text:  When Joe Biden and Donald Trump face off this evening for 90 minutes, it will make history as the first debate between a sitting president and a former president. The debate also comes unusually early in the general election calendar, before the candidates are officially their party's nominees. And in a departure from recent presidential debates, tonight's will be held in a TV studio with no audience and muted microphones when it's not the candidates' turn to speak. Greg Clugston, Atlanta.