H.264 is a video compression standard used for HD videos. In fact, it’s the most widely-used video codec today. As such, this codec can be incorporated into several container formats like MP4, 3GP, MOV, FLV, and others. Most of the time, it’s encoded with the Advanced Audio Coding (AAC) codec.
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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.