Stereophile measured wrong - who knows. |
