This came up at the Leicester 8bit exhibition. I was chatting to a musician (there was an audio installation from Virtual State), and he wondered if data cassettes were CD quality.
A data cassette is a stream of information, encoded as beeps, which are bits in on/off mode, this is then much speeded up to fit on a tape. So the audio is just zeroes and ones. This is read by an audio decoder in the micro, to convert the audio beeps back into bits.
This was also how micro programmers worked, saving to tape. This was not a pro game shop, I just started up Micro Arts at home in Clapham.
Older readers will recall the noisy screech of modems. Even older readers will remember these tapes and the tape decks.
However, the cassettes last a long time, when I opened the cardboard box in which stored Micro Arts materials, around 2019, all the tapes loaded perfectly into a PC Spectrum emulator. Apparently, as I moved about a lot (I’ve lived in many places), the oxide doesn’t get settled, so they are more reliable. That’s weird. Pretty long term when you think about it.