AdMob's acquisition of fellow mobile advertising firm AdWhile continues to provoke controversy, with Michael Chang, CEO of Greystripe, the latest to join the debate.

"We believe that it is a conflict of interest for an ad network to own a mediation company," he tells ME.

"A mediation company needs to be a separate entity to do its job of unbiased optimising of a number of ad networks. The concerns [around this deal] are most certainly justified."

The acquisition was announced last week, with AdMob promising to make AdWhirl's mediation platform - which lets developers and publishers switch between multiple mobile ad networks at will - open source and transparent, to avoid accusations of bias.

Clearly Greystripe's Chang hasn't been convinced. How does he react to the notion - suggested by AdMob last week but also by others - that many mediation firms aren't open and accountable enough?

"When we work with ad mediation companies like the folks at TapJoy, we make sure that the developer still maintains a direct link to Greystripe, both with reporting as well as a direct billing relationship," he says.

"In this way, the developer always knows where they stand and everyone is transparent, open and accountable."

As for Greystripe itself, Chang says the company is currently serving ads into more than 1,000 mobile games from more than 400 developers, and working with brands including Kia, Navy, Universal Pictures and Burger King.

"We are also building deep relationships with publishers like EA, MobilityWare, Amplified Games etc, because they trust us with their content," says Chang.

"They know that only the best advertisers will be paired with their valuable content."