What is MP3?

The format that made digital music portable, and thirty years on it's still the safest bet that a file will simply play - any device, any car, any decade-old bit of hardware. It's lossy, its patents expired in 2017, and no browser has ever been able to write one, which is why encoding it here needs LAME.

What is Opus?

The best-sounding lossy codec at low bitrates by a comfortable margin, which is why it carries the audio in nearly every voice and video call you make. Royalty-free, equally good on speech and music, and usually wrapped in an Ogg container. It always runs at 48kHz internally.