MPEG, sometimes called MPG, is a video file format popular for distributing content over the internet. It is the sister format to MP4 and MP3 because they share the same developer – Moving Picture Expert Group. MPG format uses either MPEG-2 or MPEG-1 compression, depending on the circumstance. Opening it is usually easy on any OS platform.
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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.