Why does SPDIF basically suck?

[buonassi]

35 minutes ago, jabbr said:

USB: is here to stay, particularly if we consider Lightning as a variant of USB -- In my view not the best technically going forward but will likely hold the mass market DAC space.

 

I agree that USB is here to stay.  What are the chances that USB-C becomes the standard connection type on the rear of DACs and computers in the next couple years?  Supposedly, this is the attraction of USB-C:  smaller/reversible connection, 3.1 speeds, higher power, and it doesn't matter which way you plug it in.

[buonassi]

how reliable is the widely used "J test" that ASR and Archimago use?  All of the J test measurement data I've seen all looks like it would be a non-issue the jitter levels are so close to the noise floor and 100db below the music level at worst!  The measurements would indicate that today jitter is "solved".  

 

Then you have guys like Ted Smith (PS Audio) who can easily identify jitter.  And I can hear a difference when I use my "reclocker" right before my dac (which measured very well on its own in the J test w/o a reclocker).  This improvement I hear could be due to another issue other than jitter I suppose, but I digress....

 

I'm not sure I buy that this test is the be-all-end-all to testing the accuracy of timing.  Can someone chime in as to why the J test is inadequate?

[buonassi]

So the J test is done using only one tone at 0 dbfs (or close to) and observing the effects on the rest of the spectrum.  Anyone else see the shortfall here besides me?  Even if a J test ran on a sweep, it would be better, but still wouldn't really be sufficient.  Music isn't just one perfectly oscillating sine wave.  And it's reasonable to assume that when the USB chip is presented data that doesn't repeat as nicely as an encoded sine wave, that jitter could increase substantially.

 

Or maybe I'm looking at this through the lens of a lay person, and these tests are really good proxies for real music.  But I haven't heard the any counter argument yet.