One lecture of material
CD-recordable and WORMs
The CD audio and CD/ROM standards were originally developed as a read-only medium. However, mechanical and optical technologies were rapidly developed to permit a writable CD format. These writable formats are variously called CD-R (CD-recordable), Write-Once Read-Many (WORM) or CD-WO (CD Write Once).
In spite of the fact that these names originally all meant the same thing, subtle differences between them have emerged in the culture. As a consequence, write-once read-many techniques that produce disks readable only on special machines (such as DMM or DRAW techniques) are usually referred to as WORMS. Techniques that produce disks readable on any CD-audio or CD/ROM reader (such as cyanine-based organic dyes) are called CD-Rs.
DMM techniques[1]
Direct metal mastering (DMM) techniques are the oldest of the CD writable technologies. This process creates glass master disks which can be read on special readers (thus justifying calling it a WORM disk) but are most often used as masters to create mothers and sons for small production runs of disks.
In this process, a glass substrate disk is covered with a thin (few nm) nickel flash. Then a copper layer (several hundred nm thick) is sputtered or evaporated over the nickel. A piezoelectric mechanical stylus is used to emboss pits in the disk. By correctly designing the diamond embossing tip and cleverly optimizing the write speed, it is possible to create disks that mimic conventional photoetched master disks.
Notice however, that although this technology creates disks that can be read -- the use of a glass master plate limits the number of machines that they can be read in.
DRAW technologies[2]
Like DMM technologies, Direct Read After Write (DRAW) technologies were originally developed as an alternative to conventional mastering techniques. In this process, the glass master disk is covered with a thin layer of plastic. A relatively high power laser (such as a 50 mW argon ion) is used to vaporize pits in the polycarbonate layer. A smaller HeNe laser is running right behind the argon to check for pit depth and length and adjust the feedback system accordingly.
Notice however, that although this technology creates disks that can be read -- the use of a glass master plate with no reflective layer essentially means that it can only be read in the mastering machine that created it.
CD-R systems
A number of different techniques have been tried to create a writable disk technology such that the disks can be read in any CD-audio or CD-ROM recorder[3].
Ablative: These techniques are similar to the DRAW process in that a high powered laser is used to physically ablate a thin metal film. Tellurium and its alloys are often used because of the low melting point of the metal.
Bubbling: These techniques rely on creating a bubble or blister in a material by means of heating specialized materials with a laser beam.
Optical methods: In these techniques, irreversible optical changes are made in the materials. For example, the reflectivity, index of refraction, or polarization characteristics can be changed.
Dye-based methods: The most popular current CD-R technologies are the dye-based technologies which use cyanine and phthalocyanine dyes. The dyes are sandwiched between the gold and the polycarbonate substrate. Interaction with the laser heats the dye and then creates a pit in the polycarbonate[4].
The main advantage of the dye-based technologies is that both the disks and the recorders are relatively cheap to manufacture. As of today, you can get a modest quality 2x dye-based recorder for under $2000. Top-of-the-line units run around $5000. Within a year, it is expected that 1x units will be available to the general consumer for less than $1000. There is a clear trend for dye-based recording materials to take over the consumer market in CD-R.
Archival use of dye-based recording materials.
The advent of the dye-based CD-R materials marks the first time that an economical CD recording technology was available to the general public.
One of the key applications that immediately appeared was the use of the CD-R materials for archival data storage. This type of data storage may be as simple as backing up your home PC -- or as complex as storing income tax records for all Americans. Either way, the issue of archival data storage and CD-R is a very hot issue at the present time.
There are several factors which may limit the archival lifetime of CD-R media. These are listed below in order of decreasing importance:
Aging of the phthalocyanine is a critical issue in evaluating the lifespan of CD-recordable disks. The optical characteristics of the phthalocyanine dye are critical in the recording process. A small change in the absorbance properties of this material can result in a disk error.
The photosensitivity of the cyanine and phthalocyanine dyes may be an issue in the archival stability of CD-R media. Cyanine and phthalocyanine dyes are organic materials that may be subject to damage under blue or UV lighting conditions. Although the Orange Book standard does include a "light fastness" test -- it is not clear that the spectrum of this light is a good match that found in typical office environments.
Anecdotal reports by 3M researchers suggest high block error rates when CD-R disks are exposed to sunlight. Although not formally confirmed or published -- such informal testing shows reason for concern.
Speed is clearly a critical issue for certification in an archival application. When using single and double speed access, virtually all manufacturers seem to assume that the Orange Book standard is adequate. However, at higher speeds, it is not so clear. The faster speed means higher power on the laser, and an associated higher risk with the writing process.
It is interesting to note that the recently released Yamaha Expert CD-100 recommends special media (beyond Orange Book standards) with their 4xS (4 speed) logo. Yamaha has established a complete certification program for their 4xS disks and will certify other manufacturers disks.
Additional indications of write-speed issues are indicated in tests by APEX company which suggest that the optimal writing power changes significantly with write speed[5]. Additionally, there are currently efforts underway by the Orange Book Study Group of Japan (OSJ) to include a "write-speed" specification in the subcode for the CD-R standard[6].
Both physical and logical standards for CD-recordable appear to be in a severe state of flux.
Physical standards: The current defacto physical standard for CD-recordable is the Orange Book. However, the recent introduction of the well-received Yamaha Expert CD-100 CD-recorder and the Philips CDD 522 is rapidly forcing development in the direction of 4x standards[7]. At this point, the Yamaha 4xS standard looks like the best contender for the standard of the next five years. However, Sony is also discussing another 4x standard, perhaps for disk release in 1995.
Logical standards: The current defacto logical standard is ISO-9660. However. mere are signs that this may be changing. ISO-9660 was developed in the mid-80's prior to multi-session recording capabilities. ISO-9660 does not support multi-session. Developments in multi-media technology are going to force multi-session development -- thus moving the technology away from ISO-9660. There is a new standard under discussion (Frankfurt, ISO 13490) which will support multisession. However, ISO 13490 is not currently well received.
As just one example, a key consideration in support of a new standard is support of the major industrial software houses -- in particular, Microsoft. Unfortunately, Microsoft has not indicated any plans to write a new version of MSCDEX to support ISO 13490 and has not announced any plans to support it in Chicago. Unfortunately, the problem of single versus multisession will NOT just go away. It does not appear that the ISO 13490 will triumph -- and it is not at all clear what will. This is a critical question for archival purposes, because it is possible that a new multisession standard may be more geared to multimedia applications where a higher BLER is acceptable in a tradeoff for speed, and less geared toward low BLER archival applications. Notice that the ISO or IEC standards are public domain -- but (at least currently) Kodak Photo CD is proprietary. Again -- this poses interesting questions for evaluation of disks for archival applications.
Qualifying CD-R for archival applications requires more than just qualifying the CD-R media. The problem of qualifying CD-recordable (CD-R) for archival application is somewhat different from the broader problems of qualifying CD-ROM and CD-R media alone. In archival applications -- the goal is to be able to reliably read data at some future data (typically several decades from the present). This means that qualifying CD-recordable for archival applications is a linked problem between media, recorders, readers, and software. Putting it bluntly, it is not enough for the media to have survived error-free for several decades if there is no technology that has survived to read it. This is important to consider, because several US companies (e.g., 3M and Kodak) are developing qualification standards for media alone. Valuable as these standards will be -- they do not address the archivist's problems.
CD-recordable is right on the edge of a rapid expansion phase. This generally means that formats, standards, and CD-R technologies may be obsolete in the near future. The consumer can now purchase a high quality 1x CD-recordable unit for under $2000. 4x units sell for around $5000 -- and the prices dropped nearly 30% in 1994. Philips has recently announced plans to introduce a 2x recordable with a 4x player for less than $1000 by mid-1995. This is a familiar pattern reminiscent of the period of very rapid growth following the release of the first consumer VCRs. Although this rapid growth is great for the consumer, it means that the archivist must consider the various available CD-R alternatives carefully. The hot new 4x CD-recordable technology of 1995 -- may be totally and completely obsolete in 2005. Thus, development must be planned to gracefully phase between technologies -- with the end goals of minimum error rates and maximum compatibilities -- rather than speed.
The technological development of the CD-R recorders is primarily overseas (Netherlands and Japan). As such, there is still not much effort being spent on developing CD recordable qualification standards within the US. Although there are several U.S. organizations working on development of standards for evaluating CD/ROMs (in particular, OSTA and AES) -- these are primarily oriented toward CD/ROM media.
CD-I
Compact disk interactive is a combined media which permits the simultaneous storage of audio, video, graphics, text, and data. In essence, CD-I is a multimedia CD/ROM data format.
CD-I systems are intended to be single media systems -- one where the disk contains both the program and the data. The inclusion of program data on the disk distinguishes them from the more conventional storage of audio and video format material on a CD/ROM. CD-I materials are typically intended to play on augmented television sets, rather than requiring a full PC system.
The CD-I data standard is essentially identical to the CD/ROM standard. The major distinguishing difference is that an 8-byte subheader following the regular 16-byte CD/ROM sync/address/mode header.
Byte number (starting at 0) Contents
0 0000 0000
1-10 1111 1111
11 0000 0000
12 Minutes - 74 max (+ hex A)
13 Seconds - 59 max
14 Block number within sec. (75
block/sec)
15 Mode
16-23 CD-I subheader (8-bytes)
24-2351 User data and EDC, ECC
In mode 1, the "blank" bytes in CD//ROM are moved out to make room for the header as:
Byte number Contents
0 0000 0000
1-10 1111 1111
11 0000 0000
12 Minutes - 74 max
13 Seconds - 59 max
14 Block number within sec. (75
block/sec)
15 Mode 01
16-23 CD-I subheader (8-bytes)
24-2071 User data (2048)
2072 - 2075 4 bytes of EDC
2076 - 2247 172 bytes of P-parity
2248 - 2351 104 bytes of Q-parity
CD-I also offers various levels of audio quality in order to maximize disk space utilization[8]:
Name Coding Sample rate # bits in Bandwidth Channels Size of 1
word second of
sound
CD-DA PCM 44.1 kHz 16 bits 20 kHz 1 stereo 171.1 kB
A ADPCM 37.8 kHz 8 bits 17 kHz 2 stereo 4 85.1 kB
mono 42.5 kB
B ADPCM 37.8 kHz 4 bits 17 kHz 8 stereo 4 21.3 kB
mono 42.5 kB
C ADPCM 18.9 kHz 4 bits 8.5 kHz 8 stereo 21.3 kB
16 mono 10.6 kB
Information phonetic
CD-I disks can also store video information compatible with either the US NTSC or the European PAL/SECAM format. Three standards of video resolution are supported[9]:
Name horizontal pixels vertical pixels
NTSC 360 240
PAL 384 280
Double resolution NTSC 720 240
Double resolution PAL 768 280
High resolution NTSC 720 480
High resolution PAL 768 560
Three picture encoding schemes are supported[10]:
DYUV: Delta Y-U/V coding, reduces a standard single picture to 85 kB (NTSC) 105 kB (PAL)
RGB 5:5:5: 16 bit color (5 each for R, G, and B, one for transparency). 70 kB (NTSC), 210 kB (PAL)
CLUT: Color look up table, permits 4-bit/16, 7-bit/128 and 8-bit/256 color animation. With reduction, 85 kB (NTSC) 105 kB (PAL)
MPEG: CD-I was the first consumer introduction of MPEG and a CD-I disk can hold over 60 minutes of encoded MPEG plus audio.
Photo CD[11]:
Photo CD is a special CD format developed jointly by Eastman Kodak and by Philips. The idea behind Photo CD is to provide a digital storage method for consumer photographs. The consumer would use conventional 35 mm film which would then be scanned by a Photo CD system. The digitized image would then be processed both for compression and to correct for exposure and color balance. These images would then be written to a CD-R for permanent storage. A typical CD would hold approximately 100 digitized photographs.
The Photo CD system scans images at 2048 pixels per inch. In relation to a 35 mm negative, this means 3072 pixels per line for 2048 lines. The RGB information is stored with 12 bit quantization. The encoding process is similar to NTSC, in that the color is separated into a single 8-bit luminance component and two 8-bit chrominance components.
A full Photo CD image would then include 2048 * 3072 * 24 bits = 18.87 MB. In order to reduce the file size, the chroma channels are two-times undersampled in both the vertical and horizontal directions. Thus, with undersampling, the image is reduced in size to 9.4 MB.
A clever hierarchical form is used to compress the data. Three low-resolution images are stored uncompressed. These low resolution images are Base = 768 pixels x 512 lines, Base/4 = 384 pixels x 256 lines, and Base/16 = 192 pixels x 128 lines. Higher resolution images (4Base and 16Base) are formed by successively interpolating and correcting high resolution residuals relative to the base file.
For example, to display a 4Base image, the Base image is interpolated along both the vertical and horizontal directions to obtain an uncorrected 1536 pixels by 1024 line image. The 4Base residual is then decompressed from its Huffman encoded form and used to correct the interpolated high resolution image. The result is a high resolution image at a fraction of the storage space.
Photo CD images are stored on the disk in conjunction with a construct called an Image Pac. The Image Pac contains photofinishing information, the encoded form of the image, and some microcontroller readable sectors to permit low end machines to read the image.
An Image Pack ranges from 4-6 MB in size. Additionally, the Photo CD contains an overview Pac with reduced copies of all of the image Pacs on the disk.
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