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Appeared on: Monday, February 20, 2006
Testing For The Next Generation Formats


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.


3. Quality Measures For HD DVD and BD

Assessing the quality of HD DVD and BD discs introduces new approaches to those accepted for DVD and CD and we look here at jitter, the most commonly understood quality metric.

Jitter, the difference between the idealized run lengths (pit and lands) and the actual detected run lengths, has been used in CD and DVD as a primary metric for determining the quality of the replicated disc.

When using a PRML approach, it has not been able to link regular (as used in CD and DVD) jitter directly to the quality of the replicated disc. Therefore different approaches have been taken to relate the output signals to disc quality.

HD DVD uses two methods for relating the quality of the recovered data to the disc quality - Partial Response Signal to Noise Ratio (PRSNR) and Simulated bit Error Rate (SbER) and Jitter in lead-in areas.

BD actually does use a form of Jitter measurement, but the data is first passed through a limit equalizer to improve signal quality.

Jitter on HD DVD

On HD DVD, the lead-in area has a much lower areal density. Put simply, the minimum feature size is much larger than in the data area (408nm in lead in compared with 204nm in data area). Jitter is a good measure of the pit quality, hence it is used where possible.

BD limit equalizer

BD uses a different approach. In BD, a limit equalizer is employed upon the signal. This is simply another method of boosting the signal of the smallest features (2T pit and land). The advantage of the limit equalizer, compared with the conventional equalizer used for DVD, is that it can boost the signal of the smallest features without increasing the ISI. After the signal has been processed through the limit equalizer, jitter measured in a conventional manner, as with DVD. It is very important to note than this approach is used only for evaluating disc quality; for data recovery, PRML has to be used.

As can be seen, the measurement issues for the next generation formats go substantially beyond simply working with smaller features. The coding concepts, the relationships between measurements and playability, and the links between measured effects and their manufacturing-process causes, are all unique.

In the next page you can read a brief description of the key measurements on each format and a comparison with DVD.

 

Contributed by DaTARIUS

DaTARIUS, will be presenting its BD and HD tester at the Media-Tech Expo in
Frankfurt (30 May - 1 June). For more info visit www.media-tech.net.

http://www.datarius.com/

postit@datarius.com


4. Appendix - Key Measurements on each Format
Formats Comparison DVD (SL/DL) HD-DVD (SL/DL) BD (SL/DL)
Capacity 4.7/9.5 GB 15/30 GB 25/50 GB
Wavelength 650nm 405nm 405nm
Numerical Aperture 0.6 0.65 0.85
Focused Spot Size ~1.08um ~.62um ~.47um
Focus Depth <1.8um <0.8um <0.5um
Tilt Margin/Tolerance 1 0.48 0.9
Track Pitch 0.74um 0.4um 0.32um
Indicative Pit Height ~104nm ~65nm ~65nm
Indicative Pit Width ~280nm ~200nm ~160nm
Minimum Feature Size ~400nm ~200nm ~149nm
Reference Scanning Velocity 3.49/3.84m/s 6.61m/s 4.92m/s
Data Encoding Method EFM+ ETM 17PP
Reference Channel bit Rate 26mbps 65Mbps 66Mbps
Data Detection Method Conventional Slicer (zero crossing detection) PRML PRML
Nominal Cover layer thickness 0.6 0.6 0.1/0.075
Nominal Space layer 0.055 0.025 0.025

 

BD Key Measurement Parameters What it says about the process Comparison to DVD
  HF Modulation Amplitude  Information on the depth and width of pits  I14/I14H
  HF Resolution  Information of the pit end wall angles  I3/I14
  HF Asymmetry  Information regarding the balance between shortest to longest pits, and how easy it will be to slice the data  Asymmetry
  Reflectivity  Thickness of reflecting layers  Reflectivity
  Jitter Conventional Equaliser-tilt compensated How well the data approximates the idealized data lengths Jitter
  Jitter with Limit EQ - tilt compensated (as above) Like Jitter but with a different method for digitising the data (see detailed description below)
  Random SER (R-SER) Overall quality of the manufactured disc  
  Burst Error Max Effect of localized defects/problems  
  Max total length of Burst Errors    

 

HD DVD Key Measurement Parameters What it says about the process Comparison to DVD
  HF Modulation Amplitude Information on the depth and width of pits I14/I14H
  Resolution Information of the pit end wall angles I3/I14
  Asymmetry Information regarding the balance between shortest to longest pits, and how easy it will be to slice the data Asymmetry
  Reflectivity Thickness of reflecting layers Reflectivity
  Jitter (Lead in only) How well the data approximates the idealized data lengths  
  Sber A measure of the overall quality (random errors)  


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