1.1 Disc Properties and Information Format
The recording medium is a thin metal film evaporated onto an optically polished plate glass disc 0.24" thick. Glass was chosen because it is very uniform and because its surface can be made smooth and free of scratches, pits and other blemishes by the well-known techniques of optical polishing. Starting with discs cut from twin-ground plate glass, the surface may have hundreds of small pits per square millimeter. These discs are then reground with a fine abrasive to get rid of the deepest pits. Finally, the surface is optically polished until the pit density is reduced to less than 10 per square millimeter. The disc is then cleaned chemically and transferred to a vacuum evaporator where it receives a metallic coating a few hundred angstroms thick. For recording, the disc is transferred to the mastering machine where a laser beam records picture and sound information by selectively melting the metallic coating.

Figure 1 The layout of a twenty-minute disc is shown in Fig. 1. The information [is recorded on a spiral track with a pitch of 2 micrometers] and is read from the outside in.

One TV frame is recorded per revolution of the disc. The information is recorded as a series of holes cut in the thin metal film deposited on the glass disc. The holes range in size from circles 1 micrometer in diameter at the 3" radius to ovals 1 micrometer x 2 micrometers long along the path at the outer radius. The recording has a mean wavelength of 3 micrometers.

Various techniques are available to lengthen the twenty-minute playing time of the described configuration. A forty-minute Disco-Vision record has been publicly demonstrated and the technique by which forty minutes of playing time on one side of a 12" record were achieved will be described at a later time.

To minimize the cost of the processing electronics in the home player, the information on the disc is kept in the NTSC format required by the home TV set. The relative positions and frequencies of the video, chroma and sound subcarrier are preserved when going from the program signals to the signal represented by the holes in the disc coating. Therefore, no re-formatting or re-arrangement of [the individual] signal components are required in the player and minimization of the player cost is accomplished.

The choice was made to record with frequency modulation (FM) because of its immunity to noise at low frequencies where much of the system noise is. The usual source of audio and video signals is a 2" video tape recorder. The audio signal is used to frequency modulate a 4.5 mHz carrier. This carrier and the processed video are summed and fed to a voltage controlled oscillator (VCO). This device has a center frequency of approximately 7 MHz and a deviation of ±1 MHz for a 1 volt peak-to-peak video signal. The recording polarity is such that sync tips produce the highest frequency, MHzHz, and saturated whites produce the lowest frequency, MHzHz. Figure 2

Fig. 2 shows the basic signal processing used in mastering and playback. In mastering the resulting FM signal, occupying a spectrum from approximately 2.5 to 11.MHzHz, is applied by the cell driver to a Pockels cell electro-optical modulator. The Pockels cell has incident upon it the beam from the record laser. Under the influence of the signal from the cell driver, the Pockels cell alternately passes and blocks the beam, thus allowing the beam to produce holes and lands in the disc coating. An adjustable dc bias is applied to the Pockels cell to minimize 2nd harmonic distortion that can be generated in the cutting process.

1.2 Optical and Mechanical Techniques
Details of the physical arrangement required for cutting a master are shown in Fig. 3. An argon-ion laser produces the basic "write" beam, which is modulated by the Pockels cell. Optics direct the beam onto the disc to produce the holes previously described. The rotatable Glan prism is used to adjust the average intensity of the beam reaching the disc. As shown in Fig. 3, the last few optical elements in the write beam are mounted on a carriage that is moved along the disc's radius by a motor-driven lead-screw. The objective lens Figure 3 is supported on an air bearing, which is loaded against the surface of the disc. A relatively small air flow at moderately high pressure maintains the head and objective lens at a constant distance of approximately 0.0005 inch (0.5 mil) from the surface of the disc. Fine focus adjustment is made by moving the diverging lens on the V block until optimum cutting is obtained.

1.3 Read-While-Write
Since there is no need to develop or process the master in order to read it, an optical system is included in the mastering configuration so that masters can be read while they are being cut. This feature allows the recording parameters to be adjusted while cutting and also provides continuous monitoring of the video quality of the master. Measurements of SNR and other video parameters can be completed and logged while mastering is in progress. Continuous monitoring also reveals defects in the master, which could only be detected by playing it.

The optical arrangement consists of a 1 milliwatt He-Ne laser, a beam splitter, a second diverging lens, an adjustable mirror and an adjustable dichroic mirror for combining the read and write beams before they enter the microscope objective. The two adjustable mirrors are used to position the read spot about 10 micrometers down stream from the write spot and directly on the track it has just cut. The 10 micron spacing insures that the recorded surface is in its final state at the time it is read. The return beam comes out the same way it enters (the system is retro-reflective) until a portion of it is reflected into a PIN photodiode by the beam splitter. The diode, pre-amp and discriminator are all components of the playback system described later in this paper.

Updated: September 19, 1997
Copyright ©1997 Blam Entertainment Group