This is a very ironic and also very true phrase.
Back in the early days of television the broadcast networks would always shut down at night. Yup, there was no programming after a certain time. This was gradually phased out as more people watched TV and there was a market for red eye TV programming. When the time came for the station to go off the air, you would typically get a few people (usually white, well-to-do Good American Citizens) singing along with a picture of the American flag, and the national anthem. Then the TV would go to a test pattern with a static image of color and shape patterns. You could use these color and shape patterns to adjust the picture and color quality manually on the TV set.
What I find humorously ironic with the phrase is that nothing has really changed in forty years of evolution of TV. People will still turn on a TV set and get programming that is worth less than a test pattern. The news is all sensationalistic, the sports are all endorsed and run by visa or sony. The sitcoms aren't any smarter, they are just there to entertain. They don't educate or exhibit any other altruistic properties. About the only thing worth watching is historical documentaries on public television.
Garbage In, Garbage Out applies very well to the television.