The MP4 extension indicates a file saved in the MPEG-4 multimedia format. As defined in its specification, this format can contain MPEG-4 encoded video and AAC (Advanced Audio Coding) encoded audio. MP4 media files can play in Windows Media Player provided that DirectShow-compatible MPEG-4 decoders are installed. Microsoft decided to improve the format's video compression and has so far released the following MPEG-4 video codecs: Microsoft MPEG-4 v1, Microsoft MPEG-4 v2, Microsoft MPEG-4 v3, ISO MPEG-4 v1.