THE DISTRICT of Columbia is learning the difference between cable franchises and other methods of television transmission: when a city moves to cable, what you see is never quite what you get. The picture keeps shrinking. There's a point at which the only thing to do is to turn the whole deal off -- and that is what the D.C. Council may have to do with District Cablevision, Inc., unless the picture gets a lot clearer in the next few days.

Last week, DCI President Robert L. Johnson insisted that his latest request for more concessions was not a "threat" but a statement of financial fact: without the modifications, his firm won't be able to do the job. He's got a point, but it may not be in his favor. The city should figure out exactly what it must insist on and, if DCI can't deliver, it should call off the negotiations and seek damages. That's not a threat, either. It's sound policy.

It is not the District government's responsibility to protect a franchise holder from the possibility of financial collapse or bankruptcy proceedings. If DCI or its chief would-be bankroller, Tele-Communications, Inc., of Denver, can't live up to a reasonable facsimile of what it agreed to do in the first place, then the city government has no choice but to look elsewhere, no matter that there is a tendency of all cable bidders to overpromise, to relinquish control to a parent firm and, in some cases, to fail to come up with the cash.

As Council Chairman David A. Clarke said, "We don't have the whole picture. I'm not signing any blank check with my name that doesn't involve all constituents getting cable." If DCI and TCI are serious about fulfilling their responsibilities, they should put all of their cards on the table now. Then, if every last penny or penalty can be spelled out in no uncertain terms -- with firm deadlines and cash up front -- perhaps a workable agreement can be struck. The pressure is on, but it isn't on the council. It is on DCI.