RESEARCH DEPARTMENT

COLOUR TELEVISION

THE ADAPTATION OF THE NeT.S.C. SYSTEM TO U.K. STANDARDS

PART 5~ SOME PROBLEMS ENCOUNTERED IN THE USE

OF A LOW-POWER TRANSMITTER

Report Mo. Ta 060/5

(1957/19 )

THE BRITISH BROADCASTING CORPORATION

ENGINEERING DIVISION

RESEARCH DEPARTMENT

COLOUR TELEVISION

THE ADAPTATION OF THE N.ToS.C. SYSTEM TO U~K. STANDARDS

R,F, Vlgurs

PART 5~ SOME PROBLEMS ENCOUNTERED IN THE USE

OF A LOW~POWER TRANSMITTER

Report No. T-060/5

( 1957/19)

--(w. Proctor Wilson)

This Report is the property of the British Broadcasting Corporation and may not be reproduced in any form without the written permission of the Corporation.

Report Noo T-060/5

COLOUR TELEVISION

THE ADAPTATION OF THE N,ToSoC, SYSTEM TO UoK, STANDARDS

Section

1

2

3

4

5

6

7

PART 5: SOME PROBLEMS ENCOUNTERED IN THE USE

OF A LOW-POWER TRANSMITTER

Title

SUMMARY ••••

INTRODUCTION ••

TRANSMITTER CHARACTERISTICS

2.1. Linearity. • , • • 0

202. Frequency/Response.

203. Group-Delay. • • • •

2.4. Differential Phase Distortiono

TRANSMITTER MODIFICATIONSo • • • • • ,

3.1. Video Input Equipment •• 0 • 0

3.2. . Carrier"-Frequency Drive Equipment. ,

3.3. Vestigial-Sideband Filter •• 0 ••• ,

PERFORMANCE OF THE MODIFIED TRANSMITTER. 0

4.1. Frequency/Response. 0 ••• 0

4.3. Differential Phase Distortion.

CONCLUSIONS, , , •

ACKNOWLEDGEMENTS

REFERENCES • • ,

Page

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July 1957 Report No, T-060/5

( 1957/19)

COLOUR TELEVISION

THE ADAPTATION OF THE NoT.S.Co SYSTEM TO U.K. STANDARDS

PART 5~ SOME PROBLEMS ENCOUNTERED IN THE USE

OF A LOW-POWER TRANSMITTER

SUMMARY

The report describes the work which was carried out in order to adapt a low-power monochrome transmitter to operate satisfactorily with an N,T,S.C.-type colour television signal,

1. INTRODUCTION

During the investigation by Research Department into the characteristics of the N.T.S.Co colour system adapted to U.K. standards; a series of field trials was conducted in Channel 5 and with this in view a 500 watt S.T. & C. vision transmitter was borrowed from PoI.D. An associated 125 watt sound transmitter was also used.

Preliminary tests on the vision transmitter revealed that, although the performance was adequate for monochrome~ it would be necessary to modify the trans­mitter for colour television.

2. TRANSMITTER CHARACTERISTICS

Some forms of distortion inherent in transmitteFs have a negligible effect upon a monochrome signal. These, however, can have a disastrous effect upon colour television transmissions, and must therefore be eradicated. The colour signal specification 1 requires a careful examination of certain characteristics of the vision transmitter and these are discussed below.

2.1. Linearity

In the case of a colour transmission, a degree of non-linearity which is quite tolerable for monochrome signals may lead to very visible distortion of colour saturation; this is known as "differential gain" distortion.

Fig. 1 shows that chrominance signals pertaining to the highly saturated colours may extend into regions which are both below blanking level and also above the peak-white level; hence, colour operation demands a better transmitter linearity over a greater amplitude range than monochromeo The degree of permissible non-linearity, according to the specification, is such that throughout the whole range of luminance

2

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Fig. I - Composite video waveform for saturated colours

amplitude, the chrominance-to-luminance ratio does not deviate by more than 20~ from the correct value. It has been suggested2 ,3 that, where necessary, unavoidable distortion may be corrected by "pre-distorting" the video signal.

2.2. Frequency/Response

For 405-line monochrome television, a transmitter must have a frequency/ response characteristic which is substantially uniform from 3 Mc/s below the carrier frequency to 1 Mc/s above it. In order that the correct chrominance-to-luminance ratio may be maintained when colour signals are transmitted, an additional and more specific clause in the specification requires that the transmitter shall possess a frequency/response characteristic which at or near the sub carrier frequency is substantially the same as that at the lower video frequencies; the maximum allowable deviation is ± 2 dB.

2.3. Group-Delay

If the chrominance transitions are to coincide with the associated luminance transitions, the complete radio frequency chain should be characterised by a group­delay at or near the subcarrier frequency which is equal to that for the lower video frequencies. Further, in order to prevent chrominance signal phase distortion from causing hue errors at chrominance transitions, constant group-delay should be preserved throughout the frequency band occupied by the chrominance signals. The sharp cut-off in the combined frequency characteristic of the vision r.f. and i.f. circuits in the receiver is the principal cause of group-delay distortion. The distortion due to the receiver characteristics may be corrected by the use of a compensating network at the transmitter designed to cater for what is considered to be the average receiver group-delay characteristic. The colour signal specification includes a suitable

3

characteristic for the compensator which is tabulated below:

Video Frequency Group-Delay

Mc/s J.1-sec 0'1 +0'22 1'0 +0'22 1'5 +0'22 1'8 +0'19 2'0 +0'17 2'2 +0'133 2'4 +0'085 2'6 +0'015 2'8 -0'12

3'0 -0'355

2,4. Differential Phase Distortion

The term "differential phase distortion" refers to a phase shift of the chrominance signal due to changes in luminance signal amplitude. The distortion results from a combination of non-linear circuit elements which include resistance and reactance.

An example of differential phase distortion is shown in Fig, 2, which is a polar representation of the chrominance signals of Fig, 1. It shows the relative phase angle and amplitude of each with respect to the reference burst. It will be seen that the distortion imparts a phase shift to each chrominance vector which is dependent upon the associated luminance magnitude.

-- Corrc;ct phase rglationship b&tw4I&n colours. ---- Examples of errors due to difterantial phase di stertian.

Fig. 2 - Chrominance signal diagram illustrating "differential phase distortion"

4

Experiments have shown that phase errors in excess of about 5° produce visible distortion. A maximum error of ± 10° has been permitted 1 but further improvements in the performance of equipment could lead to a more stringent specifi­cation. A problem somewhat related to differential phase distortion is the presence of the burst signal in the period immediately following the line synchronising pulse. In normal monochrome practice this period is used for clamping to stabilise black level: for colour operation it is essential that the clamp should distort neither the phase nor the envelope of the burst signal and the circuits must be designed accordingly.

Until a practical compensating arrangement has been developed, differential phase distortion will remain the major problem in the design of a transmitter for N.T.S.C.-type colour television.

3. TRANSMITTER MODIFICATIONS

3.1. Video Input Equipment

The video input equipment normally supplied with the S.T. & C. vision transmitter consists of a stabilising amplifier which accepts a standard video signal and provides an input to the transmitter modulator with a 50:50 picture-signal-to­synchronising-signal ratio. Although quite satisfactory for monochrome operation, this particular amplifier (B.B.C. Type TV/STA/2B) was found to introduce prohibitive differential phase and differential gain distortions.

After investigating the possibility of modifying the unit, it was decided that a new amplifier should be designed specifically for operation with colour signals. Such an amplifier was developed and will be described elsewhere.

In order to satisfy the requirements of the colour signal specification as regards the compensation of receiver group-delay distortion, it was also necessary to include, in the video signal circuit of the transmitter, an all-pass network having a group-delay characteristic approximating to that recommended in the table of section 2.3. Fig. 3 compares the performance of this network with that required by the specification.

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- -- - Choroctnistic os sp~cilied

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Fig. 3 - Characteristic of receiver group-delay compensator

5

3.2. Carrier-Frequency Drive Equipment

When receiving colour signals, a spurious signal may be generated by interference between the sound and 'chrominance signals at the receiver vision detector. An interference pattern may be seen, if the resultant beat, which has a frequency of some 840 kc/s, has sufficient amplitude. The visibility of pattern may be reduced substantially if the frequency of the beat bears an odd integer relationship with half the line scanning frequency: this condition will be satisfied if the frequency difference between the sound and vision carriers is an integer multiple of the line scanning frequency. A simple and convenient arrangement may be obtained if this frequency difference bears a four-thirds relationship with the colour subcarrier frequency. In both the S.T. & C. vision transmitter and its associated sound transmitter, carrier frequency is obtained by multiplying the drive frequency by nine. By changing the frequency multiplying factor to eight, the problem of locking the frequency difference between the sound and vision carriers may be simplified consider ably.

The additional equipment necessary is illustrated in Fig. 4. In this apparatus the reference burst contained within the N.T.S.C.-type composite signal is used to lock a subcarrier frequency (2'6578125 Mc/sl oscillator in a d.c. quadri­correlator4 • Continuous subcarrier is then fed to a frequency divider whose output has a frequency (443 kc/sl one-sixth that of the subcarrier, which in turn provides an input to a balanced modulator contained within the vision carrier regenerator. The second input to this modulator is obtained from the drive circuits of the sound transmitter (with a frequency one-eighth that of the sound carrier) and, by means of a suitable bandpass filter, an output is derived which, having a frequency one-eighth that of the vision carrier, is suitable as input to the frequency multiplying stages of the vision transmitter.

omposite e NT se ,ignal SUb-carrier

regenerator

7'906 Me/s Sou nd drive frequency

~66Me/. Divider unit 443 ke/s I Villon carrier 0 M regenerator

Fig. ~ - Intercarrier locking equipment

3.3. Vestigial-Sideband Filter

B-:34~M els drive ney

vision fr.que

For monochrome operation the V.S.B. filter supplied with the S.T. & C. transmitter attenuates the upper adjacent sound channel by about 25 dB. When operating with N.T.S.C.-type colour signals, however, there is considerable video signal energy in the region o,f the subcarrier. The filter should therefore be designed to provide extra protection against the radiation of those unwanted upper sidebands corresponding to chrominance signals: this may be done by providing an additional "notch" filter tuned to the unwanted upper sideband chrominance signal.

As Channel 5 constitutes the highest frequency allocation in Band I, and therefore no adjacent sound channel exists at a frequency 1'5 Mc/s higher than that of the vision carrier, it was decided to readjust the V.S.B. filter to give maximum attenuation of the upper chrominance sidebands, namely, at a frequency 2'66 Mc/s higher than that of the vision carrier.

6

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monItor TV/REC/3 Probe

0'3 0·4 0·5 0·6 0·7 O·S O·p I 2 Video frequency. Mc/s

Fig. 5 - Transmitter amplitude/frequency characteristic

4. PERFORMANCE OF THE MODIFIED TRANSMITTER

After all the modifications described above had been carried out, a series of measurements was made to assess the transmitter performance. The methods of measurement and the results obtained are described below.

4.1. Frequency/Response

This was measured by means of a video signal containing, in the intervals normally occupied by picture information, a variable frequency sine-wave test signal occupying the video amplitude range from black to white. The transmitter was operated into the normal aerial load and a signal obtained from the feeder by means of a small probe. This signal was then demodulated in a screened receiver of known characteristics whose output could be displayed on a suitable waveform monitor. As the frequency of the test oscillator was varied, the amplitude of the sine-wave in the signal fed to the transmitter was compared with that obtained from the receiver and the results plotted "as an,amplitude/frequency curve in Fig. 5. Such a characteristic satisfies the specification adequately.

4.2. Linearity

Using an arrangement of equipment similar to that used for the frequency! response measurement, the transmitter linearity was determined by means of two separate test signals. The first, a line sawtooth, was observed both before its application to the transmitter input and after demodulation by the check receiver. No observable deviation from linear-ity could be detected.

The second test signal consisted of the composite video signal representing colour bars (see Fig. 1). Again, comparisons were made between the signal fed to the transmitter and that obtained from the receiver. In this case, however, the peak-to­peak amplitude of the chrominance signal was measured for each of the colours in the bar signal: for convenience of measurement the peak-to-peak amplitude of the reference burst in the signal obtained from the receiver was made equal to that in the signal applied to the transmitter. After the measurements of the receiver output had been corrected to take account of single sideband reception, values of differential gain distortion were derived for two widely differing values of chrominance-to-luminance ratio, and the results plotted in Fig. 6. It will be noted that the degree of

7

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Fig. 6 - "Differential gain distortion" of the complete transmitter

+2

+1 Luminance signal amplitude (volts) t1 0-3 -0-2 - 0-1 0 +0-1 +0-2 +0·3 +0·4 +0·5 +o.~ +0-7

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& Bur5t

Fig. 7 - "Differential phase distortion" of the complete transmitter

distortion is dependent upon the chrominance signal amplitude; in the worst case, however, the degree of distortion is well within the specified limits.

4.3. Differential Phase Distortion

In order to determine the degree of differential phase distortion introduced by the transmitter, a vectorscope3

. was.- used_,to compare the signals at the transmitter

8

input with those obtained from the receiver. In this apparatus the chrominance components of the signal are applied to two synchronous detectors operating at subcarrier frequency. The outputs are fed to the pairs of deflector plates in an electrostatically deflected cathode-ray tube. The tube screen displays the chromi­nance signal diagram and~ for an input of the colour bar composite signal, will appear as Fig. 2. When compaTing the displays produced by the transmitter input and the receiver output, the phase of the demodulating subcarrier supplied to the vector­scope was adjusted so that, in each case? the display vector corresponding to the reference burst coincided with a given calibration mark on the screen. In this way differential phase distortion introduced by the transmitter was measured for colour bar signals having maximum chrominance signals~ Fig. 7 shows that the degree of differential phase distortion lies within the ± 10° permitted by the specification.

5. CONCLUSIONS

It has been shown that without undue difficulty the SoT. & C. vision trans­mitter can be so modified that its performance conforms to that specified for an NoT.S.C.-type colour signal.

6. ACKNOWLEDGEMENTS

The author wishes to acknowledge the assi£tance given by R.R. Minns and colleagues in re-tuning the vestigial sideband filter, and by W. Silvie and S.J. Lent in making the distortion measurements.

7. REFERENCES

1. "Specification of Colour Television Standards for Experimental Trans­missions from Alexandra Palace", B.B. C. Engineering Division, June 1955.

2. "Equipment for Producing the Transmitted Signal", Report No. 7161, Hazeltine Corporation~ November 1954.

3, Fisher J,F. 3 "Alignment of a Monochrome TV Transmitter for Broadcasting NoT,S.C, Color Signals", Proc, I.R.E., Vol. 42, No. 1, January 1954.

4. Richman D., liThe DC Q.uadricorrelator, A Two-Mode Synchronization System" ibid.

5. Schlesinger K., "The Vectorscope and its Application in Color TV, FM and Radio Navigation". Trans. 1. R, E., PGBTR-8, October 1954.

P~inted by B.B,C. Research Department, Kingswood Warren, Tadworth, Surrey