As I've mentioned in previous articles, there really is no such thing as a digital cable. All digital cables are analog cables carrying modulated signals of some kind that get interpreted as bits by the receiver, like a USB DAC. USB has become one standard for sending and receiving musical information encoded in such a way mostly because early music servers used multipurpose computers at a time when most computers supported pretty much only USB; although, before that, there was FireWire and now we have Thunderbolt, too.
One thing that distinguishes USB from AES3-based interfaces (AES/EBU, S/PDIF, and TOSLINK) is that USB cables are designed to carry enough current to power small devices without the need for a separate power supply. Unfortunately, this power can create noise in the form of RFI (Radio Frequency Interference) that can distort the data, causing lost or errant information and/or alter the latency or timing of the signal (also called jitter). Most USB cables don't have any kind of shielding internal or external and nothing of consequence to separate the power lines from the signal or data lines. Very few DAC's offer any kind of isolation on the input (one exception being the Chord Hugo TT, which uses galvanic isolation on the USB 1.2/1.3 input but not on the USB 2.0 input).
What is galvanic isolation? It's when you use a transformer to separate the connection between wires but still carry the current and/or signal, significantly reducing noise entering or exiting the cable. Galvanic isolation often gets used to break "ground loops" that create hum in, say, coaxial cable TV cables; however, it reduces noise of almost all kinds in any applicable cable as well. Passive filters such as the AudioQuest JitterBug and iFi iPurifier 2 can also reduce noise; but they don't use transformers to achieve true galvanic isolation and let a much higher percentage of the noise pass through the cable and into the input device, such as your USB DAC.
IFi makes (or at least made) a very good galvanic isolator called the iGalvanic 3.0that definitely necessitates the use of two USB cables. Plus, a number of companies make simpler but effective USB galvanic isolators mostly for instrumentation and measurement purposes that can be used for audi applications and only requires one USB cable, such as the USB2ISO USB 2.0 Galvanic Isolator adaptor.
To delve a little deeper into galvanic isolation, consider this definition: "Galvanic isolationis a design technique that separates electrical circuits to eliminate stray currents. Signals can pass between galvanically isolatedcircuits, but stray currents, such as differences in ground potential or currents induced by AC power, are blocked."
Put in much simpler language, the good stuff passes through but the bad stuff gets blocked out.
Other techniques to reduce noise include ferrite cores, or cylindrical magnets, attached at both ends of a cable that help to reduce the creation of, and to some extent the influence of, RFI or Radio Frequency Interference.
Again, all of these approaches center on the fact that all digital cables are actually analog cables and will be until someone invents a cable that transmits the bits using quantum mechanics where you actually can have discrete states of energy representing, in an absolute sense, binary data. As long as cables can only transmit and receive continuously variable levels of voltage or amperage (current), all "digital" cables will actually be analog in nature.
Plus, all electrical cables (and electricity is what flows through all analog cables) have to the potential to create electromagnetism, the nexus of RFI, particularly when two or more cables run closely alongside each other in parallel, which is why old-fashioned phone lines used a "twisted pair" of two cables, basically just insulated wires that got perpetually wrapped around one another to prevent the generation of electromagnetic energy. Even more complex geometries using many thin insulated wires braided together, such as Litz cables to minimize the creation of electromagnetism or RFI along with other artifacts, such as resistance at very high frequencies.
I won't, right now, get into the relative advantages and disadvantages of using balanced cables that run signals along two wires 180 degrees out of phase with one another or why a good shield (generally wire mesh "tubing" or even steel pipe) grounded at one or both ends helps to block and drain away RFI; however, both techniques can improve sound if and when applied correctly. Plus, there's the whole concept of a "dielectric", which can get very complex.
More to follow ...