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How many of the extra innings channels will be in HD this year?
MLB Extra Innings uses the GAME and GAME HD channels on your local channel lineup, so at present we can say that you should have the same number of HD channels as were available in the NHL package for your area. New channels may be added at any time, but I can't provide any information on future channel launches.
Local channel lineups can be found at http://ww2.cox.com/residential/tv/channel-lineup.cox and just look for "Game HD" or "MLB / NHL HD" or descriptions to that effect.