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

It is exactly four years since the European launch of CD in March 1983, which we at GRAMOPHONE celebrated with a 12-page colour feature including reviews of 50 discs and four CD players. Since then the public acceptance of CD has grown at a phenomenal rate—though slower in the UK than in Japan, Germany and the USA, for example. Here are some of the key dates:

Summer 1972:
Nippon Columbia introduce PCM (Pulse Code Modulation) digital recording which is gradually adopted by the majority of record companies for producing their original tape masters.

August 1972:
Philips give a press launch for the LaserVision disc—a technological breakthrough using a laser light beam to read tiny encoded pits on a disc and produce a colour TV picture plus sound.

March 1979:
Philips introduce a prototype Compact Disc combining PCM digital audio processing and the laser scanning technique. The main objective was to use a laser disc for high quality sound signals with the advantages of optical scanning and small dimensions.

January 1981:
PolyGram show a laboratory-made CD and initiate a manufacturing plan to produce 500,000 Compact Discs in 500 days.

April 1981:
200 journalists are flown to Salzburg from all over Europe for a major Press Presentation and Demonstration of the new CD medium. Significantly the platform party comprised the top executives of Philips, PolyGram and Sony. This unique colaboration between companies from Europe and Japan (and from both the software and hardware sides of the industry) was visible proof of the protracted behind-thescenes planning which had been necessary to establish CD as an accepted international standard. Philips had indeed developed the laser-beam CD principle but had welcomed Sony's participation in the design and production of much of the basic machine technology. Between them they had persuaded about 40 other companies (mostly in the Far East) to join the CD bandwagon. Also significant was the presence in the platform party of the conductor Herbert von Karajan. This was seen as confirming the importance of CD to the world of music. The maestro was already known to be enthusiastic about digital recording, and he must have delighted the older employees of Philips (who had so much to do with the early development of electric light bulbs) when he said that comparing CDs to LPs was like comparing electricity with gaslight!

August 1982:
the pianist Claudio Arrau inaugurates the PolyGram CD factory in Hanover which duly meets its 500,000 discs in 500 days target (and is still the world's biggest CD plant).

October 1982:
world premiere launch of CD takes place at the Tokyo Hi-Fi Show.

March 1983:
European CD launch:
PolyGram sets a 1983 target of 3 million discs, 15M in 1984.

August 1983: USA CD launch.

For a continuation of this chronology it is enough to glance at the chart in Fig. 1 comparing the UK past and projected sales (in millions of units) for CDs, LPs and Cassettes. This graph is largely based on British Phonograph Industry figures and was presented at the recent Digital Information Exchange in London by consultant Nick Hopewell-Smith. It illustrates the rapid acceleration of CD sales—much more rapid than for any other home entertainment medium in history. We see that CD is forecast to overtake LP in volume sales during 1989 and to reach about 50M discs by 1990. The chart of comparative sales values in Fig. 2 emphasizes even more clearly the high projected contribution of CD to the industry's revenues in the next few years.

Interesting figures on the number of Compact Discs bought by UK customers per player were also given—and contrasted with the fact that some 85% of UK households own an LP record-player but that the average LP collection consists of only 11 discs! It appears that 300,000 CDs were delivered to the UK trade in 1983, with 19,000 players sold— about 16 CDs per player. This is an amazing ratio considering that the available CD catalogue contained only about 450 titles, mainly classical. In 1984 sales were 800,000 discs and 30,000 players (a ratio of about 20 discs per player) whilst in 1985 the figures were 3.1 million discs and 86,000 players (plus 64,000 CD 'systems')—still around 20 discs per playing unit. Yet to be confirmed BPI estimates for 1986 are 7 million discs and 350,000 hardware sales—a drop in the ratio but indicating that about 3% of UK households now own a CD machine.

Comparing the three 'music carrier' systems
Since any progress made by CD must be to some extent at the expense of the two 'traditional' media—LP and Cassette—it is worth comparing their relative physical parameters, advantages and disadvantages.

1. Vinyl discs: have been the primary music medium since the early 1950s when they took over from the brittle 78rpm records. They are made from soft black vinyl by pressing a measured amount of the material between a pair of metal moulds or `stampers'. LPs and 'singles' are respectively 300mm (12-inch) and I75mm (7-inch) in diameter and about 1.5-2.3mm thick. LPs offer a nominal maximum playing time of 30 minutes per side, though the actual time is often less in practice and has even gone up to 40 minutes on a few recent DMM issues. The rotating speed is 33-1-rpm for LPs, 45rpm for singles. The V-shaped groove (see Fig. 3) is about 100um wide and must be played with a fine diamond or sapphire stylus of scrupulously accurate tip dimensions, tracking angle and tracking force. The spread of sound frequencies encompassed does not quite extend down to the bass-end limit of the accepted 20-20,000Hz audible range. The treble end can be very extended so far as the recorded waveform in the groove is concerned, but only the very best pickups can reproduce the full range, and this becomes restricted towards the end of each disc as the linear speed of disc under stylus slows down.

The so-called 'dynamic range' is quite a bit less than the 90 decibels existing between the quietest and loudest sound levels met in live music performances, being about 60dB at best and deteriorating, of course, as the surface noise rises due to wear and tear. The separation between Left and Right stereo channels, instead of being a desirable 50dB is only about 35dB with the best pickups and even less on run-of-the-mill record-players. Distortion (harshness due to the production of spurious high frequencies) is rather more than the desirable limit of 0.1% and shortterm speed fluctuations (descriptively referred to as 'wow and flutter') can all too often be heard as a wobble in musical pitch.

Advantages: it is clearly an advantage that so many homes are already familiar with and equipped for playing vinyl discs; collections of wellloved recordings have been built up; there is a huge catalogue of LPs and singles which is being added to every week; disc prices have stabilized and there are many bargain records.

Disadvantages: the need to track the grooves physically with a hard stylus inevitably causes wear of the tiny recorded waveforms; dust particles and scratches are `read' as signals producing loud crackles and pops; static electrical charges build up in the disc and can discharge during playing, producing yet more noise and affecting the pickup tracking force; fidelity of reproduction (frequency and dynamic range) varies from very good to mediocre depending on the quality of the player; wow and flutter can be annoying.

2. Musicassettes: first appeared around the mid-I960s and were initially of rather poor sound quality. However, as the mass-production techniques have improved (using a high-speed copying process) and better magnetic tape with Dolby noise reduction has been used, a fidelity standard approaching that of vinyl discs is possible.

The cassette measures 100 x 64 x 12mm and so can realistically be called `compact'. It is therefore a highly suitable medium for portable and in-car use. As a result of all these factors, cassette popularity now rivals that of vinyl disc, as Figs. I and 2 indicate. The narrow tape width (3-8mm) with two 0.6mm wide stereo tracks in each direction does, however, set a distinct limit to sound quality, as does the relatively slow tape speed (4.76cm/s or inches per second) and the rather high wow and flutter fluctuations. Advantages: the enclosed cassette keeps the tape free from dust; many homes are equipped with cassette machines—primarily purchased for home recording, but obviously providing an installed base for cassette buying; the cassette catalogue is very large and most new record albums are now issued simultaneously on LP and cassette; cassette prices were once higher than the equivalent LP but this no longer applies; cassette operation is easy and most machines have tape position counters and fastwind facilities; portable `Wal kmantype' and in-car players are in wide use.

Disadvantages: the tiny track dimensions mean that any dirt or dust deposits on the tape head can cause poor sound quality and momentary breaks in the music (dropouts); fidelity of reproduction and pitch consistency are rather less than those of LP and CD; locating a particular passage can be time-consuming and innacurate; tape damage can occur due to clumsy handling or lack of machine maintenance.

3. Compact Discs: measuring only 120mm (4.7 inches) in diameter by 1.2mm thick, the Compact Disc is certainly small and convenient to store, transport and design a player around. Portables and in-car players have been marketed successfully, though their ability to withstand jogging or extremes of temperature fall short of that offered by equivalent cassette machines.
In all technological respects the Compact Disc is a major advance on anything which went before. As we shall see in Part 2 "Music by numbers", the change to a digital storage process (instead of the traditional method of recording a facsimile or 'analogue' of the music signal waveform) produces very 'robust' recordings which can be protected against all the normal ravages of wear, dust, interference and stray magnetic fields.

A digital recording consists of a series of pulses (not unlike the dots and dashes of Morse code) representing the musical sounds in such a way that they can be decoded with absolute numerical accuracy even in the presence of very severe noise or damage. As a result, the digital tape masters recorded over the past 10 years or so remain in a pristine state and can be digitally copied to produce as many identical masters as may be needed. Unfortunately, until CD came along, the record companies had to transfer their masters back to analogue for issue on LP or cassette so that, whilst some residual benefits in clarity and low-noise could often be detected, the consumer 'music carrier' remained at the mercy of the old problems of mishandling and restricted playback quality.

What is needed is to keep the music signals in trouble-free digital form right up to the last possible moment when they are being fed to our loudspeakers. Only then will the consumer have the possibility of hearing the same quality of sound as the engineer intended. This is achieved in CD by using a pin-point beam of laser light and photo-processing techniques to 'print' the data stream of digital pulses directly from the master tape into a special master disc (to be described in Part 3). Figure 4 is a photo-micrograph of a Compact Disc magnified 12,500 times showing the parallel lines of encoded pulses as pits in the disc surface. The pit dimensions are almost incredibly tiny, only 0.5pm wide by 0.11 pm deep, with a track spacing or 'pitch' of only 1.6pm—much less than the 100pm pitch of LP grooves shown in Fig. 3, or the 50pm thickness of a human hair.

At the heart of the CD player is an optical system of lenses (see Fig. 5) which precisely focus the laser beam on to the disc's pit track, and then direct the reflected light on to a lightsensitive detector. The change from pit edge to 'land' can thus produce the necessary electrical change in signal to reproduce the original digital data. A digital-to-analogue converter then recreates the analogue music signals for ultimate feeding to the loudspeakers.

Advantages: linearity of frequency response covering the full 2020,000Hz audible range; wide dynamic range and channel separation—noise and crosstalk are down at around — 90dB or better; no wow and flutter; longevity—given due care, CDs do not deteriorate every time they are played as LPs and cassettes do; dimensions are small so that portable and in-car players are available; players can cue quickly to any desired part of the music and store track sequences in memory, often with remote-control; new video, data and interactive versions of CD are in the pipeline.

Disadvantages: disc prices are relatively high: the disc catalogue is small but at around 8,000 titles (3,500 pop and 4,500 classical) is growing month by month.