Old TV antennas were designed to receive UHF and VHF analog signals. Digital TV goes out on the UHF spectrum, so newer antennas are now optimized for this.
My old rooftop antenna (with amplifier) works great for Digital (and analog) TV. You're fine with an old rooftop antenna, in my opinion, as long as it's not corroded or in poor shape.
Steve H. - KE0KNH
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner