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BNC cable impedance

From http://www.canford.co.uk/Technical/BNCConnectors.aspx

Impedance

In its original incarnation, the BNC was designed for RF applications and had a characteristic impedance of 50 ohms, together with a predictable reflection factor. Increasing use in applications such as video equipment, with source and load impedance of 75 ohm and with 75 ohm cables, led inevitably to the creation of the 75 ohm BNC.

At frequencies of up to 10MHz, the characteristic impedance of the connector is completely swamped by the source and load impedance of the circuit of which it is part, thus in analogue video applications it is simply irrelevant whether a 50ohm or a 75ohm connector is used. However the increasing likelihood that the connector is being fitted in a digital video application means that it is absolutely essential that 75ohm connectors and cable are used. This requirement is becoming ever more important as the introduction of HDTV requires significantly higher bit rates so the demands on the cable and connector infrastructure increases.
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