Background Compact Disc
Part 3 - A Closer CD Encounter (May 1987)
This series of articles is aimed at non-technical readers who nevertheless would like to learn a little of the background to Compact Disc, how CD players work, how to choose and use a CD player, and what exciting new developments the CD medium still has in store. The articles have been prompted by many letters from GRAMOPHONE readers. These make it quite clear that, whilst most of the first converts to the CD format were already knowledgeable collectors of LP vinyl records or prerecorded musicassettes, who had decided to add this third 'music carrier' to their existing audio systems, there were many complete beginners attracted by the new CD medium for whom the whole hi-ti mystique was like learning a new language.
- Part 1 - Music from a silver disc (March 1987)
- Part 2 - Music by Numbers (April 1987)
- Part 3 - A Closer CD Encounter (May 1987)
- Part 4 - How CD players work (June 1987)
Part 3 - A closer CD encounter
Now that we have got the histori cal evolution of the Compact Disc and the basics of analogue-todigital conversion out of the way, we are ready to examine the disc itself more closely.
Figure 1 shows a CD in cross-section with all the important dimensions as set down in the Philips/Sony standard document. It will be seen that the disc is in the form of a sandwich and that the laser beam scans the disc from the underside. The beam must first penetrate the base layer of transparent polycarbonate to reach the thin reflecting 'mirror' layer of aluminium, which in turn has been coated with a protective layer of clear acrylic resin (lacquer). The overall disc thickness is 1.2 mm but, as the inset sketch shows, the metal layer is only 0.1pm thick and the protective coating only 30pm.
The spiralling track of digitally encoded pits begins at the inner radius with a lead-in portion which contains the so-called 'Table of Contents'. This spells out in PQ subcode all the instructions which the CD player decoder needs to memorize for subsequent cue and display operations, such as number of tracks, total disc time, whether pre-emphasis has been used, etc. There then follows an area extending over about 33mm of the disc radius carrying the programme signals in up to about 20,000 'grooves' each only 1.6p.m wide, followed by a 'lead-out'. In fact, the digital bit stream does not contain merely the stereo music signals. Every frame of the recorded track includes a myriad of the PQ subcode data already mentioned, to produce a continuous read-out of control and error correction instructions, as well as information to feed the timing display and all the user functions such as fast-search, track skip, etc.
One result of the nature of the way that the dual Philips/Sony Compact Disc format was brought to the marketplace has been the emergence of a variety of methods for manufacturing the discs and, as we shall see in Part 4, the CD players themselves. Of course the basic specification of the discs in terms of dimensions and encoding has to be strictly adhered to, but some of the detailed manufacturing stages are conducted in different ways in the various Japanese, European and (now) American CD plants—with identifiable differences in the appearance of the discs, if not in their performance.
The sketch in Fig. 2 identifies five basic stages in CD manufacture, with their individual progression running from top to bottom of the diagram.
They are as follows: (a) tape mastering, (b) disc mastering, (c) stamper making, (d) pressing and (e) finishing. Each of these raises questions of interest from the user's point of view, as outlined below.
(A) Tape mastering
The production of a tape master is shown floating at the top of the diagram, and is Indeed the only stage which is likely to be carried out at the recording studio premises, or at a specialist post-production suite, under the supervision of the studio personnel. The starting point, of course, is the studio master tape which contains all the music for the proposed CD (up to a maximum duration of 74 minutes) edited as necessary. When this is a new recording, preferably digital rather than analogue, it will certainly have been made with CD in mind, and should require no further equalization of the frequency response, ambience enhancement or dynamic range adjustments. When older recordings are to be transferred to the CD format, however—a practice which both classical and jazz CD collectors have welcomed enthusiastically—it obviously makes sense to seek out the best tape version available in the company's archives rather than use the first safety copy or LP-equalized tape that comes to hand. Even then, it is quite often found that some sound re-balancing will be necessary, or the removal of spurious rumble or studio noises which did not show up on LP but might now be all too apparent on CD.
Whatever the provenance of the music master tape, there has to be a comprehensive stage of re-mastering to create the CD tape master. Every detail of the latter's format and contents has to meet a strict set of rules. First, the CD tape master must be recorded on a i-inch PCM U-Matic video tape to the NTSC standard from a specific type of digital processor (the Sony PCM-1610, or the later PCM-1630). Second, it must carry an SMPTE time code synchronized to the NTSC video signal. Third, it must have been processed through a subcode editor which inserts all the PQ subcode data, access-begin and end-access points for each track, lead-in and lead-out details, etc.
(B) Disc mastering
The next stage is to replay this CD tape master and transfer the composite music-plus-coding signals to a CD disc master. This operation is carried out using a specialized laserbeam photographic process. Figure 3 shows the Philips version of this equipment, with the engineer on the right holding a U-Matic master tape and the other holding the master disc in its dust-excluding cartridge. The base for the master disc is a 240mm (9.5-inch) diameter, 6rnm thick glass plate. As the sequence down the left-hand side of Fig. 2 shows, the glass is first cleaned and polished, and then given a thin film of adhesive on to which a light-sensitive 'photo-resist' layer is applied. The thickness of this layer will determine the ultimate depth of the CD pits and so it has to be applied very accurately, and closely inspected (see Fig. 4) before curing in an oven.
The disc is placed in the laser-processing machine (Fig. 3) where it is automatically transferred to a latheturntable and rotated under computer control. The laser beam tracks outwards along a radius of the disc and is modulated by the master tape signals to produce a pulsed exposure of the photo-resist surface, and thus record the digital bit stream in real time. A constant linear speed of 1.2 metres per second is used, and so the rotational speed begins fast at the centre (about 500 rpm) and slows to about 200 rpm at the outer edge. In the development stage, photographic developer fluid is washed over the rotating disc and progressively dissolves away the exposed areas of photo-resist, until the glass surface is reached and the desired pattern of signal pits is achieved. A sputtered silver coating is then applied to the glass master to make it electrically conducting so that it can be used as an electrode in the subsequent electroplating processes. In fact, in the Philips design this silvered disc can actually be played on a special machine for quality assurance purposes. This does not simply check the audio performance, but track pitch, stability, etc.
As I described in our February issue (page 1193) the German company Teldec have recently described a Direct Metal Mastering (DMM-CD) technique which would appear both to speed up and simplify the preparation of CD master discs. It relies on embossing the pits directly into a copper surface. However, we must wait to see whether other companies take up this idea in preference to the standard photoprocessing method just described.
Fig. 6. Clear pressings ready for metallizing [photo: Nimbus
(C) Stamper making The procedures then follow a sequence which is superficially at least equivalent to that used in the preparation of metal matrices for LP manufacture. The first electroplating bath (see Fig. 5) deposits nickel on the silvered master disc surface until a nickel 'father' is formed. This is in effect a negative mould and could be used to press out positive discs from suitable thermoplastic material— indeed the father is sometimes used in this way when only a limited number of discs is required.
More usually, two further electroplating cycles are undergone to grow first a positive 'mother', and then grow from this one or more negative `stampers'. Typical figures that I have been given are 4 mothers from 1 father, 10 stampers per mother, and 15,000 CDs per stamper.
The type of press used varies from one CD plant to another, those originating in Japan generally employing injection moulding and Europeanbuilt presses more often using an injection/compression cycle. The thermoplastic material used (soft when heated, hard when cooled) is polycarbonate which has low vapour absorption and other desirable properties—enabling the pressing cycle to be brought down to an economically favourable 15 seconds or thereabouts. Some presses punch out the centre hole at this stage, others produce an approximately correct diameter hole to be trimmed later. Figure 6 (courtesy Nimbus Records) shows the clear polycarbonate pressings being placed on a carrier rack.
Remembering the microscopically small dimensions of the recorded pit tracks, it will be obvious that a clinically clean-air environment is mandatory. This means building a relatively expensive inner work area and the wearing of special clothing. As an alternative to this use of presses, an embossing method has been described for possible future use. This stamps CDs on a continuous roll of substrate material, which is then sandwiched between two transparent layers.
(E) Finishing It is next necessary to apply a reflective metal film to the pitted surface of the transparent pressing. New methods for this operation are coming into use, but traditionally a large vacuum chamber is employed.