When Comcast launches its worldwide rival to Netflix in 2020, one key difference will set the two services apart: digital advertising. While some international audiences can expect a Netflix-style ad-free experience, the cable giant’s initial efforts will focus on signing up more than 50 million U.S.- and Britain-based customers for the ad-supported version of the app, Comcast executives said Wednesday on a company earnings call. “Between the United States and Sky in the U.K., we think we should start and try to gain as much scale as possible with an ad-supported free consumer service,” said Steve Burke, chief executive of Comcast-owned NBCUniversal. The app will be free only to Comcast’s existing TV subscribers; others will be asked to pay a fee.