RESEARCH DEPARTMENT

A SUBJECTIVE APPRAISAL OF TWENTY-FOUR TELEVISION RECEIVERS

WITH RESPECT TO THE EFFECTS PRODUCED BY AN N.T.S.C.-TYPE

COLOUR TELEVISION TRANSMISSION

W.N. Sproson, M.A. J.G Ingleton

Report No. T-059

( 1956/11)

-(w. Proctor Wllson)

This Report is the property of the British Broadcasting Corporation and may not be reproduced or disclosed to a third party in any form without the written permission of the Corporation.

Report No. T-059

A SUBJECTIVE APPRAISAL OF TWENTY-FOUR TELEVISXON RECEIVERS

WITH RESPKCT TO THE EFFECTS PRODUCED BY AN N . .T •. S.:C .. -TYPE

COLOUR TELEVISION TRANSMISSION

Section

1

2

3

4

5

Title

SUMMA..RY ••••

INTRODUCTION .

CONDITIONS OF TEST

RESUUTS ••••••

3.1. Resolution of the Picture

3.2. Visibility of the 2"66 Mc/s Dot Pattern

3.3. Visibility of "Burst" on Flyback

3.4. Audibility of "Buzz" on Sound

CONCLUSIONS

REFERENCES

APPENDIX

1

1

2

3

3

5

8

9

10

11

12

b

PRIVATE AND CONFIDENTIAL

May 1956 Report No. T-059

( 1956/11)

A SUBJECTIVE APPRAISAL OF TWENTY~FOUR TELEVISION RECEIVERS

WITH RESPECT TO THE EFFEC:TS PRODUCED BY AN N. T .S.C.-TYPE

CDLOUR TELEVISION TRANSMISSION

SUMMARY

A series of subjective tests on twenty-four television receivers, using sixty observers for the visual tests and thirty observers for the aural tests, is described. These tests concern the compatibility of a colour transmission (British version of the N.T.S.C.) as it affects past and present black-and-white receivers. Troubles due to working with the television field frequency unlocked from the elec­tricity supply (asynchronous working) were not investigated at this stage, but all the other troubles were investigated and the general conclusion is that their effects are on the whole of small magnitude: only in a few cases did any of the observers declare the picture or sound to be unacceptable on any specific point.

1. INTRODUCTION,

It is known that various undesirable effects can result from the application of an N.T.S.C.-type colour signa1 1 to a conventional black-and-white receiver. These effects have been investigated to a limited extent2 ,3 with a few receivers, but it was felt desirable to get a broader cross section of past and present receivers. For this purpose, a number of radio manufacturers, under the auspices of B.R.E.M.A., supplied sets to Research Department. These sets have been investigated for the following features:

(1) visibility of 2°66 Mc/s dot pattern

(2) visibility of "burst" on flyback

(3) audibility of "buzz" on sound

In addition, the observers pictures produced by the receivers. picture such as those associated with not investigated at this stage.

gave their opinions of the sharpness of the Other known shortcomings of the compatible

asynchronous running of the transmission were

The receivers were supplied with an r.f. signal of the order of 2'5 mill i­volts from a 1 ohm line, the line being fed by a transformer. The receivers were isolated from each other by a 75 ohm resistance in each individual feed and the 1 ohm line was correctly terminated.

2

2. OONDITIONS OF TEST.

The picture source was a combined slide and film scanner. For all tests

except the audibility of "buzz" on sound, a 16 mm Technicolor film was used. This

places some limitation on the assessment of sharpness. Nevertheless, it was

considered that a moving picture is more typical of television programme material than static slides. For this reason a 16 mm motion picture was chosen in preference to

3 in. x 2~ in. (76 mm x 57 mm) slides, in spite of the fact that much better picture quality can be obtained from the large-sized slides. Sixty observers were used for

the visual tests, thirty of whom were non-technical. A fully-coded compatible r.f. signal was fed to the receivers: the chrominance information was automatically

switched on and off every 15 seconds. A separate indication of the condition of the

signal was given by the use of tone fed to a sound system already installed in the

room. The observers were asked to judge the sharpness of the pictures when the

luminance signal alone was present. The dots were to be assessed when the combined

chrominance and luminance signal was used. The colour synchroni sing "burst" was not

switched, so that observers could make observations on the visibility of burst on flyback at any time during the test. A copy of the questionnaire used for this part

of the work is shown below:

Receiver No.

1 2 3 4 5 6 - - - -

Good

Moderate Resolution

Poor

Unacceptable

Invisible

Visibility Just visible of 2' 66 Mc/s dot pattern Easily visible

Unacceptable

Invisible

Visibili ty Just visible of "burst" on flyback Easily visible

Unacceptable

An explanation of the terms used in the questionnaire was first given with

the aid of electronically generated colour bars. These show up the 2'66 Mc/s dot pattern in a pronounced way and are useful to illustrate this defect in a somewhat

exaggerated form. "Burst" on flyback is very much a function of the picture content, so that it is not expected that observers will agree particularly well on this item.

The sixty observers were present in four sessions. Chairs were provided near to the

twenty-four receivers, but observers were at liberty to use any distance of viewing

which they thought suitable.

____ ~~ ________________________________ ~~~ _____________ .-Id

3

3. RESULTS.

3.1. Resolution of the Picture.

The opinions of each of the four groups of observers on each receiver are shown in histogram form in Fig. 1. There is a fair measure of agreement between the different groups as to which receivers are better than average and vice-versa. More specifically we can allocate a simple numerical system to the gradings and then evaluate the correlation coefficient between the various groups. The scoring chosen was as follows:

Grade Score

Good 3

Medium 2

Poor 1

Unacceptable 0

Using this, the six correlation coefficients were calculated and are as follows:

Groups Correlation Coefficient

A andB 0'13

A and C 0'49

A and D 0'50

B and C 0'42

B and D 0'56

C and D 0'65

The correlation coefficient should exceed 0'41 for the~e to be less than 5$ probability that the agreement is due to chance4 •

It will be seen that the correlation between groups A and B is poor and non­significant: all the other coefficients are above the 5$ significance level. The best agreement, namely between groups C and D, gives a correlation coefficient of 0'65. In this connection it may be significant that these groups comprise technical staff. Groups A and B were non-technical staff.

An attempt was made to correlate these subjective judgments on sharpness with measurable objective parameters, using Test Card "c" as the test object. The contrast ratio of the image produced by each receiver was measured with an S.E.I. photometer. One skilled observer also estimated the modulation depths of the resolution bars in the reproduced images of the Test Card "C". In no case was it possible to establish a significant correlation. This result is somewhat surprising as it is generally considered that picture resolution depends on the response of the complete television channel over the range of video frequencies from 0 to 3 Mc/s and also on the contrast ratio. Picture height was another parameter that was considered both by itself and in conjunction with other parameters, but again no significant correlation coefficients were discovered. Due note must be taken, however, of the

I~~~ Group JP Group C Group 0

• .... ··m E~SI:1:l pq ... PH

G ::::

8 ~ rz U

G " ..

~~ 11 ~

=13=:~: ~':" .::.J:=::L-~" '-I..-~.J.....-l.--'-~ [;;<:-1:' ~:'. ~~:=:: FrL-m-L-'---.I..--L.....J o 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 1000 20 40 60 80 100

Percentage of observers

I~~t. Group A I Group a I Group C

.~ ~~ 51 14 P ~~

U

I~G1m~$:P1 hm 15 ~~ ~

u

17 ~II~ U Gmm

18 ~F

G """"1

25 ~ ~%:I u

f--"-:G=-i%,,-, ,-+, . m-+.l-+-+---l l;:;:;:;!;:;;;:,f,;':I*+--i I:::::m::±m= .. +-~-t--i C::m:::I=81+-+--t--i

26 M

'-_~-L--'--..l..--'--.L.....J L-l.--L-L--,L....J ::--:::-~'-:-!::-;-:'_F':-::': I:-+::-+--+.:-:-~ _ o 20 40 60 80 1000 20 40 60 80 1000 20 40 60 80 1000 20 40 60 80 100

P<l.rc<l.ntage of observers

G - Good P - Poor M - Moderate U - Unacceptable

Fig.1 Histograms showing resolution of receivers

5

fact that the various groups of observers do more or less agree on the assessment of picture sharpness.

We may summarise the observers' comments on picture sharpness by saying that only two receivers (Nos. 12 and 24) were found to be unacceptable by observers (2~ in both cases). The table below summarises the statistics relating to observations on resolution. normal manner.

One receiver was classed in grade 5, but it was not operating in a

Equivalent Assessment Total Grade Number of Receiver Nos.

Receivers Upper Limit Lower Limit in Grade

1 1~ good. 5~ good 6 15,10,5,25,2,11. and 5~ moderate

2 50~ good 1~ moderate 13 19,8,22,18,26,6,20, and 5~ moderate 17,1,16,3,9,13.

3 1~ moderate 5~ moderate 4 7,14,12,24. and 50~ poor

4 5~ moderate 100~ poor 0 -and 5~ poor

5 1~ poor 5~ poor 1 21. and 5~ unacceptable

6 5~ poor 1~ unacceptable 0 -and 5~ unacceptable

3.2. Visibility of the 2'66 Mc/s Dot Pattern.

The observers' opinions on the visibility of the dot pattern due to the chrominance subcarrier is given in histogram form in Fig. 2. In many cases the observers found the dots either invisible or just visible. The receivers which produced dots declared to be unacceptable to some of the observers were Nos. 11, 24 and 26. The dot pattern produced by the remainder of the receivers did not reach the level which any observers regarded as unacceptable. Even in the case of receivers 11, 24 and 26 it is to be noted that some observers found the dot pattern invisible.

Visibility of the dot pattern was found to give good inter-group correlation. The correlation coefficients are as follows:

Groups Coefficient

A and B 0'60

A and C 0'46

A and D 0'66

B and C 0'81

B and D 0"82

C and D 0'81

I~~~ Group A Group ;roup Group Cl

I : ... ... ~ .. I . .. ..

I JV ~ ~ EV U I ... .. " ... ... ..

2 JV i.1 ~ ~ ~I EV U

J~.~ !;;J r £

3 • 88Sl EV U

I ~ ~ f .... . ... . .... . ..

S JV ~ ~ ~ EV U I .] I

.. .. . ... ... 6 JV

~ ~ ~ EV U

I , ~~ PfJf m rn .. ...

7 JV r i2.I EV 8iI U

I·

19 i~ ~r U

I :ii

:::::;::

8 JV ~ ~ {{i:f:I ~ EV ~I

U I I ....

9 JV ~ ~~ ~ EV

U I :;:;::: ::00::: :';J

10 JV ~ ~ ~ ~ ~ ~ EV U I I I ~ BP ... ..

11 JV ~ = ~ i1i::I

EV U ~

25 bir U .. _ .. - .. -

}V~ 12 ~ ~ ~ ~ EV U

I 0'1 o 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

13 JV ~ ~ ~ ~ ~ ill EV Percentage of Observers U

0 20 40 6 10 801000 20 40 60 801000 20 40 60 80 100 0 20 40 60 80100 I _. Invisible EV - Easily visible

Percentage of observers JV - Just visible U - Unacceptable

Fig.2 Histograms Showing visibility of the 2' 66 Mc/s dot po ttern.

b

7

The system of scoring used in obtaining the above figures was as follows~

Criterion Score

Invisible 0

Just visible 1

Easily visible 2

Unacceptable 3

It will be seen that all the coefficients are above the 5% significance level (for which the correlation coefficient is 0'41). The general level of correlation coefficient is appreciably higher in this case than for resolution. Perhaps the presence or absence of a particular type of dot pattern is an easier judgment to make than an assessment of sharpness.

In the case of sharpness it was stated that no significant correlation could be found with respect to any physical parameter. The visibility of the dot pattern does, however, correlate significantly with a number of measured parameters. are listed in the following table:

Group

These

Correlation of Dot Visibility on Motion Pictures with the Following Characteristics

using Test Card "C"

Mean Value

Estimated modulation depth of resolution bars.

Est. mod. depth of 2'5 Mc/s bars.

Log (contrast ratio).

Log (contrast ratio) x picture height.

Log (contrast ratio) x picture height x est. mod. depth of 2'5 Mc/s bars.

Log (contrast ratio) x est, mod. depth of 2'5 Mc/s bars.

Picture height x est. mod. depth of 2'5 Mc/s bars.

Picture height.

A B C D

0'2.8 0'50 0'44 0'47

0°32 0'55 0'54 0'56

0'29 0'34 0'19 0'33

0'53 0'53 0"25 0'41

0'49 0'33 0'47

0'49 0'63 0'61 0'65

0"62 0'51 0'22 0'33

0'42

0'49

0'29

0'43

0'43

0'43

0'59

0'42

--8

Some of the correlation coefficients are below the 5% significance level, but many are above it. The highest figure for correlation of dot visibility is that obtained with the product of the estimated modulation depth of 2'5 Mc/s bars and picture height. All the individual group correlations for this are significant and the mean value of 0'59 is appreciably higher than any other mean value. It could be that the sets with the larger tubes happen to have high resolution and hence any correlation with picture height is a false one, as the results are in fact correlating with resolution (as indicated by Test Card "C"). The correlation coefficient of picture height and resolution is -0'02, so that there is no hidden correlation as far as picture height is concerned.

It may be asked why picture height should be considered as a parameter. If observers always choose a fixed ratio of picture height to viewing distance (of, say, 1:4) then a large tube ipso facto could not contribute to increased visibility of dots. In practice, however, observers tend to view large tubes at a distance which is relatively nearer than in the case of a small tube, and for this reason they may be more conscious of dots. It would also be true to add that they are more conscious of lines as well, but ten years of viewing have accustomed people to lines: dots constitute a new phenomenon.

It is interesting to enquire whether the observers agree that the sets with the best resolution (as estimated subjectively: not Test Card "C") are also the ones to give the most noticeable 2'66 Mc/s dot patterns. It would be reasonable to expect such a correlation: in actual fact, the correlation coefficient for each group of their opinions about dots and resolution is below the significance level. The actual figures are -0'34,0'08, -0'01, -0 0 08 for Groups A, B, C and D respectively. The result for Group A nearly achieves a significant anti-correlation: the others show alnost complete randomness.

We can summarise the observations on dot pattern by saying that 21 out of the 24 receivers were acceptable (in the sense of not being unacceptable) and receivers Nos. 11, 24 and 26 were found to be unacceptable to 2%, 4% and 13% of the observers respectively.

3.3. Visibility of "Burst" on Flyback.

The results on the visibility of "burst" on flyback do not show any significant measure of inter-group correlation. This is not very surprising for two reasons:

(i) "Burst" is not a very easy effect to explain to a non-technical observer: for a given receiver with a known rate of flyback, it is possible to tell the non-technical observer what sort of positioning and spacing to expect for the vertical bars which are the manifestation of it, but with twenty­four receivers the situation differs from receiver to receiver.

(ii) If one is using ordinary picture material (in contradistinction to a specially designed test card) even with a receiver that can show up the effect, it is a matter of chance as to what picture is showing when the observer decides to look for "burst" on flyback, and the visibility of "burst" is quite definitely a function of picture material. Dark tones

or blacks on the left hand side of the picture will show up the effect, should it be present, but of course only a small proportion of the pictures radiated satisfy this condition. It might be mentioned that a black caption with white letters satisfies the conditions very well.

9

It is not considered worth while to quote the subjective results in detail since the different groups did not agree as to which receivers show the effect. Suffice it to say that nearly all the observers found the effect either invisible or just visible. No one from Group A, B or C classed any receiver as unacceptable: in Group D one observer considered Receiver 21 unacceptable and one observer considered Receiver 26 unacceptable. The conclusion to be reached from these results is that given ordinary picture material, "burst" on flyback is not likely to prove a serious objection.

A sUbsidiary test using a test pattern consisting of five horizontal steps with density differences of 0'6 (thus giving steps of density value 0, 0'6, 1-2, 1-8 and 2-4) was undertaken. This test is the most critical it is possible to design and it demonstrated once more that most of the receivers do not show the effect at all or that it is "just visible". The only receivers to give the iieasil~ visible" category were Nos. 12, 18, 22 and 26. No receivers were classed as "unacceptable" on any of the five density steps.

3.4. Audibility of "Buzz" on Sound.

One of the possible dangers of the N.T.S.C.-type of colour transmission arises from the fact that there is an unusually large amount of energy near the outer end of the video spectrum, due to the colour subcarrier, and hence near to the sound channel. The effect of break-through of luminance (and/or chrominance) into the sound channel is to produce a low-pitched raucous noise with a fundamental frequency of 50 c/ s and with a considerable amount of energy in the harmonics. The word "buzz" is quite descriptive of the interference. This effect is not necessarily co=ected with a colour transmission, since break-through of luminance into the sound channel produces exactly the same effect. The addition of a chrominance signal may, however, increase the effect.

The test on audibility of "buzz" was performed with fifteen observers, using in the first instance a special test card consisting of eight horizontal strips of tricolour-red interleaved with peak-white. The spacing of the tricolour-red and peak-white was equal. This test card produces a strong fundamental component at 8 x 50 = 400 c/s and harmonics, which are in a part of the audible spectrum where the sensitivity of the ear is high. Chrominance and burst were switched on and off at regular 15-second intervals and the observers were asked to comment on the level and acceptabili ty of "buzz" in both conditions, i. e. with a luminance signal only and then with the full colour signal. Light orchestral music was used for setting the volume controls of the sets to a suitable level the test itself was conducted in the absence of sound modulation, leaving only the sound carrier. Proper lin~up procedure was followed to ensure that the sound-to-vi sion power ratio was tt.e one normally used.

The questionnaire used for this part of the test is shown in the Appendix. The conclusion reached was that for eight of the twenty-four receivers, the number of

-- 10

observers classifying them as unacceptable increased when the chrominance signal.was switched on. It must be remembered that this form of test card is an extreme case that could rarely, if ever, happen in ordinary programme material. Even so, sixteen of the receivers were quite satisfactory. It should perhaps also be pointed out that the buzz due to this special test card is appreciably louder than that produced by electronic colour bars.

A final test on buzz was done on the 24th November on the eight receivers which were to some extent suspect, this time using a colour transparency (a view of Morden Station) as well as the special test card. This colour transparency contains quite large areas of saturated colour and is considered typical of the more highly coloured scene which is likely to be transmitted as programme material. Fifteen more observers were used and again the colour information was switched on and off at 15-second intervals.

Special attention was paid to the tuning of the receivers (and the drift of tuning as the test proceeded) as this can affect the change of "buzz" with the addition of chrominance signal to a much greater extent than it affects picture quality. The results of this test are given below:

Receiver No. Picture

1 2 3 4 5 6 10 16 26 Material

Increase due to Addition 6 0 2 0 1 -1 0 0 0 Special test card of Chrominance in Number

of Observers finding 0 -1 0 0 0 0 0 0 0 Picture. Receiver Unacceptable.

Using the special test card, there is an increase in the number of observers who class the buzz as unacceptable an adding the chrominance signal for Receivers Nos. 1, 3 and 5. Receiver No. 10 was 100~ unacceptable in the absence of a chrominance signal.

Using the colour transparency, no receiver is made less acceptable by the addition of a chrominance signal. We may thus conclude that if a set is properly tuned, the chrominance signal will contribute no extra trouble due to buzz on sound. Such examples as have been quoted in the past must have been due to receivers which had got out of alignment.

4. CONCLUSIONS.

Picture sharpness estimated subjectively does not appear to correlate with known objective parameters of the receivers.

Visibility of dots due to the subcarrier is not found to be too serious. Three receivers were found unacceptable on this account by 2~, 4~ and 13~ of the observers.

Visibility of dots was found to correlate significantly with visibility of

-

11

2 5 Mc/s bars on Test Card "C", and with picture heiQ"ht. The hiQhest correlation coefficients were found between sub~ective assessment of the dots and the product of picture heirrht and visibility of 2 5 Mc/s bars. Contrast ratio per se did not ;Jive si~nificant correlation, althour<h the product of contrast ratio with other parameters did produce sir<nificant correlation coefficients.

Visibility of the dots did not correlate si~nificantly with the subjective assessment of the resolution of the receive'rs.

Burst on flyback is not a serious trouble with any of the receivers, althou.;h it can be made to be "easily visible" on four of the receivers by use of a suitable test card.

Buzz on sound is a feature related to the tunin.; and ali.;nment of the receiver. Provided that these are taken care of, there is no trouble due to buzz on sound for ordinary picture material with any of the receivers.

5, REFERENCES.

L Specification of Colour Television Standards for Experimental Transmissions from Alexandra Palace, June 1955. B,B,C. En.;ineerin.; Division.

2. N.T.S.C, Colour Television System: Interference on Monochrome Television Receivers by the Chrominance Subcarrier. B,B.C. Research Department Report No. T-051.

3 A First Set of SUb.iective Tests on Closed-Circuit N.T,S.C,-Type Colour Television System. B.B.C. Research Department Report No. T-057.

4. Fisher and Yates Statistical Tables for BioloQical, A.;ricultural and Medical Research. Oliver and Boyd, 1949.

12 APPENDIX

'BUZZ' TEST

Receiver No. 1 2 3 4 5 6 7 8 9 10 11 12 13

Condition A Inaudible

Just audible

Audible

Level Slightly disturbing

Disturbing

Very disturbing

Acceptability Acceptable

Unacceptable

Condition B Inaudible

Just audible

Audible

Level Slightly disturbing

Disturbing

Very disturb ing

Acceptability Acceptable

Unacceptable

Receiver No. 14 15 16 17 18 19 20 21 22 23 24 25 26

Condition A Inaudible

Just audible

Audible

Level Slightly disturbing

Disturbing

Very disturbing

Acceptability Acceptable

Unacceptable

Condition B Inaudible

Just audible

Audible

Level Slightly disturbing

Disturbing

Very disturbing

Acceptab ility Acceptable

Unacceptable

EGF Printed by B.B.C. Research Department, Kingswood Warren, Tadworth, Surrey