ManufacturingComputerised TV adjustment |
Computerised TV adjustment
Making a good product is much more complex that
it would appear at first sight and requires know-how in many areas. It is not
sufficient to have a good knowledge of electronics to make a good TV set or an
audio system. Of course, an essential part of the product is its design, both
mechanical and electrical, to make sure that the product does what it is
designed to do. But, much of the work in making a set 'good' has nothing to do
with product design, or know-how about the latest technology.
A good set is also one that is reliable and functions when it is taken out of
the box and plugged into the mains. It must also function correctly. This
reliability, in its widest sense, is incorporated into a product partly in its
design, partly in pre-production testing, but most of all on the assembly line
itself.
One of the more important aspects in the production of BeoVision TV sets is to ensure that they are able to reproduce pictures as faithfully and correctly as possible. Three basic adjustments are necessary to obtain this goal
Most other TV manufacturers carry out these
basic adjustments manually. Operators on the assembly line adjust the TV sets
using internal potentiometers and compare the result to a test. picture on a
test monitor, or some other sort of measurement equipment, before the final
assembly of the TV. The results often varies considerably, depending on the
skill of the operator. If you place a number of such TV sets next to each other
you will note that the picture quality is not consistent. Some TV sets may
display colour tints in a black and white picture, others may be lacking in
geometry with distorted lines, and others again may show a differences in colour
balance.
Bang & Olufsen has developed a highly automated technique using computer
controlled adjustment on the assembly line, to ensure optimal and consistent
adjustment. Two essential conditions for such an automated adjustment are that
the TV set in question is designed from the outset for computer aided
adjustment, and that the adjustment and measurement equipment is available. Both
conditions are met in the current TV programme and the system will be updated
with each new generation of BeoVisions.
The equipment for the adjustments include an advanced Personal Computer,
analogue-to-digital converters, colour sensors, video cameras, and not least,
some unique software programs developed in-house specifically for this purpose.
Integrated TV design
A vital component in all BeoVisions is the microcomputer, used in the finished
product for all internal adjustments, tuning, programming, switching, etc. A
small part of the microcomputer capacity is now also used for adjustments of
colour balance and geometry during assembly and prior to delivery. On the
assembly line, this microcomputer is accessed in the same way as in the home,
though the Datalink sockets, but there is also an internal data bus available.
Computer aided adjustment may seem a natural thing in these days, but a
historical view might be useful to put the method in perspective. The first
colour BeoVision, dated 1967, featured an adjustment panel with no less than 19
potentiometers and 3 slide switches, which were manually set by operators on
every TV set prior to delivery. Later on the sets had to be re-adjusted rather
frequently, in the shop and in the home, to prevent the picture quality from
degrading.
The production today
An assembly procedure starts with the mounting of the components. Electric
components are mounted on PCBs, each of which is tested before further assembly
in a TV chassis. The assembly also includes various mechanical components, the
picture tube, the loudspeakers, the cabinet, etc.
At this stage the partially-manufactured TV set is tested to make certain that
the basics function correctly. Is there a picture on the screen, do the
loudspeakers function? Brilliance, colour, contrast, colour mix and geometry are
adjusted to the standard setting and the TV is subjected to the so called
burn-in test for several hours, to prevent initial component failure, the so
called out of box failure that could occur soon after the TV set has been
delivered to the customer.
Computer aided colour adjustment
It is a paradox that a colour TV, in order to reproduce colours faithfully,
first and foremost must be able to show a correct black-and-white picture. The
reason for this is that the three main colours, red, green and blue, must be
mixed in a very well-balanced proportion to correctly render all the nuances in
a standard grey scale, from black to white.
So the first of the adjustments in the computer aided system deals with
balancing the colour. A special electronically generated test picture is
displayed on the screen, while an identical test picture is used as a reference
by the measurement equipment. The test picture, shown in figure 1, consists of
two large areas, one grey and one white.
This picture is used to adjust the current level of each electron gun, red, green and blue (RGB), to set the correct proportion of the three colours. The exact nuance of each of the two fields represents one step of the grey scale very accurately. The two steps are selected to give optimal linearity of the complete grey scale when the adjustment has been carried out (the steps use a light intensity scale where the unit of measurement is called a 'nit'. The white field measures 80 nit, the grey measures 8 nit).
Figure 2 shows the adjustment setup. Two sets of colour sensors on a rail are automatically brought to rest on the surface of the picture tube, measure the light intensity in the two fields accurately. Each sensor contains three photo cells with optical filters for red, green and blue, respectively. The filter characteristics allow only one colour to pass, which means that the intensity of the light can be measured as an electrical voltage generated by the light sensitive photo cell. The fields are now adjusted according to the voltage measured by the photo cells, until the delicate balance between the three guns is correct.
Figure 3 shows the sensors in relation to the
test picture. The levels measured by the sensors are digitised by the A/D
converters and fed to the computer, where they are compared to the levels in the
reference test picture for subsequent correction. The adjustment computer is
automatically connected to the TV in the setup, via the standard Datalink socket
and sends any required correction signal to the microcomputer in the TV set. The
RGB video amplifiers are then adjusted electronically by the microcomputer until
the two fields in the test picture show the correct light intensities.
Three adjustments, for geometry and colour, on a PCB on the picture tube socket,
cannot be made electronically as the voltages are too high - several hundred
volts. For practical reasons, a box with three motor driven screwdrivers
controlled by the adjustment computer is moved into position, which make the
adjustments automatically
Computer-aided picture geometry
adjustment
The picture geometry more obviously has to do with correct proportions between
height and width. All vertical and horizontal lines must be straight, angles
must be correct, circles must be circular and parallel lines not visibly
converge. Returning to figure 2, we also note a camera placed at a certain
distance from the screen. The camera monitors the thin white lines in the test
picture and compares them to the reference test picture. If the picture is not
correctly aligned, corrections are relayed to the TV microcomputer to adjust the
circuits that control the deflection currencies.
The geometry adjustment starts with a completely white screen picture to align
the camera to the particular type of TV being adjusted, be it LX or MX and with
different screen sizes. The adjustment setup also registers and takes into
account the small tolerances in the placement of the TV on the assembly line.
Apart from the details of the geometry adjustments shown in figure 4, the setup
also adjusts the focus. The camera monitors one of the white lines against a
black background and digitises the measurements. The digital levels correspond
to a statistical calculation around an average value and can be compared to a
reference. The measured levels are computed, compared and corrected, until they
correspond to the reference values, which gives optimal focus.
Connection and signal control
In the setup, computers are also used to control the signals and data to and
from the connector on back of the TV set. The AV link sockets are used to
display red, green and blue colours on the screen momentarily, monitored by
colour sensors. Much in the same way as sound is fed to the loudspeakers to test
them, data is sent in and taken from the Audio, Aux, Link and PowerLink sockets.
A notable element in this automatic testing is that it takes only a few minutes
from start to finish, which is probably less time than you have used to read
this article by now!
Benefits
Some of the benefits of the computer aided adjustment during production are:
Article © Bang & Olufsen 1990 |
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