[As it turns out, this is one of those posts I’ve always meant to do but never got around to it. It’s been sitting in my drafts box since March 2013, and I’m glad that I have now actually finished it! The photos were “pre-watermark” era, and this posting required a marathon amount of research effort to make.]
Before CD-RW drives became ubiquitous, there were “optical disc” drives on the market. Many of them could write to discs inside cartridges over and over, although some came in WORM (write once, read many) format as well. I always remember wanting to purchase one, although the costs were high and the availability was relatively limited. They were available from several vendors, and were standardized as ISO standards, and were commonly used for data archival.
Once the CD writer became common place and low cost, they’ve become fairly rare and obsolete, due to the high media and drive costs, it was hard to ensure there was a drive to use your discs with. They also never attained the high-speed reading and writing that their CD-R and CD-RW counterparts did, although these earlier technologies tended to have better defect management strategies leading to higher reliability.
Today, these discs are almost forgotten – but I feel that it’s worth taking a look back at these formats. Even just looking at the discs, the structure is relatively mesmerizing. Sometimes, when looking back, given the high ambition of some of these formats to replace the floppy disk, it’s surprising they didn’t get more traction and survive to the “modern” era. That being said, drives for some of these formats may still exist, although they are likely to exist for compatibility reasons rather than as a viable storage medium.
Magneto-Optical (MO) Discs
The first technology with some adoption was the Magneto-Optical disc introduced in 1985, commonly shortened to just MO. MO discs are enclosed in cartridges and are provided in 90mm and 130mm formats, at a variety of capacities depending on the generation of disc. The disc itself is made of a ferromagnetic coating which exhibits the Magneto-optic Kerr effect, whereby the polarity of reflected light from the surface rotates as a function of the magnetic field stored in the material. As a result, the discs are “optical” in the sense that a laser is used with polarization filters to read-out the data, although recording involves using the laser at higher power to heat the material past its curie point which is then recorded by an electromagnet (or fixed magnet in some implementations) riding on the other side of the disc, thus encoding the magnetic field. This is pretty much a similar system to that used by Sony’s Minidisc.
Magneto-optical discs used in computer storage use a fixed hard-sectoring scheme, with sector sizes of 512, 1024, 2048 and 4096 bytes being common, which is up to eight times the size of sectors normally used with other media (512 bytes) at the time. Such a system improves the overhead due to formatting. Recently, large sectors have seen a resurgence in the form of Advanced Format hard drives (4096 byte sectors).
MO discs can be single or double-sided, and have capacities ranging from 128Mb to 2.3Gb for 90mm discs and 650Mb to 9.2Gb for 130mm discs. The 130mm format generally required relatively large size drives which were unfriendly for consumers, but offered recordability on both sides of most cartridges (depending on their type) by flipping the cartridge over and re-inserting it into the drive. The 90mm format was more consumer friendly, about the size of two floppy disks stacked, but it offered inferior capacities in part due to being used on one side only and due to the lower recording surface area.
Magneto-optical storage systems typically had verify-after-write functionality and hardware defect management abilities, which was duplicated in some later formats (DVD-RAM, BD-RE), improving data integrity at the cost of write speed.
Original MO systems were write-once read-many, otherwise known as WORM (WO), but later systems were rewritable (RW). Later WORM cartridges still continued to be used for archival purposes, but used irreversible effects instead of pure MO (e.g. laser ablation).
In order to ensure interchangeability and compatibility between different vendors, the physical disc characteristics, as well as the data structure stored on the discs and the defect management mechanisms were very much standardized in a series of ISO standards and their corresponding ECMA standard counterparts. Standards relating to the physical discs include the following, sorted in chronological order, and then by cartridge size:
- ISO/IEC 9171-1:1990 (130mm, details characteristics of unrecorded WO cartridges)
- ISO/IEC 9171-2:1990 (130mm, details recording characteristics of WO cartridges, 512 and 1024 byte sectors)
- ISO/IEC 10089:1991 (130mm, details testing, operating, storage, physical mechanical characteristics, magneto-optical characteristics, two formats for sector/track formats, error correction codes, modulation methods and quality of signals for RW cartridges, 512 and 1024 byte sectors)
- ISO/IEC 11560:1992 (aka ECMA-153, 130mm, similar descriptions to 10089, but for WO cartridges instead, 512 and 1024 byte sectors)
- ISO/IEC 10090:1992 (aka ECMA-154, 90mm, covers RW and ROM cartridges, physical and mechanical cartridge details, formats of data embossed and user written, MO characteristics and write quality, 512 byte sectors)
- ISO/IEC 13481:1993 (aka ECMA-183, 130mm, 1Gb RW and WO, 512 and 1024 byte format)
- ISO/IEC 13549:1993 (aka ECMA-184, 130mm, 1.3Gb in RW, P-ROM, O-ROM and WO formats in 512 and 1024 byte sectors)
- ISO/IEC TR 10091:1995 (130mm, details WO formats A and B design decisions with implementation examples, 512 and 1024 byte sectors)
- ISO/IEC 13842:1995 (aka ECMA-195, 130mm, 2Gb of type RW, P-ROM, O-ROM, WO with Type B (one side unused) and Suffix -R indicating opposite side has reverse track spiral. Formatted with 512 and 1024 byte sectors)
- ISO/IEC 13963:1995 (aka ECMA-201, 90mm, 230Mb RW, P-ROM and O-ROM formats with 512 byte sectors)
- ECMA-223 1995 (90mm, 385Mb RW format with 512 and 1024 byte sectors, did not become an ISO standard)
- ISO/IEC 14517:1996 (130mm, 2.6Gb in RW, P-ROM, O-ROM, DOW, P-DOW, WO, WO-DOW formats with 512 and 1024 byte sectors)
- ISO/IEC 15041:1997 (90mm, 640Mb in RW, DOW, P-ROM, P-DOW, O-ROM formats with 512 and 2048 byte sectors)
- ISO/IEC 15498:1997 (aka ECMA-239, 90mm, 650Mb HS-1 “hyper-storage” RW, P-ROM and O-ROM formats with 2048 byte sectors)
- ISO/IEC 15486:1998 (aka ECMA-238, 130mm, 2.6Gb irreversible WORM, 1024 byte sectors)
- ISO/IEC 15286:1999 (130mm, 5.2Gb in RW, P-ROM, O-ROM, DOW, P-DOW, WO, WO-DOW modes with sector sizes 512, 1024 and 2048 bytes)
- ISO/IEC 18093:1999 (aka ECMA-280, 130mm, 5.2Gb irreversible WORM, 2048 byte sectors)
- ISO/IEC 22092:2002 (aka ECMA-322, 130mm, 9.1 Gb, RO and WO formats, 2048 and 4096 byte sectors natively, with 512 and 1024 bytes achieved by emulation on top of 4096 byte format)
- ISO/IEC 17346:2005 (aka ECMA-351, 90mm, 1.3Gb, RW type, 2048 byte sectors)
- ISO/IEC 22533:2005 (aka ECMA-353, 90mm, 2.3Gb, RW type, 2048 byte sectors)
As can be seen by the extensive list of standards, MO discs and drives came in many different generations with different densities and types. As a result, matching the media and the drive can be quite important, although newer drives tend to be backward compatible with older media (at least, to read). The standards specify many things, some rather fascinating, about the media response, physical media layout with manufacturer testing areas, VFO areas, defect lists and spare areas, disc metadata information areas including laser power and waveform requirements, reflectance, amplitude, polarity etc. Some of the nomenclature used by the standard include:
- WO – synonymous with WORM, but applied to early MO type discs.
- WORM – write once, read many – generally applied to the irreversible type.
- RW – rewritable
- DOW – direct overwrite, indicating a method of recording to rewritable discs that do not require a first erase pass
- P-ROM – partial read-only memory, indicating portions of the MO disc have been factory embossed with pre-set data, with the remainder usable for rewriting.
- O-ROM – optical read-only memory, indicating the whole disc has been factory embossed with pre-set data and can be read by reflectance mode with no reference to MO effects.
- P-DOW – partial direct overwrite, indicating portions of the MO disc are factory embossed with pre-set data, and the remainder is suitable for use in direct-overwrite mode.
- WO-DOW – write once, read many using direct overwrite mode
- Type B – indicates B-side is not to be used
- Type -R – indicates B-side has reverse track spiral, intended for dual-laser systems to access both sides of the cartridge simultaneously.
The standard also gives us an opportunity to track the development of MO storage over time, by plotting the ratified capacities by year. By comparison, the 530/650/700Mb CD-R and CD-RW only started to gain popularity in about 2000, thus the 130mm MO discs were superior in capacity, and the 90mm MO discs were of a similar capacity at the time. Despite this, MO was not highly popular in the consumer space.
It is also clear, that while MO might have been introduced to the market in 1985, it seems that it didn’t really take off until the 1990’s once the format was standardized. Major uses for the cartridges included robotic media libraries, for enterprise back-up and archival. HP offered quite a range of MO products, as can be seen from their datasheet, as did Sony in 90mm and 130mm, although it seems this may no longer be the case. Further research into looking at data sheets for drives suggests they were quite advanced with many error conditions detected, including the need for cleaning, laser wearout, discs too defective.
As I was rather fascinated by MO technology, I ordered some used cartridges so that I could have some to examine.
130mm 3M Rewritable Optical Disk 590Mb 512 byte sectors
The disc itself was supplied inside a rigid plastic casing, with a grey tab that could be pushed to release the cartridge. The cartridge spine was printed with a barcode, probably for automated library units to use, with recesses for spine labels to be put on.
Extracting the cartridge out of the case shows that no labels have been applied to the disc, and it could be possible that this disc was never used.
It is a double sided RW MO disc with part number 15175. It has a 512 byte fixed sectoring and a storage capacity of 590Mb (also known as 650Mb, possibly unformatted or RAW capacity).
The cartridge was held together by screws, so I took the screws out and opened up the insides. In the corner, we can see a disc “brake” to stop the disc from spinning when the shutter snapped closed. The disc assembly was double-sided and relatively thick and weighty, thus there would have been quite a bit of inertia when these were spinning at 30Hz (1800rpm). The spindle attachment is made by a set of embedded metal plates with a precision hole in the centre. The hard sector arrangement can be seen as embossed lines radiating from the centre, as these early MO discs used CAV.
Essentially, the disc itself is made of two discs laminated together, back to back, which explains its weight and thickness. The specified thickness in the standard is up to 3.20mm, with a mass up to 120g.
As a bonus, this disc was so complete that it even came with a set of unused labels, with a warning on an instructional label of “so not place this label on optical disk.” The ISO standards were very strict on the mechanical dimensions on the cartridge to ensure proper operation.
130mm Digital Optical Technologies DR-512
This is a “high speed” 130mm 3600rpm RW MO cartridge with 512 byte sectors. This is twice the reference speed of 1800rpm in ISO10089 (30Hz). This media may have been used with special drives that were capable of the higher speed access, but likewise, is likely to have been also compatible with other drives owing to the ISO standardization of recording and reading characteristics.
The disc came in a plastic fold-open library case.
The disc itself is in a translucent frosted plastic cartridge, secured by screws. As it follows the same standard as the 3M cartridge, it has the same shape. The disc itself also has radial embossed areas for the hard sectoring, but the media has a more “grey-blue” colour to it rather than the brown of the 3M. This indicates the use of a different metal alloy for the MO layer, and it’s likely the metadata on the disc would reflect that. However, drives complying to standards don’t need to worry – they just read and write as per the manufacturer provided metadata.
It too, was supplied with unused labels.
130mm HP 5.2Gb Rewritable Optical Disc, CC Format
A more modern MO cartridge is this one. Its accompanying standard specifies a rotation speed of 50Hz (or 3000rpm). The CC format nomenclature indicates it’s a continuous composite servo format (Format A, most common), rather than the sampled servo format (Format B). It is a 2048 byte per sector type disc. This disc was used, and was supplied bare.
Due to the more advanced technology used in this disc, we can see that the fixed pattern marks are shifted and the disc is split into zones, using logical zoned CAV access.
90mm Verbatim 540Mb Rewritable Optical Disk, 512 byte
Just when you might be thinking that I forgot all about 90mm discs, I decided to fetch a few unused ones to take apart and feature on the site. Probably not money well spent, but still worth it just to quell my fascination. This is a 540Mb disc with 512 byte sectors, which is a 640Mb disc according to the standards, with a package copyright date of 1996. Such discs operate at 50Hz (i.e. 3000rpm).
The disc is housed in a plastic jewel case that flips open like a Compact Cassette box. The above cardboard sleeve fits into the jewel case, with one side printed in colour, advertising the disc itself, and the inside marked with lines and space for entering notes.
Inside, it is provided with two labels, with care instructions and warranty information printed on the backing sheet. The labels themselves are vaguely 3.5″ floppy-disc sized.
The disc even looks vaguely like a floppy disc, but it has a height of about twice as thick, and notches and bevels in different places. There is a write protect slider, in the opposite side to a floppy disk. Similarly to the floppy and the 130mm cartridges, the disc is protected by a metallic shutter.
Unlike the 130mm cartridges, 90mm cartridges can only be used in one way, so the shutter only sides open in one way. The disc is also thinner, and the rear reflective coating is visible when viewed from the top.
The underside shows the pattern from the fixed sector size layout, with a logical ZCAV strategy resulting in the embossed data shifting around in each zone with a total of 18 bands. The underside is a brownish colour similar to the 3M disc.
90mm Fujitsu Dynamo (Gigamo) 1.3Gb
This is another disc, supplied as new, from a relatively recent generation of MO. It sports the Gigamo branding, for 90mm 1.3 and 2.6Gb MO discs. This is a 1.3Gb disc from Fujitsu, with Media ID function, allowing for a unique media ID to tie content to a given disc. It has a rotation speed of 50Hz (or 3000rpm) and 2048 byte sectors.
Similar to the disc above, it is supplied in a jewel case with a cardboard insert, with this insert featuring perforations to tear the wrap-around so that the internal disc label can be seen through the jewel case. It features an area for user written notes on the inside and a warranty statement.
Two labels are provided, with storage hints on the adhesive label backing paper.
The disc itself follows the same form factor as the other 90mm disc, instead, having a frosted translucent plastic shell.
Likewise, this disc uses logical zone CAV, with 18-bands. The media colour is a greyish-blue, like the later 130mm cartridges.
It seems that MO technology had officially hit its end of life, with Sony’s release of, and then discontinuance of, the Hi-MD disc offering 1Gb of storage in a Minidisc form factor. Hi-MD recorders offered USB mass storage compatible modes with both regular Minidiscs and Hi-MDs, offering about 1Gb on Hi-MDs and about 250-300Mb on regular MD discs formatted using the more efficient Hi-MD encoding, all with 2048 byte sectors. However, it was already too late at this stage, as Minidisc was already well fading out of relevance.
Computer based MO-storage seems to have also hit its end of life, with the last manufacturer of MO discs (Sony) claiming they have ceased production as of 2015, and no drives being produced since 2010 according to Maxoptix. Second hand refurbished stock continue to be traded and used where necessary to access records on MO media.
Phase-Change Dual (PD)
Unlike MO, Phase-change Dual (PD) discs are single sided and are more like rewritable CDs in caddies, with a capacity of 650Mb, and utilizing phase change technology similar to that employed by CD-RWs.
Phase change technology works by having a layer of crystalline material, which, when heated and cooled in different ways, they can re-form in amorphous and crystalline states which have different reflectance, thus representing the data to be stored.
Despite this, PD discs are different to CD-RWs in that they have a hard-sector format, and use a non-removable cartridge for the disc. It offered backwards compatibility in that PD drives could read CD-ROMs placed in caddies, it also failed to gain significant traction. It was so obscure, that as a child, while I was aware of MO drives, I wasn’t aware of PD drives at all.
Illustrated here is a CD-ROM caddy used by a Macintosh, which can be opened up for exchanging of the disc inside. PD drives were compatible with CD-ROMs placed inside caddies. PD discs could not be removed from their caddies.
The Phase-change Dual format is specified in ISO/IEC 15485:1997, aka ECMA-240, and specifies a RW format and a WORM format. The nominal rotational speed is 33.8Hz, or 2028 rpm. The standard specifies the use of a continuous composite servo tracking method, and the use of 10 data bands, with inner and outer test zones, and the control zone being innermost. The sector size is 512 bytes, and the discs have hardware defect management systems.
This format was much more elusive compared to MO, with very little information available. As a result, I also made it a point to buy some used PD discs to examine.
Inside, you get two disc labels, and two spine labels. It also contains a user guide, suggesting this was still in the early days of PD, which details key characteristics of the technology from a user’s perspective. This was copyright dated 1997, the same year the PD ISO standard was ratified.
At this time, 3M were changing their name from 3M to Imation, and this was another piece of material included with the disc.
The cartridge itself is grey and opaque, with a shutter that only sides in one direction due to the single sided nature of the disc. The shell itself is pressed with the PD logo and has an interesting “wavy” line on it.
Seeing as this disc had virtually identical characteristics to the next disc, I didn’t end up photographing the Imation any further.
This is a PD disc from a different vendor. Here, we can see the plastic sleeve which the disc is enclosed in, along with the edge printing design. The rear contains care information for the PD disc.
Inside the bundle, we are supplied with labels for the cartridge, and on the backing contains the short warranty statement.
For all intents, the cartridge itself looks identical to the Imation, with the exception of a different colour scheme.
Looking through the slot, we can see the disc is similar to that of a CD, utilizing the same sort of spindle hub arrangement. The zoned hard sectoring is visible, with the disc currently rotated to an angle where all the embossed sector markers “line up”. The underside has a hazy golden brown colour.
The top side is reflective, showing the reflective layer and protective over-coat, but is not used for recording.
It seems this format has completely disappeared with very few traces, indicating it was never really popular at all. There were no further PD-related ISO or ECMA standards apart from the initial 650Mb specification.
When you hear of DVD, the first thing that probably crosses your mind is DVD movies in the form of pressed DVDs and recordable and rewritable DVD’s in the DVD-R, DVD-RW, DVD+R and DVD+RW formats. Indeed, these formats are all very similar, utilizing a spiral track recording without any hard sectors, although had different densities at different stages of their life (initially, DVD-R at 3.95Gb, then 4.7Gb and finally 8.5Gb (dual layer)).
However, there was a 2048-byte hard sectored version of DVD that was intended for use with computers for storage, known as DVD-RAM. These discs were less popular, more expensive, slower and generally saw limited adoption as many ordinary drives could not read them. DVD-RAM did, however, inherit the heritage of having integrated defect management, verify after write abilities for more robust data storage and had a claimed cycle life of 10,000 to 100,000 writes, which was over 10 times the claimed cycle life of the regular -RW and +RW formats (which often could not make even 100 cycles due to physical damage).
DVD-RAM was available in both single sided and double sided 120mm varieties, at a set of speeds, launching at 1x with a capacity of 2.6Gb per side, followed by 2x, 3x, 5x, 6x, 8x, 12x and eventually 16x (DVD-RAM2) with a capacity of 4.7Gb per side. Later, 80mm versions were released as well, with a capacity of 1.46Gb per side.
Generally, discs faster than 5x have been hard to come by, with 12x and faster discs virtually unseen. DVD-RAM also had optional cartridges, with earlier drives mandating the use of cartridges. The cartridges had tabs which could be broken to release the disc inside for use outside of the cartridge, but some drives would refuse to write to the disc once it was removed from the cartridge. Later drives did not use cartridges at all. Discs were commonly used for data and video storage, with some Panasonic DVD recorders featuring catch-up recording but only on DVD-RAM discs.
DVD-RAM is standardized in several ISO standards:
- ISO/IEC 16824:1999 (aka ECMA-272 120mm 2.6Gb per side)
- ISO/IEC 17592:2004 (aka ECMA-330 80mm 1.46Gb per side and 120mm 4.7Gb per side)
- ISO/IEC 16825:1999 (aka ECMA-273 Mechanical Cartridge/Case)
- ISO/IEC 17594:2004 (aka ECMA-331 Cases for 80mm and 120mm)
Despite the rarity, high cost and compatibility limitations, I have had first hand experience with DVD-RAM, and I did end up having a collection of discs purely to test for their reliability and to archive data on. All of my discs were of the 4.7Gb variety, of 2x, 3x and 5x. Interestingly enough, DVD-RAM has been entirely solid for me, albeit slow.
The cartridges themselves are mechanically standardized, and feature a sliding shutter that slides both ways for reversable insertion. Each side is given a name, A or B. Write protect switches can be seen near the spine edge, along with an area for labelling. Pictured below is a TDK 2x 9.4Gb DVD-RAM (left) and a Panasonic 3x 9.4Gb DVD-RAM (right).
Discs can be extracted from the cartridge after breaking retainer tabs on the cartridge, which is an irreversible operation. However, the disc can be re-inserted into the cartridge, and the cartridge closed again, but the drive will understand that the disc has been removed from the cartridge before and may refuse to write to it.
Double sided discs are labelled on the inner area with printed markings identifying which side of the disc is A and B. However, to a drive, the sides are both identical 4.7Gb recording surfaces. Below is an image of the same TDK disc (left) and Panasonic disc (right) once extracted from the cartridges. Note that the TDK disc is somewhat paler in colour, and brighter than the Panasonic 3x disc. There are 35 zones for the data area on a 120mm disc.
The fixed sectoring embossed areas can be seen to make a rather impressive pattern across the disc. On a recorded disc, the data is recoded in between these marks, with a guard band zone between each zone resulting in clear concentric areas.
On an unwritten disc, as below, we can see the media is mostly a uniform colour except for the embossed areas, and a region in the centre used for manufacturing testing and servo alignment.
Discs were also available in single-sided format in jewel cases. Following are some examples of these.
Maxell 3x DVD-RAM
Maxell 5x DVD-RAM
The underside was not pictured as it looks identical to the one above.
TDK (Ritek) 3x DVD-RAM
This one had a printable top, but I can’t see why you would have that for a rewritable disc.
Discs were also made in the 80mm form factor, although didn’t find much use. These discs contain 14 data zones. The below is a Panasonic 8cm single sided bare disc that I purchased.
These discs could be used similar to hard drives, and formatted with FAT32 or UDF (commonly). The discs would write faster if hardware verify-after-write was disabled, but then losing the security that comes with the feature. Write speeds of 2x media were linear across the surface, with faster speed discs generally showing different speeds across zones, like a zoned-CAV strategy.
Optical discs in general, have been declining in popularity recently, primarily due to the rise in high capacity, affordable hard disk storage. Optical discs, by comparison, are slow, offer limited capacity and can have reliability issues. They do still have functions where immutability is concerned, and for archival usage, although cartridge-based fixed-sector style discs were only adopted by a limited number of home consumers.
Fixed sector discs offered higher reliability compared to common CD-RW, DVD+RW and DVD-RW solutions by offering better addressability for random writes, and inbuilt defect management which was drive-independent. They also utilised higher quality materials which allowed for a greater number of write cycles than their counterparts, and their cartridges served to protect them from damage. The fixed sectors were produced through embossing of the disc at manufacture time, which produces regular block shaped patterns on the media. Unfortunately, due to the complexities of manufacturing such discs and their cartridges, and their lack of popularity, even the media was too expensive and bulky for consumers. It is one reason why BluRay had abandoned its cartridge format just prior to release, as if it had persisted, it was likely to meet the same resistance from consumers.
Cartridge based optical discs continue to live on in the form of UDO catridges, which are based on blue-violet laser phase-change technology, mainly adopted for professional and archival applications as a replacement for magneto-optical technologies. They utilize 8192 byte sectors and are standardized in ISO/IEC 17345:2006 (also known as ECMA-350, first generation, 30Gb) and ISO/IEC 11976:2008 (also known as ECMA-380, second generation, 60Gb). Despite this, it seems that increasingly low-cost magnetic tape, hard drive and flash based storage solutions have eroded the market space that was once dominated by optical disc technology.
Sadly, I don’t have any MO or PD equipment myself, so actually using the media would be very unlikely. However, DVD-RAM is still supported by some DVD-writers today, and is thus not completely obsolete at this stage.