Partly due to FCC rules, television manufacturers include three types of tuners in equipment that is sold as a "television".
The tuners include:
Analog (NTSC) for Over the Air and Cable
QAM (digital cable)
ATSC (digtal over the air)
Those rules do not apply to accessories such as tuner cards, DVRs, monitors, etc.
The DTV PAL is equipped with only an ATSC tuner, that makes it practically impossible to connect an analog source to it. At present there is no consumer grade analog to digital modulators on the market. My understanding is that it's a combination of economics and politics that block the development of a consumer grade NTSC to ATSC modulator.
Hauppauge
http://www.hauppauge.com/ offers a variety of solutions that include support for ATSC and NTSC reception and recording.