The DTA only provides a lower-quality, standard-definition (SD) Ch. 3/4 RF output, while your digital cable box is probably providing a high-definition (HD) output via an HDMI or component video connection, so the DTA's picture will therefore look "fuzzy" in comparison.
However, Comcast has started to roll out an HD version of the DTA now for use with HDTV sets: »
Comcast HD Digital Adapter Video