You’re in one right now and you didn’t even know it. The United States is carved up into 210 designated market areas, which are semi-official chunks of TV viewers. Why semi-official? They correspond fairly closely to the FCCs list of television market areas (TMAs,) but no one pays attention to the FCC here… they’re far more interesting in ratings and ad revenue and those come from Nielsen, who created DMAs.
DMA’s are important because of the FCC-sanctioned idea of exclusivity, which says that if you own a TV station, you have the right to demand that no other station in your DMA show the same programming. There can only be one ABC, one NBC, et cetera. Even some of the syndicated programs (like Ellen) are protected by this doctrine. You probably know what your DMA is, as they’re named either for the largest city in the region (like Detroit) or for the two or three largest cities (like Albany/Schenectady/Troy New York.) If you’re interested in how your area ranks, here’s the list of markets by size, courtesy of TVB.
Most of us don’t terribly care what our DMA is; it’s good to get our local news and sports. But some DMAs are really “overdrawn” like the Los Angeles, CA media market which reaches almost to the Nevada border, far beyond where broadcast signals can travel. Even if lowly Baker, CA (home to the world’s tallest thermometer, don’t you know) wanted its own ABC station, they wouldn’t get it — Los Angeles’ KABC has dominion over a huge area about the size of New England, and that includes Baker. Exclusivity means that the folks in Baker are just left out … in the heat (they couldn’t really be out in the cold, they’re in the middle of Death Valley.)
DMAs are the most important part of the TV business, because ratings books are separated by DMA. Yes, people, it’s 2014 and you can still get your ratings in a book if you want. You can also download them if you pay for the privilege. Your ratings book tells you how many people watched your shows last quarter, and that helps you set ad rates. It’s a big fancy racket, really — Nielsen ratings are the only way that ad rates get set, and ad rates are the only way TV stations get money from advertisers. TV stations do get income through other means like retransmission contracts, but those contracts are set based on ratings too.
DMAs dominate the business of broadcasting, and that in turn, dominates how much you pay for pretty much everything. A little scary, right?