M4R (MPEG-4 Ringtone) is an audio container format belonging to the MPEG-4 specification. M4R files are commonly referred to as iPhone Ringtone Files, as storing ringtones is the main purpose they were engineered for. In fact M4R is the default ringtone format for the Apple iPhone.
In their essence M4R files are not much different than M4A files, with the only exception being that audio data stored in M4R files can't be longer than 30 seconds in duration and may be copy-protected.
Most M4R files are created with the Apple iTunes app from songs purchased from the iTunes store, although there are a number of alternative ways for creating perfectly valid M4R files. However, utilizing these is rarely necessary, because the iTunes player is really flexible and allows users to even convert popular audio formats like MP3 to the M4R audio file format.
Contrary to M4A files, which can utilize a couple of different encoding schemes, M4R files are strictly limited to using only the Advanced Audio Coding (AAC) compression scheme.
The AAC scheme is well-known for providing superb audio quality at a reduced file size and said to become the successor of the widely popular MP3 encoding format.
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