"Real World" is what human beings can actually hear.
Welcome to Sound Science. Sorry about your misconceptions.


Also, USB 2 is quite robust, if you don't want to use the optical.
USB audio can suffer less from jitter (depending on how it's implemented) than toslink optical audio, but it's more susceptible to line noise. This is why the better USB cables that are made for audio have the power transmission component of the cable broken out or heavily shielded from the data transmission component. The type of USB cable does seem to make a difference to my ears, although I haven't tried to the ridiculous (>$150) high-end ones. This is just what I have noticed from playing around with different DACs and cables, so my findings are 100% unverifiable but that's my experience. I have also noticed improvement using a modest real glass toslink cable compared to a cheap plastic toslink cable when using optical.
The Macbook Air does not have the optical/analog combo jack found in all other Macs (except Mac Pro where they are separated), but it does offer HDMI output as an alternative to USB audio.

Data transmission over USB and the way in which A DAC streams audio from a computer are different. Data transfer uses bulk mode which does not care about the timing at all. Cheaper USB transports and DAC chips use adaptive transfer mode in which the USB receiver is reveives the clock from the computer, and attempts to fix any timing errors (jitter). Some more recent or expensive USB transports use asynchronous tranfer where the USB device controls the rate at which the packets are tranfers with it's own clock, usually with some sort of buffer to attempt to ensure that if a packet is not sent in time the stream is not interrupted. The Hiface uses bulk mode transfer which in theory means packets can be resent if they are not received correctly. Bear with me if I have not described things correctly.
Some USB transports draw power from the USB port, so the cable may contribute the the rective impedance of the power being supplied, or pick up noise from the surroundings, or crosstalk between the conductors. Most USB cables are effective enough to ensure that non-timing critical transfer modes work without error. At this point people who beleive they can predict how equipment will behave for whatever reason will say "but asynchronous USB is buffered to compensate for variation in the latency between when when the USB transports requests a packet and when it arrives, and they are correct in theory. In practice things are not that simple in my experience and you may feel free to take my experience with a grain of salt if you think you know everything there is to know about digital audio.
Me personally I know enough to know that I in fact do not understand everything to do with digital audio, and to trust my ears and judgement instead of my limited understanding of the technologies at hand, or what studies I have not actually closely studies might be used to make claims about what will or will not affect sound quality.
So what you are saying is the DAC receives its clock from the computer. How exactly does it work? USB2.0 runs 488MHz and the DAC runs 44KHz. What if I have a USB1 or USB3, won't that really screw up the system?
Let's assume if indeed you get your clock from the computer. Let's say my music is recorded at 43KHz (there will be tolerance and I am exaggerating the difference) and you are running 44KHz. Won't that cause buffer under run?
People are confused by synchronous transmission and synchronous timing. Synchronous transmission is synchronous to a transmit schedule which gurantees bandwidth and latency that's important in real time communication. Synchronous timing refers to clock synchronization which is necessary to recover data.

USB audio can suffer less from jitter (depending on how it's implemented) than toslink optical audio, but it's more susceptible to line noise. This is why the better USB cables that are made for audio have the power transmission component of the cable broken out or heavily shielded from the data transmission component. The type of USB cable does seem to make a difference to my ears, although I haven't tried to the ridiculous (>$150) high-end ones. This is just what I have noticed from playing around with different DACs and cables, so my findings are 100% unverifiable but that's my experience. I have also noticed improvement using a modest real glass toslink cable compared to a cheap plastic toslink cable when using optical.
Wouldn't it be cheaper simply to cut the power line? It might be interesting to DBT a USB cable with and without the power line since they're the same in every other aspect.
Ya know... I really like you guys, but I think I am going to need some therapy pretty soon. :-0
Just for fun.... here's a nice hypothetical question for you guys that believe in your hearts-of-hearts that USB cables don't sound better. Please humor me for just a second here... I'm really curious.
What would you say "IF" (note the big, and bolded IF).
1 - We all got together at my house... and drank a couple of beers (always a good plan... and maybe a pizza too... I like pepperoni)
2 - We ran a 100% fully legitimate DBT to you full specifications.
3 - The result of said DBT was that, "Yes" the high-end USB cable was identified as significantly better every single time. DBT passed with flying colors (remember the IF).
4 - The beer in step 1 had no influence whatsoever in the results of step 3.
What conclusions and thoughts would you have to say, if this were indeed the result?
And yes, I really do believe in my heart-of-hearts, that on my system, in my living room, on my music, that I really could deliver this result.
But coming back to our individual realities in the real world... I again suggest that unless you have heard a really good cable (Locus Design Polestar and Axis in my case), then I don't think that there is very much that you can tangibly contribute to threads such as this. The same goes for interconnects, speaker cables and power cables. On the other hand, if you really have indeed listened to a specific cable change (USB Cables in the case of this thread), then your experiences (confirming that you can hear improvements or conversely, that you can't) truly are invaluable to the our community and to our understanding of what is worthwhile and what isn't.
If a cable does deliver an improvement, then it becomes a simple question of whether that amount of improvement is worth it to you, as you prioritize your short and long term financial situation and goals. If yes, then buy it; if no, then send it back. It's pretty simple really. Therory is great, but in a thread like this where all of us are really pretty much professional amatures at best, the back and forth theory debates fail to bring much real value to the table in terms of a specif cable being considered is worthwhile (or not). And with that... I'll be at the therapist if you need me any further :-)
Take Care... Happy Listening!

