Does USB cable quality matters (affects in someway the signal) on audio interface (or DAC)?

Question

I was wondering this for quite a while. Usb cables are purely digital signal so in theory no one should care about its quality if it does the job well and transfer the signal to the end (ones and zeroes). But then again... Why do some cables have double shielding for interference? And when we are talking about recording and audio interfaces I started thinking that perhaps interference could bring noise to the recorded signal or some 'impurities'? I know that gold coating is just a cheap marketing trick and practically is not important at all. Cable resistance - this I do not know (on digital cables).

So how (if it does) a good quality, double shielded, cable "improves" the sound?

Is there ANY advantage of using it over a cheap, single shielded (or not even shielded at all..) cable when we are talking about high quality DAC, Audio interfaces and other devices which transfer signal that is video, audio or anything familiar. Should I run and purchase a good quality usb when I'm recording? Or good quality cables are only good for reliability?

Haven't solved the mystery myself. I know that you guys are always helpful and will solve it :)

P.S. I am asking specifically for audio interfaces or DAC. I know that if you'd pay 50$ for an iPhone cable it would be simply dumb as it does not matter expensive or rather cheap. Audio interfaces and specifically audio recording is what I am not sure about.. (and didn't quite found the information I was looking for on the internet or forums)

Answer 1

It's worth having a USB cable with ferrite 'stoppers' on it, to attenuate conducted emissions along the cable. This will reduce the chance of hash from the switch mode power supply and other PC generated interference 'getting into' the audio circuits of the ADC that's doing the recording. With an electrically noisy PC, and a cheap ADC, a cable without stoppers might produce breakthrough interference on audio. However, part of the cost of a good ADC should be high immunity to conducted interference.

The ferrite stoppers add a few pence (cents) to the cost of a cable, and most cables I've seen have them (the cylindrical lump close to the connector at one or both ends of the cable), only the very cheapest cables do not.

Once a cable has these stoppers, all cables either work, or are broken, there are no differences in data quality. If it can transfer a file, then it can transfer your highest quality recording. Golden ears cannot tell the difference between two identical files.

I wish I could dream up snake oil like expensive 'audio grade' USB cables and sell them for a fortune to audiophools, but I'm just an honest engineer!

Answer 2

As USB is a digital signal, no, interference will not cause the audio to change. But USB audio is normally delivered via isochronous data transfer (think UDP), a best effort non-error correction protocol. USB uses Manchester encoding on differential twisted paired data lines to reduce the effect of any interference, as well as the shielding. USB does have error checking (CRC) for transmissions, but as the audio is real time transfers, any corrupted packet is simply dropped/ignored.

You would hear audio drops if enough interference happens on the line, but no static or similar analog noise. Normal interference shouldn't affect it much if at all.

A sufficiently sub-par or damage cable may drop too many packets, or cause the line to drop to a lower speed, to make it usable. The same cable for bulk transfers, like copying a file from a hard drive, may result in corrupt transfers or transfers that take much longer than normal.

Answer 3

Yes it can. Even if you have lets say isolation barrier between USB procesor and DAC chip (inside the "DAC box"), EMI modulated on USB data transfer (and thus heavilly corolated) will get thru and onto I2S data lines to the DAC modulating his silicon substrate with jitter and affecting the audio quality. This is true for each UAC standart transfer (asynchronnous, isynchronnous and adaptive).

On the other hand...USB audio cable is the last thing you want to mess about and its a "voodoo" in lot of senses. There is technical (measurable) as well as hearable difference (both of them are very small), but you are better saving those USD500 for better DAC, then spending it on a cable. Double shielding isn't really going to help you much. It has some technical backround, but doesnt solve the problem and its mostly to sell cables for ridiculous prices.

Thing that usually helps is a USB hub between PC and DAC which is powered from external power supply. It repackes the USB frames resulting in lower jitter and has much cleaner supply for the DAC.

Answer 4

No. That shielding is ineffective against conducted EMI from the computer anyway which is the major source of interference. Unless it's in the audio spectrum, you won't be able to hear nor measure it though.

Shielding in general is a way to sell audio-whatever at a higher price. Think balanced XLR with transformer for ground-lift in one end. Now change the cable and listen again. Did the shielding make the audio warmer, more emphasis in the midrange or clarity that just wasn't there for the hi hats?

As for the USB part of it, the specification requires sheilded cables but this has nothing to do with audio.

Answer 5

Besides EMI issues as explained in the other answers, there is also jitter, but it is rather subtle.

Isochronous audio transfers allow the DAC/ADC master clock to be either the host, or the device. This depends on the chosen mode which is a decision made when implementing the device:

• synchronous, where the stream’s clock source is linked to the USB’s clock, such as through the use of USB frames;
• adaptive, where the stream’s clock source varies according to the buffer requirements of the stream.

In these two cases, the device has to recover the master clock from USB frame timings or data rate. This is usually done via a PLL, and its implementation may be good, or not. The clock created by the PC is usually very jittery, so the ability of the PLL to create a clean clock is important.

• asynchronous, where the stream uses a clock independent from the USB clock source;

In this case the device runs its own clock (presumably clean) and sends feedback packets to the PC to keep it in sync. This is the preferred mode for high quality.

In both cases, the influence of USB cable quality on clock jitter seems very dubious. Implementation issues like PLL or local oscillator quality, or noise leaking from the CPU/USB chip into sensitive parts would matter much more. I would not put a lot of trust in claims of USB cables making a huge difference in jitter.