When you watch a long DVD movie, you may notice a hiccup two-thirds of the way through, when the DVD player's laser jumps from the first layer of the disc's data to the second. Most DVD videos are one-sided, two-layer discs that can store the film plus extras and cinematic audio, but DVD burners for computer or home theater have until now recorded only onto single-layer discs. Sony is first with a two-layer burner, the $199 DRU-700A for Windows PCs, while Iomega and Dell are about to introduce products with double-layer DVD burning. Since a one-layer disc generally fits less than its promised two-hour maximum (owing to data taken up by the disc menu and variables of video compression), the double-layer discs are the only ones that can guarantee the room required for, uh, personal copies of feature-length films. But some players have trouble recognizing the new format, and the discs cost $12 each more than many DVD movies on sale.