GLOBAL MASTERS
Maybe they're not quite masters of the universe, but Philips and Sony are at least masters of the Compact Disc's population explosion. Not only are they the license-holders of the format--collecting 30 on every CD made-and major manufacturers of discs; they also make the mil lion-dollar machines to which most CDs owe their birth.
After Philips and Sony cleverly invented the CD, the next clever thing they did was invent the technology needed to manufacture the damn things. Although much of the manufacturing chain could be assembled from existing technology, such as injection molding, metallization, and label printing, the first link in the chain required a wholly new system. Philips and Sony thus independently developed CD master recording systems; then they installed them in their disc-manufacturing facilities, and sold them to subsidiaries or other companies. Most of the billions of pits in your CD collection were originally etched by a Philips LHH-0400 or Sony DMC-1200 master recorder. Let's take a look at mastering technology and why its mastery puts these companies in the driver's seat.
CD mastering begins with a video cassette and a piece of glass. The PCM audio data contained on a master tape will be transferred to the glass master, where it will be represented as pits. All CDs are ultimately derived from that master disc.
The master tape, a 3/4-inch videocassette, carries all of the audio, subcode, and time-code information to be converted to a channel bit stream and fed to the master disc recorder. The audio data is contained in the video fields as PCM data, while the subcode and time-code information is stored on the longitudinal (formerly audio) tracks.
The subcode includes control data such as table of contents, track numbers and indices, track lengths, text information, pre-emphasis (on/off), dig ital copy (prohibited or permitted), number of audio channels (two or four), and catalog number.
The glass master disc, about 240 mm in diameter and 5 mm thick, is washed, lapped, and polished. An adhesive is applied, followed by a coat of photoresist applied by a spinning developer machine. After inspection and cleaning, the plate is tested with a laser for optical dropouts; any burst dropouts in reflected intensity will because for rejection of the plate. The plate is cured in an oven and is then ready for master cutting; if stored, it has a shelf life of several weeks.
Disc mastering itself is accomplished with a laser mastering machine which exposes the photoresist on the master glass disc. The mastering ma chine is composed of two units, a control rack and a lathe. The control rack consists of a minicomputer with video terminal and floppy-disk drive, U-matic video transport, PCM audio processor, and diagnostic equipment. The master tape is loaded in the video transport, and the CD encoder uses the tape's subcode, time-code, and digital audio data to carry out multiplexing and CIRC encoding. The encoder generates the channel bit-stream signal; in addition, it outputs signals used for automatic quality control.
A controller provides for automatic system operation, and recording operations are stored on floppy disk. Recording parameters such as linear velocity, master identification number, and program length are entered via controller keyboard. A video display shows indication of process status.
The master glass plate, coated with photoresist, is placed on the lathe. The channel bit stream is input to the master recorder; it is used to intensity-modulate a laser which creates a cutting signal corresponding to the data on the original audio master tape. It is this "cutting" laser which creates the spiral track, in real time, as the master tape is played through the PCM processor.
Another laser, which does not affect the photoresist, is used for focus and tracking. In making the spiral data track which extends outward across the disc, the disc's rotational speed and the linear motion of the sled that carries the focusing optics must be very precisely controlled. To obtain frictionless motion, air bearings are used for both mechanisms. Although the optics are similar to those found inside consumer CD players, the mechanisms are built to even more exacting specifications, especially in terms of isolation from vibration. The entire cutting process is accomplished automatically, under computer control.
After exposure in the master cutter, the glass master is developed by an automatic developing machine; a laser monitors pit depth and stops development when proper engraving depth has been reached (that is, when the etching reaches the glass substrate).
The pit depth thus depends on the thickness of the photoresist layer. The optimum data signal would result when there is maximum difference between the light reflected from the pits (seen as bumps, from the laser side) and the surrounding land. This is achieved when (a) the pit depth is one-quarter of the laser pickup's apparent wave length, and (b) when the pit width is such that the intensity of the light reflected from the pit's bottom equals the intensity of the light reflected from the land between the pits. These two conditions cause interference between the incoming and outgoing light for maxi mum contrast. In practice, pit depth and width specs must be modified a bit for a more robust tracking signal.
The developed master plate is transferred to an electroplating room; there, a silver coating is imparted onto the glass master to result in a metal "stamper." This CD master disc can be played on a system for masters to assess aural and measured quality; it also serves as a reference to evaluate the quality of the final production discs.
A negative "father" of nickel is then made from the master disc, and positive "mothers" are made from that; each mother can then generate a number of negative nickel molds from which the actual discs are made.
Injection molding is typically used to produce the finished commercial discs. A polycarbonate material is melted, then injected in the disc mold.
After molding, a layer of metal (usually aluminum) is placed on the disc surface to provide reflectivity; this is accomplished by sputtering or by evaporation in a vacuum chamber. The metallization layer is then covered by a photo-polymerized plastic layer applied by a spin-coating machine. This protects the metallization from scratches and oxidation. The label is printed upon this plastic layer. Following inspection, the disc is finished.
Even though the entire production chain is long, at least the mastering process sounds fairly straightforward.
However, most CD mastering facilities will probably testify to the difficulties involved. The equipment for a mastering system requires a modest-sized book for specification. The main items include a resist master preparation system, a master recording system, a developer system, a master disc player system, and disc master electroplating equipment. Other items (from a long list) include microscopes, ovens, chemical-preparation equipment, diagnostic and test equipment, glass ware, cleaning and protective materials, desks and hoods, audio monitoring equipment, and dust-free paper.
Site requirements are carefully specified too. Clean air is critical. CD pits are among the smallest manufactured formations--about the size of a smoke particle. One stogie could put your factory out of business. Therefore, the en tire mastering process must be carried out in a clean-room environment, with the size and number of particles in the air strictly regulated. Temperature and humidity, as well as ambient air-pollution levels, must also be specified. In the Philips system, the glass disc moves from one process stage to the next in a sealed cartridge. In each pro cess step, the disc is automatically re moved from the cartridge and then re turned to it. This minimizes manual contact and air exposure, thereby reducing the risk of disc contamination.
Vibration would be disastrous to the cutting process. The laser-beam recorder is mounted on a massive baseplate (Sony uses cast iron, Philips uses granite) with a pneumatic vibration-isolation system. Other considerations include a clean electrical system, demineralized and hot water, compressed air, filtered air, nitrogen, and exhaust of contaminated air.
The facility could be housed in a clean room of 200 square meters. However, many disc manufacturers are specifying substantially larger rooms, with an eye toward additional mastering equipment to accommodate production of audio discs, CD-ROM discs, and future formats.
The total cost for the highly specialized CD-mastering equipment is in ex cess of $2 million, plus clean-room construction costs. (The rest of the production chain costs a lot more.) Even so, there's a long line of customers eager to purchase mastering equipment. Philips is happily accepting orders for its turnkey system, but a system ordered today will not be delivered until 1988. That's what I mean about being in the driver's seat.
(adapted from Audio magazine, Aug. 1986)
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