11S-VHSjCVBS-TO-RGB CONVERTER

PART 1: INTRODUCTION

Although the technical advantages of the Super-VHS video systemare weil proven, many owners of an S-VHS video recorder balk at

the expense of a compatible monitor or TV set with separateluminance and chrominance inputs. This article describes an

obvious missing link in the apparently ever-incompatible tleld ofvideo equipment connections. An advanced circuit is discussed

that converts S-VHS or CVBS (composite) video signals into RGBcomponents. The upshot is that you can use your existing monitor

with an RGB input (i.e., with a SCART or Euro-AV connection) tobenefit from the improved picture resolution offered by an S-VHS

video recorder. This month we discuss the basics of the videostandards involved.

The compatibility issue has played a stg-nificant role in the development of boththe NTSC and the PAL TV transmissionsystems. In both ca ses, there were tWQ

conflicting aspects: on the one hand, exist-ing monochrome TV sets were not to beaffected by colour transmissions; on theother hand, existing bandwidths of about5 MHz for the luminance (brightness) sig-nal were to be mainta ined.

The compatibility requirement auto-matically dictates that the black-and-white information (Iuminance or 'Y'signal) must also be conveyed in colourtransmissions. The Y signal forms the surnof all basic colours, red (R), green (C) andblue (8), but only as far as their relativebrightness is concerned. From perceptionexperiments, the brightness appears todetermine the overall sharpness of the pic-ture. Hence, the luminance bandwidthmust be as large as possible (up to 5 MHz)for monochrome as well as colour TV sets.However, this raises the problem of whereto put the colour Information.

Colour components andtransmissionAny colour can be reproduced on a pic-ture tube by actuating in the correct pro-portion the basic colours it is cornposed of.The final colour is obtained by controllingthe intensity at which the RGB pixels atthe inside of the picture tube light up. Tothe human eye, the three individual basiccolours in a pixel group appear as one,composite, colour or hue at a particularsaturation.

The need to convey R, G and B, is,therefore, obvious. Since the surn of the

H. Reelsen

equivalent luminance values of all three isalready contained in the Y signal, onlytwo further stgnals. R-Y and B-Y, aregenerated by means of CI differential oper-ation with the Y signal. R-Y and B-Yaretherefore referred to as the colour dif-ference signals. Before these signals aretransmitred. they are given relativebrightness factors. The resulting chromin-ance signals may be written as

U = 0.49(B- Y)V = 0.88(R- Y)

and the luminance, Y, as

Y = 0.3R + 0.59C + 0.116

The RGB intensity information requiredto contral the respective electron guns inthe picture tube is obtained from the R- Y,B- Y and Y Information with the aid of anaddition operation in a matrix circuit.

A problem that remains to be solved ishow to include the colour difference stg-nal in the bandwidth already occupied bythe Y stgnal. without causing interferenceon monochrome TV sets, or reducing thepicture sharpness on colour sets. At thispoint, design engtneers are in a position toprofit from a characteristic of human cye,namely its reduced ability to resolve col-our contours as compared to brightnessvalues. This means that the colour infor-

ELEKTOR ELECTRONICS SEPTEMBER 1990

S·VHS/CVBS·TO·RGB CONVERTER IU.

a

I:I

Uu

I I I Ib

Uv f-O---<>--..o,

I Ic

Y :. luminance = O.3R + O.59G + 0.11 BU : O.49(B-Y)V: O.88(R-Y)U + V "" chrominanceThe amplitude of a subcarrier fc is modulated with U and V:

sin 21tfct modulated with U ~ U sin 2nfctand

cos 21tfct modulated with V --+ V cos 2ftfclThe complete PAL signal, X.

X = y + U sin 21tfcl ± V cos 2Jtfct

fl m AI

, 1\,f\I I, ,

I ,c ,,

i,

\l V, I

I: II, I II I . ,

V1 ! I

I VI,I V: V I, Uu coswt ......-,

d

I I Ue sinc t

e

//

/

IIIL_

Fig. 1. Signal waveforms resulting trom quadrature modulation of the colourdifference signals uc = O.49(B-Y) and u, = O.88(R-Y). Drawing'a' shows the quadrature-modulated signal Ua•while 'b' and 'e' show the modulation signals Uu and Uv, which for clarity's sake are forrnedbya sinusoidal waveform and a rectanguJarwaveform respectively. Drawings 'd' and 'e' illustrate how these signals are modulated on tothe 90-degrees shifted carriers. The waveform shown in drawing 'a' is the result of adding the signals in 'd' and 'e'.

mation may be transmitred at a relativelylow bandwidth without signifieantly de-grading the overall sharpness of the pic-tu re. In the PAL system, the eolour (orchrominance) bandwidth is about 1 MHz.

The eolour differenee signals are read-ily embedded in the frequency spectrumof the Y signal by making use of the facttha t the spectrallines of the Ysignal occurat even multiples of the line frequency(15,625 Hz). Also, the amplitude of thesespectral hnes decreases with frequency.

The colour difference signals modulatea subcarrier of which the frequency, le, isan odd multiple of the line frequencydivided by four, plus the picture refreshfrequency (see Ref. 1):

je: 1135 X (15,625/4) + 25 (Hz)

This causes the spectrallines of the colourdifference signal to be slotted in betweenthose of the Y stgnal. The eolour subcar-rier frequency is set at 4.43361875 MHz,and the colour difference signals arequadrature-amplitude modulated(QAM). The B-Y and R-Y componentsmodulate the amplitude of the colour sub-carriers of 0 degrees and 90 degrees re-spectively (see Ftgs. 1d and Le). Thecarrier itself is suppressed, so that it hasan amplitude of nought in the absence ofa colour difference stgnal. This is done tokeep the ptcture free from interferencecaused by the otherwise continuouslypresent subcarrier .

In order to eliminate the risk of phase

ELEKTOR ELECTRONICS SEPTEMBER 1990

shifts in the transmission path, the phaseof the R- Y component is inverted everyother picture line. Details of this operationpeculiar to the PAL system may be foundin Refs. 2 and 3.

The use of amplitude modulation withsuppressed carrier requires a phase- andfrequency-synchronized subcarrier at thereceiver side. In a IV set, the modulatedR-Y and B-Y components are recoveredfrom the chrominance subcarrier with theaid of a 4.433-MHz quartz crystal osci11a-tor whose phase and frequency are cor-rected every 64 I1s by a 2-115long burstsignalslotted into the rear porch in theblanking period at the end of every pic-ture. The burst consists of 8 to 11 cyc1esofthe colour subcarrier frequency and fol-lows the Une sync pulse as shown in Fig. 2.A phase eomparator is used to keep thecrystal oscillator synchronized to the re-ceived burst, which also contains the PALswitch signal for the Hne-by-line R- Yphase reversal. This arrangement ensuresthat the R-Y signal in the receiver is in-ver ted in synchronism with that at thetransmitter side to ensure that the de-modulation operatton can work correctly.

Pitfalls ...In practice, the 'packaging' of the lumin-ance and the chrominance informationinto a single CVBS (chrominanee-video-blanking-synchronisation) signal is notwithout problems. Since the colour sub-carrier falls in the spectrum of the lumin-

ance signal, it causes a finely patternedtype of interference known as moire. Lu-minance circuits in all modern TV sets aretherefore fitted with a 'colour trap', whichis a relattvely simple filter that removesmost of the moire effects with the excep-tion of those occurring at areas with sharpcolour transitions. Here, large phasejumps give rise to subcarrier sidebandsthat fall outside the stop band of the 4.43·MHz colour trap. Unfortunately, Ysignalsin this stop band are also suppressed,which results- in reduced picture resolu-tion because some of the high-frequencycomponents disappear. Incidenta11y, mostmonochrome sets also contain a colourtrap to eliminate motre.

The (possible) interference betweenchrominance and luminance also worksthe other way around: since the lurnin-

lw1!',elevel'00%

linesynch.onlsallon pulse

~.

BUrstblanklng j

level 30%

52~:;ec

Ipicluf(' s;gnal/ 981l 2

hOfi~ontalblanklng

Fig. 2. Structure and timing of a compo-site video signal (PAL standard).

RADIO AND TELEVISION

ance band includes the frequency rangefor the colour subcarricr. high-frequencyY stgnals ean cause interference in the fre-queney range around 4.43 MHz. The re-sult is a quasi-random type of patterningand colouring in and around picture areasof fine detail. Notorious examples of thishappening can be seen virtually everyevening in jackets, shirts or ties of peopleon television.

Standard VHS videorecordersSame 15 years aga, during the develop-ment of the VHS video system, a lumin-ance bandwidth of 3 MHz was deemedsatisfactory for VeRs considering thetechnical limitations imposed by thedrum head speed and the tape consump-tion. In the original VHS system, the col-our subcarrier is mixed down to 627 kHzto keep it weIl way from the lower end ofthe speetrum of the Y informa tion. whichis recorded as a frequency-modulated(FM) signal (see Fig. 3)

The FM recording improves the signal-to-noise ratio of the Y signal and makes itlargely independent of amplitude vari-ations of the tape signal. The frequencysweep ranges from 3.8 MHz to 4.8 MHz.

Returning to the colour information,this is recorded as an analogue signal in'helical scan' mode (Ref. 3). The differentfrequencies used allow ready separationof the two signals. However, the band-width of the eolour inforrnation is inevit-ably reduced to about 500 kHz. The resultis noticed as 'smeared' eolour transitions,to which the reduced (3-MHz) luminaneebandwidth adds an impression that thepicture is blurred.

These imperfections of the originalVHS system were soon recognized byVeR manufacturers. Their answer, theHQ video recorder, was based on smallimprovements to the recording methodand a better edge definition of the Y sig-nal. The resultant picture quality im-provement was marginal and not really astep forward. It was, however, the bestthat could be achieved given the need forcontinued compatibility. Clearly. real im-provements to the picture quality offeredby VCRs could be achieved only by chang-ing some of the standards.

The Super-VHS systemThe bandwidth of the recorded video sfg-nal was increased significantly (at theexisting relative speed of 4.85 m/s be-tween the tape and the head) by virtue oftwo teehnological developments. First,new metallurgie techruques allowed thesize of the air gap of the video head to bereduced. Second, tapes with a very highmagnetic particle density became avail-able.

To maintain compatibility with olderVHS recorders, the 5-VHS system is basedon the same method of colour recording(see Fig. 3). However, the frequency

Chrominanceband

627kHz

Chrominanceband

6271<:Hz

6 7 MHz

o 2L4 ~65.'SMHz

400 Iines

7 MHz

Fig. 3. Typical standard-VHS and S-VHS spectra.ln both cases, the quadrature-modulatedcolour signal is recorded with the aid of a carrier which ls mixed down to 627 kHz, while theluminance signal (Y) is recorded in FM. S-VHS recorders use a luminance carrier frequencyof 5.4 MHz and a trequency sweep of 1.6 MHz. This ofters a bandwidth of 5 MHz tor the Ysignal, as opposed to about 3 MHz tor the standard-VHS video recorder.

sweep of the Y signal is shifted up to aband from 5.4 MHz to 7.0 MHz to give amuch higher noise margin. At the sametime, the frequency of the FM sub carrierallows the luminance signal to be re-corded at its full bandwidth of about5 MHz.

In most standard VHS video recorders,the chrominance and luminance signalsare processed separately until they arecombined to give a CVBS signal with allthe previously mentioned rtsks of runninginto trouble w ith interference.

Bycentrast. the S-VHS system is basedon separate chrominance and luminancesignals right up to the two associated out-puts on the VCR. Evidently. this separ-ation is not perfeet when , for instance, aTV programme is record ed, since then thechrominance and luminance componentsmust be extracted from the composite sfg-nal before they can be recorded, playedback and fed separately to a monitor. Theprocess of extracting the components haspitfalls as described before. Not so, how-ever, with video sources that do supplythe components separately. Examples tn-clude S-VHS carneras, some prerecordedS-VHS video tapes and MAC decoders.

Connection problemsSo far, so good. A look at the rear panel ofthe TV set, however, reveals that there isat best a SCART connector, which doesnot allow luminance and chrominancesignals to be taken in separately. The TVset is, therefore, not S-VHS compatible.This unfortunate discovery forces ownersof S.VHS recorders to connect the monitorand the recorder via a CVBSlink, forgoing

most of the advantages of better picturereprod uction offered by the new videosystem.

Considering the cost of an 5-VHS com-patible monitor, the only way to benefitfrom the separate chrominance and lu-minance signals supplied by S-VHS re-corders and other video sources is toconvert these to RGB signals that can beapplied to the existing monitor or TV setvia its 5CART input. Next month's secondinstalment of this article discusses a cir-euit to accomplish this. In addition, thecircuit provides a colour transition im-provement (CTn function, and is capableof converting CVBSto RGB.

From composite to RGBAlthough most standard video recordershave a SCART socket, this rarely suppliesRGB signals. Likewise, most set-top TVtuners and indoor units for satellite TVreceptton supply a CVBS (cornpositevideo) signal only. A problem arises whenthis equipment is to be connected to ahigh-resolunon colour monitor with anal-ogue RGB Inputs. or a TV set w ith aSCART (Euro-AV) input. In both cases,the converter to be described next monthcan link this equipment and ensure optt-mum picture quality. Q

References:1. Chrcminance-Iocked clock generator.Elektor Electronics July / August 1988.2. Video line seleetor . Elektor Electron icsApril 1990.3. Video Handboote (second edition). by R.van Wezel. Published by Heineman New-nes, ISBN 043492189 O.

ELEKTOR ELECTRONICS SEPTEMBER 1990

:[TIONS CORRECTIONS CORR~400-watt laboratory powersupplyOctober 1989 and November 1990A number of constructors of this popularproject have brought the following problemsto our attention.

1. The onset point of the current limit cir-cuit lies at about 3 A, which is too 10w. Solvethis problern by repladng TI with a TypeBC517 darlington transistor, and R20 with a82kQ restetor.

2. Depending Oll thecurrent transfer ratioof the optocoupler used, the trans formerproduces ticking noises. This effect, which iscaused by overshoot in the pre-regulationcircuit, may be traced with the aid of an 05-

cilloscope monitoring the voltage across C26at a moderate load currcnt. The capacitormust be charged at each cycle of the mains

frequency I and not once every five cycles.The problern is best solved by reducing theamplification of the regulation circuit. Re-place RI7 with a 39 kQ resistor, and createFeedback by fitting it between the base andthe coUector of T3. Also add a resistor inseries with the optocoupler. These twochanges are illustrated in Pigs. 1 and 2.Lower R 16 to 10 ill, increase C24 to 10 IlF,and increase Rl5 to 270 kQ.

3. Excessive heating of the transformer iscaused by a d,.c. component in the primarywinding. This is simple to rernedy by fittinga cepacftor of any value between 47 nF and470 nF, and a voltage rating of 630 V, acrossthe primary connections. This capacitcr isconveniently mou.nted on to the PCS termi-nal block that connects the tmns formet to themains.

4. One final point: when using LED

1

original

+ 12V

."."

220.

'~~ IC5

2 ~ CNY17-2

."270.

mOdJli"d

... 1 ~~ KJ52 ~CNY17.2

Y--O--J

2

OVMs for the voltage/current indicatiou,their ground line must be connected to thepositive terminal of Cu.

Hard disk monitorDecember 1989In some ca ses, the circuit will not resetproperly because the CLEARinput of IC3A iserroneously connected to ground. Cut theground track to pin 3 of TC3,und use a shortwire to connect pin 3 to pin 16 (+5 V).

Microprocessor-controlledtelephone exchange

October 1990In some ca ses, the timing of the signals ap--plied to ICl7 causes a latch-up in the circuit,so that the exchange does not detect the stateof the connected telephones pmperly. Solvethis problem by cutting the track to pin 1 ofICI7, and connecting pin 1 to ground (a suit-able point is the lower terminal of C6).

The text Oll the fitting of wires on theBASIC computer board (page 19, towardsthe bottom of the right-hand cclurnn) shouldbe modified to read: 'Finally. connect pin 60fK2 to pin 70110 (Y7 signal).'

5-VHS/CVBS-to-RGBconverter (2)

Ocreber 1990The capacitor marked 'C37', next to R2·1Ollthe component overlay (Pig. 7b and ready-made printed circuit board), should bemarked 'C39'.

In case they are difficult to obtain locally.inductors type 119-LN-A3753 (LI) and 119-LN-A5783 (U) may be replaced with the re-spective types 119-ANA-5874HM and119-ANA-5871 HM, also lrom Toko, lnc.Suggested suppliers are Cirkit DistributionLtd., and C-I Electronics.

EPROMsimulatorDecember 1989Counters 10 and IC4 may not functionproperly owing to a too low supply voltage.This problern may be solved by replacingren with a 7806. Alternatively, use BAT85diodes in positions 01 and 02.

Programmer for the 8751November 1990The ready-programmed 8751 for this projectis available at E3S.25 (plus VAT) underordernumber ESS 7061, not unde.r order numberES55951 as stated on the Readers Servicespages in the November end December 1990issues.

ELEKTOR ELECTRONICS .IANUARY 1991

VARIABLE A.C. POWER SUPPLY I

AC POWER SUPPLY

o

EB EB EB

900104-F

Fig. 6. Layout of the front panel foil (shown here at about 70% of true size). This toll is available ready-made through the Readers Services.

This means that the supply can not be set toan output voltage ofO V. Portunately, the ac-tual minirnum output volta ge is quite low ata value smaller than 0.5 V.

The regulation circuit is powered by themains transformer via rectifier D3-C2. Com-ponents R9, 04 and T4 reduce the rectifiedvoltage to about 33 V, which can then besafely appLied to the input of a 12-V regula-ror, Kz.

ConstructionThe construction of the power supply isstraightforward if you use the printed-cir-cuit board shown in Fig. 4. The optocoupler

6-metre band converterApril 1991, p. 38-43The components list and the inductor over-view in the top left hand corner of the circuitdiagram should be corrected to read:

Li, L2 = 301KN0800.Capaeiter C16 (4.7 pF) must not be fitted onthe board.Finally, a few constructional tips:

Fit a 10 nF cerarnicdecouplingcapacitorat junction L7-R36.Fit a 18 kQ resistor between the base ofT3 and ground. This reduces the Q factorof L2, and prevents too high stgnal levelsat the base of T3.For irnproved tuning. inductor L9 maybe replaeed by a Toko Type113KN2K1026HM.

Multitunction measurementcard tor PCs

January and February 1991We understand that the 79L08 (lC17) is nolonger manufactured and, therefore, diffi-cult to obtain. Here, the lC rnay be replacedby a 7908, whieh, although physieally larger

ELEKTOR ELECTRONICS JUNE 1991

is a Imme-made device. The construction isshown in Fig. 5. Use a length of PVC tubtng.a (black) spool of a photographic film, or cuta piece of thiu. black decoration cardboard tosize and roll it into a tube.

The printed-circuit board enables theLDR and the LEDs to be connected in twoways, allowing you to deterrnine where therespective component terrainals exit fromthe optocoupler assembly.

The heat-sinks of T4 and lC2 need not beIarge - in rnost cases, two pieces of alumi-nium of an area of about 15 mm' will do.Makc sure that the heat-sinks da not touch.Alternatively, lC2 and T4 may be mountedon a common heat-sink, provided both corn-

CORRECTIONS

than the 79L08, is pin-compattble, andshould fit on the peB.

Dimmer tor halogen lightsApril 1991, p. 54-58In the circuit diagram of the transmitter,Fig. 2, pm 14 of the MV500 should be shownconnected to pin 13, not to junction R1-R2-C2.The relevant printed-circuit board (Fig. 6) isall rtght.

RDSdecoderFebruary 1991, p. 59Line AO between the 80C32 contral boardand the LC display is not used to reset thedisplay, but to select between registers anddata.

We understand that the SAF7579T andthe associated 4.332 MHz quartz crystal aredifficult to obtain thraugh Philips Compo-nents distributors. These parts are availablefrom C-I Electronics, P.O. Box 22089,

ponents are fitted with insulating washers.The power supply is housed in a metal

enclosure Type LC850 from Telet. Thedimensions of the blue-and-grey seff-ad-hesive front-panel foil for the supply (Fig. 6)are geared to this type of enclosure ..

The two toroid transformers stated in thecomponents list have two 15-V secondarywindings, which must be connected in seriesto obtain the required 30 V.

The power supply has only one adjnst-ment. Connect an a.c. voltmeter to the sup-ply and set an output volta ge of 25 V. Next,adjust Pr until the moving-cotl meter indi-cates the sarne value. •

6360 AB Nuth, Holland. For prices and or-dering information see C-T's advertisementon page 6 of the May 1991 issue.

S-VHS-to-RGBconverterOelober 1990, p. 35-40Relays Ret and Re2 must be types with a coilvoltage of 5 V, not 12 V as indicated in thecomponents list. Constructors who have al-ready used 12-V relays may connect the coilsin parallel rather than in series.Suitable 5-V relays for this project are the3573-1231.051 from Günther, and theV23100-V4305-COOO from Siemens.The components list should me modified toread:

6 33nF C57-C62