Ferrite bead in cable or in device?

Question

I am wondering, I made a MIDI circuit and was advised to use ferrite beads in my device for MIDI input/output. It is a similar to the following circuit (only the L1/L2 ferrite beads are important for my question):

However, in most USB cables, there is a ferrite bead inside the cable.

If it is advised to use ferrite beads with MIDI, why aren't ferrite beads standard inside the cable? Or otherwise, why do USB cables use ferrite beads, and they are not within the USB device/adapter?

Below is an example of a USB cable having a ferrite bead (I assume) on both sides; btw, most generic USB cables do not have them, this cable is for charging a mobile telephone.

Answer 1

Ferrite beads around a cable raise the impedance for common mode signals. That is the main reason that they are external to any device, on the cable.

Common mode signals can be present on differential pairs and in this case a common mode choke inside a device would suffice. But common mode signals can just as well be present on e.g. the ground signal towards the cable. In that case only a bead around the whole cable section will help.

And in case you are not aware: only a few microvolts of common mode signal suffice to violate EMC compliance rules in case they are present on a cable. This is because the device ground (e.g. metal case or PCB GND plane) in combination with the cable combine to an almost perfect antenna.

Answer 2

A USB cable sends a single serial data stream and therefore the ferrite Core near the input suppresses any Electromagnetic Interference. Midi-Din Cables are Balanced and run one machines expecting balanced inputs so any EMI is filtered out by the input circuitry.