HD service requires an HD converter to accept the different types of signals that are transmitted with this programming. These converters look similar to standard converters and function the same way. There is no additional cost for an HD converter.
An HD converter gives you access to additional channels that are only available in HD. The standard definition channels you watch with a regular converter are still displayed in standard definition, but many of these channels have a separate HD version on a different channel number. These HD channels become available if you have an HD converter.