The No Hiss Shush
Of course, it didn't quite sound like the original, but it sure was quieter. Already the first trade-off of reality for efficiency had been committed.

This trend continued as Dolby and competing systems found new ways to take out tape hiss and other extraneous sounds, mainly by manipulating frequencies where the trouble lay and where our listening perceptions didn't. These noise reduction systems became ubiquitous and made the cassette revolution happen by allowing the tiny and usually noisy cassette to go hi-fi.

Where's The Codecs?
By this time you may be wondering what analog noise reduction has to do with codecs, so it's time to look at the leap the recording industry made when it went digital. Digital, in its better moments, reproduces sound much better than analog and entirely bypasses tape hiss. It does it by taking a single sample of the signal 44,000+ times a second and reduce it to bits. That's a lot of data to cover just one second, and it takes up a lot of space, especially when you transmit it.

So, using the technologies Dolby had pioneered, researchers noticed that taking out a lot of frequencies the listener didn't pay much attention to in order to gain a quieter signal also saved recording space. Sound was recorded, dismantled and trimmed to a minimum size, and then reassembled at the other end during playback.

A Codec Was Born
Since bits are bits, of course, it worked for video, too, but differently. It was a different illusion, and thus had another, though similar, set of rules.

The size of an audio signal can be shrunk by cutting out sound areas people don't pay much attention to, and size of a video signal can be shrunk by cutting out the visual areas people don't pay much attention to. What might those be?

In a word, it's the gorilla. Cut the gorilla, keep the game, halve the file size. People won't know it's missing, poor hapless hominid.