Jitter on Mac Mini's optical output

Discussion in 'Computer Audio' started by doraymon, Dec 4, 2016.
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  1. doraymon
    Hi,
     
    I use a Mac Mini Late 2012 to feed my external DAC and headphone amp.
     
    I couldn't find anywhere a measurement of the Jitter on the optical out of this little Mac.
     
    Anyone can pint me in the right direction?
     
    Thanks folks!
     
  2. doraymon
    Nothing folks?
     
  3. PolkManiac
    I've got the same question, were you ever able to locate information on this?
     
  4. doraymon
    Unfortunately not.
    Apparently Apple does not release this information and from what I understand the measurement of jitter is not an easy thing to do.
     
  5. pinnahertz
    Did anyone even bother to google it?
     
    Enter "mac mini optical out jitter"....this is the First hit: 
    https://forum.audiogon.com/discussions/jitter-of-new-mac-mini-optical-line-out-improved
     
    Now, the caution is, it's another forum, and there are lots of people just spewing random opinions with absolutely no basis.  But I might put a bit of stock in the authoritative post that quotes actual numbers. Clearly somebody did some measurements. 
     
  6. PolkManiac
    I spent over an hour scouring the internet, that post is the first thing I came across but as you see it's from a 2011 model and we both have the 2012 model. These numbers obviously vary greater by model year (that's the whole point or that post), so that unfortunately doesn't help us. Unless of course you can confirm that the hardware is the same on the 2012 model...

    All that being said I've now moved on as I've discovered the optical output on our model is limited to 24/96.
     
  7. pinnahertz
    So, you're saying they took a problem, improved it, then made it worse again?  To improve jitter there would have to be a deliberate design change.  They generally don't go backwards in hardware design.  Generally...meaning never.
     
    Great to hear you've moved on.  I don't see 24/96 as a "limit" though.
     
  8. PolkManiac

    Sure they do, companies cut corners and de-content stuff all the time.  Hell, Apple did it themselves, with the newer model of the Mac Mini in question, which is still not as fast as the older model I have.  Pretty much all products are a result of compromise and are packages of emphasis, if they decided that the extra performance wasn't worth the extra cost or some other choice they had to make they'd take it out.  USB is backwards from Firewire in terms of audio, but here we are these days with basically no firewire products...
     
    I don't intend on debating the merits of higher resolution files and whether or not we can hear a difference, I DO, however, intend on playing back all the 24/192 content I bought at it's native resolution.
     
    buonassi likes this.
  9. pinnahertz
    Ok, fair enough, since neither of us have hard evidence of the jitter performance of the actual model.  However, it wouldn't surprise me at all but that the newer chip sets involved in the optical and USB signal path had higher performance at lower cost, and therefore would stay put in newer models until even cheaper stuff came along.  The general trend of performance and cost in electronics is for newer products to have higher performance at equal or lower cost.  The exception seems to be in CPUs where performance and speed have flat-topped, and so the only way to save cost is to negatively compromise.  At least in the short view.  But for all other analog and digital chips, as we progress forward performance always goes up, cost levels or drops. 
     
    I would question your statement that USB is "backwards from Firewire in terms of audio", but since you don't state the version of either, we'll just let that drop.  There are plenty of situations where the reverse is true.  
     
    Your efforts to play your 24/192 content at the native resolution are commendable, but may be misguided if the original masters were not 24/192.  And very, very few are.  Perhaps you've noticed as I have that everything I'd really want to hear in hi-res was not actually recorded that way, and the versions sold are just up-samples from the original CD masters, or perhaps an intermediate master at 24/48.  If you been able to accumulate a substantial library of original, native 24/192 stuff of mainstream artists playing music you actually want to listen to (as opposed to demo pieces), congratulations.  You've succeeded where everyone else has failed.
     
  10. doraymon
    Exactly.
    A lot of fuss about nothing...
    Back to my point there is no measurement nor official data from Apple.
     
  11. buonassi
    I just bought a schiit eitr because I didn't have faith in the tosslink out on my late 2012 mac mini. Shame there aren't any readily available measurements. I suppose I can do some listening tests between these three setups once I get the eitr in place:

    1. USB out to iFi iUSB3.0 nano to USB input on DAC
    2. USB out to Eitr to spdif input on DAC
    3. tosslink out to optical spdif on DAC

    I have to imagine that if the optical were as good as the other methods, folks wouldn't be spending money on these other solutions....right?
     
  12. doraymon
    There are two main effects to be considered, one is jitter the other is RF noise which affects only the USB.
    Depending on how sensitive to jitter your DAC is your strategy and priorities may vary.
    In my case with a Dave (completely immune to jitter as the internal high quality clock is the one ruling, no matter what gets in) the issue is only RF noise.
    That's why I went for an optical connection from my MacBook Pro late 2013 and it's the best sounding solution in my setup.
    The best advice would be test and than chose with your ears, as I did.
     
    Last edited: Jun 2, 2018
  13. buonassi
    I was going to mention what you just summarized in my next post.

    In the case of your Dave, it buffers all digital inputs including optical spdif (into RAM), then reclocks the buffered spdif before sending it on through the conversion process, which most DACs don't do AFAIK. You'll notice the clocks inside a typical DAC are implemented by means of the USB PCB (next to the USB chip itself), so I'm not sure how you'd access them for other digital sources. Again, your Dave is an exception to this. Most of the time, you are at the mercy of the clocks in the host (or bridge converter) that is sending the DAC its signal. In general, the spdif protocol is not asynchronous, so once it leaves the host (ie MAC) that's as good as it's going to get. This speaks to jitter. By materials used in optical cables, there is inherent galvanic isolation, so no "noise" can enter the DAC.

    The issue you have already pointed out with USB is the "noise" entering in on both the power conductors and data conductors AND ground pin plugged into the DAC. Jitter as well if the USB chip and clocks aren't of sufficient quality.

    I will say right of the bat that I can't tell jitter apart from noise. And usually the effect is subtle to me. So much so, that if I didn't have a highly resolving system, I probably couldn't tell the differences between input methods! Whether there is noise or jitter, the results seem to be the same for me: less precise imaging and instrument separation, less authoritative transients, perhaps some muddy treble splashiness, and an overly hollow and loose sound to the music presentation. It doesn't sound "bad", but it just isn't as "in focus" and "right sounding". When you eliminate or greatly reduce these artifacts, it just relaxes and sounds "right" and not forced. It's hard to put it into words

    Having said all that - and presenting this next section with the caveat that this is for my DAC (Metrum Amethyst).......round one goes to the iUSB 3.0 reclocker/noise canceller toy from iFi. It definitely helps and is an easily audible improvement over the tosslink on the Mac Mini. The optical wasn't nearly as bad as I was expecting, but it's noticeably not as "in focus" as the iUSB. Surprisingly, it only sounded slightly worse than a direct USB connection! And so much so, that if I wasn't able to switch quickly, I'd never pass a blind test. So the tosslink is not bad at all, hanging in there against the decent HiFace2 XMOS 8 core USB interface of the Metrum.

    Here is the ranking so far for my Mac Mini 2012 late and my Metrum Amethyst:

    1. iFi nano iUSB3.0 as a bridge between Mac and DAC
    2. USB direct connection
    3. Optical Spdif/Tosslink direct connection

    stay tuned for updated results with Schiit Eitr. Gonna be a while because I'm out of town most of next week and I want to allow at least a day or so of "settle in" for the new Eitr bridge.
     
    doraymon likes this.
  14. doraymon
    Yes, it's the same for me. In both cases the first thing I perceive on a "cleaner" setup is more and better bass, less brightness and a pleasant smoothness which makes the sound less fatiguing on the long run without diminishing the detail retrieval and transparency of the system.
     
  15. pinnahertz
    Hard to put into words would be correct. None of what you describe can be attributed to noise or jitter, and the fact that you've identified those qualities as related to them, AND are making fully-sighted and biased listening tests is why you're getting the results you are.

    Unless noise is extremely poor, it will be fully masked by any audio. Listening to noise on a digital-silence test track, with the gain way up, is the only way you or anyone ever could evaluate DAC noise audibly.

    The sound of jitter is no so easy to describe because of how it is caused. It's a variance in clock frequency, true, but what kind of variance? In essence its frequency modulation of the audio signal by some other unknown signal which could be a single frequency, multiple frequencies, or noise, or a mix of all of them. The results are easily seen by looking at the spectrum of a mid-band tone like 3 or 5kHz. Ideally the tone would show as a single frequency, but jitter will place "sidebands" above and below the single tone. The amplitude of those sidebands and their distance from the fundamental affect jitter audibility, but jitter would have to be very poor indeed to be audible with any broader spectrum audio signal. The spectrum resulting from jitter varies depending on the spectrum of the modulating signal -- the cause, so we can't actually describe the audible effects of jitter simply at all.
     
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