Optical Recording

A crucial factor in the success of the CD player turned out to be a new type of laser, as Lou Ottens had predicted. By 1979 the life of solid-state lasers, initially only a few seconds, had already been increased to 100 hours. Specific fundamental research was however still necessary to bring about further improvement.

There were three materials that were suitable for solid-state lasers. All three were very promising, but a choice had to be made because the three different materials created three different colors of laser light. The shape of the pits and the design of the lenses have to be geared precisely to the color. Which material would be most suitable for further development? The choice was a matter of weighing up uncertainties. In the end, gallium arsenide (GaAs) was chosen. Looking back, this was a fortunate choice because it turned out that this type achieved the longest life and was easiest to produce in large quantities.

It was agreed that the laser should shine through the CD, just as it did in the Philips video disc. A transparent plastic disc would be used for the CD, which would have an imprinted pit pattern and a reflective metal coating on one side. The information would be read from the other side, through the transparent disc. This meant that dust could never get near the pits. The plastic disc, with a thickness of 1.2 mm, would protect the information. Lenses could be used to focus on the pits without the dust particles becoming visible. The same happens with dust on the lens of a photographic camera or scratches on a window; if there are not too many of them, they fade just enough so as not to appear on the photograph.

This method for reading the CD thus ensured that the CD was not sensitive to dust, scratches or fingerprints. The disadvantage, however, was that the laser light had to be directed very precisely at the pit track and had to be focused with extreme accuracy. That is why a special objective lens was required to correct the deviation caused when the laser light shines through the plastic. The aspheric objectives that had already been developed earlier for the video disc could be put to good use here. Furthermore, irregularities in the rotation of the CD had to be taken into account. If the CD is a tiny bit off-centre, the pits bump up and down. The variation may well be as much as a millimeter, whilst the depth of field of the lens is no more than half a micrometer. The solution is a rapidly moving lens and an electronic control that ensures that every irregularity in the rotation of the CD is followed. The electronics are linked to the electronic error correction. If the signals contain too many errors, the scanning mechanics must be adjusted.

It was not always the technical arguments that won when choices had to be made. For example, the playing time of the CD was determined posthumously by Ludwig van Beethoven. Philips engineers had always based their work on a playing time of an hour, a few minutes longer than a double-sided LP. This meant that the existing repertoire could easily be issued on CD and, with a diameter of 11.5 cm, the CD would come very close to achieving Lou Otten’s ideal compactness.

However, Sony vice-president Norio Ohga, who was responsible for the project, did not agree. "Let us take the music as the basis," he said. He hadn’t studied at the Conservatory in Berlin for nothing. Ohga had fond memories of Beethoven’s Ninth Symphony (‘Alle Menschen werden Brüder’). That had to fit on the CD. There was room for those few extra minutes, the Philips engineers agreed. The performance by the Berlin Philharmonic, conducted by Herbert von Karajan, lasted for 66 minutes. Just to be quite sure, a check was made with Philips’ subsidiary, PolyGram, to ascertain what other recordings there were. The longest known performance lasted 74 minutes. This was a mono recording made during the Bayreuther Festspiele in 1951 and conducted by Wilhelm Furtwängler. This therefore became the playing time of a CD. A diameter of 12 centimeters was required for this playing time.

In this way the specifications of the CD were determined by means of intensive contact between Philips and Sony. In June 1980 these were recorded in the Red Book, so called after the favorite color of the Philips project leader Joop Sinjou. The Japanese standardization committee, which had had a hand in bringing Philips and Sony together in this way, cautiously expressed a preference for the CD system over the digital techniques used by JVC and AEG. From that moment onwards Philips and Sony were to work separately on the development of their own CD equipment in accordance with the standard that had been agreed upon, but with one common aim: to win over the world for the CD.