You're thinking of MPEG-2 Part 3 (also known as MPEG-2 BC) in Layer 2, a backwards-compatible extension to MPEG-1 Part 3 that allows multichannel audio. MPEG i

You're thinking of MPEG-2 Part 3 (also known as MPEG-2 BC) in Layer 2, a backwards-compatible extension to MPEG-1 Part 3 that allows multichannel audio. MPEG itself has a FAQ entry on "When should I use AAC rather than MPEG-2 BC?" [1]; of course the point is moot in context of DVDs. This Apple documentation article [2] lists the supported codecs and lists the relevant one as "MPEG-1 Layer 2 Audio"... close enough. It talks about how AC-3 enjoys broad support worldwide, but MPEG-1 Layer 2 is only mandatory in PAL regions. It includes language to suggest that AC-3 may not play on very old players in PAL regions, implying that at some point AC-3 wasn't mandatory in PAL regions -- this is corroborated by empirical observations elsewhere, like this forum thread from 2004 [3]. The book 'Spatial Audio' [4] by Francis Rumsey mentions that MPEG-2 BC was supposed to be multichannel audio in PAL regions, but this push was later abandoned. Dolby has lobbied extensively in favor of its own formats. This news article from MIT in 2002 recounts [5] an interesting situation where money exchanged hands between Dolby and MIT after MIT's vote to support Dolby AC-3 in a TV broadcasters consortium's recommendations to the FCC. [1] http://mpeg.chiariglione.org/content/when-should-i-use-aac-rather-mpeg-2-bc http://mpeg.chiariglione.org/content/when-should-i-use-aac-r... [2] https://documentation.apple.com/en/dvdstudiopro/usermanual/index.html#chapter=5%26section=3%26tasks=true https://documentation.apple.com/en/dvdstudiopro/usermanual/i... [3] https://forum.videohelp.com/threads/164582-MPEG-Audio-Vs-LPCM-Vs-Dolby-Digital-(AC-3)-What-is-b https://forum.videohelp.com/threads/164582-MPEG-Audio-Vs-LPC... [4] https://books.google.com/books?id=b3kqBgAAQBAJ&pg=PA108&lpg=PA108&dq=mpeg-2+bc+dvd https://books.google.com/books?id=b3kqBgAAQBAJ&pg=PA108&lpg=... [5] http://tech.mit.edu/V122/N54/54hdtv.54n.html http://tech.mit.edu/V122/N54/54hdtv.54n.html