Digital TV converter boxes only work for over-the-air antenna TV reception, transmitted in "ATSC" format. They are specifically designed NOT to decode digital cable signals, which are transmitted in "QAM" format.
Note also: most new TVs manufactured since 2007 have a "Clear QAM" tuner, which MIGHT receive some of the "non-encrypted" QAM channels in your local Comcast market. Comcast personnel will not provide you any technical support for using "Clear QAM" tuning, as they prefer that you rent one of their set-top boxes. In any event, "Clear QAM" channels are typically the "limited basic" tier, i.e. the standard-def versions of your local CBS, ABC, PBS, Fox, PBS, CW, and independent OTA channels, plus a few public, educational and government channels. In many markets, the high definition feeds of those channels are encrypted.
Keep in mind that most of Comcast's cable channels (CNN, ESPN, Root Sports, Fox News, etc.) are "encrypted" QAM, so the "Clear QAM" tuner won't allow you to view those signals.