The puckish god of cable has a knack of giving us essentials that we had no inkling we needed. The notion of a 24-hour all-sports, all-news or all-rock channel was improbable a decade ago. But imagine life without ESPN or CNN or MTV now. Well, get ready to yuck around the clock, because the HBO folks are putting the Comedy Channel on-line this week. (MTV will unveil its all-comedy channel, Ha! TV, next spring.) On the Comedy Channel a crew of hosts (CJs?) will introduce classic comedy films, videos, clips and stand-up routines, as well as original programming. If it follows the pattern of other specialty carriers, the newcomer will take a while to establish its style and compile enough resources to fill the day. Right from jump street, though, the Comedy Channel is a tempting rest stop during zapper migrations. Let’s keep an eye on it.