The video version of Sweeney Todd is the show which The Entertainment Channel hopes will make pay-TV consumers sit up and take notice of this new cable service, which is having some trouble getting to market. Potential viewers must first convince their local cable system operator to carry this service.
The RKO/Nederlander production of ''Sweeney'' was shot in Los Angeles during the final days of the national tour of the musical. Angela Lansbury, who starred in the original Broadway production, stars in this as well, and manages to bring comic relief and startling animation to a sobering and impressive production.
Some of it may take alert watching and listening - but it is a unique viewing experience, one of the first in a long line of videotaped major productions.
The Entertainment Channel constantly proclaims that it is not ''cultural'' (after all, word is out that CBS Cable, a self-described cultural channel, has already lost somewhere between $20 million and $40 million in its initial year). But, if Sweeney Todd ain't cultural, I don't know what is.
I saw the original production and I previewed the videotaped production recently - and it comes through as even more of an opera and less of a musical comedy than on stage. This astounding three-hour production will not be aired until Sept. 12 so there is still time to decide whether or not you want to pay the approximately $10 a month The Entertainment Channel costs.
In New York (where the cable systems do not carry The Entertainment Channel) the service has engineered what amounts to a promotional coup - it has been arranged that Group W Cable, Manhattan Cable, WQXR-FM will present the production free, on a stereo simulcast, at the same time as it is nationally. Call it entertainment or culture, if that doesn't lure large numbers of viewers, then ''Laverne and Shirley'' is what viewers deserve.