USB Cable Design

High End Audiophile USB Cables

electro and magnetic flux around wires

There are many ways to design a USB cable. The USB 2.0 specification is ostensibly a set of recommendations. Indeed, some design principles are better than others. The basic design requirement is that a USB cable should communicate the digital signal traversing it faithfully and without signal degradation. For most computing purposes, an average consumer-grade cable will be successful in doing this. Not so with digital audio. The digital audio flow model is unique, and it's not the same as your mouse or keyboard. Computer audio (CA) is a whole different beast.

As described in other articles on this web site, audio USB is a stream that traverses the cable in real-time, without the ability to request a resend if there are errors. See the article about the USB Specification. There are numerous things that can cause errors to the digital stream; and a poorly designed cable will sound differently than a better designed one.

The challenge of getting our hi-fi music data faithfully from the music server to the DAC involves a complex set of engineering principles. The responsibility lies with the computer, the USB cable and the DAC. We will address only the cable itself in this article. The data flow model for USB audio is called isochronous communication. What that means is that the digital square waves traveling down the pair of data wires in a USB cable are sent in a carefully timed pattern. While the data is carried in packets, it essentially is a continuous stream with a few bits set aside for the protocol and data organization. The villains in this scenario are noise buildup and timing jitter.

The job of the cable architecture is to protect the data from outside noise, not introduce noise or jitter of its own, and facilitate the endpoint firmware to reduce any noise that may have accumulated. This is ultimately not too difficult to accomplish. Especially if good digital engineering principle are followed, and careful attention is given to the electromagnetic compatibility of the design. A few basic guidelines should be followed by the user of the cable also, such as not to route the cable next to unshielded power cables, or near the inductive field of a transformer.

Remember these two important points: a.) isochronous means 'equal time' and b.) noise is unwanted voltage. These two issues must be addressed in the cable design to insure that the digital data reaches its destination on time and free of noise.

While USB provides for 5 volt power distribution, we ideally want to eliminate that feature if possible. If we must include a 5 volt lead in the cable, then it should be shielded and that shield should be grounded on one end. The specification calls for shielding the two data leads, also. This is not strictly needed, and in fact should be avoided. There are numerous reasons for this. The positive and negative data wires are supposed to be twisted together. This helps to eliminate inductive noise from building up in the data wires, and provides a method of keeping the leads in close proximity to each other. Metal shielding of the data wires would only confine the electric field emanating from them, thus contributing to the accumulation of noise voltage into the data. While unshielded data leads may not meet USB specifications, for the short distances, this should not be an issue. Unless you are operating radio receivers in the immediate vicinity, in the megahertz ranges, such as in Ham radio.

The twisted pair of data wires need to be exactly equal in length, and symmetrically twisted. If the twisted form is not carefully achieved, there will be timing inconsistencies in the form of signal phase shifting. This will cause digital jitter, or a smearing of the audio sound that we hear. While careful consideration of wire metallurgy is important, it is not as important as wire architecture and wire gauge. A high quality wire, preferably silver coated, of a sufficient gauge, is important to provide a good pathway for current travel. The digital signal is a timed sequence of square waves. Each pulse rise or fall is transcribed as a digital '1' or a '0'. These pulses are a voltage in the range of 3.2 to 3.5 volts. Since noise is a voltage itself, which is caused by stray inductance, the buildup of capacitance or a combination of both (EMI), that voltage combines with the data signals. Given that accumulated noise is inevitable, it's important that the noise infiltrates each wire in the twisted pair equally. This way, the DAC can better filter out that noise current, and recapture the original square waves to decode them into sample values.

USB cables are better when they are shorter because of the possibility of accumulated noise. Therefore, choose a cable that is sufficiently long enough for your application, but not significantly longer. Data traversing a cable can be subject to reflection, or bounce back of the signal. However, reflection is much less important than reducing electric and magnetic interference (EMI). The specification suggests wire gauges in the range from 20 AWG to 28 AWG. We feel that a thicker gauge, with accurate twisting will provide the best balance of sound. Data wires that are too thin tend not to preserve bass frequencies, and wires that are very thin will cause music to sound tinny. Some listeners may perceive that as more detail, but after prolonged listening it can be fatiguing. Of course don't forget we are talking about digital signals, and not analog. So any changes in audible sound is due to how accurate the digital signal has been transported and decoded on the receiving endpoint. Think square wave pulses and mathematics, and not analog sine waves.

There are many DACs on the market, and some require 5 volt power to handshake with the computer and or operate, and others don't need it. Therefore it's very useful to provide a variety of cable designs. The two cables described elsewhere on this site, allow a user to match the needs of their music system to the best cable configuration. One should be able to choose a cable with or without the 5 volt Vbus, or to disconnect the Vbus and add their own high quality 5 volt supply for their DAC.

The wire dielectric should be as inert as possible, so that it does not store or conduct electricity and thus has a low dielectric constant. Short of having no dielectric at all, which actually isn't a good choice due to oxidation, Teflon (PTFE) is a really good choice. The cable connectors should be of high quality, and ideally have gold plated contacts, if not connector housings. The gold plated connectors are primarily to suppress oxidation, which would interfere with making good electrical contact at the fitting. Remember that gold is less conductive than silver, and silver is slightly more conductive than copper. But, gold is the best choice for resistance to oxidation and corrosion.

Finally the cable should be adequately flexible so as not to stress USB connectors in the DAC or the computer. The finishing touch is for a cable to look classy when sleeved with a quality cloth covering. We consider the USB cable to be an integral part of a hi-fi computer audio system.

Ken Matesich