Well, that's another matter. If what you're talking about is grabbing like Over The Air (OTA) TV that comes freely, then you'd need an OTA tuner.
Or are you saying you have an OTA tuner already? If that's what you're saying, that you already have an OTA tuner, and that it has composite (YRW) and component (RGB+RW) on it, then you need a capture card.
Obviously component is preferred over composite basically always, but, for standard def, I'm not really sure how that matters.
Which is it you're wanting?
Chuck