The Toshiba model in question is a Tivo / DVD Player combo, so you would find a zillion threads about it in the Tivo forums, not this one.
It is a standalone PVR, encoding from analog sources only so it will not record in HD. It doesn't "convert the signal to analog first"; you have to feed it an analog signal to begin with. In order to record from a HD channel, you would need an HD cable box; the cable box would convert the signal to analog NTSC (480i), the Toshiba then would take that, digitize & MPEG encode it & store to disk.
If you want to actually record in HD you need a integrated digital tuner/DVR, such as the Motorola 6208/6412, SA 8000HD for cable, or the DirecTV HR10-250 HD-Tivo model for satellite/OTA.