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How Laserdiscs Are Produced

There are three basic steps to disc production; pre-mastering, disc mastering and disc pressing. Each stage is separate, so much so that the pre-mastering stage can be completed before the programme ever reaches the disc pressing plant. All programmes destined for disc are either originated on film or videotape. Film programmes are routinely transferred to one of the professional gauges of videotape first.

The essence of pre-mastering is to ensure that the picture and sound is put on the tape to the required technical standard and with all the ancillary test/level signals. Additionally, there are special requirements for the production of CAV discs in order to ensure still frames without stability (jitter) problems.

All this can be done in-house by the programme owner, providing they have their own facilities. The completed tape will them be shipped to the pressing plant if it has not been done in-house, for translation into the disc master, from which the individual discs will be pressed. 

Writing Discs 

Incoming tapes are checked prior to disc mastering. Originally, master tapes were requested to be supplied with a 5-bit cue code, inserted electronically along their length, which was then converted during the disc mastering process to 24-bit address codes that make up the LD control and display signals (frames and chapters etc.). In practice, it turned out to be difficult for facility houses to encode the signals to the necessary standard and all such information is now put onto a computer disc and fed to the disc-mastering machine.

The master disc recorder is in effect an over sized and over powered disc player, except that instead of reading the disc it uses a similar laser to write the signal onto a specially coated blank glass disc. The laser in a LD player is typically in the range of 1.5 5.0mW, but in order to burn the pits that constitute the signal into the master disc a much more powerful 100mW laser is used.   

Although the current laserdisc format is often referred to as being a digitally encoded signal this is not quite the case leastways not in the PCM on/off switching sense. The signal consists of a whole series of discontinuous pits and lands. In the sense that these signals are not joined it could be described as sort of digital but it is more correctly analogue Pulse Width Modulation (PWM). 

In the diagram the top waveform represents the composite video signal direct frequency modulated (FM) on a top carrier frequency of 7.1MHz. The two audio signals (left & right) are modulated onto sub-carriers at 684kHz and 1066kHz and are combined with it in such a way as the resulting clipped FM signal becomes an analogue code for the modulated signals. In this the pit represents the video part of the signal and the spacing between them 9lands) determines the audio signal. Both the length of the pits and the space in between them varies independently, in accordance with the strength of the signals.

Were a person to have both the time and patience to do so, it should be possible to count up to 25 billions pits on just one laserdisc side.

Laserdiscs are made from two separate pieces of plastic bonded together. In the mastering stages described below we are only dealing with what is effectively one side of what will eventually become a double-sided disc. 

To write the master they start out with an optically polished flat glass disc that has been coated with a layer of photo-resist there are similarities between the process and that used for the manufacture of printed circuit boards. The coated disc is clamped into the mastering machine and the signal fed from the master tape into the laser writing head, which traces the pit spiral out from the centre of the disc. The blue light laser pulses on and off exposing a series of holes in the coating. This is a continuous process and once started the disc has to be written without a break. If for any reason a problem arises mid-process the master has to be aborted and the process re-started. The writing is done in real time and regardless of the actual playing time the entire disc surface is written to avoid having the outer area of the disc appear blank i.e. with the sort of mirror finish you might find on the blank side of a one-sided disc. 

The master recorder is a pretty beefy machine but it is still kept well away from foot traffic and other disturbances to avoid vibration. Its even insulated from the specially prepared floor by additional air cushion suspension. 

Processing 

Once written the glass disc has to go through a series of chemical processes etching away with solvents the exposed areas of the photo-resist, before depositing a layer of protective silver onto the surface. This actually results in a playable disc but only one. For those who require discs quickly or maybe need to try out a complicated interactive programme these glass master discs can be finished off here and placed on a conventional laserdisc player  (albeit at a price of around 2000 a disc), but for mass replication it is necessary to continue these chemical stages and grow another disc, but this time a mirror-image stamper. 

The silver glass master is placed in solution and by means of deposition a more substantial nickel-plating is built up over a period. This layer of nickel is then thick enough to strip away from the piece of glass. Its a tricky step as the nickel must separate without any deformation or creasing (its only a thin sheet of metal). The nickel stamper sheet undergoes a series of leaning steps top remove the non-waste silver coating. 

All these manufacturing steps take place in highly controlled clean room conditions. The tolerances involved require a degree of air purity far greater than would be considered in any hospital operating theatre and the cluster of rooms in which all these master making stages take place is under a constantly controlled pressure. When you go through the door there is a noticeable flow of air outwards. This ensures no contaminants from the outside world creep in. Needless to say, all the protective clothing and special footwear is worn inside these clean rooms.

Stampers can become damaged and there several are actually produced. An approximate figure is 25,000 pressings from a single stamper before it needs to be retired. The process of running off sub-masters beforehand  (that would involve treating the first nickel stage as the father and growing a mother from this and then producing a series of sons) is presently avoided due to the subsequent signal losses involved. 

 

Disc Pressing 

Disc pressing (replication) is a totally separate process from the mastering and stamper making. While this is still a clean room environment, there is less of a requirement for special clothing and the general level of activity is noticeably higher. Injection moulding is by far the commonest method of pressing discs world-wide. Injection moulding allows the continuous flow production rather than the batch working of the previous 2P process. Quality control is more effective (for example it is possible to start monitoring the output 15 minutes or so after the presses start running, rather than the period of hours that would be involved before. 

Come this pressing stage, raw granules of polymethyl methacrylate (Perspex or Plexiglas) are heated and forced under pressure into a mould consisting of two halves that are pushed together at the same time. The injection molding machine are pretty massive pieces of equipment compared to the size of the disc they have to handle. But such are the enormous pressures involved that their bulk is necessary. They operate in horizontal mode and are rather like oversize bench vices the two halves of the disc mould being where the jaws would normally be, and behind are the shafts that push the two faces together as the granules are injected. The nickel stamper is mounted on one jaw face, backed up by a very precisely engineered steel support plate, for which there is a matching back plate on the opposing jaw. The two faces are brought together and the molten granules fed in between them, via a cylinder that comes through the back of one plate, through where the spindle hole of the disc will be. The heated plastic fills the mould as the plates are forced together and the press visibly heaves off the ground in the process. As the jaws of the press are pulled apart a clear plastic disc is left between them with the recorded signal embedded into one of the faces. 

A robot arm dives in between the jaws and lifts the disc out, placing it on raised support pins on an adjacent conveyor belt. The speed and temperature of the press governs the flatness of the discs as well as maintaining the integrity of the recorded signal, and it is after the discs are removed from the press that monitoring for flatness can be carried out and adjustments made accordingly.

The conveyor belt transports the disc halves through to the next stage the addition of the reflective coating. It is this coating that allows the laser in the player to read the pattern of pits on the disc. The reflective coating stage is added using a horizontal, continuous coating enclosure. The discs go in one end transparent and come out the other end metalised, with the signal side coated in silver. 

At the same time, one of the adjacent presses has been producing an equivalent number of other sides to the disc and the output of the two lines can be brought together and the separate faces of the disc bonded to each other. For this a toffee coloured hot adhesive is applied and the two halves sandwiched together and then the labels added on the wrong way round as the discs are read from underneath i.e. Side 1 label is actually on Side 2 of the disc.

Some additional finishing goes on with the disc having its outside rim shaved smooth and given a slight bevelled profile. There is a bank of specially modified players, which monitor the signal quality of the disc at the end of the line. The surface is inspected for any visible faults that may affect playability. This area is close to the bagging and sleeving department where plastic and cardboard meet, receive their baptism in shrink wrap, and go on their way to meet the outside world.