I am looking at getting a DVR. Do I have to use a DVR issued by Comcast? If I purshase one not through Comcast if possible, do I have to get them to do anything to get the signal for cable in order for it to work?
This is not the case. Comcast, like all MSOs, is required to support CableCARD. Consequently, there are two DVRs, currently available for sale in the United States, which you can buy, and they will work with your Comcast service: The TiVo HD DVR, and the Moxi HD DVR.
The TiVo HD DVR is available two ways: You can buy one for about $250 and then pay about $12 per month extra for service on it, or you can purchase it for about $550, and service is included for its lifetime.
The Moxi HD DVR is only available one way. It costs about $700, and service is included for its lifetime.
In each case, you'll need a CableCARD installed by the service provider, to activate it to receive the services that are part of your subscription. Comcast includes the price of the CableCARD in their digital packages.