Adding a PPA

I'll use the Handbrake video encoder in this example but it will work for any PPA providing you have the correct id string. In this case that is "ppa:stebbins/handbrake-releases"

Add the PPA to your apt repository sources:

sudo add-apt-repository ppa:stebbins/handbrake-releases

If you do an apt-get update now you will probably get an error like this:

W: GPG error: http://ppa.launchpad.net raring Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6D975C4791E7EE5E

Add the key like this, replacing the key at the end of the command with the one from your previous key error output.

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 
6D975C4791E7EE5E

You should be able to update again without errors.