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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M4R is a filename extension for AAC format, which is developed by Apple Inc. It is typically used to save custom iPhone ringtones from Apple iTunes. M4R can only be created from iTunes purchased songs and is usually DRM protected. It can open on iTunes, VLC Media Player, Apple QuickTime Player, and more.