On DVDs there are several audio formats that can be used. You may see AC3 (aka Dolby Digital), LPCM (uncompressed), MPA (MPEG-Audio), or DTS (Digital Theater Surround). Some of these formats are mandatory, meaning they can be included as the sole audio stream on a disc because every player is required to support them in order to use the official DVD logo.
Mandatory formats for DVD differ for PAL and NTSC discs. For PAL the mandatory formats are MPA, AC3, and LPCM. For NTSC they're only AC3 and NTSC. Every disc must include at least one mandatory audio format.
HD DVD and Blu-ray also have mandatory audio formats. For HD DVD, mandatory formats are AC-3, LPCM, Dolby Digital EX, DTS, Dolby Digital Plus, and Dolby TrueHD. For Blu-ray they're the same, except that Dolby TrueHD is optional instead of mandatory.
Mandatory audio
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