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 reached 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 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 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 on to 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. It’s even insulated
from the specially prepared floor by additional air cushion suspension.
Processing
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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. It’s
a tricky step as the nickel must separate without any deformation or
creasing (it’s 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.
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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 3Mooth
and given a slight beveled 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 bapti3M in shrink wrap, and go on their way to
meet the outside world.