Compact Disk (CD)[6]
- Introduction
- CD Drive: basic design
- Compact disk (CD): basic design
- Data encoding
- Making CDs
- Spin rates and Data transfer rates
- Interfaces
- Multi-beam technology
- White Papers on compact disk technology
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Spin rates and data transfer rate
The X ratings of CD-ROM drives are based on comparison with the first generation drives with the data transfer rates of 150 KB/s or 1X. Today's drives operate at more then 32X boosting data transfer rates beyond 4.8 MB/s, and the improvement has mostly come from the increase in spin rates. The other components have mostly remained unchanged. It seems at this point, that further increase in spindle speed may be impractical due to loss in drive performance.
Previously, CD-ROM drives (slower than 12X) were designed on the basis of the constant linear velocity (CLV) principle, where the angular speed of the drive (rpm) was continuously adjusted following the read head to keep the laser spot moving over the disk surface at constant velocity. This provided uniform spacing of the pits along the track and a constant data transfer rate independent of head positioning over the disk. At some point, this principle was sacrificed to keep up with the need for faster and faster motors, which is much easier to achieve with the constant-angular speed motors.
Therefore, the newest CD drives operate on constant angular velocity (CAV) principle, like hard drives, for example. Now, the transfer rate is a function of the data radius. This also means that the average data transfer rate of the drive is much lower than the drive's maximum rate specified by its X-rating. This could be partially dealt with by using larger memory buffers, but large memory will also increase the cost of the drive. As an alternative, a hybrid CD drives could be introduced with a constant angular velocity closer to the center of the disk and constant linear velocity closer to the outer diameter of the disk. This should be possible thanks to the digital signal processor chips controlling the drive, which could be reprogrammed relatively easily.
The first 1X CD-ROM drive had the data transfer rate equal to 0.15 MB/s. Therefore, 2X drives had 0.3 MB/s rate, 4X drives - 0.6 MB/s, and so up to 12X. For faster drives, which operate on CAV principle, one needs to specify two data transfer rates: minimum and maximum. The next table summarizes this:
| Drive speed rating | Principle | Minimum rate (MB/s) | Maximum rate (MB/s) | Average rate (MB/s) |
|---|---|---|---|---|
| 1 X | CLV | 0.15 | 0.15 | 0.15 |
| 2 X | CLV | 0.30 | 0.30 | 0.30 |
| 4 X | CLV | 0.60 | 0.60 | 0.60 |
| 6 X | CLV | 0.90 | 0.90 | 0.90 |
| 8 X | CLV | 1.20 | 1.20 | 1.20 |
| 10 X | CLV | 1.50 | 1.50 | 1.50 |
| 12 X | CLV | 1.80 | 1.80 | 1.80 |
| 12 X to 20 X | CAV | 1.80 | 3.00 | 2.40 |
| 12 X to 24 X | CAV | 1.80 | 3.60 | 2.70 |
| 12 X to 32 X | CAV | 1.80 | 4.80 | 3.30 |
| 12 X to 48 X | CAV | 1.80 | 7.20 | 4.50 |
Interfaces
Most CD-ROM drives come with ATAPI Interface (enhanced IDE), which can be directly connected to the motherboard. External CD-ROMs requires a SCSI interface. Read more about interfaces here.