Overview

A sampling of one hundred recorded discs for each brand is divided into six groups (Table 2). Each group of discs is exposed to one of six stresses, which are combinations of temperature and relative humidity as described in section 4.3. After each period of incubation, each disc shall have its BLER (in the case of CD type media) or inner parity (PI) errors (in the case of DVD type media) measured (Fig. 1). Data collected at each test point for each individual disc will be used to determine a time to failure (usually measured in hours) for that disc. The times to failure for discs at each stress are fitted to a log-normal distribution to determine a mean time to failure for that stress. The resulting six mean times to failure are regressed against temperature and relative humidity according to an Eyring acceleration model. This model is then used to estimate the distribution of lifetimes at a preferred usage condition.

Fig. 1: Example of a testing and incubation timeline for the six stress conditions.

Method

The process of accelerated aging requires that discs in each stress condition be ramped from ambient conditions to that stress condition in an environmental chamber, maintained at that stress condition for a particular period of time and then ramped back to ambient conditions. This incubation cycle will occur a minimum of four times and possibly more during the course of testing. The ramp duration and conditions must be chosen to allow sufficient equilibration of absorbed substrate moisture. Large departures from equilibrium conditions can result in the formation of liquid water droplets inside the substrate or at its interface with the recording layer. Gradients in the water concentration through the thickness of the substrate must also be limited. These gradients drive expansion gradients, which can cause significant disc curvature.

Table 1 — Temperature and RH ramp profile

T amb and RH amb are room ambient temperature and RH, T inc and RH inc are the stress incubation temperature and RH, RH int is the intermediate RH that, at T inc , supports the same equilibrium moisture sorption in polycarbonate as that supported at T amb and RH amb (see Table 2).

Fig. 2: Graphically portrays one incubation cycle at 80 ºC and 85 % RH.

A system independent of the chamber control system shall be used to monitor the temperature and the relative humidity conditions in the test chamber during the stress test for independent verification of every chamber conditions.

Conditions

The stress conditions used in this study are shown in Fig. 3. They include the five stress conditions specified by the ISO 18927:2002 standard, with an additional stress at 70 °C and 70 %RH. Table 2 specifies the temperatures, the relative humidity (RH) , the initial incubation cycle durations, the minimum number of incubation cycles and the sample distributions for each stress condition. If necessary, more incubation cycles may be added or the duration of the incubation may be increased to determine the time to failure of the media at any specific stress. A separate group of specimens is used for each stress condition. This procedure constitutes a constant-stress test plan. All temperature stress points have an allowed range ±2 ºC and all relative humidity stress points have an allowed range of ±3 %RH.

Fig. 3: Stress conditions of acceleration aging.

Table 2 - Summary of stress conditions

The intermediate RH (RH int ) in Table 2 is calculated assuming a 25 ºC and 50 % RH ambient. If the ambient conditions are different, the intermediate RH to be used is calculated using the equation:

RH_int = (0.24 + 0.0037T_amb) / (0.24 + 0.0037T_inc) * (RH_amb)

where

T_amb is the ambient temperature in units of ºC ;
T_inc is the incubation temperature in units of ºC ;
RH_amb is the ambient RH.

The stress conditions tabulated in Table 2 offer a sufficient combination of temperature and RH to satisfy the mathematical requirements of the Eyring model to demonstrate linearity of either natural logarithm (ln) of the max BLER (ln(max BLER)) versus time in the case of CD type media, or ln(max PI errors) versus time in the case of DVD type media, and produce a satisfactory confidence level to make the desired conclusions.

Specimens

Test specimens shall be nominally identical with regard to substrate groove structure, layer structure, coating composition, recording capacity, and age prior to test initiation. Media should be chosen from different lots and production lines in order to be representative of normal process variations. All discs shall be maintained within the manufacturer's shipping and at ambient storage conditions prior to recording. Table 3 and Table 4 describe the recordable CD and DVD media chosen as test specimens for this study.

Each Brand discs will be recorded disc-at-once with a single session recording. In other words, the disc shall be recorded such that the entire disc is written to in one pass without turning off the laser. Any CD type disc that, after recording, meets the Yellow Book (Philips-Sony Compact Disc Read Only Memory System Description) specification (also described in the ISO/IEC 10149 or EMCA-130 standards) can be included in the remainder of this study. Likewise, any DVD type media that, after recording, satisfies the DVD specification (described in ISO/IEC 16448:2002 or ECMA-267, third edition, April 2001 standards) can be included in the remainder of this study . Any specimen that exhibited unusual error rates immediately after recording is not included, and care should be taken to ensure that initial error rates were consistent through out the full diameter of the test specimen.

For CD-R discs:
Since the emergence of CD-R technology, a number of different dye types have been used as the recording layer, including Cyanine, Phthalocyanine, and Metallized Azo, or their hybrids [7]. However, due to current conditions in the dye market, a single dye type has become the most popular in CD-R media and is based on the Phthalocyanine dye. Seven large samples of several popular brands were tested in this study. Due to limitations with the CD analyzer, it is not possible to include CD-RW type media in this study.

For recordable and rewritable DVD discs:
There are several different types of recordable DVD available, including DVD-R for general, DVD-R for Authoring, DVD+R, DVD-RW, DVD+RW, DVD-RAM and more recently dual layer DVD. Not all types are compatible with each other but could potentially be used in archival applications. In this study, we will examine DVD-R for general, DVD-RW, DVD+R and DVD+RW. Fifteen large samples of DVD-R, DVD+R, DVD-RW, and DVD+RW, encompassing several popular brands, were used for this study.

For CD-ROM discs:
For comparison, 100 CD-ROM discs and DVD-ROM discs divided in to six groups are included and will be subjected to the same stresses and test conditions as the recordable discs. All CD-ROM discs are provided by LC, while NIST provides all DVD-R, DVD+R, DVD-ROM and CD-R media.

For the shelf time test:
Media included in the shelf life study are the same brands of media used in the aging studies. Discs from each brand are split into six groups for each stress condition. After every aging period, one blank disc from each brand will be recorded and tested to determine its useful shelf life. Attempts to achieve one successful recording will continue until all the media is used up.

Parameters for Analysis

BLER (block error rate) for CD-R
The ratio of erroneous blocks to total blocks measured at the input of the first (C1) decoder (before any error correction is applied) as defined in ISO/IEC 60908. The maximum BLER is measured as the biggest number of BLER at anywhere on a disc.

Inner Parity Errors for DVD
Maximum of PI errors are less than 280 as defined by the DVD specification , or ISO/IEC 16448: 2002 or ecma-267 , third edition, April 2001.

Measuring & Collecting Data

This study uses the measured maximum BLER for recordable CD and CD-ROM and the measured maximum inner parity (PI) errors for recordable DVD as a high-level estimate of the quality of the media at the time of measurement. The purpose of measuring BLER and PI errors is to establish a practical estimation of a compatible system's ability to read the recorded data without uncorrectable error s occurring .

However, the correlation between actual loss of information and the measured maximum BLER and PIE actually depends on several factors including the playback system as well as the media quality. A BLER of 220 and the PI errors of 280 are conservative levels chosen by the recordable CD and DVD specifications as a predictor of the onset of uncorrectable errors and thereby end of life.

Media recording and initial data
There are many formats and speeds of discs (for example, 4X, 8X, 16X…) in the market and yet no unified standard for the drive write strategy. Therefore each recorder drive manufacturer develops their own drive to satisfy to the write/read requirements of recordable CD and DVD discs. As such, there is no guarantee that the recorder used to burn a disc is fully compatible with that disc. This can, and often does, lead to very high error rates immediately after recording the disc, which in turn can make aging analysis very difficult. It is therefore important to ensure that initial error rates are low by ensuring that the recording drive and the media are as compatible as possible[8,9].

To ensure recorder drive and media compatibility, four Disc Burning Stations (DBS) were built in the NIST laboratory with high-end computers and new recorder drives. Each drive must have the latest update to its firmware to ensure its best performance.

To determine the recorder with the highest level of compatibility with each brand of media, a limited number of recordings will be made for each brand by each recorder in the four DBS. The test specimens will then be burned as needed by the most compatible recorder and initial error rates monitored to ensure that the recorder continues to operate correctly.

Disc Burning Stations
In an effort to ensure the integrity of the results, all disc burning station (DBS) computer configurations should be the same. For this investigation, each disc burning station (DBS) consists of a Pentium-4 processor workstation with the following specifications:

Intel Pentium-4 Processor 3.6 GHz w/ 800mhz FSB,
1GB DDR2-533 MHz dual channel RAM,
Seagate 120 GB 7200 rpm SATA hard drive w/ 8 MB cache,
Microsoft Windows XP Pro w/ doc and media.

All test specimens will be indexed so as to associate it with the particular DBS and DVD recorder drive used to record that brand. The recording drives used in this study include the Sony DRU-510A, 700A , 800A, Pioneer DVR-A09 , Plextor PX-716SA and NEC ND -2510A . The drive chosen for the particular brand of media will be the drive shown to be most compatible with that media.

Analyzers
In order to monitor the change in the error rate during the aging, the discs are analyzed after each incubation cycle using optical disc analyzers. A CD analyzer capable of reading BLER (in the case of CD) and a DVD analyzer capable of reading PI error (in the case of DVD) are used.

For testing DVDs:
CATS SA300 DVD-R/RW PRO and CATS DVD+R/RW PRO from Audio Development are used to analyze the DVD media in this study. These systems measure recorded and unrecorded DVD discs and include the Pulstec SDP-1000 optical drive. All relevant parameters including inner parity error (PIE), outer parity error (POE), jitter, asymmetry and more are measured using this system.

For testing CDs:
CD CATS SA3 Advanced can measure all relevant parameters of CD discs. All measurements are performed according to optical disc industry standards. All relevant parameters including correctable BLER (including E11, E12, E21, and E22), uncorrectable E32 errors, jitter, asymmetry and more are measured using this system. Unfortunately, this system is not capable of testing CD-RW type media.

Collecting data
Test specimens are not analyzed in their entirety but analyzed at five test bands throughout the diameter of the test specimen. DVD test points are located at the following radii (in millimeters): 24.00-25.20, 31.95-33.15, 39.90-41.10, 47.85-49.05, and 55.8-57.00, from the inside to the outside of the disc. CD-R test points are at the following times (in minutes:seconds): 00:00-03:00, 19:00-22:00, 38:00-41:00, 57:00-60:00, 76:00-79:00.

Incubation Requirements

The temperature and relative humidity levels used in this study are chosen to ensure that there is no change of phase of the moisture within the environmental chamber over the test temperature range, and such that no condensation occurs on cooler sections of the chamber such as observation windows, cable ports, wiper handles, and so on. Any droplets caused by such condensation could damage the media and/or influence the error rates. Furthermore, the temperature of the testing environment must not be so high so as to cause any plastic deformation within the disc structure.

Practical experience shows that 80 ºC and 85 % RH is the upper limit for control within most accelerated test cells. Table 2 shows the complete stress conditions used in this study.

Disc samples shall be uncovered and placed vertically within the chamber. Discs shall be aligned so that their surfaces are parallel to the chamber airflow. A space of at least 2 mm shall be maintained between discs.

Environmental Chambers
A Parameter Generation and Control Inc (model: 9131-3119) 30 cubic feet environmental chamber is used to control precisely the temperature and relative humidity through various combination settings of temperatures (5 ° C - 85 ° C) and relative humidity (10% - 95%). The controlled accuracy is ± 0.2 ° C for temperature and ±0.5% for relative humidity.

Incubation time requirements
Since there are six stress conditions to be implemented, the media from each brand will be split into six groups (see Table 2). Depending on the error rates observed as the testing progresses, one to three chambers would be required for this study. Based on initial incubation cycle durations and the number of media to be tested, all initial incubation cycle testing can be complete in the timeline shown in Fig. 4. The most severe stress (80 ºC, 85 %) will be conducted first and tested after incubation to determine if the incubation durations should be increased and to what extent. In such a case where the incubation time is increased extensively, additional chambers can be used. Increasing the incubation durations should not increase the total duration of the study since the limiting factor is the number of analyzers available to test the large numbers of media. There are two analyzers available for testing DVD media and one available for testing CD media.

Fig. 4: Initial incubation cycle testing timeline.

10-Step Outline

The following is a brief outline of the steps required to estimate the life expectancy of information stored in a Recordable DVD system as a function of temperature and RH, as taken from the ISO 18927:2002 standard for CD-R and applies to this study also.

1) Determine the failure time for each specimen.

2) Determine, for each stress, the median rank of each specimen, and plot median rank versus failure time on a log-normal graph.

3) Verify that the plots for all stresses are reasonably parallel to one another. The log standard deviation for each stress may be calculated using standard techniques or estimated from straight lines drawn through the plots.

4) Calculate the log mean for each stress.

5) Regress log mean, temperature, and RH for all stresses using the reduced Eyring equation. Calculate the estimated log mean for the standardized temperature (25 ºC and RH 50 % RH). 6) Determine the acceleration factor for each stress.

7) Normalize all the failure times by multiplying each failure time by the acceleration factor for its stress.

8) Combine all normalized failure times and censored data into one data set. For this entire set, make one composite log-normal plot.

9) Estimate the log mean and the log standard deviation at the usage conditions from this plot, or the combined data.

10) Calculate confidence intervals for the survival function.

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