Once upon a time, somebody suggested to producer Steven Bochco that his television series "Hill Street Blues" would make a great Broadway musical.

That never came to be, but now we have the new ABC series "Cop Rock," which is as gritty and strong as "Hill Street" ever was - and adds a musical element as the characters have a tendency to burst into song.The crack dealers do a rap number. The jury does a gospel number. The mayor does sort of a show tune. A husband and a mother do plaintiff ballads.

There's just music busting out all over. And quite a bit of it is quite good.

And the straight dramatic segments of the show are certainly up to anything on TV today, although they're nothing you'd want your young kids to watch.

The major problem is that there's just too much contrast between the two genres. It's one thing to have a light, cheerful musical. But mixing music with drugs, crime and murder just doesn't work.

Several of the numbers are so jarring in their context that it's almost ludicrous. You don't know whether to laugh or simply watch in stunned amazement.

Almost lost admist all the hype about the music is that this is an extremely violent, often tasteless show. In Wednesday's pilot, a cop is gunned down by a drug dealer, a drug dealer is out-and -out murdered by a police detective, and a drug-addicted young mother sells her baby.

So how did this show end up on the air? Ther are two reasons.

First, networks are looking for anything new and different to attract viewers. This is a gamble along the lines of "Twin Peaks."

Second, this has Bochco's name attached to it. In addition to "Hill Street," he's produced "L.A. Law" and "Doogie Howser, M.D."

He's become such a power in Hollywood that he can do almost anything he wants.

And, as "Cop Rock" demonstrates, that kind of power can be dangerous.