From Wireless and Electrical Trader for 20th March 1948, the following describes the reasons behind the design of various parts of the then recently inreoduced teast card 'C'.
How to Make Full Use of the Morning Test Signal
DEALERS and engineers in the television service area will be aware that a new test card ("C") is being radiated by the B.B.C. from Alexandra Palace daily for one hour from 10 a.m. to 11 a.m. The introduction of this card came about as a result of the close liaison which B.R.E.M.A. maintains with the B.B.C. on technical matters of common interest relating to television. It was designed jointly by the two organizations with a view to overcoming the limitations of test card " A " from the point of view of those concerned with the design, development, and maintenance of television receivers.
The notes below have been prepared by B.R.E.M.A., and have been approved by the B.B.C.
The pattern approximates in mean signal to that of the average picture. The general background of the whole pattern is made mean grey to enable both positive and negative high frequency overswing, and and similar effects, to be observed at the correct setting of the brightness level and in the form in which they are usually most noticeable on picture transmissions.
Areas of mean grey background are left between all sections of the test pattern to enable "following" effects to be observed and in order to avoid, as far as possible, interference between different tests.
The main frequency and contrast range tests are confined to the area of the
pattern within the centre circle where the focus quality should be a maximum.
Subsidiary focus tests are provided in the corners of
An outer border of black and white sections similar to that used in Test Card "A " has been retained.
The individual test sections and their uses are dealt with in more detail below.
The two frequency test patterns within the centre circle consist of five frequency gratings corresponding to fundamental frequencies of 1.0, 1.5, 2.0, 2.5 and 3.0 Mc/s. They are arranged vertically for ease of intercomparison and are provided with white reference areas at the top and bottom to aid in assessing the reproduced level of modulation in the grating. The two patterns are reversed vertically relative to each other to reduce effects of non-conformity of cathode-ray tube focus and effects arising from other parts of the whole test pattern.
In use in receiver checking, referring to the left-hand pattern, the top three freuencies, 1.0, i.5, 2.0 Mc/s, should certainly be resolved, and, in the later designs of receiver, the 2.5 Mc/s pattern also, although with reduced intensity of modulation. It is unlikely that significant resolution of the last pattern will normally be obtained, since the frequency is outside the range for which most receivers are designed.
Additional diagonal frequency gratings are provided in the comers of the pattern and extend over that part of the picture area where focus variation is most significant. The equivalent horizontal definition of these gratings corresponds to a fundamental frequency of about i Mc/s and should be well within the response of the amplifier circuits. The variation of cathode-ray tube focus, or optical focus in projection systems, over the picture area can, however, still be judged by observation of the sharpness of the lines of the gratings.
The majority of the pattern is covered by a white square grid on the grey background. This provides a means of judging scan linearity over the majoi part of the picture area for both directions of scan. In addition, a more critical test of linearity over the central area is provided by a centre circle of slightly larger diameter than that on test card "A"; the grid is therefore omitted from the area inside the circle.
For perfect linearity of scan the circle would be accurately circular and all the grid meshes square and equal in size. A close approximation to this can usually be obtained with present receivers (see also section 5 below).
The pattern is surrounded by a border of alternate black and white sections, the length of each section being half that of the mesh of the linearity grid.
The outer edges of this border represent the boundaries of the transmitted picture, and therefore have an aspect ratio of 5-to-4¹. Under correct scan amplitude adjustment these outer edges should just fill the receiver mask. In practice it may be found that it is not possible to fulfil this condition exactly with optimum linearity in the centre of the picture, as judged by the circle. In this case it is probably preferable slightly to overscan in either the horizontal or the vertical direction in order to maintain central linearity.
The black and white border sections on the right-hand side of the picture, immediately preceding the line synchronizing impulses, also afford a critical test of separation of synchronizing impulses from picture signal.
Incorrect adjustment of the synchronizing separator or limitation of frequency response in the vision channel will tend to cause horizontal displacement of parts of the picture information, e.g., the contour of the circle, corresponding to the positions of the black and white sections down the height of the pattern.
The central contrast wedge provides five tone values varying between full white at the top to black at the bottom. It is not at present possible to specify the' brightness of the intermediate tones exactly, but with satisfactory receiver operation they should all be reproduced as definite steps in brightness.
For satisfactory receiver operation the brightness and contrast controls should be adjusted so that the scan is just not visible on the black square, while the white square represents the maximum brightness available from the tube at satisfactory focus quality.
If one of the intermediate tones is missing, or the grading appears unequal, it will in general be necessary to reduce the contrast, and reset the brightness to give the correct black level.
Two vertical bars, one white and the other black, of about 0.25 microsecond width, are provided on either side of the centre circle. These provide in effect a pulse test of the whole system and enable the response to isolated detail approaching the maximum resolution of the system to be judged.
In addition, these bars provide a means of checking the pressure of spurious reflection signals such as those arriving at the aerial by multipath transmission.
Amplitude and phase distortion at the low frequency end of the video spectrum give rise to background shading over the picture area in the form of horizontal streaking effects. Such effects, however, are infrequent as a form of receiver distortion and could only occur where one or more stages of video amplification with unsuitable L.F. time constants are employed; such effects may also be caused by faulty D.C. restoration. The fault is, however, more likely to occur at the transmitter, due to the difficulty of maintaining accurately a perfect L.F. response of the transmission system.
An adequate test for practical purposes is provided by the black bar on a white ground positioned .above the centre circle, and in addition, the black and white areas on either side of the centre circle.
The grid pattern has been made to correspond to a full white signal in- order,to provide an additional check on the variation of focus quality over the picture area at maximum cathode-ray tube modulation. For this purpose, the lines of the grid have been made as narrow as permissible without appreciable introduction of the interference effects on horizontal lines, inherent in the line scanning process.
|Notes||1.||The aspect ratio changed to 4-to-3 two years later. This new ratio remained the standard for over 40 years until picture quality began to deteriorate with the introduction of wide screen TV's.|
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15th October 2004