What is Tilt?
Tilt occurs when the plane of a CD or DVD disc is not perpendicular to the incident laser beam. Standards define either angular deviation between the incident and reflected beam or tilt as the angle between a surface and the reference plane of the disc, established by the clamping zone surface.
ISO/IEC 10149 contains two different tilt specifications for CD discs. Angular deviation of the reflected beam in a radial direction must not exceed ±1° 36' when the incident beam is normal to the reference plane, controlling tilt of the information layer. In addition, the angle between the entrance surface of the disc and its reference plane must not exceed 0° 36'. The difference between the two reflects non-parallelism of the entrance surface and the information layer. (Angular deviation of 1° 36' is equivalent to a 0° 48' angle between the information layer and the reference plane.)
ISO/IEC 16448 for DVD read-only discs requires that the angle between the incident and reflected beams be 0.80° maximum in the radial direction and 0.30° maximum in the tangential direction. These limits are equivalent to a 0.40° radial angle and a 0.15° tangential angle between the information layer and the reference plane.
What Causes Tilt?
Warped discs are a common cause of tilt. Short production cycle times may produce discs that are still hot and deformable upon removal from the mold cavity. DVD discs are manufactured as two half-thickness substrates, each of which are more susceptible to deformation than the more rigid 1.2 mm thick CD substrate.
What is the Result of Tilt?
In the absence of tilt, the incident laser beam is focussed by the objective lens of a drive into a small, well defined spot on the information layer. Although the diffraction limited spot is fuzzy, it is capable of resolving the sub-microscopic data marks as shown in the figure that follows (the spot is depicted to the right of the information layer).
Although tilt results in intensity loss because some of the reflected beam is not collected by the objective lens, the major problem is coma, as shown in the next figure.
Coma is an aberration of the focussed spot that adds a comet-shaped tail. It occurs when light from different parts of the objective lens does not intersect the information layer at a common point.
Why is DVD Tilt Tightly Specified?
Crosstalk between tracks can result when a radial coma tail spans the track pitch that is 1.6 µm for CD and 0.74 µm for DVD. Intersymbol interference results when a tangential coma tail spans a 3T data length; 0.833 µm for CD and 0.400 µm for DVD. Higher data density explains the tighter tilt tolerances for DVD, especially in the tangential direction because intervals between data are very small along the track.
Is Disc Tilt Always Measured?
Tilt measurements are challenging for two reasons. First, the required accuracies are only a few minutes of arc. Second, the test requires special instrumentation, and usually cannot be conveniently measured by a read drive. Many manufacturers can afford the expensive, dedicated equipment, but some do not routinely conduct tilt tests or do not have the capability.
Is Tilt Compensation Possible?
Some DVD drives attempt to compensate for tilt. This can be accomplished by pivoting an existing drive component such as the spindle motor. Newer methods utilize an LCD element in the beam path to predistort the beam. Neither method perfectly compensates for tilt, and low tilt discs are always desirable.
What Are Other Sources of Tilt?
Tilt can occur when the clamping surface of a drive is misaligned. Even a perfect disc is then mounted with tilt.
Tilt can also be caused by a misaligned objective lens. Rotation about an axis perpendicular to the laser beam path will create an angle between the incident beam and a perpendicular to the disc. Coma then results just as it did for a tilted disc. Lens tilt can occur if the drive manufacturer does not properly align the axis of the objective lens.
The objective lens is suspended by a voice coil mechanism that is driven by the servo that maintains spot focus. Drive vibration at high rotational speeds can be transmitted to the objective lens that can rotate at harmonic or sub-harmonic vibrational frequencies. Resulting rotation about an axis perpendicular to the laser beam does not defocus the spot, but does produce tilt and coma. Such effects are noticeable for many CD’s recorded at speeds in excess of 12X, and result in poor readability in single beam drives used in car players and some audio equipment. Very high tilt will also adversely affect three-beam drives that are commonly used in computers.
Tests at Media Sciences have shown that limit values of CD tilt produce frequent E22 and E32 uncorrectable errors and severe burst errors in single beam CD drives. These are not observed in high quality three-beam CD drives, although very high jitter values are observed when tilt is high.
Since 12X CD spindle speeds approximately correspond to 4X DVD spindle speeds, vibrational problems, tilt, and resulting coma may appear as DVD drive speeds increase. Compensating methods that could be included in the drive may not be compatible with price pressures.
Summary
Tilt is important to CD and DVD interchange because resulting coma in the focussed laser spot produces intersymbol interference and crosstalk, both of which can degrade interchangeability. Tilt can result from a warped disc, normally as a result of disc manufacturing methods. Tilt can also result from misalignment of drive components or from the effect of internal vibration on the objective lens of a drive. Each of these elements must be properly managed in order to assure high quality and successful disc interchange.