A record for the compact disk

A record for the compact disc: The electronics industry wants to sell us digital recording equipment - The record companies vetoed digital tape / Now the industry has developed recordable discs that they like even less


22 July 1989 by BARRY FOX
Magazine issue 1674. Subscribe and save


ELECTRONICS companies are rushing to perfect a partner for the compact disc; a CD that records as well as plays. One of the main incentives, besides the opportunity to sell an eagerly awaited product, is that the industry does not need to agree a single standard before it begins to sell us the new recording system. For the first time in the history of consumer electronics it has only to devise the best system for the job. In the search for a recordable CD that can be sold as cheaply as magnetic tape, researchers in Japan, the US and Europe have come up with a range of discs that all work differently. The common standard that they all aim for, however, is the ability to play back their recording on a conventional CD player.

The non-recordable CDs, which have been in the shops for about six years and whose annual sales will soon exceed those of vinyl LPs, store their data as a spiral of pits pressed in the surface in the factory. A laser 'reads' these data from the underside of the disc, as bumps. This is handy for the development of recordable CDs; the discs can use pits or bumps or even spots of different optical density or colour, depending on which method a manufacturer finds most suitable, as long as the playback laser can interpret them as pressed pits.

The electronics industry used to assume that any new recording medium for the home had to be capable of both erasure and re-use, in the same way as magnetic tape is. But in practice most people who buy blank audio and video tape seldom re-use it; they make a recording and keep it. The industry now reasons it can produce two types of recordable disc; a relatively cheap version for single recordings, the so-called 'write-once' discs or CD-Rs, and another for multiple recordings, the erasable discs or CD-Es.

The technology for making both versions of recordable disc has been known for 10 years, but the discs produced so far are expensive and used almost exclusively by the computer industry. With rival computer firms unable to agree on a standard, equivalent to the CD player in the consumer industry, recordable discs have had limited commercial success.

The first generation of CD-Rs has a thin coating of a reflective metal, such as tellurium. A recording laser, modulated by digitised sound, video or computer signals, burns patterns of holes in this coating. When the disc is replayed, with a lower-power laser, the holes are the only area of the surface that does not reflect light, and so they look to the playback laser like the pits of the pressed disc. Philips and 3M, the Dutch and American manufacturers, have been selling 'write-once-read-many-times' computer discs, known as WORMs, for five years. Although reliable, they cost up to Pounds sterling 100 apiece, far too expensive for most homes.

The first CD-Es rely on magneto-optical technology. A magneto-optical disc is coated with a compound of cobalt and a rare earth, such as gadolinium. These constituents are carefully mixed so that the coating records patterns of magnetic spots that can rotate the plane of polarisation of any polarised light that the surface reflects.

To make a recording, the disc spins in a powerful, constant magnetic field while a laser, whose intensity is modulated by the signal being recorded, is focused into a small spot on the disc's surface. The signal switches the laser on and off, rapidly heating spots on the surface as it does so. These spots lose all their previous magnetism and then, as they cool, they become strongly magnetised by the constant field. In this way the coating ends up with a pattern of magnetised spots that represents the recorded signal. The snag is that a laser must erase a recording before the disc can be overwritten with a fresh recording. In an alternative system, the laser beam remains at constant strength while the signal being recorded modulates the strength of the magnetic field. This is technically harder to arrange, but it allows a new recording to overwrite an existing one directly; it wipes the disc during the recording process.

To play back the recording, the player unit focuses a low-powered beam of polarised laser light on to the surface of the disc. The different patterns of magnetic spots on the disc rotate, to varying degrees, the plane of polarisation of the light that the surface reflects. When this light passes through a polarising filter, the shifts in the plane of polarisation appear as changes in intensity of the light. A light-sensitive detector records the fluctuations and converts them into electrical signals.

Until recently, disc designers faced a dilemma. Coatings that can retain a recording for a decade cause only a slight shift in the plane of polarisation, around 0.1 degrees, which makes the playback equipment expensive; those that cause a greater shift tend to lose their recordings within a few months. Canon, a Japanese manufacturer, claims to have got round the problem. It has developed a disc with two coatings, each incorporating a different rare earth element. The top layer has a short magnetic life but it gives a strong polarisation shift; the bottom layer has less effect on light but it retains a strong magnetic field. Canon says that the two layers complement one another; the top one simplifies the equipment needed to play back the recording while the bottom one continually refreshes the magnetic state of the top layer, like a magnet in contact with soft iron. Canon says recordings on its discs last 10 years and, at Pounds sterling 50 apiece, are the cheapest available; shop prices would be much higher, however, and thus far too expensive to attract anyone who wants to use the disc at home to store an hour of music.

The industry says that magneto-optical discs will always be expensive because of the technology involved in laying the coating as a thin film from a vapour in a vacuum. This has forced manufacturers to look for an alternative. Discs for a system called 'phase change' look likely to fit the bill.

A phase change disc is coated with a compound of tellurium with, for instance, antimony and germanium. The laser in the recorder, which the signal modulates, is not strong enough to burn permanent holes in the surface, as it does in a WORM recorder. Instead the laser emits brief pulses that melt pits in the crystalline coating, which cool rapidly as amorphous spots on the surface. These spots are less reflective than the surrounding crystalline metal, and a low-powered laser can read them and still leave them amorphous. The recording is erased by heating the coating with longer pulses from a laser beam, which causes the spots to cool more slowly and revert to their crystalline state.

Phase change discs are made in the same way as WORMs are. Manufacturers pour a drop of the liquid coating material on to a spinning disc to disperse the metal before it cools. There is no need for expensive vacuum deposition. Blank discs should eventually cost just a few pounds; for the moment, however, manufacturers cannot mass-produce them and those that have been available as laboratory samples have performed unreliably after a few hundred record/erase cycles.

In Japan, Matsushita, the world's largest consumer electronics company, is hedging its bets. Like most electronics firms Matsushita is developing magneto-optical discs. It has also bought a manufacturing licence from an American inventor, Stanford Ovshinsky, and his company, Energy Conversion Devices of Troy, Michigan: the pair hold the master patents on phase change technology. Earlier this year Matsushita demonstrated a 9-centimetre disc that uses a phase change coating to store, erase and re-record 280 megabytes of digital information, which is equivalent to half-an-hour of stereo sound. Ovshinsky describes Matsushita's achievement as 'a real milestone'.

One major problem remains with both magneto-optical and phase change discs: they do not reflect as much light as conventional CDs, which are coated with highly reflective aluminium. This means that the new discs cannot yet be used in ordinary CD players. Also, the coatings are easily damaged and the disc must be protected in a caddy from which it emerges only when loaded into a player.

To get round the problems, manufacturers have come up with a polymer coating that changes colour, or physically deforms, when a laser beam hits it. Hitachi, a Japanese manufacturer, has tried using a compound of silver and zinc that turns pink above 300 Degree C. When the coating is heated again it reverts to its original silver colour. Philips has been experimenting with organic dyes that change colour under laser illumination. Two dyes, SQS (a squarylium nucleus with thiopyrylium end groups) and TPMP (tetradimethylaminophenyl pentamethine perchlorate), are particularly suitable because they absorb infrared light at the wavelength emitted by a solid-state laser (around 800 nanometres) but they do not change colour in daylight.

Mainly for political reasons - the desire not to offend the record industry - Philips will no longer discuss its technical progress on the technology of recordable CDs. Other electronics companies are also reluctant to divulge what they are doing. So it was big news in April 1988 when the Tandy Corporation of Texas announced Thor, which the company described as a 'major breakthrough in optical media - the first CD-compatible record and erase technology'. Tandy said it would be selling the general public a CD recorder, costing under $500, within 18 months: it has since extended the deadline to two years. Tandy refused originally to say how Thor would work but later admitted that it had a licence to use patents, which cover the technology of dyes in polymers, owned by Optical Data Incorporated of Beaverton, Oregon.

The patents explain how a blank disc is coated with two layers of polymer, each with a different coloured dye. The recorder has two lasers, each of a different wavelength. To make a recording, one beam passes through the top layer of the disc coating but is absorbed by the lower layer. This heats and melts, forming tiny bumps whose size and density are modulated by the signal being recorded. The second laser erases the recording: the top layer blocks it and heats up, which causes the bumps to melt back into a flat layer.

This hypothesis of the process intrigued the industry, so much so that PdO, the joint venture of Philips and du Pont, the American chemicals giant, also took out a licence on the patent. The joint venture wanted to see if Tandy's disc really would reflect light like a conventional CD and thus work in an ordinary CD player. PdO claims that the disc performed satisfactorily after one erasure. It seems, however, that once deformed the surface could not recover its original flatness. After the first deterioration, PdO claims the performance of the disc was constant for a few hundred record/erase cycles.

In November 1988, Taiyo Yuden, a Japanese chemicals company, began to sell recordable CDs that played on standard equipment. The disc is not erasable, as Tandy's is, and this has kept its price down to a few dollars. Sony recently signed a deal to work with Taiyo on developing the technology. Taiyo's disc is bright green and there is, again, some doubt about how it works. Taiyo has said that an ordinary blank disc of polycarbonate is coated with a polymer that decomposes to form a gas bubble when it is exposed to an 8-milliwatt laser. (This is four times as powerful as the laser used in a conventional CD player.) The bubble deforms the polycarbonate substrate to create a pit similar in dimensions to the pits formed by moulding molten polycarbonate in a conventional CD pressing factory. Tests carried out by Philips, however, suggest that there is no such deformation; it seems more likely that the disc is coated with a coloured dye that bleaches under a laser's spotlight.

Taiyo claims the shelf life of a blank disc is at least 10 years; it says the disc will last at least another 10 years after a recording has been made. The only proviso is that the discs should not be left in direct sunlight for long periods. The company also suggests that the management of copyright is straightforward because the discs can record only once. It says that copyright royalties can be included in the price of a blank disc.

Not surprisingly this did not appease the record companies, which only recently blocked the sale of digital audio tape. (DAT was once seen by its manufacturers as the natural partner of CD, as magnetic tape is of vinyl records.) On hearing about Taiyo's disc, the International Federation of Phonogram and Videogram Producers (IFPI) announced that its board 'reacted vigorously to the reported development of CD-R (write once) and CD-E (erasable) compact disc systems . . . (which) represent an even greater potential threat to copyright owners than DAT.'

In an internal memorandum, written for the IFPI when Taiyo announced its write-once disc, Philips predicted that mass production of CD-R could start in 1990. It also estimated that a combined CD player and CD recorder would cost less than Pounds sterling 100 more than a CD audio player. The blank discs, says Philips, will cost between Pounds sterling 3 and Pounds sterling 5.

The IFPI's reaction convinced the electronics industry that it must now keep quiet about CD-R as a domestic product. Both Tandy and Taiyo are lying low. Sony is anxious to say that it has no plans to sell a domestic version of the Taiyo recorder, and that the system will be used industrially to let companies produce small quantities of music or data discs. The companies with an interest in CD-R technology will talk only about its use as an industrial tool. But in the competitive world of electronics, a gap in the market is not left unplugged for long. The only questions now are how much will digital recording equipment cost and when will it be available.

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Recordable discs spin from the bonds of digital tape

DIGITAL audio tape (DAT) captured the public's imagination when it was first proposed five years ago. The fascination was the idea of a tape cassette with the profile of a credit card, but capable of recording four hours of stereo sound with the same quality as compact discs. By the same token, the idea of people being able to make perfect digital copies of CDs onto tape, and then making perfect 'clone' copies of the copies, horrified the record industry.

The record companies, represented by the International Federation of Phonogram and Videogram Producers (IFPI), lobbied against DAT at government level. Unwilling to risk an international trade war over something as trivial as a tape recorder, the Japanese electronics industry, which has made all the running on DAT, soft pedalled on the new format. It deliberately crippled the format, making DAT unable to record digital code streaming at 44.1 kilohertz, the same frequency used for CD. This prevented direct digital dubbing from CD to DAT. Even this did not satisfy the record industry. The Japanese industry bowed again and agreed not to sell DAT recorders to the general public outside Japan.

This apparent climbdown caused electronics companies round the world to reconsider the recordable disc technology that they have been developing for industrial clients over the past 10 years. But knowing full well how record companies will react to the sale of recordable CDs to the general public, the electronics industry is playing its cards close to the chest.

When both sides began to meet for 'round table' discussions in December 1986, only DAT was on the agenda. Nowadays these meetings, which take place every few months around the world, include discussion of all forms of digital recording. And there is still an impasse. The electronics industry has pinned its future on technology that records digitally, and the record industry remains radically opposed to equipment capable of doing just that.

Technical systems exist that prevent or limit digital cloning. They rely on extra bits of code, or 'flags', introduced into the stream of digital data. These flags have no audible effect on pre-recorded signals but they instruct a digital recorder with matching circuitry to switch off. This makes the recorder unsaleable. But things could be changing. The record companies fear recordable discs so much that they may agree to the sale of DAT in the hope of blocking the disc.

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Wrinkles to iron out for a perfect clone

PRERECORDED, or pressed, CDs start with a 'table of contents', or TOC, that lists in digital code the number and length of sound or video tracks. The player uses the code to search out selected tracks and passages. Manufacturers have not yet agreed on how a CD recorder will add a TOC to a blank disc. Some prototypes require a full recording to be made in one go, with a space left at the start of the disc to add the TOC as soon as the recording is finished.

Another idea is that CD recorders will store information for the TOC in a temporary memory while recordings are made.

In this way one disc could be used to make several short recordings, for instance to make a compilation of music selected from different sources at different times.

Write-once recorders will also need a 'rehearse' function. This will let the user make a dummy run to avoid wasting space on the disc with false starts. Most companies making CD players and analogue tape recorders already offer these facilities to simplify dubbing from disc to tape.

In a CD player, the laser tracks the spiral of pits under instructions, known as servo control signals, derived from the pits themselves. A blank disc has no pits, so some other system is needed to guide the laser used to make the recording.

Manufacturers can give the blank disc a spiral groove that the laser tracks during recording. The pit-shaped deformations are then formed inside or alongside the groove, a technique known as Continuous Composite Recording. Alternatively, a blank disc is made with a skeleton of pits that provide servo control signals for the recording laser. This technique is called Sampled Servo and it is still at an early stage of development. The only constraint in either technique is that the final track pattern must match the pattern on a pressed disc.