Shielded twisted-pair
Overview
The term "shielded twisted-pair" is used to describe a type of cable used for balanced analog interconnects and certain digital interconnects; most commonly an AES3 interconnect. The two signal carrying conductors are twisted together to increase noise immunity, and an over-all shield encloses the twisted pair of signal conductors.
Basics
There are basically two types of shielded twisted-pair cable used in audio:
- Analog
Analog shielded twisted-pair cable is typically constructed with 22-26 gauge wire in the pair. For line level audio, 22 gauge offers low enough DC resistance for long cable lengths, while the smaller 24-26 gauge wire is more often used for low-level microphone cables. The shield is connected to ground to provide further immunity from induced noise as well as a low-impedance path for induced currents to return to a place where they will not be combined with the audio signal.
The audio signal appears with equal amplitude but opposite polarity on the pair conductors, which allows the differential amplifier in a balanced input to amplify the desired audio signal while cancelling out the vast majority of induced noise. In some exceptional cases; this type of cable is used to carry unbalanced audio in adapter cables, because it allows separation of the shield and the audio signal return conductor for increased noise immunity.
- Digital
The AES3 digital audio standard calls for 110 Ohm shielded twisted-pair cable to be used with XLR connections. Because the digital audio signal contains frequencies in the megahertz; the electrical characteristics of the cable are much more critical than at audio frequencies. The characteristic impedance of the cable must match the termination at both the sending and receiving end of the cable. Various methods are used to keep the twisted pair at a constant distance from each other, including twisting two non-conductive fibers with the conductors. The signals appear with a voltage of 2-7 volts peak-to-peak and opposite polarity; and signal transformers are used at the sending and receiving ends. These transformers provide additional noise immunity by limiting bandwidth to many octaves above AC power “hum” frequencies and removing a path for DC currents.