WASHINGTON — For the fourth time in two weeks, broadcasters sped up their timetable for the rollout of digital TV, but it is still not clear if their frantic efforts are enough to please FCC chairman Reed Hundt, who wants at least three digital signals in the top 10 markets within the next 18 months.In a proposal sent to the FCC Thursday, broadcasters promised to deliver three or more digital signals in seven out of the top 10 markets in the United States within that timetable. The latest offer from broadcasters is a dramatic improvement from their first offer which, after two years, left Los Angeles as the only market with three or more digital TV signals. It had been widely expected that on April 3, the FCC would officially hand out an extra TV channel to every sta-tion in the country. Those channels will eventually be used by broadcasters to deliver a digital signal with CD-quality sound and razor-sharp television images in a new widescreen format Hundt’s aides at the FCC maintain that unless the nation’s top stations begin broadcasting digital signals in the next 18 months, TV set makers will abandon their efforts to put the next generation of sets in stores in time for the Christmas season of 1998. While lawmakers such as Senate Commerce Committee chairman John McCain (R. Ariz.) have supported Hundt’s bid for the FCC to draft a rule setting a swift timetable for stations’ switch to digital, other FCC commissioners aren’t in agreement. On March 27 the FCC published its agenda for the Thursday commission meeting and digital television was not included. However, FCC staffers and industry lobbyists remain optimistic that a compromise can be finalized in time to hold the final vote Thursday.