What is the RAD format?

The RAD file type is primarily associated with Rhapsody by RealNetworks. Rhapsody is powered by RealNetworks’ caching/streaming technology. This technology splits (or distills) audio files into two asymmetric pieces during the encoding process. The larger piece (called the Residual Audio Data, or RAD) contains 99% of the audio data and is streamed and cached (in encrypted form) to the subscriber’s hard drive during initial song playback. The remaining 1% of the file (called the Essential Audio, or EA) is streamed from RealNetworks’ servers and united (or rendered) with the larger file at playback, enabling subscribers to hear the sound. This process is nearly instantaneous and is invisible to the subscriber. A proprietary file called RADFILE.RCF holds the cache. On first play songs appear to stream from the Internet; on second play during a session, from the cache.