http://www.minidisc.org/minidisc_faq.html#_q1
Audio was originally encoded in lossy ATRAC format at 292kbps. Later revisions moved to ATRAC3 coded, and allowed two lower bitrates to extend the recording/playback time.

MD originally transcoded MP3 to ATRAC when transferring them to MD. It wasn't until the later Hi-MD format that you could store MP3 natively on an MD and play it back. Hi-MD also had larger capacity 1GB discs, so you could store a lot of MP3s on one disc:
https://en.wikipedia.org/wiki/Hi-MD#...upport_for_MP3


The other cassette format was Digital Compact Cassette DCC, which was somewhat backwards compatible with analog cassettes as, due to the similar form factor, DCC players could also play back analog cassettes. DCC, like MD, used lossy audio compression, in this case PASC. Only DAT had uncompressed PCM.
https://en.wikipedia.org/wiki/Digital_Compact_Cassette

BTW, if anyone has interest in the various rare and "failed" audio formats, the Techmoan channel on Youtube has a lot of interesting videos:
https://www.youtube.com/user/Techmoan

I like this one about the Audio Craft CC-1 cassette, which basically pulls out the reel from a compact cassette for even more compact storage. To play it back, you'd put it into a cassette adapter, similar to playing a VHS-C back on a regular VHS VCR.

2 hours and 40 minutes for an MD-80 minute disc (recorded in mono).