VOB (Video Object) is a digital video container format usually found in DVD video media. VOB contains DVD video, audio, menu, and text data such as subtitles all multiplexed together. It typically includes MPEG-1 Part 2 or MPEG-2 Part 2 video codec encoded with AAC audio. Although this format is free, some copies might be encrypted.
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M4B is an audio filename extension for AAC format, which is developed by Apple Inc. It is mainly used on podcast and audiobook files. Typically, M4B files contain audio and other metadata like images, hyperlinks, and chapter markers. M4B files are DRM copy-protected and can be opened on Apple iTunes and QuickTime Player.