RMVB (RealMedia Variable Bitrate) is a digital variable bitrate extension for the RM (RealMedia) container format. Unlike the standard RM format that streams content in CBR (Contant Bitrate), the RMVB, as the name suggests, streams local media in VBR (Variable Bitrate). This file format uses a .rmvb filename extension.
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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.