I ran into this post recently, and would like to add some thoughts.
The fun thing about designing a S/PDIF device is that the goal is perfectly well defined. There is really one one design parameter, which is to get the jitter as low as possible. You're not faced with design trade-offs, as with, say, designing a DAC, or speaker, or anything that is voiced. In that case, you may make a dozen trade-offs (say, trading higher THD for a simpler, cleaner circuit, etc.).
The irony, though, is that despite the fact that there is a single, final design parameter - the jitter - most audio companies do not actually list this number. Probably for some devices, this is simply because the numbers are not exactly impressive. But even the absolute best companies do not list figures, due in a large part, I think, to the difficulty of having a single accurate number, which universal across systems and devices, and thus is not misleading.
Keep in mind that audio jitter can be measured using a number of different techniques, some of which are completely unhelpful (such as the cycle-to-cycle jitter), and most of which cannot be directly compared to each other. Different techniques integrate over different bandwidths, for instance, or are frequently stated as RMS rather than peak-to-peak (these numbers do not directly convert). Moreover, because the jitter on modern audio devices is often extremely low, the resolution of the measurement setup itself comes into play, and can add a significant offset to the final number.
The S/PDIF protocol itself throws in another complication, because it's not actually the jitter on the S/PDIF signal itself that you care about – it's the jitter on the recovered clock on the DAC itself (or actually, on the final analog output signal), which can vary widely, depending on things like the transmission cable and the receiving circuitry on the DAC.
As an example with some numbers, check out the jitter measurements made by John Atkinson for the Bridge and a number of similar competing devices (here). JA measured jitter on the S/PDIF signal itself, as well as measuring the jitter in the reconstructed analog waveform through a number of DACs via the J-test. You can see how much these numbers vary: the Bridge through the Musical Fidelity X-24K DAC measures 185 ps peak-to–peak (quite low), whereas the measurement with the older Assemblage DAC-1 gave a full 1,100 ps jitter.
That's a factor of six difference, with the same S/PDIF source. Even more interesting, the various S/PDIF sources cannot even be ranked consistently from first to last, since a device that performs well on one DAC may perform notably worse on another. (BTW, this test didn't include results from a DAC which employs asynchronous sample rate conversion, through which different sources would presumably give very close to the same result. We don't employ ASRC in any of our devices, since we feel the drawbacks outweigh the benefits, but it is a useful technique if you are starting with a jittery source and a D/A chip that is highly susceptible to jitter. These types of DACs also make it very difficult to A/B compare two S/PDIF devices, since they whole point is to eliminate any variation resulting from different sources!)
Which jitter number to use is a matter of debate, which is why, as I mentioned, most companies don't use any.
But by far the most misleading term used to describe audio jitter is is "jitter free." I pretty much cringe whenever I hear this term, as it's physically impossible. EVERY digital signal will have jitter, even if it's in the picosecond (or femtosecond? attosecond?) range.
"Jitter free" or "zero jitter" is simply physically impossible, and whoever uses these terms is either content to feed you a line of total marketing BS, or they just have no idea what jitter is, and how to measure it correctly. Either way, I'm absolutely stunned that any legitimate audio company would ever claim their device is "jitter free," and correspondingly at least mildly surprised that this practice would be accepted by consumers (and even some reviewers). If you make a device, and don't know how to measure jitter ... you can't just guess "zero" (the one number that's guaranteeing to be incorrect), and be done with it.
Anyway, moving on ...
The focus on the USB cable on the Bridge has been a bit of a head-scratcher for us. The whole point of an asynch design was to minimize or eliminate any jitter from the USB line. Ironically, a large part of the Bridge design was centered around improving the cable at the other end of the device, the S/PDIF end - by getting rid of it. No cable, no reflections. I guess the focus on the USB cable is just a byproduct of the fact that our device is like 95% USB cable by length, and most of the stuff that really matters (like the power supply circuitry, the clocks, the layout), people just don't see.
Okay, if you've read so far, and are wondering if I'm going to say which USB to S/PDIF converter is the best, here is the answer: NONE. The best is to take the signal directly from the USB to I2S receiver chip, as is done with the high-end asynch USB DACs. In these setups, the jitter is essentially limited by the ability to get a low-jitter master clock circuit (itself quite an art). Encoding the digital signal to S/PDIF, sending it across a cable, and then decoding it only adds jitter. Depending on the receiver circuit in the DAC, this can be significant.
Some comments by manufacturer removed. -Editor
Also, one other thing - we have the 60 day return policy for a reason, because sometimes people need to try something out to really know what it's like. If you want to compare the Bridge against any other device, please feel free ... if you choose something else, no problem. We have nothing to hide, and our ultimate interest is just to make listening to and experiencing music more enjoyable.
Hope this helps. I generally try to stay out of these discussions, and let them evolve on their own. Feel free to email us (contact at halidedesign dot com), with any questions or comments, it's always nice to hear from people in the audio community.
Best,
Jonathan
Halide Design
