ANYONE ADDICTED to Grade B detective movies or paperback police procedurals knows how a confession is taken. When the villain finally breaks, the lieutenant tells his sidekick, the rumpled and overweight sergeant, to "get your notebook and take this down." Eventually, at 3 a.m. and after many cardboard containers of coffee, the sergeant types out the confession on an old manual Smith- Corona, and the murderer signs.

What seldom happened in the movies but what frequently happens in real life is that the accused has second thoughts about the confession. If he wants to change his story, he can claim that it was coerced, or that he was drunk when he gave it. Or he can deny that he was given his Miranda warnings or had waived his right to a lawyer. The police in New York City have come up with a way to avoid these challenges to the validity of a confession, and it has proved remarkably effective. They videotape all confessions in homicide and other serious felony cases.

The project began eight years ago with a $95,000 federal grant that was used to purchase equipment and hire two cameramen. When the tape begins to roll, the suspect is seated in front of a clock that runs through the entire sequence, assuring the viewer that the tape hasn't been altered. At the beginning of the interview, the Miranda warnings are read and waived by the suspect. The camera records his demeanor and physical condition as well as his own story detailing his participation in the crime.

The procedure has been extraordinarily successful, according to a story in The New York Times. Most defense lawyers who view the taped confessions are able to persuade their clients that a guilty plea makes more sense that an attempt to repudiate the confession. Those who go to trial learn the hard way, says the Bronx district attorney who started the program. For, once the tape is shown to a jury, a conviction is obtained in virtually every case.