If after you have everything set up properly, you find that your pcHDTV card will not capture Analog NTSC signals properly, you may need to rebuild the card's drivers from newer source. As of 2008 Feb 14, Mythbuntu's 7.10 kernel still contains code which will give you problems switching back and forth between NTSC and ATSC. Mythbuntu 8.04LTS, on the other hand, does contain the newest code and, furthermore, compiling and installing the fixes shown herein will break the DVB drivers. So, if you have 8.04LTS and it still does not work, you have the s.o.l. blues. Basically, this card is a piece of crap when it comes to Analog NTSC (many people in the pcHDTV/Ubuntu forums complain of it not working). If you still want Analog NTSC (now that the rest of us have moved on to HDTV), maybe you should try the Haupaugge WinTV-PVR-150 cards.
Anyway, under the earlier releases of Mythbuntu, to fix the DVB drivers, you need to compile the v4l-dvb tree yourself and install it over your current kernel modules. Follow these steps:
sudo apt-get install gcc linux-headers-`uname -r` mercurial
cd your/build/directory
hg clone http://linuxtv.org/hg/v4l-dvb
cd v4l-dvb
make
sudo make install
sudo reboot
The second step should change to your local build directory (whatever that is), where the third step will clone a copy of the v4l-dvb build directory. If you want to put this in your home directory or some other inncuous spot, the build can be done without root priviledges. You only need root priviledges to install the resultant build.
Once the build/install is done, you need to reboot for the new drivers to be loaded. It should fix problems with the pcHDTV card. It is known to work on Mythbuntu 7.10, for example.
One more thing. Should you apply these changes to your system and they hose the DVB drivers, you can recover by firing up the package manager and reinstalling these packages:
linux-image-ver-generic
linux-ubuntu-modules-ver
Note that, despite what the comments tell you about using linux-image-generic instead, reinstalling it won't cut it because it does not force the dependant packages to be reinstalled (droppage). You must pick the two packages mentioned above, with the highest version numbers, and reinstall them individually.