Digital marketing continues to grow for TV marketers when it comes to tune-in promotion for TV shows.A study from Mixpo and Promax BDA, by way of eMarketer, says digital advertising now represents 24% of a TV marketer's tune-in advertising budget in 2014 -- up from 18% a year earlier.Digital seemingly gained at the expense of the two biggest ad categories when it comes to TV program promotion. Radio -- still the biggest ad platform for TV marketers -- declined to 30% from 32%, while cable TV advertising dropped to 25% from 29%.Research also noted that in addition to digital, print advertising for TV show advertising also grew 6% in 2014 from 2% the year before. Outdoor advertising slipped to 7% share from 8%.Some 77% of U.S. broadcasting and syndication executives say that one major benefit was that digital tune-in advertising can reach viewers via their mobile devices, according to the study.Estimates are that U.S. adults will spend 2 hours, 51 minutes with their smartphones and tablets, according to eMarketer.More positive news for digital for TV promotion: There is an above-average completion rate for video tune-in advertising on smartphone and tablets -- 87.7% and 82.4%, respectively -- according to fourth-quarter 2013 research by Rhythm NewMedia.