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Internal and External Transfer Rate
The transfer rate of a drive refers to how quickly data can be moved from the surface of the CD and into the computer. This is the primary statistic that measures how efficiently you can actually get your data off the surface of the compact disk when you need it. Since many applications involving CDs require the transfer of large blocks of data, the transfer rate of the drive is an important performance metric.
There are two different factors that make up transfer rate: the internal transfer rate and the external transfer rate. There is some confusion in terminology surrounding transfer rate, because different companies tend to focus on one or the other (the problem is more pronounced with hard disks). In a nutshell, the internal transfer rate measures the speed at which data can be actually read from the disk and into the CD-ROM drive's internal controller. The external transfer rate measures the speed at which data can be moved from the control over the CD-ROM interface and into the rest of the PC.
The external transfer rate is rarely an important factor with CD-ROM drives (unlike hard disks) because their internal transfer rates are so low that even slower interfaces are rarely a limiting factor. The theoretical maximum transfer rate of a 12X drive for example is only 1,800 KB per second, and most any SCSI or IDE/ATAPI interface can handle this with no difficulty. It is only at the very high end drives that the interface becomes more of a concern, and even here it isn't much of a concern compared to high-end hard disks that have much higher transfer rates from the disks themselves.
The internal transfer rate is in most respects the "true" transfer rate of the drive, since this is the rate that data can actually be read from the disk. The external transfer rate is more of a limiting factor; the data cannot be fed to the PC any faster than the interface's transfer rate, but having a faster interface does you no good if the drive itself is slow. This is explained in much more detail in the section on hard disk external transfer rate. CD interfaces are explained here.
Unlike hard disks, where far too much focus is on the external or interface transfer rate, most manufacturers of CD-ROMs quote the true internal transfer rate of the drive, although even here there are caveats. As explained in detail in the section on hard disk internal transfer rate, the quoted specs are always for ideal, peak transfers, and will never be achieved over a period of time in the "real world", due to overhead and other imperfections (but there is no zoned bit recording for CDs as there is for hard disks; see below). Also, the real world is a mix of random accesses and sequential transfers, and the transfer rate measures only sequential transfers.
Conventional CD-ROM drives use constant linear velocity (CLV). Since there is more data at the outside of the disk than at the inside, they make the disk spin slower when reading the outside of the disk so that the transfer rate over the entire surface of the disk is identical. This means that the theoretical internal transfer rate of the drive can be calculated easily by looking at the "X" rating of the drive and multiplying it by 150 KB/second, the transfer rate for a standard 1X drive. This will be the transfer rate for an entire CD, no matter what part of it is being used.
The newer constant angular velocity (CAV) CD-ROM drives do not vary the speed of the disk depending on where they are reading. They spin the disk at the same speed all the time (thus the name). This means that when reading the outside of the disk, where there is more data per revolution, they will have a (much) higher transfer rate than when they are reading the inside of the disk. (This is exactly the way it is for hard disks as well). The difference is substantial because the ratio of the data density of the outside of the disk to the inside is about 2.5 to 1.
The manufacturers of course only state in the specifications the "big number", the transfer rate when reading the outside of the disk. Since a CD has its data recorded from the inside out, this means on a disk that is half-full, none of the data will ever be read back at 24X when you are using a CAV 24X drive. The data at the very inside edge will be read at the equivalent of less than a 10X drive. Do not be fooled by this!
This table shows a summary of the transfer rates of different types of drives:
As you can see, in many instances you are far better off with a good 12X CLV drive than with a 16X CAV drive, due to the fact that a 16X drive is really a "6X to 16X drive". Even the 24X drive isn't so impressive when you really understand how it works.
The last entry in the table shows the performance of a mixed CLV/CAV drive such as Plextor's 12/20Plex. This drive uses CAV when reading the outside edge of the disk to get maximum transfer rate as good as that of a standard 20X CAV drive. It then switches to CLV when reading the inside edge of the disk, giving it a transfer rate 50% higher than a regular 20X CAV drive when reading the starting edge of the disk. Overall, this drive will give you transfer rate performance probably exceeding that even of the 24X CAV unit.
Next: Processor Utilization