The Digital Fact Book - Quantel

CConsolidationClearing continuous space on a disk store to allow consistent recording. This generallyinvolves the moving of data on the disks to one area, leaving the remainder free so thatrecording can proceed track-to-track – without having to make random accesses. The largerthe amount of data stored, the longer consolidation may take. Careful consideration mustbe given to large-capacity multi-user systems, such as video servers, especially when usedfor transmission or on-air.The need for consolidation arises because of the store’s inability to continuously recordtelevision frames randomly at video rate. This is taken care of by Quantel’s FrameMagic.Recording can take place over small, scattered areas of the store so there is no need forconsolidation.Disk tracksFragmented dataConsolidationConsolidated dataFree space available for recordingSee also: Defragmentation, FrameMagic, True random accessConstant bit rate (CBR) compressionCompression systems that are used to create a fixed rate of output data. This is usually to fitwithin a given bandwidth such as that available on a video tape recorder or a constant bitrate transmission channel. With video, the amount of useful information contained in thematerial varies widely both spatially and temporally – with movement. For example, a footballmatch with crowds and grass texture as well as fast panning cameras typically containsfar more information than a largely static head-and-shoulders shot of a newsreader. Usingconstant bit rate means that the quality is altered to reduce the information to fit the fixedbit rate. In the football case, the grass may go flat during a camera pan, with the texturesuddenly reappearing when the camera is still.48