From The TV IV
Bandwidth is a selection of frequencies used to transmit data over an electromagnetic wave. Depending on the method in which the carrier signal is modulated, the amount of data that can be sent over the frequencies varies. An analog signal doesn't have as many modulation methods as a digital signal, allowing digital transmissions to send more data through the same amount of bandwidth. For example, an analog 480i TV signal requires the same amount of bandwidth as any HDTV signal.