WMA

A WMA file is usually encapsulated in an ASF (Advanced Systems Format) container format that consists of a single audio track. It is basically based on MDCT, or Modified Discreet Cosine Transform, in which the bit stream of the file is composed of super frames. Each super frame consists of 1 or more frames of 2048 samples. Each frame in turn contains several blocks, which may be 64, 128, 256, 512, 1024, or 2048 samples long after being transformed, via the MDCT, into the frequency domain. The transformed samples are re-quantized in the frequency domain, and then finally are coded.

WMA is a versatile lossless audio format that is capable of both VBR (Variable Bit-rate) and CBR (Constant Bit-rate), thus allowing allocation of more bits (less compression) to the more complex parts of an audio track. This gives better and uniform audio quality across the track. WMA also supports multichannel and high resolution audio. It can sample audio signals at up to 48 kHz (48,000 times per second).

WMA also supports DRM (Digital Rights Management) through a combination of elliptic curve cryptography key exchange, block ciphers, stream ciphers, and the SHA-1hashing function. This allows downloading of music from such services as Yahoo Music, Napster, and Virgin Digital. WMA is also an ideal format for ripping audio CDs, giving the same audio quality as the original audio track.

WMA files, with file extension ‘.wma’, are one of the most popular audio codecs today. They can be played using Windows Media Player, Winamp, Real Player, besides a large number of portable music players and set-top DVD players.

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