Data is read from the CD-ROM at a certain speed. There are two principles used reading from a CD-ROM:
CLV
Constant Linear Velocity was used in the early generations of CD-ROM drives. It implies that the data track must pass under the read head at the same rate, whether in inner or outer parts of the track. This is accomplished by varying the disk rotation speed, based on the read head's position. The closer to the center of the disk the faster the rotation speed to deliver the same constant stream of data.
CAV
Constant Angular Velocity. It is not very smart to change the rotational speed of a CD-ROM all The time, as the CLV drives do. Therefore, in more modern and speedy drives, the CD-ROM rotates at a constant number of rounds per minute. This implies that the data transfer varies; data read from the outer parts of the CD-ROM are read at very high bit rates. Data from the inner parts are read slower.
Let us look at a modern 40X CAV drive. It rotates constantly with a whopping 8900 RPM. This drive will deliver 6 MB per second when reading from the outer tracks. Reading from the inner tracks it only delivers 2.6 MB per second. An average will be 4.5 MB/sec.
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Problematic readings
The CD-ROM disk has to read in random pattern. The read head must jump frequently to different parts of the disk. You can feel that. It causes pauses in the read function. That is a disadvantage of the CD-ROM media. Also the faster drives can be rather noisy.
Within the next years the CD-ROM and DVD drives will merge into one unified drive type.
Rotation speed and data transmission [top]
There are different generations of CD-ROM drives. Here you see their data.
CD-ROM type Data transfer rate Revolutions per minute outermost - innermost track
1X 150 KB/sec 200 - 530
2X 300 KB/sec 400-1060
4X 600 KB/sec 800 - 2,120
8X 1.2 MB/sec 1,600 - 4,240
40X CAV 2.6 - 6 MB/sec 8,900 (constant)
40X40 multibeam 6 MB/sec 1,400 (constant)
