Online video ad insertion and tracking firm ScanScout has raised $5.1 million in the first tranche of a third round of funding, per MHT, citing a regulatory filing. The Boston-based company plans to raise $8.1 million in total once the round is complete; new investors were not disclosed, but ScanScout has raised $9 million over two rounds from General Catalyst Partners, First Round Capital and a group of angel investors. Time Warner (NYSE: TWX) also took an undisclosed stake in the firm in 2007.

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Founded in 2005, ScanScout’s technology scans through video clips to make sure that the content is “brand-appropriate”; it also runs a contextually-targeted video ad network. Former About.com co-founder and CEO Bill Day joined the company last September as CEO.