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 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.