But ESPN won't specify who they are. But this article speculates HULU because ESPN is part owner of HULU. This is from John Ourand, who has always been skeptical about the ACC Network.

These passages are worth note;

Magnus said negotiations have been helped by the ACC’s on-field successes, which include the two most recent national championships in football (Clemson) and college basketball (North Carolina).

“The advantage the ACC has over most, if not all, conferences is they couple high-quality football with the highest quality men’s and women’s basketball,” Magnus said. “It’s pretty formidable.”