RM (Real Media) is a multimedia format developed by RealNetworks. It features a .rm filename extension and is typically multiplexed with RealAudio and RealVideo when streaming content online. The stream is usually in constant bitrate (CBR), although a file container for variable bitrate (VBR) is now available, called Real Media Variable Bitrate (RMVB).
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M4V is a digital video container format developed by Apple Inc. It is pretty similar to the popular MP4 in terms of structure, but M4V files can sometimes be DRM protected. Today, M4V is the primary video format for iTunes Store videos and can multiplex together multiple audio and video data.
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