It's not often you actually witness a TV star being born, but watching the first 13 episodes of NBC's benchmark cop drama allows just that. As dogged Baltimore detective Frank Pembleton, Andre Braugher doesn't even appear until 15 minutes into the pilot. The moment the galvanic actor steps onto the screen, though, he owns it. By the sixth installment, ''Three Men and Adena,'' Braugher pulls off a tour de police force as Pembleton and his rookie partner, Tim Bayliss (Kyle Secor), conduct a fierce, fruitless interrogation of a suspected child murderer (Moses Gunn). The episodes collected on Homicide: Life on the Street -- The Complete Seasons 1 & 2 also offer evidence that the series' original ensemble was its strongest, before such substantial actors as Ned Beatty and Jon Polito were replaced by relative lightweights. The extras are skimpy (e.g., only one commentary track from exec producers Tom Fontana and Barry Levinson, on the pilot), but the shows themselves supply plenty of dramatic heft.