Search

Termination has two meanings in the context of audio. One application is simply the type of connector applied to a cable or wire, so a cable might be terminated in a 3-pin XLR plug, for example.

The second application refers to the electrical characteristics of some specific types of audio interface where the cable acts as a 'Transmission line'. This typically occurs where the cable is long relative to the wavelength of the signal it is carrying, so it most commonly applies to digital audio, video signals, and radio-frequency connections — although it can also apply to analogue audio cables when more than a few kilometres long.

When a cable operates as a transmission line it is said to have a 'characteristic impedance' — essentially the impedance presented at its input if the cable was infinitely long. To transfer a signal along the cable efficiently, it is vital that a end of a transmission line is terminated with the same impedance as its characteristic impedance. If it is left unterminated, the end of the cable behaces like a mirror, and signal energy is reflected back along the cable where upon it will interfere with the source signal. 

S/PDIF and Video cables normally require a 75 Ohm termination. RF cables usually require either 50 or 75 Ohms, depending on their construction. AES3 requires a 110 Ohm termination and RJ45-style Ethernet cables require 100 Ohms.

The ‘resistance’ or opposition of a medium to a change of state, often encountered in the context of electrical connections (and the way signals of different frequencies are treated), or acoustic treatment (denoting the resistance it presents to air flow). Although measured in Ohms, the impedance of a ‘reactive’ device such as a loudspeaker drive unit will usually vary with signal frequency and will be higher than the resistance when measured with a static DC voltage.

Signal sources have an output impedance and destinations have an input impedance. In analogue audio systems the usually arrangement is to source from a very low impedance and feed a destination of a much higher (typically 10 times) impedance. This is called a ‘voltage matching’ interface. In digital and video systems it is more normal to find ‘matched impedance’ interfacing where the source, destination and cable all have the same impedance (eg. 75 Ohms in the case of S/PDIF).

Microphones have a very low impedance (150 Ohms or so) while microphone preamps provide an input impedance of 1,500 Ohms or more. Line inputs typically have an impedance of 10,000 Ohms and DI boxes may provide an input impedance of as much as 1,000,000 Ohms to suit the relatively high output impedance of typical guitar pickups.