Because a CD-Writer is a real-time device, it places special demands on a computer and its peripherals. For a CD Recorder to successfully create a compact disc, it must receive an uninterrupted flow of data that must be transferred at a speed equal to or greater than the recording speed of the CD Recorder. For CD Recorders operating in single-speed (1x) mode, data must be transferred at a continuous rate of 150 KB per second for data, and ~172 KB per second for audio. For CD Recorders operating in double-speed (2x) mode, data must be transferred at a continuous rate of 300 KB per second for data, and ~344 KB per second for audio. For CD Recorders operating in quadruple-speed (4x) mode, data must be transferred at a continuous rate of 600 KB per second for data, and ~689 KB per second for audio. Data transfer rates in the range of 150 to 600 KB per second are well below the data transfer rates claimed by hard disk manufacturers. However, slow CPU or SCSI bus performance and sub-optimal hard disk performance may combine to reduce the actual sustained data transfer rate below the transfer rate requirement of double-speed or quadruple-speed CD Recorders.