AVI (Audio Video Interleave) is a digital multimedia container format developed by Microsoft in 1992 for its Video for Windows project. This file format saves both video and audio data in its container that supports synchronous audio-with-video playback. And like DVD format, AVI allows multiple audio and video streaming as well.
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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.