A filtered D-sub is a connector that has been designed to reduce high-frequency interference from EMI (electromagnetic interference) or RFI (Radio frequency interference). The simplest way to achieve this is through a C or feed-through filter which incorporates a single capacitor placed between the ground and the signal line of the contacts. This then acts as a low pass filter meaning low frequencies are able to pass through with a minimal effect on the signal strength while the higher frequencies relating to EMI and RFI are attenuated. This means that any interference from inside a product is not released out into the surrounding environment and/or any external interference is not transferred to the inside of the product.