About WMA files : WMA (Windows Media Audio) information.

Windows Media Audio (WMA) is an audio data compression technology developed by Microsoft. The name can be used to refer to its audio file format or its audio codecs. It is a proprietary technology that forms part of the Windows Media framework. WMA consists of four distinct codecs. The original WMA codec, known simply as WMA, was conceived as a competitor to the popular MP3 and RealAudio codecs. WMA Pro, a newer and more advanced codec, supports multichannel and high resolution audio. A lossless codec, WMA Lossless, compresses audio data without loss of audio fidelity. And WMA Voice, targeted at voice content, applies compression using a range of low bit rates.

The first WMA codec was based on earlier work by Henrique Malvar and his team which was transferred to the Windows Media team at Microsoft. Malvar was a senior researcher and manager of the Signal Processing Group at Microsoft Research, whose team worked on the MSAudio project. The first finalized codec was initially referred to as MSAudio 4.0. It was later officially released as Windows Media Audio, as part of Windows Media Technologies 4.0. Microsoft claimed that WMA could produce files that were half the size of equivalent-quality MP3 files; Microsoft also claimed that WMA delivered "near CD-quality" audio at 64 kbit/s. The former claim however was rejected by some audiophiles. RealNetworks also challenged Microsoft's claims regarding WMA's superior audio quality compared to RealAudio.
Newer versions of WMA became available: Windows Media Audio 2 in 1999, Windows Media Audio 7 in 2000, Windows Media Audio 8 in 2001, and Windows Media Audio 9 in 2003. Microsoft first announced its plans to license WMA technology to third-parties in 1999. Although earlier versions of Windows Media Player played WMA files, support for WMA file creation was not added until the seventh version. In 2003, Microsoft released new audio codecs that were not compatible with the original WMA codec. These codecs were Windows Media Audio 9 Professional, Windows Media Audio 9 Lossless, and Windows Media Audio 9 Voice.

A WMA file is in most circumstances encapsulated, or contained, in the Advanced Systems Format (ASF) container format, featuring a single audio track in one of following codecs: WMA, WMA Pro, WMA Lossless, or WMA Voice. These codecs are technically distinct and mutually incompatible. The ASF container format specifies how metadata about the file is to be encoded, similar to the ID3 tags used by MP3 files. Metadata may include song name, track number, artist name, and also audio normalization values.
This container can optionally support digital rights management (DRM) using a combination of elliptic curve cryptography key exchange, DES block cipher, a custom block cipher, RC4 stream cipher and the SHA-1 hashing function. See Windows Media DRM for further information.

Windows Media Audio (WMA) is the most common codec of the four WMA codecs. Colloquial usage of the term WMA, especially in marketing materials and device specifications, usually refers to this codec only. The first version of the codec released in 1999 is regarded as WMA 1. In the same year, the bit stream syntax, or compression algorithm, was altered in minor ways and became WMA 2. Since then, newer versions of the codec were released, but the decoding process remained the same, ensuring compatibility between codec versions. WMA is a lossy audio codec based on the study of psychoacoustics. Audio signals that are deemed to be imperceptible to the human ear are encoded with reduced resolution during the compression process.
WMA can encode audio signals sampled at up to 48 kHz with up to two discrete channels (stereo). WMA 9 introduced variable bit rate (VBR) and average bit rate (ABR) coding techniques into the MS encoder although both were technically supported by the original format. WMA 9.1 also added support for low-delay audio, which reduces latency for encoding and decoding.
Fundamentally, WMA is a transform coder based on modified discrete cosine transform (MDCT), somewhat similar to AAC, Cook and Vorbis. The bit stream of WMA is composed of superframes, each containing 1 or more frames of 2048 samples. If the bit reservoir is not used, a frame is equal to a superframe. Each frame contains a number of blocks, which are 128, 256, 512, 1024, or 2048 samples long after being transformed into the frequency domain via the MDCT. In the frequency domain, masking for the transformed samples is determined, and then used to requantize the samples. Finally, the floating point samples are decomposed into coefficient and exponent parts and independently huffman coded. Stereo information is typically mid/side coded. At low bit rates, line spectral pairs (typically less than 17 kbit/s) and a form of noise coding (typically less than 33 kbit/s) can also be used to improve quality.
Like AAC and Ogg Vorbis, WMA was intended to address perceived deficiencies in the MP3 standard. Given their common design goals, it's not surprising that the three formats ended up making similar design choices. All three are pure transform codecs. Furthermore the MDCT implementation used in WMA is essentially a superset of those used in Ogg and AAC such that WMA iMDCT and windowing routines can be used to decode AAC and Ogg Vorbis almost unmodified. However, quantization and stereo coding is handled differently in each codec. The primary distinguishing trait of the WMA Standard format is its unique use of 5 different block sizes, compared to MP3, AAC, and Ogg Vorbis which each restrict files to just two sizes. WMA Pro extends this by adding a 6th block size used at 88.1/96 kHz sampling rate.
Certified PlaysForSure devices, as well as a large number of uncertified devices, ranging from portable hand-held music players to set-top DVD players, support the playback of WMA files. Most PlaysForSure-certified online stores distribute content using this codec only. In 2005, Nokia announced its plans to support WMA playback in future Nokia handsets. In the same year, an update was made available for the PlayStation Portable (version 2.60) which allowed WMA files to be played on the device for the first time.