Seattle-based online video advertising software maker Mixpo reported Tuesday that it is offering a free trial of its VideoAd platform. Mixpo said it is extending the offer for a limited time to local television broadcasters and cable operators but did not specify the length of the trial or offer period. The free trial, Mixpo explained, covers standard fees associated with creating, delivering and optimizing a VideoAd campaign. Founded in 2005, Mixpo has raised $10.5 million in venture financing. Its backers include the Madrona Venture Group, Yaletown Venture Partners, Growthworks Capital Ltd., the Working Opportunity Fund and SpringBank TechVentures. Mixpo's flagship product, the VideoAd platform, helps advertisers upload photos, videos, music and other items to create and run their own video advertising.