But TV stations and wireless-microphone vendors raised concerns about interference even as the FCC conducted tests on prototype devices. The NAB said the FCC tests gave no proof that their signals would be protected from interference. Wireless microphones have long operated in the TV spectrum without FCC licenses, and microphone vendors raised many of the same concerns as TV stations.

During FCC tests, a handful of prototype devices failed, but they generally stopped working for various reasons and didn't cause interference.

Microsoft applauded the FCC's report. "We urge the commissioners to come to a decision quickly and adopt rules that will allow all Americans to realize the full and enormous potential white spaces have to expand broadband access in underserved, urban, and rural areas and to enable a new wave of innovation and Internet services and products," Anoop Gupta, Microsoft's corporate vice president for technology policy and strategy, said in a statement.

All five devices submitted to the FCC for a second round of testing, starting in October, "were able to reliably detect the presence" of a clean digital television signal, the FCC report said. The detection sensitivity varied considerably, however, the report said.

The report noted other problems as well. The devices often had difficulty sensing TV signals in adjacent channels. "This could impact significantly the ability of the devices to reliably detect TV signals within stations’ service areas," the report said.

The FCC tested for wireless-microphone interference in a laboratory and in field tests. It tested three devices for interference with wireless microphones during field tests at a professional football game in Maryland and a Broadway play in New York. While the devices were able to detect wireless-microphone signals when no other signals were present, the "detection threshold sensitivity of all of the devices was severely impacted" when a TV signal was nearby, the report said.

A prototype device from Philips indicated that channels were occupied even when they weren't, and a device from the Institute for Infocomm Research "indicated several channels as available even when the microphones were on," the report said.