RESEARCH DEPARTMENT

A FIRST SET OF ,SUBJECTIVE TESTS ON A CLOSED-CIRCUIT

NoT.S.C.-TYPE COLOUR TEtEVI'SION SY'STEM

Report No. T=057

( 1955/22)

THE BRITISH BROADCASTING CORPORATIQN

ENGINEERING DIVISION

RESEARCH DEPARTMENT

A FIRST SET OF SUBJ£CTIVE TESTS ON A CLOSED-CIRCUIT

NoT.SoC-TYPE COLOUR TELEVISION SYSTEM

Repo rt No. 1=057

( 1955/22)

-WeN. Sproson. M.Ao (w. Proctor Wi1son)

This Report is the property ot the British BroadcastiDg CorporatioD aDd Bay DO.t· be reproduced or disclosed to B third party iD aDY tora without the writteD per.1ss1oD ot the CorporBt1oD.

Section

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3

4

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Report No. T-057

A FIRST SET OF SUBJECTIVE TESTS ON A CLOSED-CIRCUIT

N.T.B.~.~TYPE CDLOUR TELEVISION SYBTEM

Title Page

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INTRODUCTION .. 8 '" I) III '" " I) * • ~ .......... '" ••• • ' .... :It '" •• '" ,. ..... '" ~ <» :t '" '" SI :It ,. .. '" ,. :t 110 • :t (9 III .. @ •

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3~1. Colour Rendering Properties "'~~ ••• "' •••••• ~\lI •••••• 8"' •••••• ~'.... 2 3.2. Visibility of Dots in the Compatible Black-and-white Picture •• 4 3<1>3. Reverse Compatibility .• ":&.~~ •• 8 •• 1iI~:e •• ~ .•••• ~~ ...... ~ ••• ~....... 6 3.4. Sharpness of the Coloured Pictures ••••••••••••••••••••••••••• , 6 3.5. Noise on the Coloured Picture ..••.•••••••••• ~ •••••• ~~ .••••. 8.. 7

3.6. Brightness of the Colour Television Picture ••••••••••••••••••• 8 3.7. Size of Colour Television Picture ••••••••••••••••••••••••••••• 9 3.8. Preference for Colour or Black and White ••••••••••••••••••••••

DETAILED TESTS ON NOISE .e ••••••••• a •••••••••••••••••• "' ••••••••••••••

4.1.

4.2.

Chrominance Gain and Fringe-area Noise •••••••••••••••••••••••• Choice of a Suitable Notch Filter for the Luminance Channel ••• of the Transmitter Pare Noi se Versus Bandwidth of Notch at 20, dB Attenuation ••••• Parc Noise and Change in Chrominance Gain •••••••••••••••••••••

TABLE 1 " .... ,. .... " '" • 0 ... '" ...... :I ............ " ..... " .. ,. " " • 0 • '" .. .

APPENDIX I

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September 1955 Report No. T-057

( 1955/22)

A FIRST SET OF SUBJIECTIVE TESTS ON A CLOSED~CIRCUIT

N.T,S0C,~TYPE COLOUR TELEVISION SY:STEM

SUMMARY

A set of general appraisal tests has been carried out on a British version of the American N.T.S.C. colour television system using 61 observers. The results of this show a satisfactory subjective reaction to the various known shortcomings of the N.T.S.C. colour television system. A further set of more detailed tests relating to noise and definition has enabled us to assign numerical values to the exchange of fringe-area signal-to-noise ratio with chrominance gain in the interests of better compatibility and non-overload running of the transmitter. The optimum value has also been determined for a "notch IJ filter to be used in the luminance channel of the transmi tter to improve "parc"* noise without any substantial degradation of the colour or black-and-whi te picture.

1. INTRODUCTION.

The subjective tests which form the subject of this report were done on a British version of the American N.T.S.C. colour television system. The colour sub-carrier is located at approximately 2'66 Mc/s and the I and Q components of the chrominance signal are given bandwidths of 1'0 and 0'4 Mc/s respectively. The tests were done with direct connection between signal coder and decoder so that modulation on to a radio frequency carrier wave was not employed; all tests being conducted at video frequency. The "burst" of S cycles of sub-carrier frequency was not used at this stage of the work. The signal source was a combined 16 mm motion picture colour telecine and slide scanner (accommodating both Leica-sized slides and standard 3i in. x 3~ in. (S' 26 cm x S' 26 cm) lantern slides), The two coloured picture displays incorporated R. C. A. tricolor kinescopes giving pictures of si ze approximately S~ in. x 11 in. (21'6 cm x 28 cm). The limiting resolution of these picture tubes is about ~ Mc/s and this must be remem~red when certain of the results are considered. The compatible monochrome picture was shown on a high grade domestic black-and-white receiver of which the video channel was used but not the r.f. channel. This receiver was adjusted to be about 3 dB down at 2'66 Mc/s and 4 dB down at 3 Mc/s. These figures were considered to be typical of a good domestic receiver although clearly not up to the standard of a laboratory monitor. For one test only, where definition was of the highest importance, a laboratory monitor was also used.

*"Pare" noise is an abbreviation of American origin denoting "het.erodyned cross-talk from lUminance into the chrominance channel110

2

The tests are divided into two halves, the first of which concerns general appraisal and was done with a total of 61 observers in five sessions. The second half described in Section 4 was concerned with certain aspects of noise and definition of the colour and compatible _black-and~white pictures and a much smaller number of observers was used. The observers for the first half included more than 30 non­technical staff. In the second half, only engineers were used because it was felt that relevant results would be obtained more quickly in this way.

2. GENERAL APPRAISAL TESTS.

Each observer was handed a questionnaire, a copy of which is shown in Appendix I. The answers to the questions will be dealt with in the same order as on the questionnaire. The seating arrangements and display of monitors is shown in Fig, 1. The monitors from left to right are

o 0-5

ill Bleck and

white monitor

4 \ ,

1-5 Metres

ROWIDDDDDDD ROW2DDDDDDDD

Fig. ! = Arrangement of monitors and seats

i. a colour monitor using a 15 in. (38 cm) tricolor kinescope

ii. a high grade domestic black­and-white receiver* showing a compatible black~and~white picture without interfering dots due to the chrominance signal

iii. a high grade domestic black~ and~white receiver* showing the compatible picture including the dot interference due to the chrominance signal

iv. another colour monitor using a 15 in. (38 cm) tricolor kinescope (tinted face-plate)

v. an optical monitor which is essentially an optical projector giving a picture of a duplicate colour transparency to that used in the picture originating slide scanner. This monitor was used for Question 1 only.

The observers were seated on two rows of chairs. Clearly the distance of the observers to any of the five monitors must vC),ry over a considerable range, but it is hoped that the varying conditions include all those of normal domestic viewing.

3. RESULTS.

3.1. Colour Rendering Properties.

The observers were shown five colour slides each of which was available in duplicate form. Adjacent frames of 35 mm motion picture colour film were used as

*The two black-and-whit.e receivers were of' the same type and the responses of the video circuits were adjusted to be virtually identical.

3

duplicates as these must be almost identical in colour balance and would differ in geome~rical arrangement of subject matter only if there were rapid movement in the film concerned. In point of fact, the two adjacent frames were virtually identical in every case. One picture was shown on the two colour television monitors and a comparison picture was sho~ on the optical monitor. The black~and~white pictures on monitors 11 and III (Fig. 1) were not displayed for this question.

It was explained to the observers that this question concerned colour rendering only and that, as far as it was possible to do so, they were to ignore any other differences between the pictures such as sharpness, brightness or geometrical distortions.

r;;----lldcnlical b

ti:::lldanlicai b

Acceptoble as a reproduction Dt'finihlv diffncnt

AcclEptoblll os 0 re roduction Definitely different

tildentic:al b ACCEptable as 0 re reduction

Ddinitcly diffennt

c;--) Idlntical b Acceptable· cot 0 re roduction

Definitely difftr.nl

ra---, Identical b Acceptable os a re roduction

Definitely different

-a--1ldln1.icol b Acceptable os a reproduction

Definitely diftafl!nl

Slide I

Slide 2

Slide 3

Slide 4

Slide 5

Mean values

o 10 No" of l...' -------..1.,--::-0:::' _____ ::.2.:,0 ___ " ___ ~3J.'O::....-____ ..,_-~4P observers

20 40 60 DJ. of observers

Fig. 2 = Hi stograms of answers on colour accuracy

The answers for the 61 observers are shown in histogram form in Fig. 2. Commenting first on the average results for the five slides," we see that a few observers (4%) regarded the television picture as the same as the optical one (category (a) in the questionnaire) but among the remainder there was a roughly equal division of opinion as to whether the reproduction via t"elevision was acceptable as a reproduction or noticeably different. 45% placed the slides in category (bl--~ slightly different but acceptable as a reproduction, and 51% placed the reproduction in category (cl~~noticeably different. The reactions to individual pictures vary. The least satisfactory was slide No. 3 and the colours which were observed to be wrong were those of scarlet gloves and flesh colour. Slide No. 4 was also not reproduced well; the observers' comments show that flesh tones, colour of hair and the blue-green of a dress were the inaccurate colours. Slides Nos, 1, 2 and 5 were regarded as satisfactory by more than 50% of the observers.

The colour errors of the system have been calculated from the measured characteristics of the film scanner and the colour television tube. The results of this are shown in Fig. 3, Additionally the number of "just noticeable differences" between the reproduction and original have been worked out on the basis of MacAdam'sl

4

Oo9r---,---,----.---,----,---,-_-,-_-,

:x

Fig. 3 = Chromaticities of twelve co]our filters and thei r reproductions

data. It should be emphasised that one "just noticeable difference" is determined in MacAdamls experiments with two juxtaposed semi-circular patches of colour. For ordinary non-critical colour comparisons it would probably be more reasonable to regard ten "just noticeable differences" (j.n.d.) as a perceptible step~-although, even then, not very obviously perceptible. Table 1 gives the MacAdam differences for 12 colours (6 hi g h 1 y sat u rat e d an d 6 desaturated). Red and magenta 0

are not well reproduced~-hence the scarlet gloves of slide No. 3 were poorly regarded. Cyan is reproduced with 24 j.n.d. IS and tricolour blue with 95 j.n.d. IS.

None of the slides contained a blue of the saturation of tricolour blue. The desatur.ated colours are reproduced fairly well (i.e. 10 j.n.d. 's or less) with the exception of the green. Adverse

comments on the rendering of flesh tones may well be due to an increase in "sit" level during the tests due to warming up of the monitors. There is no theoretical reason why flesh tones should not be adequately reproduced.

question. It may be argued that this test of colour accuracy has asked the wrong

Viewers in the home will rarely be given an original with which to compare the colour television reproduction. To that extent, the question about colour rendering has been made much more critical. 50~ of the observers found it satisfactory.

Even so we observe that approximately A further point is that if a duplicate

slide is not shown, some of the observers might object to the colour television picture because of errors in the colour transparencies quite apart from reproduction by television. It was desired to avoid recording comments on the merits of colour photography and to restrict the answers to the objective performance of the colour television channel.

3.2. Visibility of Dots in the Compatible Black~and-white Picture.

For this question, the gain controls of the colour television monitors were turned down to zero and the two identical black-and-white receivers were used. Monitor No. 11 (Fig. 1) was fed with luminance signal only and hence had no dot interference pattern. It was used as a comparison picture and the observers were told that it was typical of black-and-white pictures received in the present television service. Monitor No. III (Fig. 1) was fed with the composite luminance and chrominance

5

signal* and thus gave rise to a picture which had an interfering dot pattern everywhere except in the white-grey-black areas of the original coloured picture. The observers were asked to comment on the visibility of the dot pattern according to the scale

(a) not perceptible

(b) just perceptible

(c) perceptible but not ~oying

(d) definitely annoying.

Three Leica-sized slides were ~sed followed by one minute of a travel film ~16 mm Kodachrome). Comments on the visibility of dots were separately listed for each slide and the moving pictures.

The results are shown in histogram form in Fig. 4. Slide No. 6 depicted a rural cottage and could be regarded as having colour s

Not p&rceptible

No. erceptlble

No' 2rceptibl:z

20

1 Slide 6

Slide 7

Slide 8

Motion picture

of intermediate saturation. Almost 70% of the observers were unable to p3rceive dots, 19% gave the "just perceptible» category for their answers and 11% the "perceptible but not annoy::'ng" category. No one found the. dot structure "definitely 0 IL-____ ~-, ____ J-__ .-19 , I , ~ ",0 50 No. of observers , ,

6'0 annoying". Sli·"l.e No. 7 was one of the N.T.S.C. slides prepared by Eastman Kodak and contains a large area of sat 11rated red. The perceptibili ty of dots would appear

20 40 80 ~/o of observers

Fig. It - Histograms of answers on visibil ity of dots

to have slightly diminished according to the resultz although we know that dots are more predominant where there is a large component of the chrominance signal---colours of high saturation. It is possible that some of the observers confused noise with the regular dot pc,ttern that they were actually being asked about. Slide No. 8 was a transparency of the grounds of Kingswood Warren and had a fairly low saturation. Once more, about 70% of the observers could not perceive any dot~· two observers (8%) classed the dots as definitely annoying otherwise the distribution is very similar to that for slide No. 1.

The moving picture showed up dots to a greater extent than the static slide~ possibly because for a certain speed of motion of the subject matter the dot uawl can become much more obvious** in the same way that interlsce appears to be lost during vertical movement in the scene. 55% of the observers were unable to see dots: 30% stated that the dots were just perr.eptible: 13% gave the dots as perceptible and only one observer (1.6%) claimed that the dots were definitely annoying.

~but not with the ftburst W for locking the pbase of the local oscillator in the colour

:r e c e i v.e r.

##As L.C .. Jesty has often remal-ked.

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If we count the "just perceptible" category as being satisfactory, we can state that 85~ of the observers found the pictures satisfactory and 98~ of the observers gave an answer in categories (a), (b) or (c), that is "not annoying".

3.3. Reverse Compatibility.

For this question, the observers were asked to compare the black-and-white "colour" picture obtained on a colour television monitor in the abs_ence of chrominance signal with the compatible black-and-white picture as shown on monitor No. III (Fig. 1). The judgment was to be based on general appraisal of the picture. The contrast range of the black-and-white receiver (No. Ill) exceeded that of the colour television monitors but this was considered fair as each type of receiver was operating under its own optimum cond~tions. The colour temperature of the screens was not the same and it is possible that this factor influenced the judgment of observers. The material used for this question was a further minute of the 16 mm travel film.

b SIi t.I bettar Some 05

Much worse 511 htl worst

o 10 20 30 40 o~~r~:rs '~----------~'--rl --------~'----_.,----~,--------~,--~,

20 40 ro % of obserWlrs

Fig" 5 - Histograms of answers on reverse compatibility, colour receiver compared with monochrome

The results are shown in Fig. 5. The largest group of answers (55~) were in category (d), i.e. picture on the colour receiver was considered to be slightly worse than the picture on the black-and-white recei.ver. 10~ of the observers considered the colour receiver picture to be slightly better than the monochrome as against 33~ Who thought that the picture on the colour monitor was much worse than the black-and-Whi te one.

3.4. Sharpness of the Coloured Pictures.

The chrominance channels of the colour monitor were switched on again and a comparison of the sharpness or definition or resolution of the colour picture was asked for, the standard beihg the black-and-white monitor No. 11. Technically it is known that the colour tubes on which the present experiments w?re conducted have not as high a limiting resolution as the ordinary monochrome tubes. The question was asked in order to find out how this difference was assessed subjectively. not to bias the observers the questionnaire gave five categories as follows:

(a) much better than

(b) slightly better than

In order

colour picture (c) about the same as the black-and-whi te picture

(d) slightly worse than

(e ) much wor se than

The picture source was 16 mm Kodachrome which is known to have a limited resolution. Had the test been done with the 3 in. x 2i in. (7'6 cm x 5'7 cm) slides the answer might have involved a regrading towards category (el. However, programme

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material which is of no higher definition than this particular 16 mm Kodachrome copy may well be transmitted for a considerable part of the time.

Much blttr;r Slightly betttr

A out the ,ame

e: Much worse

9

511 htly worst

40

60 !

No. of observers

% of observers

Fig. 6 - Histograms of answers on sharpness, colour receiver compared with monochrome

The results (Fig. 6) show that 5% regarded the colour picture as being much

better than the black-and-white; 6% regarded it as being slightly better; 18% found

the two pictures much the same for sharpness. The largest group (61%) consisted of

those who found the colour picture slightly worse, whilst 10% classed the colour

picture as much worse than the black-and-white. The overall conclusion from these

results is that, with 16 mm film as the source, the majority find the colour picture

slightly less sharp than the equivalent black-and-whi te picture. (Note that the

comparison was made with monitor No. 11 operating on the luminance signal only.)

3.5. Noise on the Coloured Picture.

For the benefit of the non-technical observers, an intentionally noisy

colour picture was first shown. Pictures were then shown in which noise equivalent

to that encountered in fringe-area reception had been added to the 200

composite signal and various values of

chrominance gain at the transmitter

were used. Overall ~hrominance gain

was kept constant by appropriate

adjustment of the receiver chrominance

gain control. A range of 9 dB of

chrominance gain was used in steps of

3 dB. The purpose of the experiment

was to find whether (in the interests

180

160

140

120

of better compatibility for the black­

and-white receiver and non-overload~ 100

condition of the transmitter) it isw

possible to reduce the'chrominance gain

at the transmitter, The purpose of the exercise was not explained to the

observers who were told only that four

different conditions of picture were

being shown. The signal source for

this question was a 3 in. x 2i in. (7'6 cm x 5'7 cm) slide (Bathing Girl)

because a substantially noise-free picture

can be generated in this way (in contra­

distinction to 16 mm film where the

picture was certainly not noise-free).

The results for this test

have been plotted in graphical form

(Fig. 7). The numerical values are

80

60

40

20

o

Limits shown with each ~oint are :!: standard deViat on.

Max. score 61 obse rYers

..E!>~~~ f---- ---- f---- ----

~ ~ ,.... .-/

jp to pbna ---- r--- ---- -----=---

j p threshold ---- ---- ---- ----r·----

-9 -6 -3 o ChrominaflCI gain (trcnsmiUir)t dB

jp=JU$'t plfrct'piib(f, pbnQ=pcr"p\ibl~ but noi ann"Ting, do.dfflMllly annoying.

Fig. 7 - Effect of chrominance gain on visibil ity of noise

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obtained by. weighting the observers' comments according to the following scale:

Grade Score

(a) not perceptible 0

(b) just perceptible 1

(c) perceptible but not annoying 2

(d) definitely annoying 3

The graph (Fig. 7) also shows the standard deviations. The horizontal dotted boundary lines represent conditions equivalent to each half of the total number of observers answering according to adjacent criteria. From this it is clear that the 61 observers taken oas a whole did not find any significant change in the noise

ass 0 cia t e d ,wi t h the t r an s m i t t e.r ~ chrominance gain settings of 0 dB I

Limits shown with each point arc ± standard deviation

11 observers

Maxo score

30

pbna to da f----- ----fo'----- ---- ----

10

o

jp to pbna

~ ----- -----".0 ___ - ----

~ ~

j p threshold 1=---- ---- ----r:--------

-9 -6 - o ChrominanCl' gain (tranlmittcr), dB

Fi g. 8 - Effect of chrominance gain on visibility of noise

(standard N. T. s. C. ) ~3 dB, -6 dB and -9 dB.

If we consider the results of one of the sessions in which all the observers were engineers, it is seen (Fig. 8) that a trend is clearly visible. A further point is that the engineers, who mi~ht be expected to be more critical, gave the pictures a lower noise rating, From this it is suggested that the non-technical staff were net very clear about what was being asked for. The only other explanation would be that non-technical staff have lower differential (and higher absolute since they rated the noise higher) visual acuity than engineers in view of the much clearer trend in Fig. 8 than that in Fig. 7. This seems an unreasonable hypothesis.

In view of the apparent unreliability of general observers on the question of noise, a further and much more detaile~ set of tests relating to noise was done with a small group of engineers. These tests are described in the second half of this report (Section 4).

3.6. Brightness of the Colour Television Picture.

The photometric brightness of the peak white obtainable with the present R.e.A. colour picture tubes is about 10 ft-lamberts. It was desired to find out what degree of ambient lighting is permissible with these colour tubes and so Question 6

9

was asked about the subjective brightness of the colour picture under three conditions of room lighting. With full room lighting the photometric brightness of the tube faces due to room lighting only was 1 ft-lambert for monitor No. I and ~ ft-lambert for monitor No. IV (with tinted face). Two effects have to be considered

i. a desaturation of the picture due to adding white light

ii. a raising of the level of adaptation due to the distribution of brightness in the whole visual field.

There is little doubt that both factors influenced the judgment of the observers.

Wi th a stan·dard lamp alone at the rear of the room, the desaturating brightness was down to 0'06 ft-lambert for monitor No. I and 0'03 ft-lambert for monitor No. IV. With no room lighting the desaturating brightness due to daylight leaking through the curtains had reached a level below the lower limit of the S.E.I. photometer, namely 0'01 ft-lambert.

The observers' comments are shown in histogram form in Fig. 9. With full room lighting, no one regarded the picture as too bright, 72% of the observers found it satisfactory and 28% too dim. With one standard lamp on, 15% found the picture too bright, 75% satisfactory and 9% too dim. With no ambient light, the number finding the picture too dim was reduced to 6%, the remaining 94%

Too dim

Q Too bri ht b IL----J Too dim

Q Too bri ht

Sati5tacto.rr.

Satls factory

b Satisfactory

Full room lights on

Standard lamp

No ambient lighting

o 10 20 30 40 50 No. of observers ~I ______ ~I~ ____ ~I __ ._--~J----,_~J ____ ~J

2b 4'0 6b 8'0 % of observers

Fig. 9 - Histograms of answers on brightness of colour picture

finding the picture either satisfactory or too change of subject content in the 16 mm film

bright. It should be mentioned that made this question perhaps a little

difficult to answer. Such changes, however, will presumably take place in actual colour television transmissions. The conclusion which emerges from this test is that a limited amount of ambient li~hting is acceptable in the present stage of tube development.

3.7. Size of Colour Television Picture.

The size of picture is approximately S~ in. x 11 in. (21'6 cm x 28 cm). Larger tubes are known to be in production in the U.S.A. but a question on size of picture was considered to be of interest. The result of this shows that SO% of the observers considered the size to be satisfactory.

3.S. Preference for Colour or Black and White.

This question asked the observers which type of picture they preferred and the result shows that 73% of the observers prefer colour. It is of interest to note that a not negligible proportion, 27% prefer black-and-white pictures on the basis of the pictures shown during this particular demonstration.

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4. DETAILED TESTS ON NOISE.

4.1. Chrominance Gain and Fringe=-area Noise.

The question on noise included in the general questionnaire covered only one set of variables out of a whole possible range. Four levels of transmitter chrominance gain were used, but at only one setting of receiver noise (fringe-area noise) estimated to be at ~26 dB, (rms noise level below peak~white picture signal). The range of test was extended by using four different levels of fringe-area nO.ise. The noise was generated by a photomultiplier followed by a standard head amplifier and was then added to the composite signal. Control of the noise level was achieved by controlling the light flux emitted by a small tungsten lamp run off a bati;ery--some of the light flux reaching the photomultiplier surface. The noise level was adjusted to what was considered to represent fairly bad fringe=-area reception (as judged on the colour receiver) and known neutral density filters were then interposed between the· light source and the photomultiplier. In this way noise levels of ~26, ~28, ~30 and ~31 dB were generated. Sixteen presentations of a 3 in. x ~ in. (7·63 cm x 5·72 cm) slide (Bathing Girl) were shown to nine observers. These sixteen versions resulted from the application of the above noise levels at each of the four transmitter chrominance gain settings; 0, ~3, ~6, ~g dB relative to the standard N.T.S.C. value of chrominanc&-to~luminance ratio.

The observers were asked to state the visibility of the noise on the scale used in Question 5. Before showing any particular combination of noise level with

30r-------------r-----------~

Max. score

2S~------------~----~~

pt.!!-..a ____ ~---

20~------------~----~~~~

-6dS chrominance gain

l0r-------------~~~p+_+_--~

-3, r.m.s. nol~e/d.a.p. signal, dB

jp=just per~rpibtt. p=prfuptiblt. o=annoying.

Fig •. 10 = Assessment of fringe=area noise

-25

transmitter chrominance gain, a standard more or less noise-free picture was shown each time. Th4s was done to give the observers a comparison level to return to each time in the hope that their .cri teri a might not change during the tests. The various combinations were presented in random order.

The results are shown in Fig. 10. Each curve represents the ~isibility of noise, at a constant transmitter chrominance gain for a range of noise levels. It is immediately apparent that a reduction of 9 dB in transmitter chrominance gain is equivalent to a much smaller change in the overall signal~to~noise ratio. This is due to a large extent to the constant luminance principle 2 employed in the N.T.S.C. system. The noise in the chrominance channel affects chromaticity far more than luminance and it is now well known that the annoyance value of random chromaticity changes is much less than that of random luminance chanl<"es3

•

If we plot transmitter chrominance gain against signal~to~noise ratio for a constant subjective criterion we obtain Fig. 11 which shows

11

that a reduction of transmitter chrominance gain of 9 dB can be offset by a 1'6 dB increase in signal-to-noi se ratio. Such an exchange may well be worth considering since it implies that a 3 dB (for example) reduction in chrominance signal interference with monochrome reception could be achieved by a mere ~ dB reduction of apparent signal­to-noise ratio on the colour receiver display tube; thus one might be completely free of the objections raised to the compatible picture such as the dot interference pattern (which we found to be only slight in the present tests) and chrominance signal interference or buzz on sound.

o,-----~------~~----~~--_.

; -3r-------t-------~--_7b-_+------~ 8. ~

j u -6r-------i-----~~--------+_------~

r.m-s. noiSII/d.o.p. signal. dB

Fig. I! = Chrominance gain versus signal-to-noise ratio at constant

cri terion 4.2. Choice of a Suitable Notch Filter for the

Luminance Channel of the Transmitter,

Cross-talk from luminance into the chrominance channel of a colour receiver is a feature of band sharing which can be a serious disadvantage, particularly if the spectrum of the noise associated with the picture signal source rises with frequency. A component of noise in the luminance signal at or near the frequency (2°66 Mc/s) of the chrominance sub-carrier 2 will be heterodyned by the synchronous detectors of the receiver chrominance channel down to a low frequency. Since the human eye is more responsive to large-area chromaticity effects than to fine detail colour changes, this noise component, although ideally only a colour change at constant luminance, nevertheless becomes more annoying. The effect can be greatly exaggerated if the spectrum of the noise in the luminance channel rises with frequency because of the enhanced perceptibility of the high frequency noise when translated to a low frequency component. Noise due to this cause is sometimes called "parc" noise. A method of reducing this parc noise* by diminishing the degree of band sharing is to use a notch or band-elimination filter in the luminance channel in the transmitter (see Fig. 12 for block schematic diagram showing where the transmitter notch filter is inserted),

Fig. J2 - Block schematic to show position of notch filter in signal source (see Sect i on IJ. 2)

*Following the 1954 colour television demonstrations by Marconi's Wireless Telegraph Company at which the disadvantages or band sharing were shown, AoVo Lord proposed this use or a notch rilter in the transmitter luminance ehannelo

12

As will readily be seen, there are two fundamentally opposite factors which have to be considered

i. loss of definition due to reduction of bandwidth of luminance channel

ii. improvement in noise due to removal of low frequency components in the chrominance channel of the colour receiver.

The reduction in definition was first investigated and this was done on both a good domestic black-and-white receiver (monitor No. III, Fig. 1) and also on a high-grade laboratory monitor.

The following table shows the attenuation bands available on a special filter unit~

Bandwidth for 6 dB

Attenuation Depth of Notch at Centre Frequency

± 50 kc/s

± 100 kc/s

± alO kc/s each available at

6 12 20 40 dB ± 300 kc/s

± 400 kc/s

The centre frequency was made to coincide with the sub-carrier frequency; hence we have a total of 20 combinations of bandwidth and attenuation. Preliminary observations made close to the screen of the cathode-ray tube showed that the effects of the various notches were not very marked and for thi s reason only six of the notches were investigated as follows:

Bandwidth for 6 dB

Attenuation Depth of Notch in dB

± 50 kc/s 40

± 100 kc/s 12 40

± 200 kc/s 40

± 400 kc/s 12 40

Using the domestic receiver, two pictures were shown:

slide 6: Rural Cottage

slide 9: Bathing GirL

For each picture six versions were shown and each version was preceded by the picture without the notch filter, the purpose of this being to act as a standard

13

of reference. Five observers were used seated in the arc of a circle at a distance of approximately four times the picture height.

The degradation of picture quality due to either loss of definition or "rings" was rated on the following scale:

Grade Score

imperceptible o

just perceptible 1

perceptible 2

marked 3

The rural cottage scene contains railings which happen to have a pattern frequency very close to that of the colour sub-carrier; hence the insertion of a notch filter should 15 Mox.scor"-r----,------,--------,

easily be noticed. Fig. 13 shows that the threshold of perceptibility has been slightly exceeded when a bandwidth of ± 100 kc/s at a depth of 40 dB is used in the case of the rural cottage scene. For

p to m

the Bathing Girl slide the threshold of JOi----i----I----I----P

perceptibility was not quite reached even with a ± 400 kc/s notch of 40 dB depth.

The domestic receiver had been adjusted to have a response of -3 dB at 2"66 Mc/s and -4 dB at 3 Mc/s. It was decided that for this experiment a receiver with a uniform response/frequency character­istic should also be used so that a further test was done with a high grade laboratory monitor. used.

In this case three slides were

slide 6: Rural Cottage

slide 9: Bathing Girl

slide 10: River Cam Scene

8 '"

jP~L_

°O~--~5~O~--~JO~O---720~O~--~400 Half bandwidth of notch. lecls

Jp=just rarcrptlblt, pzptrctirtiblt, m .. morktd.

Fig. 13 ~ Effect of bandwidth of notch filter on degradation of picture

quality (domestic receiver)

The results for this are shown in Fig. 14 where it will be seen that slides Nos. 6 and 10 gave approximately the same result. The score of 5 which is equivalent to everyone finding the degradation just perceptible is reached with ± 75 kc/s bandwidth. Slide No. 9 does not achieve this score until a bandwidth of ± 300 kc/s has been reached.

Inspection of Figs. 13 and 14 shows somewhat less than a half-criterion difference between the effects of notch filters on a domestic receiver and on a laboratory monitor. Choice of a ± 200 kc/s 40 dB notch filter would result in a

14

15 Malt score -,------,------,-----,

IO~---+_---~---_f~~~~

jp \0 P

°O~---5~O~--~,O~O---~20~O~--~400

Half bandwidtn of notch, kcfs

Fig. lij - Effect of bandwidth of notch filter on degradation of picture

quality (laboratory monitor)

"perceptible" deterioration of picture quality to 75% of critical viewers and a "just perceptible" deterioration to 25~ on a laboratory monitor, The comparative figures for a domestic receiver of good quality are 20% of critical viewers noting a "perceptible" deterioration and 70% a "just perceptible" deterioration. It may be concluded from the above that a ± 200 kc/s notch filter has as wide a bandwidth as is permissible without undue loss of picture quality.

The effect of the depth of the notch is seen from Figs, 15 and 16. As might be expected, the degradation for a given bandwidth increases quickly up to 10 dB, but at 20 dB the effect has almost reached it~ maximum,

4.3- Parc Noise Versus Bandwidth of Notch at. 20 dB Attenuation.

Having found that a bandwidth of ± 200 kc/s at a depth of 40 dB does not produce too great a degradation of picture quality, it was decided to investigate the effect of all the available notches at 20 dB. The use of ± 400 kc/s is almost equivalent to using a lo'-'F-p ass filter and was included

(in spite of considerable loss of definition) because there has been a tendency to suggest the use of such a filter.

The observers were seated at six times picture height and pictures with various signal-to-noise ratios were assessed with various notches. The signal-to­noise ratio was changed by inserting neutral density filters between the flying-spot scanning tube and the colour transparency. The overall signal level was kept constant by altering the amplifier gain. The following table gives the various signal-to-noise ratios and the notch filter bandwidths.

Density of D.A.P. Sign al-to-rm s Notch Bandwidths Neutrc:.l Filter Noise Ratio dB kc/s

0 31

0'3 28 each at

0'7 24 ± 50 ± 100

1-0 21 ±200 ± 400

1'5 16

The noise was assessed on the

usual scale except that a fifth

cri terion of "very annoying" was

found to be necessary with some of

the presentations. Prior to the

showing of any particular combina-

15

15 Max. score ----;,-------,-------,------

t i on , th e no i s e- f re e (31 dB) 10i-----t------t----:=±:::;;r==--i picture was shown without notch.

The c0mbinations were presented in

random order. In actual fact two

separate experiments were done but

the results have been combir~d in

Fig. 17 as there was an intentional

measure of overlapping between the

two.

The overall result from

Fig. 17 is that a ± 200 kc/s notch

improves the signa1-to-noise ratio

of the picture by approximately

2~ dB and a ± 400 kc/s notch by

approximately 4 dB.

As a compromise between

loss of definition (or rings) and gain in signa1-to-noise ratio it

is considered that a ± 200 kc/s

notch with a 20 dB attenuation at

subcarrier frequency is the

optimum.

4.4. Parc Noise and Change in Chrominance Gain.

In Section 4.1 changes

of chrominance gain were investi­

gated for fringe-area noise. To

complete the present series of

experiments the effect of change

of chrominance gain on the

visibility of noise for various levels of source noise was

investigated. If the signal

source had a uniform noi se spectrum (e.g. no derivative equalisation

with the present si~na1 source)

then a change of chrominance gain would be expected to be equivalent

to a change in signal-to-noise

ratio in exactly the same way as

for fringe-area noise. The

10

± 100 kc/s Rural cottaga

±400 kc/s ~a\hing girl

±IOO kc/s 8athing girl

20 30

Attenuation at 2-66 Mc/s, dB

Jp=jus1 prrcrptibJr p=pcrcf'ptlbl .. m=>morJct'd

Fig. 15 - Effect of depth of notch at fixed bandwidth on degradation of picture quality

(domestic receiver)

15 Max,scoro ---,-------.,------,-------

± 100 kc/s Rivar Cam

±IOO kc/s Bathing girl

Att.nuaticn at 2'66 Mc/s, dB

Jp=jU5't pt'ruptiblr p",pcrct'ptlblf' m=-morlc,d

30

Fig. 16 - Effect of depth of notch at fixed bandwidth on degradation of picture quality

(I aboratory receiver)

40

16

.0 MD~SCOrQ------~--------------;---------------r-------------~

a to VD

---0-- Oke/s _.-I!r-.- ±200kc/s ----x---- ±50 kefs _ ....... - ±400 kefs --0-- ±IOOkefs

15~------------~r--------------t--------------~~~~~~~~

p to a

~ ~ 10r-------------~--------------~----_7~~~,£+__.I_--------~

jp to p

• -25 -20 -15

r.m.s. nois./d.a.p. signal, d8

jp_ Just prrU'ptibl, p_pnc.'ptiblt' o.onnoyinq va. nry anflftyifW]

Fig. 17 ~ Effect of various notch filters on visibility of noise

20'r------------r----------~:=~==~--~~~~~------

1'= I

!lJ cIOr--------------+--~~~~~~~--------------~--------~~~ 'g

4

Fnqu,ncy, Mc/s

Fig. 18 - Frequency characteristics of the Video Control Units

response/frequency characteristics of the video control units for the derivative equaliser settings used in this test are sho"I-ID in Fig. 18 from which it will be seen that there is a "lift" of approximately 16 dB at 2· 66 Mc/s on all three channels. It will be appreciated that the initially uniform noise spectra will, due to these

equaliser characteristics, take on the shapes shown in Fig. 18. The note "y = 1"

17

shown in the figure implies that the equalisation was applied before gamma correction, as it should be.

This experiment consisted of presenting a picture with various signal-to­noise ratios obtained in the manner described in Section 4.3 together with the 0, -3, -6, -9 dB settings of the chrominance gain at the transmitter. A random order of presentation was again used. Each combination was preceded by the standard more or less noise-free picture (31 dB signal-to-noise ratio and standard chrominance gain). The ± 200 kc/s 20 dB notch filter was used throughout this test.

20 :1110. score---,-------r-------,----,---tr-,--,

,~,,---J-----l------0-- OdB(two sets of points) --0-- -6dB ----x---- -3 dB ---1>--- -9 dB

15r-------~-------+----~~~~L--~~-~

pto 0

§ l0r-------+-------t~--~--~~~~----~ '"

o

l>/ j p threshold • ---7- o

~3~5~-L---1r-d~~~--L----_~25~------~_2~0---------JI5

r.m.s. noise/tl.o.f>. signal, dB

jp=jus1 p.ruptiblv p=pi'rcllptiblr a,.annoyi1\9 vo=vrry annoying

Fig, 19 = Effect of chrominance gain and source noise on visibility of noise

The results are shown in Fig. 19 and it will be seen that the curves are more widely spaced than was the case with Fig. 11. Whereas a 9 dB change in chrominance gain was equivalent to only a I' 6 dB change of signal-to-fringe-area noise ratio, in the present test the 9 dB change in chrominance gain is equivalent to about 6 dB change in signal-to-source noise ratio.

5. CONCLUSIONS.

From the general appraisal tests described above we conclude that, excluding propagation phenomena and the effects of the reference colour burst signal, the

18

British version of the N.T,S,C. system is satisfactory in most respects. Visibility of dot pattern is not troublesome; the colour receivers give b1ack~and-white pictures that are judged to be "slightly worse i1 than the monochrome receivers; the definition of the colour picture is judged to be somewhat better than "slightly worse i1 although not in the category "a.bout the same as" the monochrome receiver; the brightness of the picture is sufficient to permit a limited amount of ambient lighting and four~ fifths of the observers considered the present tube size to be satisfactory. Colour rendering was critically tested by direct comparisons and was found to be satisfactory by half the observers,

The more detailed tests have shown that

i, a. 3 dB change of transmitted chrominance gain is equivalent to a i dB change of fringe~area noise

ii, a ± 200 kc/s 20 dB notch filter in the transmitter luminance channel gives a "just perceptible" loss of definition when viewed under ideal conditions and an improvement of approximately 2! dB. in signa1~to~noise ratio

i ii, a 3 dB change in chrominance gain is equivalent to a 2 dB change in source noise when there is aperture correction to the extent of some 16 dB at sub-carrier frequency,

iv, The signa.l~to-noise ratio requirements for co10ur~signal pick~up devices having noise spectra which rise with frequency will be more stringent than those for b1ack~and~wnite practice,

6. RECOMMENDATIONS.

1, It would seem that a 3 dB reduction in transmitted chrominanc~tcrluminance ratio would increase compatibility by much more than it would deteriorate the colour receiver signal-to-noise ratio and is, therefore, recommended.

ii, The use of a. ± 200 kc/s 20 dB notch filter in the transmitter luminance channel enables a 2! dB gain in signal-to-noise ratio to be achieved with only slight loss of resolution and is therefore recommended during colour transmissions.

7, ACKNOWLEDGMENTS,

This report represents the combined efforts of· the whole of Television Group and is in many ways the cUlmination of the first stage of the colour television programme. The author is very grateful to Dr, R.D.A. Maurice, who guided the work and suggested some of the experiments in Section 4. The experiments made use of three complete colour television video channels of which the N.T.S.C, system is shown in Fig. 12. In general, the pick-Up and display devices were designed and built under Mr. C.B,B. Wood's supervision, and the signal processing and coding and decoding equipment were built to Mr. A.V. Lord's design and under his supervision, The author is indebted to his three colleagues for their collaboration in many valuable discussions in the course of this work 0

8. REFERENCES,

1. MacAdam, D.L., "Visual Sensitivities to Colour Differences in Daylight", Journal Optical Soc. Am., VoL 32, p. 247, May 1942.

2. Loughlin, B,D., "Recent Improvements in Band-shared Simultaneous Color­Television Systems", Proc, I.R.E., VoL 39, No. 10, October 1951,

3. Bailey, W.F., "The Constant Luminance Principle in N.T.S.C. Color Television", Proc. LR.E., VoL 42, No. 1, January 1954.

TABLE 1

Chromaticity of Original and Reproduced Colours and MacAdam j.n,d. 's*

Chromaticities

Colour Original Filter Reproduction j.n.d. 's

x y Y x y Y

Red 0'682 0"318 0'134 0'659 0'340 0'214 25'4

Green 0'238 0'713 0'2)2 0'205 0'609 0"199 18'1

'1J (!)

Blue 0"146 0'044 0'025 0'141 0"093· 0"041 95'0 +> '" ~ ::l 1;) Cyan 0'112 0'252 0'130 0'155 0"229 0"168 24'1 CIJ

Yellow 0'496 0'498 0'645 0'486 0'458 0'576 17"0

Magenta 0'406 0'163 0'123 0'487 0'252 0'210 60"0

Red 0'391 0'358 0'599 0'400 0'348 0'606 10'3

Green 0"375 0'471 0"686 0'336 0"399 0"631 26'4 '1J (!)

1;) Blue 0"287 0"296 ~

0'656 0'285 0"301 0"414 5"7 ::l +> III Cyan 0'286 0'318 0'799 O''Z77 0'314 0'778 7'3 Ul (!)

~

Yellow 0"353 0'383 0"853 0'345 0"358 0"838 9"3

Magenta 0'316 0"292 0'714 0"323 0'299 0"740 4"4

*just noticeable differences

19

20

APPENDIX I

Row ...... (J it • ID ... ID eo e " III \9

QUESTIONNAIRE ON COLOUR TELEVISION DEMONSTRATION

1. Accuracy of colour rendering; for each slide state whether the television picture is

Slide 1

(a) the same as the optical one (b) slightly different but acceptable as a reproduction (c) noticeably different; if possible name the colours

which are wrongly reproduced

Slide 2 Slide 3 Slide 4

2. Do you find the dots in the compatible black-and-white picture

Slide 6

(a) not perceptible (b) just perceptible (c) perceptible but not annoying (d) definitely annoying

Slide 7 Slide 8

Slide 5

Moving Pictures

3. Reverse compatibility; do you find the black-and-white picture produced by the colour monitor

(a) much better than (b) slightly better than (c) about the same as the picture on the black-and-white monitor (d) slightly worse than (e) much worse than

4. Sharpness (definition) of the coloured picture; do you find this

(a) much better than (b) slightly better than (c) about the same as the black-and-white picture (d) slightly worse than (e) much worse than

..

5. Noise on the coloured picture; do you find this

(a) not perceptible (b) just perceptible (c) perceptible but not annoying (d) definitely annoying

Condition A Condition B

6. Brightness of the coloured television picture;

Condition C

With full room lighting, do you find the picture

With subdued room lighting, do you find the picture

With no room lighting, do you find the picture

7. Do you consider the size of the colour television picture

(a) satisfactory (b) unsatisfactory

8, State which you prefer

(a) colour (b) bl ack- and-whi te

If no preference, say so.

9. Any other comments.

21

Condition D

(a) too bright (b) satisfactory (c) too dim

( a) too bright (b) satisfactory (c) too dim

(a) too bright (b) sati sfactory (c) too dim