In: Root » Computers and technology » Storage Devices
Developed jointly by Philips and Sony Corporation, the CD-Digital Audio system was first introduced in the United State in 1983. The standard CD-DA disc holds up to about 70 minutes of stereo music with a range equivalent to today's FM radio station-the high end goes just beyond 15 KHz; the low end, nearly to DC. The system stores audio data with a resolution of 16 bits, so each analog audio level is quantified as one of 65,536 levels. With linear encoding, that's sufficient for a dynamic range of 96 decibels, that is 20log(162). To accommodate an upper frequency limit of 15 KHz with adequate roll-off for practical anti-aliasing filters, the system uses a sampling rate of 44.1 KHz. Under the Red Book standard, this digital data is restructured into 24-byte blocks, arranged as six samples of each of a pair of stereophonic channels (each of which has a depth of 16 bits). These 24 bytes are encoded along with control and subchannel information into the 588 optical bits of a small frame, each of which stores about 136 microseconds of music. Ninety-eight of these small frames are grouped together in a large frame, and 75 large frames make one second of recorded sound. In CD-DA systems, the large frame lacks the sync field, header, and error correction code used in CD ROM storage. Instead, the error-correction and control information is encoded in the small frames. The necessary information to identify each large frame is spread through all 98 bits of subchannel Q in a given large frame. One bit of the subchannel Q data is drawn from each small frame. From the subchannel Q data, a sector is identified by its ordinary playing time location (in minutes, seconds, and frame from the beginning of the disk). The 98 bits of the subchannel Q signal spread across the large frame is structured into nine separate parts: a two-bit synchronization field; a four-bit address field to identify the format of the subchannel Q data; a four-bit control field with more data about the format; an eight-bit track number; an eight-bit index number; a 24-bit address counting up from the beginning of the track (counting down from the beginning of the track in the pre-gap area); eight reserved bits; a 24-bit absolute address from the start of the disk; and 16 bits of error correction code. At least nine of ten consecutive large frames must have their subchannel Q signals in this format. In the remaining large sectors, two more subchannel Q formats are optional. If used, they must occur in at least one out of 100 consecutive large frames. One is a disc catalog number that remains unchanged for the duration of the disk; the other is a special recording code that is specific and unchanging to each track.
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