We continue our Blu-ray disc recording tests. We used some of the latest BD-R/RE SL and DL media for these tests. Each disc was burned at the highest allowed recording speeds as well as in lower speeds. The LiteOn iHBS112 drive supports 12x burning for specific BD-R SL and 8x burning for BD-R DL media. For 12x recoding, we used BD-R discs certified for 6x. And that because there is no physical specification for 12x speed recording for BD-R media. However, most Blu-ray writers support burning at 8x and 12x on 6x-certified media.

We also used Erik Deppe' s Opti Drive Control software for testing the writing quality of each recorded BD-R/RE disc. The software could give you an idea of the writing quality of each disc. For BDs, the following parameters are measured:

• LDC (Long Distance Code): number of parity errors on LDC codewords per ECC block (32 sectors)
• BIS (Burst Indication Subcode): number of parity errors on BIS codewords per ECC block (32 sectors)

On a good disc, the average LDC should stay below 13 and BIS should stay below 15, according to the author of the software.

If you are interested in the data recovery methods of the Blu-ray discs, continue reading below. If not, skip the following text and proceed directly to the tests found later on this page :)

Blu-ray 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 (the 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 BD, the minimum 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 BD, the Partial Response- Maximum likelihood (PRML) method is used for recovering the data from the signal.

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.

The Blu-Ray disc is more sensitive to burst errors compared to the DVD system. Therefore, the error correction system of Blu-Ray disc should be able to cope well with long burst errors, rather with single (random) errors.

The maximum number of errors that can be corrected depends on the number of parity symbols added. For each two parity symbols added, one error can be corrected. But Blu-Ray uses a more efficient approach to correct the burst errors. It uses a burst indicator mechanism that can detect bursts of errors before the correction starts. The advantage of this method is actually the prior knowledge of the error locations on the decoding process.

These burst indicator used in the Blu-Ray format is called picket code. The pickets are columns that are inserted in between columns of the main data at regular intervals. The main data is protected by a Reed Solomon code, while the pickets are protected by a second independent Reed Solomon code. When decoding (reading), first the picket columns are corrected. The correction information can be used to estimate the location of possible burst errors in the main data.

A BluRay Disc Error Correction Block (ECC Block) can store 64 Kilobytes of user data. This data is protected by the Long Distance Code (LDC) which has 304 code words with 216 information symbols and 32 parity symbols giving a code word of length 248. These code words are interleaved two by two in the vertical direction such that a block if 152 bytes x 469 bytes is formed as shown in the picture above.

A Blu-Ray Disc ECC block contains 4 equally spaced picket columns. The left most picket is formed by the sync pattern at the start of each row. If the sync pattern was not detected properly, that can be an indication for a burst error similar to the knowledge that a symbol of a picket column had to be corrected. The other three pickets are protected by the so-called Burst Indicator Subcode (BIS). The BIS code words are interleaved into three columns of 496 bytes each. Both LDC and BIS codes are decoded by the Reed Solomon decoder.

Reference: BD-ROM Physical Specifications

For the first test we used a Panasonic BD-R SL for 6x (MEI RA1) disc. We burned it at 6x and 12x. Here are the results:

- Media Info

- Writing @ 6x (5.95xx average) in 15:55 min

The drive burnt the disc at 6x using the P-CAV strategy in 15:55 minutes.

- Quality @6x

It looks like both the LDC and BIS remain below the 13 and 15 limits, respectively. As such, this could be consider as a good burn.

- Writing - 12x (8.96x average) in 11:39 min

We burned the same disc at teh maximum supported speed of 12x:

The drive burned the disc at 12x using the CAV strategy in just 11:39 minutes.

- Quality @12x

This time the average LDC was higher 13. Both the LDC and the BIC rise after the 17.5GB mark of the disc and towards the end of the data area, as the recording speed was also increasing to reach the maximum 12x.