The traditional usage of 50 Ohm BNC connectors on 75 Ohm cable in analog video and analog telephony central office equipment has little distortion effect on the signal at frequencies below 300 MHz.
However, digital signals in video and telephony applications have necessitated the usage of 75 Ohm connectors with 75 Ohm cable. Mismatched connectors cause attenuation of the digital signal resulting in slower rise time of square waves. This distortion of the signal can cause transmission errors. Also, impedance mismatched connections cause reflections of the signal returning to the source known as return loss. Perhaps one mismatched connection will not have a noticeable effect on system performance, but multiple mismatched connections in the link between the source and destination have a cumulative effect causing possible distortion of the transmitted signal.
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