USB cable hype

At least my post is seeking a discussion which has value, even if you don't think so. On the other hand your post has no value. Troll.

Axle, asynchronous DAC controls the timing. Data coming from the computer is placed in the buffer. Every frame computer adjusts number of samples in the frame based on buffer under/overflow signal from the DAC. DAC takes data from the buffer and writes it into D/A converter using internal stable clock. Because of that jitter does not even apply here. It is possible tough that ambient or computer electrical noise can enter DAC thru the cable. USB cables carry power that is not needed and can be source of such contamination. Ethernet is pretty much the same story - data comes without timing in packets so cable should not matter, but people reported improvement when moving to better shielded cables. I suspect that the same thing takes place - cable picks-up ambient electrical noise and injects it into DAC affecting internal clock thus jitter. Jitter converts to noise in frequency domain.

I don't have USB DAC so my observations are only theoretical. I assumed that DAC is asynchronous. Synchronous DACs, where computer controls timing supposed to be pretty bad since computer clock is very jittery.
The main problem is jitter. Clock jitter is a form of signal modulation (similar to FSK) that produces sidebands. These sidebands are at very low level but also very audible since they are not harmonically related to root frequency. Eventually with many frequencies (music) jitter produces a lot of sidebands resulting in noise that is proportional to signal level and hence undetectable without signal.

"My OP asked about need not preference for expensive USB cables. This could have been posted more clearly. Need is about functionality."

"Having said that, what is good enough? Is it the cable that costs another $50 or $500? I think the cable industry intentionally does us a disservice of keeping us in the dark in order to sell into that ignorance."

If you're just concerned about the function of USB, and things like maximum cable lengths before the signal starts to degrade, there really isn't any industry conspiracy. USB specs are published like any other format. Now if a private company wants to go out and make high end, expensive USB cables, who's to stop them? And why would someone stop them? (I'm assuming the people making these high end cables are not breaking any laws, such as copyright infringement, or similar type of offense.). So, as long as you stick to whatever standards the format requires, you shouldn't have any problems from a functionality standpoint, regardless of cost. Even a cheap cable should be fine. I found this website for you to look at. Its a business that sells cables, but they go over the requirements for the different versions of USB cables (USB 1, USB 2, USB 3, etc...)

http://www.yourcablestore.com/USB-Cable-Length-Limitations-And-How-To-Break-Them_ep_42-1.html

Thanks Sd542. I was originally thinking functionality only because bit errors should be zero in the audio range. But digging deeper, I realize that may not be true. Some USB cables may result in fewer errors than others. In which case, it's not only about functionality but also about SQ.

Cable manufactures aren't doing anything illegal. But I would call it unethical to mislead the customer for profit.

Hi Kijanki, I agree with everything you said except for this one thing: "...jitter does not even apply here".

Just because the source clock isn't used inside the DAC doesn't mean that it has no effect. The DAC clocks data only after it reaches the DAC. The source clocks data up to that point. Source clock jitter combined with cable jitter may err a bit before it reaches the DAC.

For short cable runs, bit errors induced by the cable are unlikely. For long cable runs, bit errors are inevitable. Therefore, the longer the cable and more marginal the source, the more the cable matters for both function and SQ.

Of course, as you mention, a cable may also help SQ by isolating the signal from various noises.