Correcting picture faults 

1 November 2021 tbs.pm/73933

Cover of 'Television Explained'

From ‘Television Explained’, first published in 1947; this, sixth, edition published 1957

GREAT CHANGES HAVE TAKEN PLACE in the design of television receivers since the publication of the fifth edition of this book, owing mainly to the introduction of an alternative television programme and the consequent necessity for design features that enable the non-technical user to tune his receiver from one transmission to another. This involves switched tuning, automatic gain control, an increased use of flywheel synchronization and dual aerial systems, and the differences in technique are accentuated by the location of the alternative programme in the very high transmission frequency region of Band III.

All the new features, as well as those of the older receivers, are explained in simple terms in this sixth edition, but they involve major changes in receiver design, and as a result it has become necessary to revise the book entirely. The five preceding editions have proved very popular, however, and every endeavour has been made to preserve the original style, which has demonstrated its popularity by the consistent sales of large printings in the five successive editions since it was first published in 1947.

The present writer, as a new co-author, feels some diffidence in omitting any of the original text, which has proved so successful in the past, and wherever possible it has been retained, new material being added to it as necessary. Nevertheless, the differences between the old receivers and the new are so far-reaching that several chapters have had to be completely rewritten, and others added, although throughout the new text the original author’s authoritative and easily assimilated method of presentation has been followed as nearly as possible in order to preserve the existing character of the book.

As in previous editions, in explaining the principles of television receiver circuits, a knowledge of ordinary sound radio receivers is assumed, but no previous knowledge of television circuits is necessary for an understanding of the text. The complete receiver is described stage by stage, and it is hoped that the method adopted of splitting this up into a number of more or less self-contained separate units will help the reader to understand the complete receiver more easily.

The book is intended to be of assistance to students of television, to radio service engineers who wish to embark upon television service work and want to understand the principles and circuits involved, and to the knowledgeable owners of television receivers who would like to find out how their sets work.

E.A.W.S.

Dorset House, London, S.E.1.
September, 1956.

 

CORRECTING PICTURE FAULTS

The B.B.C. tuning signal, both as transmitted and as received, is shown on the facing page so that the reader can compare this with the various faults illustrated later. It will be seen that some pictures with the older tuning signal have been retained from the previous edition, as the Editor considered they showed certain faults particularly well.

 

BBC 'batwings' tuning signal: normal

The B.B.C. tuning signal, as transmitted

 

BBC 'batwings' tuning signal: as seen on a TV

A reasonable reproduction of the tuning signal as received on an average receiver when the controls are adjusted properly

 

BBC 'batwings' tuning signal: too bright

Too much brightness, showing flyback lines at the top, and too little contrast

 

BBC 'batwings' tuning signal: too much contrast

Too much contrast, giving a “soot and whitewash” effect

 

BBC clock: poor vertical hold

Effect of frame slip through loss of frame hold. Adjust frame (or vertical) hold control

 

BBC clock with lines across

Effect of partial line slip. Adjust line (or horizontal) hold control

 

Completely destroyed picture

Here the picture is slipping badly in a horizontal direction. Adjust line (or horizontal) hold control

 

BBC 'batwings' tuning signal: squeezed top and bottom

Picture too shallow. Adjust height (or frame amplitude) control until the pattern is circular.

 

BBC 'batwings' tuning signal: stretched top to bottom

Picture too deep. Adjust height (or frame amplitude) control until pattern is circular

 

BBC 'batwings' tuning signal: squeezed left and right

Picture too narrow. Adjust width (or line amplitude) control until the circle is correctly shaped

 

BBC clock: stretched left and right

Picture slightly too wide. Adjust the width (or line amplitude) control until the circle is of the correct shape

 

BBC 'batwings' tuning signal: fuzzy

Picture out of focus. Adjust focus control

 

BBC clock: squashed at the top

Picture slightly compressed at the top. Adjust frame linearity (vertical form) control, and then the height control is necessary

 

BBC clock: tilted 33deg anticlockwise

Picture not level. Rotate the scanning coil unit on the neck of the C.R. tube inside the receiver until lines are horizontal (using mechanical levelling controls if provided)

 

BBC clock: shifted up and left and peeling in the corner

Picture displaced upwards and to the left. Adjust mechanical or electrical shift controls to centre picture. If corner shadow persists, scanning coils should be pushed forwards

 

Test card C

The special “C” test card, which is transmitted out of normal transmission hours for the benefit of the radio industry in testing receivers.


TELEVISION TEST CARD “C”

 

THE TELEVISION TEST CARD “C”, which is radiated by the B.B.C. and I.T.A. services daily during the morning trade period, can be of great value in the setting up of a television receiver, and also in obtaining useful information on its visual performance.

The card was introduced as a result of close liaison between the British Broadcasting Corporation and the British Radio Equipment Manufacturers’ Association. This happened before the introduction of an alternative programme, but the I.T.A. transmitters also radiate the test card.

It was designed jointly by the two organizations to overcome the limitations of the original test card “A” from the point of view of those concerned with the design, development and maintenance of television receivers. The following explanatory notes will permit the maximum benefit to be obtained from the card:

1— GENERAL

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 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 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 central circle where the focus quality should be a maximum. Subsidiary focus tests are provided in the comers of the pattern.

An outer border of black and white sections similar to that on the tuning signal pattern forms the edges.

The individual test sections and their uses are dealt with in more detail below.

2— HIGH-FREQUENCY RESPONSE

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 frequencies, 1·0, 1·5, 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.

3— FOCUS UNIFORMITY

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 1 Mc/s and should be well within the response of the amplifier circuits. The variation of cathode-ray tube focus over the picture area can, however, still be judged by observation of the sharpness of the lines of the gratings.

4— LINEARITY OF SCAN

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 major 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 central 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).

5— PICTURE ASPECT RATIO

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 now have an aspect ratio of 4 to 8. 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.

In the case of receivers sold before April, 1950, the tube mask will have been made suitable for the original aspect ratio of 5 to 4. When such receivers are correctly adjusted according to the new 4 to 3 aspect ratio, a small gap will be present at the top and bottom of the picture, provided the border just comes within the sides of the mask. These gaps are quite normal. Receivers issued since April, 1950, will, of course, have masks suitable for the 4 to 3 aspect ratio.

6— SYNCHRONIZING SIGNAL SEPARATION

The black and white border sections on the right-hand side of the picture, immediately preceding the line synchronizing impulses, 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; for example, the contour of the circle, corresponding to the positions of the black and white sections down the height of the pattern.

7— CONTRAST RANGE

The central contrast wedge provides five tone values varying between full white at the top and black at the bottom. It is not 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.

8— PULSE RESPONSE AND SPURIOUS ECHOES

Two vertical bars, one white and the other black, of about 0·25 microsecond width, are provided on either side of the central 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 presence of spurious reflection signals such as those arriving at the aerial by multipath transmission.

9— LOW-FREQUENCY RESPONSE

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 above the central circle, and in addition, the black and white areas on either side of the central circle.

10— MISCELLANEOUS POINTS

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.

It will be realized from the above description of the features of the “C” test card that its use enables a very good idea of the performance of a receiver to be obtained merely by inspection of the reproduction of the card on the screen. Since the card is transmitted intermittently each morning, ample time is available to make adjustments to a receiver and to note their results. The ordinary tuning signal, transmitted for five minutes before each programme, is obviously not so useful in this respect, and is only intended to give time for setting up a receiver by means of its normal controls.

 

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