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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The M4R is an iPhone ringtone file that is essentially a renamed AAC (m4a) file. Not all iTunes Music Store songs can be used to create M4R ringtones. In order to create an M4R ring tone from an iTunes song, the song must allow for M4R ringtone creation. An M4R ring tone is automatically transferred to a user's iPhone when the iPhone is synced with the user's computer.
M4R is essentially a renamed AAC (M4A) file - it is similar to MP3 but compresses the digital audio files further. It is part of the MPEG-4 standard and is specified in the ISO/IEC standard 14496-3. It is a lossy algorithm much like AAC and MP3. M4R supports auditory masking - essentially discarding data with minimal loss of quality.