AC-3 is the most common version containing up to 6 total channels of sound, with 5 channels of full-bandwidth of 20 Hz - 20 kHz-normal range for speakers (front left and right, center, front, right rear and left rear) and one dedicated output channel for low frequency sounds known as low frequency Effect, or subwoofer. The Dolby Digital format also supports the use of Mono and Stereo. The AC-3 is one of the so-called perceptual compression formats. What it does, basically, is to remove all parts of the original sound, encoded analog, which can not be perceived by the human ear. In this way, it is achieved that the same information is smaller and therefore take up much less physical space. Once achieved compression of the original wave, you can add more information that was not previously possible.
RealAudio is a proprietary audio format developed by RealNetworks and first released in April 1995. It uses a variety of audio codecs, ranging from low-bitrate formats that can be used over dialup modems, to high-fidelity formats for music. It can also be used as a streaming audio format, that is played at the same time as it is downloaded. In the past, many internet radio stations used RealAudio to stream their programming over the internet in real time. In recent years, however, the format has become less common and has given way to more popular audio formats. RealAudio was heavily used by the BBC websites until 2009, though it was discontinued due to its declining use. BBC World Service, the last of the BBC websites to use RealAudio, discontinued its usage in March 2011.