I don't have the option of adding a card due to limitations with my case and need a USB tuner for comcast cable (basic) that supports QAM. I'll be using this in mythbuntu. I've seen a few of them but it looks like they can only do ATSC and not QAM. From what I've read QAM support is what i will need for the few HD channels that are available. Any help would be greatly appreciated!

WinTV-HVR-950Q appears to be one option but it looks like the driver support is iffy.

If your restrictions are only the case, you could also use the HDHomeRun, which is a network tuner.

thanks, I am going to try that :)