eMarketer’s latest ad spending forecast projects continued, if moderate, growth for U.S. TV ad spending, which will remain larger than digital ad spending through at least 2017. But digital video ad spending is growing much faster, especially on mobile devices, it says.

According to eMarketer’s forecast, U.S. advertisers will spend $66.35 billion on TV (broadcast and cable combined) this year, up from $64.54 billion in 2012 and set to rise to over $75 billion by 2017. That’s a compound annual growth rate of 3.7% between 2011 and 2017 – far short of the growth of digital advertising spending, but enough to keep total television ad dollars far above the amount going to the entire digital ecosystem, including all ad formats served to PCs and mobile devices.

Digital video ad spending is growing fast, though. eMarketer estimates spending on video ads served to PCs and mobile devices will reach $4.14 billion this year, more than twice 2011 levels. By 2017, spending will more than double again, to $9.06 billion.

Much of that growth is coming from mobile, including tablets. Mobile video will account for just 12.6% of all digital video ad spending this year, or $520 million. But it’s growing much faster than desktop-based digital video ad spending, at a pace of 112.4% vs. 35% for online video this year. eMarketer expects growth for both types of digital video to moderate in coming years, but predicts the mobile portion of the pie will continue to increase at a faster rate. By 2017, 29.7% of all digital video ad spending will go toward mobile ads (including ads served to tablet devices).

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