A way to interface between two machines or signals. In video and audio, coupling can be direct – called DC coupling, through capacitors – called AC coupling, using transformers – called inductive coupling, by opto devices - using optocouplers or using RF (radio frequency) transmitters and receivers. DC coupling preserves the original DC levels of the signals and insures maximum signal flatness. However, erroneous DC levels, if they exist, are transferred between the machines and may cause damage. AC coupling practically isolates the two machines or signals from their respective DC components, but has the problem of poor low frequency response and signal breathing effect. For example, video switchers and DAs may have AC or DC coupling either at the inputs, outputs or both. A smart system allows the user to decide which way the signals should be coupled to and from the DA or switcher. Transformer coupling is problematic in the video field, mainly because of bandwidth considerations, as the transformer should perform equally in all relevant frequencies, and magnetic core transformers hardly ever do so, especially when a true flat response is needed. Transformer coupling is used in the audio field mainly for maintaining proper balanced signals, for impedance matching and for ground isolation - as ground loops may be quite harmful in audio. It is used in video as well for similar reasons. Opto-coupling in video is mainly used to link devices in remote locations, by using fiber optics technology instead of coaxial connections, thus extending substantially the range of operation. RF connection is mainly used in radio, TV and cable transmissions, in microwave links - node to node for ENG and in satellite transmissions. RF modulators and demodulators are needed to maintain the link. RF offers the longest connection range of all systems, crossing continents, oceans and even outer space.
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