A visually impaired person watching CBS’s Survivor cannot see the ousted member’s torch extinguished, doesn’t know what Bart writes on the blackboard in The Simpsons’ opening sequence, and can’t laugh at the antics of Eddie the terrier on Frasier. But thanks to Descriptive Video Service, he or she can understand that the silence on an episode of ExxonMobil Masterpiece Theatre’s “Our Mutual Friend” means that Bella is gazing at the fire with tear-filled eyes after scorning a suitor, that the splashing on Nature signifies grizzly cubs out for a swim, or that Arthur doesn’t look much like a real aardvark. DVS, a service of WGBH, Boston, has been around since 1987, and in those 13 years PBS has been the lone broadcast network to regularly carry programs that are accessible to the blind. By tuning their TV sets to the secondary audio programming (SAP) channel, those who have trouble seeing can enjoy Mystery!, Nova, The Living Edens, The American Experience, Wishbone, Mister Rogers’ Neighborhood and a smattering of other programs in which a narrator fills the pauses in dialogue to describe the characters’ features, clothing, and gestures; setting and scenery; action sequences and other purely visual elements. DVS, which operates under the aegis of WGBH’s Media Access Group, provides video description not only for public television but also for some films shown on the Turner Classic Movie Network, in IMAX theaters, and some first-run film releases, such as Titanic and the upcoming The Patriot.