PC Hardware in a Nutshell, 3rd Edition

Data Transfer Rates

CD-DA discs record music as a digital data stream. The analog music is examined or sampled 44,100 times per second (the sampling rate) using 16-bit samples, for a data rate of 88,200 bytes/s. Multiplied by two channels for stereo, that means the CD stores 176,400 bytes for each second of music. Each second’s data is stored as 75 physical sectors, each of 2,352 bytes.

Music data need not be completely error-free because an occasional flipped bit will be inaudible, which means the entire 2,352-byte capacity of each physical sector can be used to store actual music data. The same is not true for computer data, for which every bit must be correct. Accordingly, CDs store computer data using 2,048 bytes/sector, with the remaining 304 bytes in each physical sector allocated to error detection and correction data. This means that a computer CD running at the same speed as an audio CD delivers (75 × 2,048 bytes) per second, or 150 KB/s. This data rate is called 1X, and was the transfer rate supported by early CD-ROM drives.

Later CD-ROM drives transfer data at some integer multiple of this basic 150 KB/s 1X rate. A 2X drive transfers (2 × 150 KB/s) or 300 KB/s, a 40X drive transfers data at 6000 KB/s, and so on.