H.264 is a video compression standard used for HD videos. In fact, it’s the most widely-used video codec today. As such, this codec can be incorporated into several container formats like MP4, 3GP, MOV, FLV, and others. Most of the time, it’s encoded with the Advanced Audio Coding (AAC) codec.
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AU is an audio format standard in early web pages NEXT systems. It is also called u-law or Sparc-audio format. Typically, AU files can store audio data and annotation blocks. Initially, it was a headerless audio format, but newer versions have a header, thanks to the six unsigned 32.bit words.