The use of this technique is therefore best reserved for critical inputs likely to be fed from long cables. Serial digital interfaces using coaxial cable normally employ a coding technique which removes DC and audio frequency components from the data. The power frequency and other low-frequency noise due to an earth loop is therefore outside the frequency range of the wanted signal and can be rejected by filtering. Given that digital video processing involves both useful and spurious RF signals in the 100 MHz to 500 MHz frequency range, the EMC need to earth the screen directly to the case of the equipment is overwhelming. We therefore commend the following principles to the industry.
Cables have direct connection to the screen available at both ends.
Equipment output connectors always bond the cable screen to the equipment s case. Equipment input connectors preferably bond the cable screen to the equipment s case but otherwise provide a low RF impedance connection. Signal outputs are always low common-mode impedance.