Fewer people are playing video games than last year, according to a new report from market tracking firm the NPD Group.

About two-thirds of Americans, 211.5 million, play video games in the U.S. That's about 12 million fewer, a 5% decline, than last year, NPD Says.

The number of mobile and digital gamers grew over 2011, while 2011's largest gamer category, core gamers, who focus on playing traditional console game systems, declined by 2%.

Overall, mobile gamers grew to make up 22% of the gaming population, an increase of 9%. Digital gamers, who primarily acquire games through free and paid downloads, rose 4% to account for 16% of all gamers.

Core gamers dropped 2% to 21% of the gaming population. But core gamers spent the most money on games -- $65 on physical games in the past three months -- than any segment of gamers. On average, gamers spent $48 on physical games and $16 on digital games for PCs, consoles or portables over the past three months.

"Given the long life cycles of the current consoles and the increasing installed base of smartphones and tablets, it's not surprising to see a slight decline in the Core Gamer segment," said NPD industry analyst Anita Frazier. "It's the revenue contribution of the Core Gamer segment that continues to outpace all other segments, and remains vital to the future of the industry."

Gamers are more likely to have spent money on downloadable content or in-game microtransactions than last year, with about 14% so vs. 11% in 2011. Core gamers (27%) and digital gamers (23%) were the most likely to buy digital content.

All other gamer segments had declines: Family+Kid Gamers (15% of total, down 7%); Light PC Gamers (13%, down 2%); and Avid PC Gamers (13%, down 2%).

Gamer segments are shifting, Frazier says. "Looking across the total gaming audience, we see a tremendous impact from mobile gaming, particularly on smartphones and tablets," she said.

The slight decline is not a reason for the industry to panic, she says. "There is so much change occurring in the industry, coupled with the aging console systems," Frazier says. "I do believe new hardware systems will bring a much-welcomed stabilizing force and bring more gamers back to bigger gaming experiences."