Transfer Rates and Drive speeds

Transfer Rate

The transfer rate is a measurement of how fast a CD-ROM drive can read data off the disc. The faster the drive can read data, the faster it can deliver it to the computer for processing (limited by the speed of the bus connection of course).

CD-DA calls for a standard disc layout of 2048 bytes per block (or sector), with a read speed of 75 blocks per second. As such we can see that 2048bytes/block x 75blocks/per second = 153600, or 153.6KB/s. Rounded off, this equates to about 150KB/s. First generation single speed drives transferred data at 150KB/s. Since data does not need to be read in real time like audio information, you can easily increase the data transfer rate by simply spinning the disc faster. "Double speed" or 2X drives transferred data at 300KB/S. Back in the day, multimedia (games and movies) required a minimum 4X drive, which transfers data at a rate of 600KB/s.

Unlike audio CD information, digital CD information does not need to be read in real time. If you continue to increase the spin speed of the disc, you can therefore increase the data transfer rate. This is feasible so long as the controller and the laser are able to keep up. The laser and controller must work fast enough to keep a steady stream of data going.

Most current 50X CD-ROMs sold today use CAV drive technology that keeps the disc spinning at a constant overall speed. Transfer rates are near 7.5MB/s, which is a steep improvement from the pokey 150KB/s of the first generation single speed drives.

Even with the massive advancement in CD-ROM speeds, they are still rather slow as compared to hard drives, which can reach up to and over 20MB/s for IDE, or even up to about 80MB/s for SCSI. Optical drives are still turtles compared to the hard disk rabbits.

Access times

Drive manufacturers advertise their drives according the maximum speeds the drive can attain. This rating assumes prime conditions in the system in order for the drive to attain and sustain its highest transfer rate. In reality however, your computer will rarely be in a condition to allow such speeds ever to be reached. The rating also does not take into account the time it takes for the drive to seek from one track to another; it assumes a continuous read operation is taking place.

The "average seek time" is defined as the mean (average) amount of time it takes the drive to seek a new track approximately half way across the disc's readable area. First generation single speed drives had an average seek time of about 400ms. Newer 40X-50X drives have an average seek time of 90-80ms. This is quite a bit faster, but still slower than even the slowest hard drives.