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Field-Sequential Colour Television Receiver

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Field-Sequential Colour Television Receiver Wireless World, Seplember 1971 443 Field-sequential Colour Television Receiver I-Introduction and basic principles by T. J. Dennis, B.A. All systems of colour TV in general use higher than the flicker-fusion rate of the normal broadcast colour transmissions. today have as their display a system eye, a highly variable quantity found on A standard PAL decoder4 provides whereby the three primary coloured average in the region of 30Hz. the three (narrow band) colour difference pictures are spatially superimposed, For this reason it is normally considered signals. These are then switched in turn whether by projection of the red, green and necessary to increase the basic field rate to the grid of a monochrome c.r.t., the blue images using the Schmitt system, by from that of, say, a 50 field monochrome change taking place during the field blank­ the use of three c.r.ts and half-silvered standard to 150 fields per second in order ing period (see Fig. 1). The luminance mirrors or with the three pictures on one to maintain the original luminance rucker (wideband) signal is fed to the c.r.t. cathode c.r.t. whose screen consists of triads of rate. This demands a trebling of the signal as usual, after its passage through a 60005 independently controllable phosphor dots, bandwidth, other factors being constant. delay tine. This is practically the only as in the R.C.A. Sbadowmask' tube. Noting the discouraging comments of major modification needed to the mono­ All three systems are capable of others on the subject of f.s. systems retain­ chrome receiver which is the source of all J excellent results, but are difficult and ing the existing monochrome field rates , the signals used. The net result is that the elCpensive to set up. For example, in the it was decided to attempt to bUild and set can be made to display, field sequen­ projection system complex distortions have operate such a unit, to work from the tially, the black-and-white equivalents . of to be introduced into the scanning wave­ forms to correct for the fact that the ' Goldmark, P. C., et aI., ' Color Television', Part I, "Sec T. D. Towers on principles of colour TV, lan.­ projectors cannot be co-sited. Much the Proc. I.R.E., Vol. 30, pp. 162-182, April 1942. Dec., 1967, Wireless World. same problem is encountered with Shadow­ mask tubes, hence the joys of convergence adjustments. Any system using separate electron sources is prone to grey-scale l.um. ---------------------------;~~ C Q'i~·~d., tracking errors. The Shadowmask is able to reproduce G-Y Line a range of colours because the spatial flyback puiS., colour resolution of the eye is poor: close to a screen the dots can be easily perceived, but the overall impression is still one of the 8-Y B uffer Chromo c.r.t . additive colour resultant. Temporal colour amplifl.,r output grid resolution is equally weak, as may be seen by rotating a disc carrying segments of, 4 5 say red and blue, when the colours rapidly merge to magenta as the speed of rotation is increased. This is the basis of the R-Y field-sequential process, whereby the three coloured images are presented to the eye in turn. It is the oldest form of colour display a version having been demon­ 2G strated by J. L. Baird in 1928.2 In the author's opinion it is capable within its limitations, of giving results of the highest quality. Coil Perhaps the major of these limitations 12 is caused by the eye itself: perception of luminance, or brightness, changes in time, as well as in space, are particularly good. 50Hz Colour input While a rotating disc of red and blue will wh.,el appear magenta, it also carries a marked brightness rucker due to the luminance +12V difference between red and blue. Flicker only disappears when its frequency is 6 Coarse­ 'Herold, E. W., 'Methods suitable for TR color vel. 1) kinescopes,' R.C.A. Review, Vol. l2, Sept. 1951, P 445 et seq, " TV in natural colours demonstrated', Radio Fig. 1. Block dlagram offield-sequential colour receiver eqUipment. The input is provided News. Vol . 10, p. 320, October 1928 by a normal monochrome receiver. 444 Wirele s World, September 1971 the red, green and blue images of a colour on the screen (not per field). when display­ transmission. ing any but saturated primary colours, will A disc carrying sections of primary red, differ in shade. However since the colour green and blue filters rotates at 16jr.p.s. detail resolving power of the eyc is poor, in front of the C.r.t. Its rotation is phase the eJTect could only be seen within about locked to the field sync pulses to ensure 12in. of the 14in. C.Lt. used. Bearing in that when the red picture is being scanned, mind that these initial tests were on 405 the red filter is in place, and so on. lines, with a 625-line colour picture at lt may be noted that two-thirds of the normal viewing distance, the effect is available colour information is wasted in unnoticeable. _ this system, but it should also be recalled After the encouraging results obtained that considerably more than two-thirds of with the colour bars. a PAL decOder was the energy imparted to the electron beams built, with slight modifications notably in in the Shadow mask tube is dissipated as elimination of dependence on the line out­ heat in the Shadow mask ! put stage of the receiver: an additional Phase lock of the colour wheel is re­ sync separator was added, the line pulses quired, to ensure that the correct filter is obtained being used to trigger a mono­ in place at the correct time. This is achieved stable and produce an accurately timed by a simple feedback system using a signal burst gating pulse. The burst gate itself derived from a coil wound on aU-shaped was in the fonn of a four-diode bridge, all transformer limb and mounted in front of of which will be discussed more fully next a bar magnet fixed to the centre of the wheel. month. The coil output waveform is square in At first the decoder was operated without form, with slow sinusoidally changing a delay line; i.e., in the PALoS mode. edges. A foUr-diode bridge is used to gate Oscilloscope examination of R - Y for the through an 8ms portion of this wavefonn, colour bars with careful adjustment of L6 16G. AI. 'I. in. AI. which has a manually controllable d.c. of the May 1'969 W. W. article enabled potential superimposed on it for coarse results to be obtained which did not differ speed adjustment. The bridge output is appreciably line by line. Stability over integrated, amplified, and with suitable d.c. long periods, however, was not good due level adjustments, used to drive the motor to mechanical vibration and thermal armature via a 2N3055 emitter follower. changes. Hanover bars were then obtained. Assuming phase lock, the gating pulse Addition of a PAL delay line effected a is placed symmetrically about the mid­ complete cure. Adjustment of the R - Y, B - Y and point of the positive going edge of the Bar feedback waveform. H the motor speeds magn .. t G-Y drives to the sequential switches up for any reason, the waveform reaches a enabled colour pictures to be obtained higher level than it would normally when whose fidelity was indistinguishable from sampled, and the integrator output moves Shadowmask results, with the advantages of in a positive direction. Because of the full luminance bandwidth (a notch mter has inverting amplifier, the armature voltage is been found unnecessary; some commer­ reduced, and the motor slows down. By cial receivers do not include them), similar reasoning it can be shown that a and total elimination of the necessity for reduction in motor speed will also be com­ complex convergence and grey-scale pensated. Not surprisingly, the sy tern tracking adjustments. With the latter even oscillates about its stable position when any 5in. if the filters do not give an exact white, velocity transient is applied; settling time there can be no failure, since the same gun from switch-on is about 20 seconds in the G .. , ati n .. filter is used for all three pictures. Problems prototype, but this is immaterial as it takes will arise, however, if any attempt is made the line timebase considerably longer to to provide switched compensation for warm up on the displaying set. Programme filters of wildy incorrect characteristic. switchings, when field sync may be'inter­ As mentioned above, field-sequential rupted, tend to upset phase lock, but this systems working at low field rates suffer effect has not been found troubleseome. V.in. from luminance flicker effects. Another problem is colour fringing, obtained when there are differences between adjacent Results Fig. 2. Front view (a) and section (b) of a simple cowur wheel. fields' i.e., when the scene contains Before embarking on the construction of movement. a PAL decoder, a generator was built to Fortunately, both have proved a far less produce the 4f, 2f and f, where f = line serious drawback than was expected. frequency squarewaves needed for the least well presented as on all colour sets. Perception of flicker depends on many blue, red and green, respectively, signals The reddish tint obtained when observing factors including background light level, of the standard colour bars, viz. white, a white object through the wheel (due degree of dark adaption and size of the yellow, cyan, green, magenta, red, blue and to the red filter having excessive trans­ field under consideration.
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