About CDs

Characteristics

The substrate material:	polycarbonate
Disc diameter:	120 mm
Disc thickness:	1.2 mm ± 0.1 mm
Metal covering:	Aluminum or gold
Weight:	~14g
Smoothness tolerance:	± 0,6°
The accuracy of the track position:	± 0.05 µm
Information deepness:	0.11 µm
Reading Speed:	200 – 500 rev/min
Rotation Sense:	anticlockwise
Reading direction:	from the inside out
Scan speed:	constant between 1.2 and 1.4 m/s
Number of tracks:	20,000
Playback time:	80 min/700 MB
Sampling frequency:	44.100 Hz
Encoding:	16 bit linear (amplitude levels 2 16 = 65,536)
Frequency response:	20 – 20.000 Hz
Dynamic:	98dB theoretical
Harmonic distortion:	0,0012
Wow flutter:	below measurable limits.

The history of the CD

1938	The English engineer Alec Reeves invents the encoding system of the future CD, called PCM (Pulse Code Modulation), which had no practical use at that time.
1958	American physicists Arthur Schawlow and Charles Townes invent the laser.
1972	At a press conference of the Philips Research Laboratories in Eindhoven is presented the VLP system (video long play), which would later become LASERVISION. The disc with the diameter of 30 cm provides a playback time of 30 minutes on each side. Both the audio and the video column were encoded in analog format.
1978	In May 1978, Philips presents for the first time the compact disk, which then had a diameter of 11 cm using PCM coding (Pulse Code Modulation) in words of 14-bits and did not use any error correction system.
1978	In autumn 1978, starts the selling of the new system LASERVISION, in the United States.
1980	Philips and Sony announces the beginning of the collaboration for the development and trading of the CD. Sony's main contribution was equipping the CD with error correction.
1982	Philips and Sony publish the standard of the CD under the name Red Book.
1982	In October, the first CDs and related reading devices are launched in Japan. In that moment the CD had the known form, meaning the 12 cm diameter, uses the encryption in PCM, 16-bits/44.1 kHz and has a very strong error correction. The playback time is of 74 minutes.
1983	The CD is also launched in Europe (England, France, Netherlands and Germany).
1985	Philips and Sony publish the standard for CD-ROM under the name Yellow Book.
1986	Philips, Sony and Toshiba launches the first CD-ROM readers for computers.
1987	Philips launches on the market the CD Video with discs having three diameters: 12, 20, 30 cm offering 6,2 cm x 20 and 2 x 60 minutes of image and sound. The image is analogical and the sound is digital, in the parameters of the cds.
1988	Philips and Sony publishes the standard for interactive CD under the name Green Book.
1989	Appears the first CD-R record unit that costs $ 100,000.
1991	Denon launches on the market the first CD-Recordable device.
1992	Appeared on the market in England the first blank CD-R-S manufactured by Taiyo YUDEN known as the That's.
1993	It’s launched the Video CD, the first optical disc format on which the image was digitally encoded in MPEG1 system.

A brief theory

The specific properties of the laser light (Light Amplification by Stimulated Emission of Radiation) are:

Monochromaticity; On the one hand the wavelength is defined very precisely. On the other hand, the sunlight or the light bulb with incandescence has a very wide spectrum of frequencies.
The coherence; each package of light energy's in phase one towards another. Because of this reason the laser light is more intense.
Focusing; the laser light is generated as a fascicle that can be focused with high accuracy.

A laser beam projected on a glossy surface is reflected back to the laser source. Let us assume that the distance between the source and the surface is an integral multiple of the wavelength of the laser. In this case, the reflected wave is in phase with the direct wave. If a small area of the same surface is higher than a quarter of the wavelength, the reflected wave will not be in phase with the direct one. On this dephasings that are translated through intermittent reductions of the reflected light intensity, is based the operating principle of the CD.

These bumps that the laser reads on the surface of the CD, are actually recesses (pits) of 0,13 microns on the opposite surface, that is the equivalent of a quarter of the wavelength of the reading laser that is of 780 nm.

The laser fascicle, as focused as it may be, is not perfectly punctiform. Due to the diffraction phenomenon on the surface of the CD is formed a spot of light made of concentric circles, whose intensity decreases toward the periphery.

So that the CD reading takes place under optimum conditions, the standard for the CD, elaborated by Philips and Sony provides recesses width of 0.5 micron and 1.6 micron distances between tracks.

Signal Encoding on the CD

Because the sound cannot be registered as such, it must be converted into an analog form of energy. In the telephone case, is the electric current; at the gramophone is the mechanical movement of the needle, while to the magneto phone, cassette recorder and video recorder we are dealing with the magnetic flux. The deficiency of the analogue signal is that along with the proper signal; appear to the registration distortions and background noises. No matter how much we try filtering the signal, it’s impossible to realize it completely. The solution is passing from the analogical system to the digital one. In this case the sound is converted into binary code that is a sequence of numbers 0 and 1.

The coding with high accuracy of an audio signal requires sampling at a frequency of at least twice the maximum of the audible frequency. Normally it is of 20.000 Hz, so the sampling should be at least 40,000 times per second. All above mentioned standard sets this number at 44.100 Hz.

For each sample, the corresponding amplitude sound must be encoded in binary system. For minimal distortion, it is necessary that the scale of possible values be as high as possible. The agreed standard for a high-fidelity sound are the words of 16 bits. This means 216 meaning 65.536 amplitude levels. So the transfer rate of information needed for a stereo signal is 44,100 x 16 x 2 =1.411.200 bits per second.

Only with the appearance of the optical disc it became possible storing a large amount of information, with relatively limited financial resources.

CD and CD-R. Similarities and differences.

Similarities:

Information once written cannot be erased or rewritten;
Storage capacity are identical;
Registers both data and audio channels;
Are realized in series, on high productivity automated machines;

Differences:

CD	CD-R
The information is registered once with the fabrication of a metal mold.	The information is registered by the user.
The information does not alter in time only with the massive scratching or destruction.	The information alters over time, due to the continuing influence of the reading laser on the organic substance which in its turn changes the structure of the polycarbonate. The Sunlight and the heat accelerates the process.
Ideal for large print runs (500 pcs.)	Ideal for unique products
Is obtained by injection of hot polycarbonate (350°C) in a mold that contains the information engraved. Thereafter is covered with a layer of aluminum or gold with the thickness of 55 nm and then with one of lacquer. ,	It is obtained by applying on the un-engraved polycarbonate disc, of an organic paint layer and on one of gold. When it’s irradiated by the laser fascicle of the recording device, the polymer contained by the paint reacts with the polycarbonate, forming protruding. They are later interpreted by the reading fascicle laser. ,
The face on which the reading is made is silver or gold	The face on which the reading is made is green or blue

How to produce a CD

Unexpectedly, the production of a CD is not different from the classic LP. It is involved just by a much advanced technology.

Creating a series of a CD involves the existence of the original (the model) which follows to be replicated. It is produced either by a musical studio (CD audio), or a production software firm (CD-ROM). The support of this “original” is usually a CD-Recordable.

The Data from this are passed through an encoder, that generates the signal used by the laser, that engraves what will become the mold. The surface on which it is made is a very fine and uniform photosensitive layer. Where the laser beam exposure takes place, it produces a chemical change by which the photosensitive film becomes soluble to the solution that is used to the further development. Therefore, the photosensitive layer disappears to that place. So, on the glass disc it results a surface, where the “burned” places by laser appear as recesses.

After the information is transfered in this way, over what remained from the photosensitive layer, glued to the glass surface, is made a silver or nickel deposit, to make it electrically conductive. Then by electrolysis it takes place the deposition of a thicker layer, that subsequently detached by the glass surface, represents the so-called “father”. This is the negative image of the future CD.

Also by electrolysis, with the help of the "father" are produced the "mothers" and from these the "sons". The latter are also called molds (in english are called stampers) and are actual used to fabric the CDs.

Serial production is made by injecting the polycarbonate at a temperature of 345 ° C in an enclosure in which a surface is actually the "son" mold.

The disc comes out of the injector perfectly transparent, but with the information already “imprinted” on one side. At the next-generation injectors, as in ODS’s case, a CD comes out of the injector at every 3,6 seconds.

In order to be subsequently read by laser, over the surface, with the information is deposited a 50 nm aluminum layer. The process is called sputtering, that is splashing and is made into an enclosure with an advanced void, into an argon environment.

In order to protect the metallic layer, is applied over it a transparent lacquer, that dries to the exposure to a UV lamp.

At the end of the technological process it take place an optical verification with the help of a scanner with a dedicated soft, and with it’s help the CDs with imperfections are sorted separately.

The productivity of the manufacturing line with which ODS is equipped is of 80,000 CDs per day, or in other words 2.400,000 per month. The productivity is so high that during the listening of a CD, the fabric can multiply in 4.000 copies.

In order for the CD to be complete, on the lacquered screen the printing is made through the serigraphic method. The polychromatic image which is to be printed, is decomposed into basic screen printing colors (yellow, magenta, cyan and black). These are transposed on linear photographic film at a scale of 1:1, then on the 4 screens, that are used to the consecutive printing colors. The printing is made on a specialized machine that loads, centers, prints the four colors and downloads automatically the CDs. The productivity is of one CD at every second.

For the quality certification, the CDs are tested randomly at the beginning of the series and then at every 1000 pieces. The checking consists in measuring the analog and digital parameters and is based on the Red Book (audio CD) and Yellow Book (CD-ROM) standards imposed by Sony and Philips. This operation is made with equipment produced by the Austrian company Koch.