M4A files are a type of audio file developed and popularised by Apple. Since 2007, music purchased through the iTunes store has been in m4a format, therefore accounting for over 15 billions files worldwide. MP4 and M4A files are often confused, and the two are quite similar, being both based on the MPEG-4 codec. However, M4A is a file comprising solely of audio, whereas MP4 may also contain video.
M4A files are superior to MP3 in terms of the scale of compression and audio quality. The M4A file uses Apple's codec and resides within the MPEG-4 container. The main benefit of M4A is that files are compressed but are lossless. This means they can be decoded back to the original quality they were at the point of compression. Another benefit of M4A files are that do not carry any Digital Rights Management (DRM) protection associated with other files meaning they are less restricted.
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AC3 is an audio file specifically developed by Dolby Digital to provide surround sound audio. The format is used for audio on many DVD and Blu-Ray media. In order for the file to be correctly decoded and output, a DVD player with a Dolby Digital compatible home cinema system is required. It became the de facto standard for surround sound, and has held that position since its inception in 1992, with it coming to home cinema in 1995.
AC3 allows for separate audio to be defined for each of the 6 speakers in the standard cinema and home cinema setup. This produces the effect of surround sound to the person listening to the audio file. The six channels consist of right front, center, left front, rear right, rear left and a sub woofer.
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