Editing Windows Media Audio (section)

===Windows Media Audio===
'''Windows Media Audio''' (WMA) is the most common codec of the four WMA codecs. The 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.<ref name="Broadcom"/> Since then, newer versions of the codec have been released, but the decoding process remained the same, ensuring compatibility between codec versions.<ref name="Broadcom"/> 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&nbsp;[[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.<ref name="Broadcom"/> WMA 9.1 also added support for low-delay audio,<ref name="Low-Delay Audio">{{cite web |url=http://msdn2.microsoft.com/en-us/library/aa390496.aspx |title=Windows Media Format 11 SDK Low-Delay Audio |publisher=Microsoft MSDN |access-date=2007-08-16 |url-status=live |archive-url=https://web.archive.org/web/20071024183517/http://msdn2.microsoft.com/en-us/library/aa390496.aspx |archive-date=2007-10-24 }}</ref> which reduces [[Latency (engineering)|latency]] for encoding and decoding.

Fundamentally, WMA is a transform coder based on [[modified discrete cosine transform]] (MDCT), somewhat similar to [[Advanced Audio Coding|AAC]], [[Cook Codec|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 several 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 coding|huffman coded]]. Stereo information is typically [[Joint stereo#M/S stereo coding|mid/side]] coded. At low bit rates, [[line spectral pairs]] (typically less than 17&nbsp;kbit/s) and a form of noise coding (typically less than 33&nbsp;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, 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 88.2/96&nbsp;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 player]]s, 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.<ref name="nokia">{{cite web|url=http://www.microsoft.com/presspass/press/2005/feb05/02-14NokiaCollaborationPR.mspx|title=Microsoft and Nokia Collaborate to Help Ensure Consumers Can Enjoy Digital Music Anywhere|website=[[Microsoft]]|url-status=dead|archive-url=https://web.archive.org/web/20070831104239/http://www.microsoft.com/presspass/press/2005/feb05/02-14NokiaCollaborationPR.mspx|archive-date=2007-08-31|access-date=2007-08-15}}</ref> 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.<ref name="PSP Review">{{cite web |author=Carnoy, David |title=Sony PSP review |date=2005-03-23 |url=http://reviews.cnet.co.uk/gamesconsoles/0,139102149,39188324,00.htm |access-date=2007-08-16 |url-status=dead |archive-url=https://web.archive.org/web/20070810200119/http://reviews.cnet.co.uk/gamesconsoles/0,139102149,39188324,00.htm |archive-date=2007-08-10 }}</ref>