Cable Passing Through Metal: What Is It?

[cool dude]

Hi,
My camera USB cable has this cylindrical container close to the end where the USB connects to the camera. When I opened it up, the cable was going through two holes in the metal. The metal wasn't perfectly black, and it wasn't smooth.
What is that piece of metal for? I don't know much about electrical engineering.
Thanks.

 

[chroot]

It's a ferrite choke, to kill high-frequency electromagnetic noise on the line. Arguably, they don't do much under normal circumstances.

- Warren

 

[cool dude]

Thanks. I notice the same thing in other devices as well. Why isn't there one at the end close to the computer?

 

[berkeman]

The choke presents a high common-mode impedance to any RF signals on the cable, so it effectively looks like a break in the line for RF common mode signals. RF common mode signals are what cause radiated electromagnetic interference (EMI), which shows up as noise on broadcast TV receivers and radio receivers. It usually doesn't matter where the "break" is placed in the line for EMI purposes (but not always).