Frequently Asked Questions About Compact Discs

Please contact Media Sciences if your questions are not answered on these pages.

CD discs work great in our test drive. Why do they fail in some 40X and higher speed drives?

Quality is defined by successful interchange, not by functionality in one or two drives. Powerful error correction methods are used in CD formats that can flawlessly read a disc containing a 1 mm hole. 1X test drives might indicate that this disc works great, but problems can become fatal at higher speeds.

Flaws become serious above 40X speeds for many reasons. One is the channel bit rate of 4.3 MHz at 1X that multiplies to 173 MHz at 40X. The same powerful error correction methods that tolerated the 1 mm hole at 1X may not be available in higher speed drives. Inexpensive integrated circuits used for error detection and correction are speed limited. When used in 40X and higher speed drives, their error correction capability may be only one-half their 1X capability.

Although spindle speed is a maximum of 580 rpm in a 1X drive, it becomes 13,950 rpm at 24X and 27,900 rpm at 48X. Hard drives run in sealed enclosures at lower speeds. CD drives are not sealed. Minor physical unbalances in the CD disc can be ignored at 1X speeds. These same unbalances result in large radial forces at high speeds that wear out spindle bearings and cause radial displacement of the disc. Such displacements strain servo systems that must maintain proper focus and radial position of the laser spot to within a fraction of CD track spacing values (0,0016 mm or 0.000063 inches).

Radial, focus, and spindle speed servo designs may be compromised in high speed drives. All CD drives must properly play CD-Audio discs which require 1X spindle speeds. Therefore each servo loop in a 40X reader must function over a 40:1 range. Compromises can be avoided by switching analog loop filter components, but this may not be done for cost reasons. Digital servos may be required to achieve proper performance.

Vibration is always an issue in CD players, and rubber bushings and sound-dampening ceramics or fiberglass are used to control the effects of vibration. High spindle speeds and disc unbalance often result in excessive vibration, higher jitter, and increased error rates.

Center hole diameters of the disc are allowed to vary between 15.0 mm and 15.1 mm. The disc is mechanically clamped during rotation, and the radial servo compensates for any eccentricity resulting from the spindle-to-center hole misfit. Stable mechanical clamping at 1X may become unstable at higher speeds. Sudden radial motion of the disc may exceed acceleration specs of the radial servo, resulting in loss of lock, mistracking, and severe read errors. Discs with 15.02 mm center holes might not have this problem at 40X while those with 15.09 mm center holes could be unusable at high speeds.

Comprehensive quality measurements are necessary to confidently predict successful operation of CD discs in a wide variety of drives. In the absence of proper quality methods, users should be surprised when the discs actually work in high speed players, not when they fail.