Real Audio is an audio format. It was first introduced by RealNetworks in 1995, the current version being Real Audio 10. Files of this format usually have the extensions of RA, RAX, RM or RAM. Real Audio is often used as a streaming audio format (can be played at the same time as it's downloaded) due to its ability to conform to low bandwidths. This makes Real Audio a common format for many internet radio stations.
RealAudio was developed as a streaming media format, meaning that it can be played while it is downloaded. It is possible to stream RealAudio using HTTP. In this case, the RealAudio file is retrieved similarly to a normal web page, but playback begins as soon as the first part is received and continues while the rest of the file is downloaded. Using http streaming works best with pre-recorded files. Some alternative protocols have been developed which work better for live broadcasts. In many cases, web pages do not link directly to a RealAudio file. Instead, they link to a .ram (Real Audio Metadata) or SMIL file. This is a small text file containing a link to the audio stream. When a user clicks on such a link, the user's web browser downloads the .ram or .smil file and launches the user's media player. The media player reads the pnm or rtsp URL from the file and then plays the stream.
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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