http://en.wikipedia.org/wiki/USB I think we need to pay attention to USB 3.0 as a must have for audio interfaces and USB bus powered controllers. The above article has many very interesting point about things that apply to our needs. Most is way down the page when they start talking about USB 3
USB signals are transmitted on a
twisted-pair data cable with 90
Ω ±15%
characteristic impedance,
[99] labeled D+ and D−. Prior to USB 3.0, these collectively use
half-duplex differential signaling to reduce the effects of electromagnetic
noise on longer lines. Transmitted signal levels are 0.0 to 0.3
volts for low and 2.8 to 3.6
volts for high in full-bandwidth and low-bandwidth modes, and −10 to 10 mV for low and 360 to 440 mV for high in hi-bandwidth mode. In FS mode, the cable wires are not terminated, but the HS mode has
termination of 45 Ω to ground, or 90 Ω differential to match the data cable impedance, reducing interference due to signal
reflections.
USB 3.0 introduces two additional pairs of shielded twisted wire and new, mostly interoperable contacts in USB 3.0 cables, for them. They permit the higher data rate, and full duplex operation.