If your overlaying text on live video then your probably going to need some external hardware no matter what microcontroller you use. I have seen cheap(ish) SD (standard definition) versions (http://www.decadenet.com/bob4/bob4.html) but not HD (high definition) versions. Only cheap(ish) thing I can think is to use a Raspberry-Pi and purchase a standalone HD DSK (Down Stream Keyer) or a PC with a Decklink Studio (DeckLink | Blackmagic Design) I'm sure there are probably dedicated video overlay chips that someone else can suggest.