1. Drives - Critical For Measurement
Blu-ray Disc (BD) and HD DVD both present us with new formats and new manufacturing
processes. For the test equipment manufacturer the challenge is to integrate
the technologies and demands of the new formats into measurement systems that
fully inform the disc producer on the quality of the media being produced as
well as enabling them to establish links between product quality and the manufacturing
processes.
DRIVES – CRITICAL FOR MEASUREMENT
Repeatability, reproducibility and
Accuracy
As with DVD and CD testing, our key goal when designing a BD/HD DVD test system
is to ensure a high degree of repeatability, reproducibility and accuracy (in
that order) and this starts with the drive. The drive produces an analogue
HF signal, which is then processed with processing techniques that are highly
reproducible. Although electrical testing of optical media aims to be ‘testing’ the
disc, what we are really measuring is how the drive interacts with the disc.
If the drive itself creates additional defects within the analogue HF signal,
these could easily be misinterpreted as a disc problem.
The key design requirements for drives are specified in the physical format
specifications for HD DVD and BD, and it is critical that the drives used have
a very high level of reproducibility – with the absolute minimum of variations
between samples.
Although drive calibration may work for a narrow range of discs (disc that
themselves are similar to the calibration disc), it can never compensate for
significant differences between drives with the full gamut of discs seen in
a “real-world” production environment.
If the drives are not reproducible then it will be impossible to have a reproducible
test system to accommodate and correctly quantify the range of manufactured
product. To do this, close attention has to be paid to the quality and consistency
of the drive’s component parts and the consistency of key elements such
as the actuator, optical components, laser diode, etc.
Within the optics, failing to control the focused spot quality will have a
huge influence on the HF signal produced by the drive. With the high NA lens
used for Blu-ray, we must also add spherical aberration correction into the
drive to compensate for the differences in the layer thickness when switching
between layers.
A high quality of mechanical stability, especially from the spindle motor,
is also required –especially for the higher rotation speeds required
from the new formats. It is vitally important not to add to the disc’s
playability problems, errors due to spindle motor runout, excessive vibration,
or unstable sled movement.
2. Blue Laser - Understanding The Key Differences From DVD
Compared with CD and DVD, the blue laser formats present a range of different
technologies that need to be incorporated into the measurement tools.
Modulation Methods
Modulation methods are the means of encoding the source
data to the channel bit data recording on the disc.
The modulation scheme used for HD DVD is Eight to Twelve Modulation (ETM),
where 8 bits of data are converted to 12 channel bits on the disc. This
will follow the run length limit RLL (1,10) ie no run lengths (pit or land)
shorter
than 2 channel bits and no run lengths longer than 11 channel bits. With
sync bits, this can result in run lengths up to 13T.
The modulation scheme used for BD is 17 Parity preserve-Prohibit repeated
minimum run-length transition (17PP). Parity preserve (P) refers to the method
of ensuring the DC content in the data is zero. 17 refers to the minimum and
maximum allowable run length limits (RLL) for data features (e.g. the marks/spaces,
pits/lands) on the disc. So for BD we have a minimum data feature size of 2T
and a maximum data feature size of 8T for the data (compared with 3T to 11T
for the data features on DVD). Due to sync frames, feature lengths of 9T can
occur.
Digital Errors on BD and HD DVD
For HD DVD, we have a similar metric for digital errors as on DVD, which
is Parity Inner (PI) and Parity Outer (PO).
For BD, we have Random Symbol Error Rate (R-SER), Burst Error Max, and Max
total length of burst errors.
Data Recovery Methods: Partial Response - Maximum Likelihood (PRML)
For CD and DVD, the method for data detection was based on a zero crossing
point method, using a conventional slicer. Basically, when the analogue signal
(output from the optical pick-up) crosses a reference level, it indicates
a binary transition. This method has its limitations, notably when the feature
size (smallest pit/land) is less than the spot size, the modulation of the
light is relatively small (smaller the pit/spot size ratio the smaller the
modulation). Thus, the conventional slicer can create data with non-exact
mark/space lengths – otherwise known as jitter, and when this jitter
becomes greater than 0.5 of a clock cycle it becomes a bit error.
For HD DVD and BD respectively, the minimum feature/spot radius ratios are
0.88 and 0.85 of the ratio of DVD. This reduced resolution (low modulation
of light by 2T pits) means that it is much more difficult to have effective
data detection using a conventional slicer. Boosting the high frequency part
of the signal has limited effect because this also increases the InterSymbol
Interference (ISI), which is where adjacent pits and lands interfere with each
other. This is mainly a problem with the shortest run lengths, particularly
those that are smaller than the spot size.
Hence for HD DVD and BD, the Partial Response- Maximum likelihood (PRML) method
is used for recovering the data from the signal. It is not specified because
the specifications deal with the manufacture of the disc, not the drive. This
method is also more commonly used in hard drives.
Partial Response (PR) equalization is used to limit the effects of ISI, and
then a sequence of bits is evaluated to define the most likely sequence of
bits, based upon known allowed sequences. This is the Maximum Likelihood (ML)
detection and uses a Viterbi algorithm to determine the ML sequence.