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When using a DVB-T tuner the option 'Add Missing Channels' is missing under Settings > TV > Guide.
Digital TV channels cannot be manually added to the Media Center Guide so this option is not displayed. Also analogue channels (such as a VCR output) cannot be used once you have selected to use a Digital signal. This is the case even if the tuner card you have is a hybrid PAL/DVB-T or you have a stand alone analogue tuner.
There is no work around to this issue in Media Center if you wish to receive DVB-T without using a Set Top Box. If you have an analogue capable tuner card you can access analogue channels by using 3rd party TV software or reconfigure Media Center to use an analogue signal through Settings > TV > Set Up TV Signal.