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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Developed by the Signal Processing Group at Microsoft, the WMA file format is part of the Windows Media framework and was first released commercially in 1999, with support for playback of WMA encoded files as part of Windows Media Player. Creation of WMA files did not come until Windows Media Player version 7. The original codec was targeted as a direct competitor to the MP3 and RealAudio formats, and has achieved a broad level of adoption thanks to support for playback on numerous DVD players, Nokia mobile handsets and Playstation portable devices.
In almost all circumstances WMA files are part of the Advanced Systems Format (ASF) container, a proprietary container format developed by Microsoft for both digital video and digital audio. Every WMA file contains an audio track encoded in one of four mutually distinct codecs - WMA, WMA Voice, WMA Lossless or WMA Pro - WMA is the most commonly found of the four, but is a lossy codec, with the ability to encode audio signals sampled at up to 48 kHz. WMA Pro is an improved version, allowing sampling up to 96 kHz, but has achieved little hardware and software support. WMA Lossless is designed to compress audio signals with no loss of quality from the original source (up to 96 kHz) and is used in some Windows Mobile devices as well as the Logitech Squeezebox Touch. Finally, WMA Voice is a lossy codec optimized for low-bandwith voice playback applications, with mono sampling support up to 22.05 kHz - it is perhaps most well known for being used by the BBC World service for streaming Internet radio.
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