Computerised 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
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
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!
Some of the benefits of the computer aided adjustment during production are:
|Article © Bang & Olufsen 1990||
BeoTech © 2006 All rights reserved