USB

USB-speakers are a well known and very popular example of USB audio.

How it works

The audio is routed to the USB.
This is a matter of choosing the USB audio device in your media player.
The onboard sound card is bypassed, in fact you don’t need a sound card at all.

The USB audio device is your (outboard) sound card.

The USB audio device can be a protocol converter, a USB DAC or a amp with USB input

This is explained in the USB Audio section.

USB cables

Cable length between full speed devices is limited to 5 meters. For a low speed device the limit is 3 meters.

As the signal degrades proportional to the length of the cable, a short cable is often recommended. Other says this put the PC to close to the USB-DAC.

A short cable

Audiophile USB cables

USB audio is gaining momentum.
Like analogue interconnects, now you have audiophile USB cables.
Audiophile USB cables are as controversial as analogue interconnects.
It is hard to imagine that one cable deliver better bits than the other but jitter comes to cable manufacturer’s rescue.

The Limitations of digital audio processors and cables create timing errors known as jitter, which remove portions of the audio signal and replace them with noise and distortion. Cables tend to round off the square waveforms of the signal, making them less clear to the processor, thus increasing jitter. This rounding effect varies greatly among cables and a truly superior digital audio cable can make great improvements in sound quality.
http://www.wireworldcable.com/categories/usb_cables.html

Another manufacturer talking some marketing bull shit?
They do have a point.
Digital is indeed sending fully analogue electrons over a wire.
And indeed, the block pulse degrades with the length.
A good digital cable is one who minimizes this degradation.

This is an easy test.
Connect your high speed USB device, e.g. a hard disk using your audiophile grade USB cable. If hi-speed mode (480 Mbps instead of 12 Mbps, the old USB 1 standard) fails, it is a bad DIY digital cable not even compliant with USB 2 standards.

More details can be found here.

$0.78 - Monoprice USB 2.0 A Male to B Male 28/24AWG Cable - (Gold Plated) - 3ft

€ 11,90 LaCie 1.2 m

$85.00 - Wireworld Starlight 6 (1m)

Silver-clad oxygen-free copper

$125.00 - Kimber Kable MBUSAG (1m)

Pure Silver signal conductors

(6.1%) Silver-plated copper power conductors

(6.1%) Silver-plated copper drain and shield wire

High-Density polyethylene signal conductor dielectric

$1149 - Locus Design Nucleus

1-3ft, $120 each additional foot after three

Do you smell something fishy?

Wireless USB

Today USB can also be done wireless.

Consonance PC Wireless Digital Box

Wireless USB (50m radius)

More about connecting USB devices can be found at USB.ORG.

Extenders

If you have to bridge a long range, there are USB extenders over CAT5 and optical.

Gefen USB 2.0 LR USD 499.00

Extends high-speed USB devices up to 100 m over Cat5 cable.

This sets you back $499.00, a networked low power PC is cheaper.

Opticis M2-100-10 USB Extension Cable

Complete optical electronic isolation with remote power.

Stretch your USB connection with the Opticis Optical USB Extension Cable. Works on all USB 2.0 host system ports at USB 1.1 speeds.

Length from 3 to 50 m.
Extender and galvanic isolation in one.

Adnaco USB 3x 480 Mbps Over Fiber Optic USD 385.00

The Adnaco-R1USB is a remote standard-USB host which can operate at distances up to 250 meters from the location of your computer. Powered by our PCIe over-fiber-optic technology, the R1USB was designed to best suit in many applications to realize the USB host interface. The R1USB provides 3 USB downstream ports compliant with the USB2.0 specification. All downstream facing ports can handle high-speed (480 Mbps), full-speed (12 Mbps), and low-speed (1.5 Mbps) interfaces. Each port supports high-power devices (500 mA). The R1USB is connected to a host computer equipped with the Adnaco-H1 card by a multi-mode fiber optic cable. The R1USB works with Original USB systems, peripherals and cables.

Isolator

There are claims that sound quality improves by having a full galvanic isolation between the PC and the DAC. Some manufacturers of USB DACs use opto-couplers to eliminate the noise from the USB connection.