There has been a lot of talk in the past year about a renaissance in podcasts, particularly in the wake of “Serial,” which helped bring new listeners into the decade-old medium. But as CMO Today reports, advertising dollars have yet to flow into podcasts in a big way, as many big brand marketers remain unconvinced they will get their money’s worth. For now, much of the roughly $35 million in podcast ad spending is comprised of direct-response advertisers that hawk promo codes and can more easily track their return on investment. In order for big advertisers to take the plunge, podcast insiders say, the industry needs to scale up and sort out a series of measurement issues. For one, the industry transacts based on “unique downloads,” but it’s unclear to marketers whether people actually listen to the ads in the podcasts they download. Call it listenability, or podcast’s “viewability” problem.