Comcast Corp. is experimenting with a "double down" for its main high-speed-data product in two test markets, opening up customers' downstream throughputs from 1.5 megabits per second to 3 Mbps at no extra cost.

The MSO has been running an open-ended test of the turbo-charged service in Knoxville, Tenn., for about one month, and it has recently added Pittsburgh for a 30-day trial.

The tests are part of Comcast's efforts to peer around the broadband-service corner, according to Greg Butz, vice president of marketing and business development.

"Really, what we are focused on here is to prepare for what we envision as the future broadband experience — how we think folks should expect a broadband Internet experience to work," he said.

"When you think about that experience you are trying to create and what people expect from a broadband experience, and where we are evolving with content and applications, that is what this is about," he added.

The response among Knoxville customers has been positive, according to the MSO.

Customer messages praised the increase, but Comcast is mum as to whether it will expand the 3 Mbps service beyond the two market trials.