SEASON PREMIERE: Up to now, this PBS series has been somewhat of a disappointment, with both too broad a focus and with too poorly chosen a selection of clips. This season opener, however, may reverse the tide, because its subject is precise enough to allow for an intelligent and in-depth examination. The subject? The transition from standup comic to sitcom premise, a topic that includes Bob Newhart, Bill Cosby, Jerry Seinfeld, Ray Romano, Roseanne Barr, Robin Williams and others. Check local listings.