Unlike a quality program like Seinfeld, Hogans Heroes or even Friends which were shot on film, as said the Cosby Show was shot on SD videotape of the time, which was very much SD. With film you have the option of going back to the original HD film, re-editing it and ending up with pretty decent HD. With a film shot TV program of the time the original material was shot with a film camera but was then edited on SD videotape for airing. With a program that was originally shot on videotape and then edited on videotape, you have no quality source to go back to
When I said "quality" program I'm not knocking The Cosby Show but rather how much money the producers wanted to spend recording it. Many sitcoms or other 1/2hr programs of the time were "shot on videotape, in front of a live audience" as they used to say in the audio over credits on All In The Family, which is a highly respected show but unfortunately the producers decided to save money, that and they probably wanted the "live audience" effect over a canned laugh track.
I've never seen one but I'd love to see a list of all the major 70s-90s TV programs listing which ones were shot on videotape vs film. I'd guess most all the 1hr dramas would be film based and most 1/2hr sitcoms would be videotape but it would be nice such a list.
Note at some point in the 00's producers started using HD video, which is the best of both worlds. I really doubt anyone is using film for TV programs anymore, heck even movies meant for the theater are now being shot on video although I doubt they use tape but rather HDDs? for storage.
I'm open to being corrected by anyone who knows more about the whole film/videotape shooting thing, I'm just repeating what I've read in various threads mostly here on AVS