Before we start, let's be clear that this is not a tutorial in how to build an oscilloscope. Tennis for Two is supposed to display on a 'scope, so beg, borrow, or buy one if you don't have one handy. Older low-end analog scopes like mine (a Hameg!) usually go for $50-$150, and if nothing else, you can always make a Scope Clock out of it later.
There are three parts to the electronics that we're building. First, there is the AVR microcontroller-- the brains of the outfit. The specific variety that we're using is the ATmega168, the same chip used in (for example) the Arduino platform. Secondly, there are two handheld controllers that connect to the ATmega168 microcontroller. Each handheld controller has a knob and a button. Third, there is the digital to analog converter that takes the output from the AVR and uses it to drive the scope.