World Analogue Television Standards and Waveforms World Analogue Television Standards and Waveforms Specifications of all the analogue television transmissionstandards defined by CCIR, and waveforms of the 405-, 525-, 625-and 819-line standardsRelated sections: | E-mailme | Home Page | 405-Line Standard | Test Cards | Teletext |World TV section: | Overview | Line Standards | Colour Standards | CCIR Systems | Bands | Radio Channels | UK |This page: | Contents | Timeline | Scanning |Interlace | AspectRatio | Resolution | Gamma | Colour | Levels | Transmission| Bookmarks | HE INFORMATION presented in this sectionhas been compiled from several modern and historical sources and,errors and omissions excepted, the intention is to give a summaryof the various standards at the time that they were current.Nevertheless, it is hoped that present-day standards are alsoaccurately accounted for, and to this end any corrections wouldgratefully received (please E-mailme with any comments). Thanks are especially due to Mark Carver, Steve Palmano andPeter Vince for helpand advice. Written sources consulted include [Electronics and]Wireless World and [Practical] Television magazines,textbooks by Benson KB and Whitaker JC, Carnt PS and Townsend GB,Holm WA, Hutson GH, Kerkhov F and Werner W, and technicalpublications from BBC, EBU, IBA and ITU.I am particularly indebted to Peter Vince for recently spotting certain anomoliesin the ITU document BT.470-6 from which many of the details in these pages were taken.It has been superseded by BT.1700 and BT.1701,and the values quoted in these pages are now verified by those, and by SMPTE 170M-1999in relation to the NTSC standard. Many of the NTSC parameters feature recurringdecimal fractions, and I have indicated these throughout with square brackets, for examplefSC = 3 579 545.[45]HzContents World Analogue Television Standards and WaveformsThis page:OverviewTimelineScanningInterlaceAspect RatioResolutionGammaColourComponent Video LevelsTransmissionPage 2:Line StandardsHistoryNaming of PartsUnitsAmplitudesHorizontalTimingVerticalTimingLine and FieldSynchronisationSummaries of Features:405-lines525-lines625-lines819-linesPage 3:Colour StandardsHistoryNTSCSECAMPALSECAM IVMACExtendedPALPAL PlusPage 4:TransmissionCCIR TransmissionSystemsBroadcastingBandsRadioChannels, system by system, country by countryUK Band Planand Aerial GroupsMissing Channel1|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Timeline  OME SIGNIFICANT dates in the comings andgoings of television line standards, colour standards andtransmission bands. Starts and ends of official services are inroman type, while other landmarks and experimental services are initalics.1930s1930 United Kingdom: Start of Baird 30/12.5experimental television service from the BBC Brookmans Park mediumwave radio station in London: vision 356.3m, sound 261.3m (1 Apr1930)1933 United States of America: Start of RCA 240/24experimental television service on 45MHz from station W2XBS on theEmpire State Building1935 Germany: Start of 180/25 public television serviceby the German Post Office in Berlin (22 Mar 1935)1935 France: Start of 441/50 television service in vhfBand I from Eiffel Tower in Paris (26 Apr 1935). Closed after ayear and reopened as 445/50 (1 Jul 1937). Changed back to 441/50 in1942 for German Wehrmacht service. Closed in Aug 1944 and re-openedby the French in Oct 19461936 United Kingdom: Start of Baird 240/25 and EMISystem A 405/50 public television service in vhf Band I by the BBCin London (2 Nov 1936)1936 United States of America: Start of RCA 343/60experimental television service on 45MHz from station W2XBS on theEmpire State Building1937 United Kingdom: Closure of Baird 240/25 publictelevision service by the BBC (7 Feb 1937)1939 United States of America: Start of RMA 441/60experimental television service1940s1941 United States of America: Start of System M 525/60public television service (1 Jul 1941)1943 Germany: Closure of 441/25 public televisionservice by the German Post Office in Berlin due to allied bombing(Nov 1943)1949 France: Start of System E 819/50 television servicein vhf Band III from Eiffel Tower in Paris (Dec 1949)1950s1950 West Germany: First test transmissions of SystemB 625/50 television service from Hamburg on vhf Bands I and III(Jul 1950)1951 United States of America: Start (and closureafter five months) of RCA 441/120 experimental field-sequentialcolour television service1952 United States of America: Start of System M 525/60transmissions in uhf Bands IV and V1954 United States of America: Start of System M/NTSC525/60 first-ever public compatible colour television service (1Jan 1954)1954 United Kingdom: Demonstration of System A 405/50 NTSCcolour by the Marconi company to press in London (May 1954)1954 United Kingdom: Start of System A 405/50 NTSCcolour out-of-hours test transmissions in vhf Band I by the BBC inLondon (7 Oct 1954, and then regularly from 10 Oct 1955)1955 United Kingdom: Start of System A 405/50 service invhf Band III by the ITA (22 Sep 1955)1956 France: Closure of 441/50 television service in vhfBand III from Eiffel Tower in Paris due to transmitter burning out(Jan 1956)1957 United Kingdom: Start of System A 405/50 NTSCcolour test transmissions in uhf Band IV by the BBC in London (11Nov 1957 until 1960)1960s1961 Republic of Ireland: Start of public televisionservice by RTÉ using System A 405/50 and System I 625/50 invhf Band I (System A: 31 Dec 1961, System I: May 1962)1962 United Kingdom: Start of System I 625/50monochrome test transmissions in uhf Band IV by the BBC inLondon1963 United Kingdom: Start of System I 625/50 NTSCcolour test transmissions in uhf Band IV by the BBC in London (Feb1963 until 1964)1963 United Kingdom: Start of System I 625/50 SECAMcolour test transmissions in uhf Band IV by the BBC in London (Mar1963 until 1964)1963 France: Start of System L 625/50 television servicein uhf Band IV from Eiffel Tower in Paris (late 1963)1964 United Kingdom: Start of System I 625/50 secondprogramme (BBC2) by the BBC in London in uhf Band IV (20 April1964)1965 West Germany: First demonstration for press of625/50 PAL colour in Berlin (Feb 1965)1965 United Kingdom: Start of regular System I 625/50PAL colour out-of-hours test transmissions on BBC2 by the BBC inLondon in uhf Band IV (24 May 1965)1966 United Kingdom: PAL adopted officially as UK coloursystem (3 Mar 1966)1967 United Kingdom: Start of System I/PAL colour testtransmissions of scheduled programmes on BBC2 (1 Jul 1967) followedby full service (2 Dec 1967)1967 West Germany: Start of first official European fullcolour service on System B/G/PAL (autumn 1967)1967 France/USSR: Start of full 625/50 SECAM III colourservice simultaneously in France and USSR (1 Oct 1967)1968 France: TF1 first programme duplicated on System L819/50 on uhf (Low power transmitters - only example of non-625 or-525 services on uhf)1969 Belgium: Closure of RTB (French language) System E819/50 service on vhf - replaced by System C 625/50 +ve mod asalready used by BRT (Flemish language) service (Mid-Feb 1969)1970s1971 Luxembourg: Closure of System E 819/50 service onvhf - replaced by System L/SECAM 625/50 +ve mod colour service asused on uhf (1 Sep 1971)1973 United Kingdom: Start of regular System I 625/50teletext test transmissions by BBC Ceefax (23 Sep 1973) followed byIBA Oracle1977 United Kingdom: Experimental System I 625/50 BBCCeefax and IBA Oracle teletext transmissions declared officially inservice by Home Office1977 Belgium: Closure of System C 625/50 +ve mod serviceon vhf - replaced by System B/PAL 625/50 -ve mod colour service asused on uhf (25 Apr 1977)1980s1982 Republic of Ireland: Closure of System A 405/50 vhfservices by RTÉ, transmitters being changed one-by-one toSystem I/PAL during 1978-82 (last one, Letterkenny, Donegal, closed23 Nov 1982)1984 France: Closure of TF1 first programme System E/L819/50 transmissions on vhf/uhf - vhf bands re-engineered forSystem L/SECAM 625/50 transmissions of new fourth programme CanalPlus1985 United Kingdom: Closure of last-ever System A405/50 transmitters by BBC and IBA on vhf, duplicated sinceNovember 1969 in System I/PAL on uhf (England, Wales NorthernIreland: 1 Jan 1985, Scotland: 2 Jan 1985)1985 Monaco: Closure of last-ever System E 819/50transmitter by Télé Monte Carlo in Monaco on F10 -replaced by System L/SECAM 625/50 (mid-1985)1990s1990 Eastern Europe/Africa/Asia: Start of changeoverfrom SECAM to PAL in many OIRT member countries and elsewhere1998 United Kingdom: Start of DVB-T digital terrestrialservice (15 Nov 1998)2000s2003 Germany: Start of closedown of analogue terrestrialservices to be replaced overnight, region-by-region (beginning withBrandenberg/Berlin), with DVB-T2008 France: Start of first European High Definition (1920x1080/50i) free-to-air terrestrial digital television servicevia TNT (Télévision Numérique Terrestre) (30 Nov 2008)2010s2012 United Kingdom: Proposed date of closedown of final transmitters(including Crystal Palace in London) in the UK uhf System I 625/50 analogue networkwhich opened with Crystal Palace on 20 April 1964|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Scanning  Oscilloscope trace of a line of video containing a staircasepattern (screen shot inset) LL ANALOGUE television systems work in thesame way - only the precise characteristics are different. Theimage is dissected in much the same way that you are reading thistext. A sampling device scans across from left to right, and fromtop to bottom, in a series of near-horizontal lines that arearranged in a rectangle to form a raster. The output fromthe sampling device comprises a constantly changing voltage whichat any moment represents the brightness of a given point in theimage.This video stream is punctuated at the start of each scanningline by a line synchronising pulse, and at the start of each frame,or field, by a series of field synchronising pulses. The syncpulses are separated from the picture information in both time andamplitude. In a standard 1Vp-p (into 75 ohms) 'composite' (mixedsyncs plus picture) video signal, the peak white amplitude is+700mV, whilst black and blanking levels sit at 0V and the synctips reach -300mV. In the 525-line standard however, peak white is714mV and sync level is -286mV. In addition, in the US (but not theJapanese) version black level is 54mV above blanking level.The most basic difference between television standards is thenumber of lines per field and the number of fields per second, asdetermined by the line and field scanning frequencies. Details maybe found in the line standardssection of this page.|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Interlace HUNDRED years ago or more, the movingpicture industry satisfied itself that the persistence of visioneffect in the human eye and brain was such that by projecting aseries of still images at the rate of sixteen or eighteen persecond, an illusion of smooth movement was created. A few yearlater, with the introduction of synchronised sound, the projectionrate was increased, and standardised at twenty-four pictures persecond, partly to give smoother lip movement, and partly toincrease the writing speed available for the optical sound-track.However, sixteen, or even twenty-four, complete blackouts persecond between pictures creates too much flicker for the humanbrain to tolerate. For that reason a shutter is placed in the lightbeam of the projector that interrupts it forty-eight or seventy-twotimes a second, giving a flicker-free impression of smoothmovement.It would be a simple matter to incorporate such a scheme into amodern television system. All it would require is that each frame,as it is received, is written into a digital field store (anintegral part of every digital tv receiver) and read out again two,three, or even four times at a much faster rate.Unfortunately, digital field stores were not available in thenineteen-thirties when analogue television was being developed.Instead, having determined that at least 24 complete frames persecond, each with at least 240 scanning lines, were required toprovide a watchable picture, the designers came up with aningenious method of reducing the flicker rate. Using an odd number of lines per picture they simply doubledthe field frequency, whilst keeping the line frequency the same.So, in the 405-line standard, the scanning beam reads202.5 lines over the complete height of the image, then half-waythrough line 203 it jumps back to the top of the image to read the202.5 lines that lie in between the first set.In this way there is still a full 405 lines per picture (thoughin practice, because the video signal is time-shared betweenpicture information and synchronisation pulses, there are actuallyonly 377 lines of picture per frame, not 405), but there are nowfifty fields per second projected onto the tv screen to reduceflicker. As a bonus, because of the way the television cameraworks, each picture line contains information from the last 1/25second period since it was last scanned, and 1/50 second earlierthan that for the lines immediately above and below, and so this2:1 interlaced system of alternate staggered fields has theeffect of smoothing out movement even further.Interlaced scanning - he called it 'intermeshed' - was first proposed by Randall C Ballardin an RCA patent of 19 July 1932.In it he described a system using a Nipkov disc having 81 holes.Ironically, now that receivers with digital field stores arewith us and it is possible to increase the displayed picturerepetition rate beyond fifty per second, the presence of 2:1interlace causes huge problems for tv set designers and createsnasty motion artefacts on the screen, which are diffcult toeliminate.All 'standard definition' (ie between around 400 and 900 linesper picture) analogue television systems incorporate 2:1interlace.|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Aspect ratio HE ASPECT ratio of a picture is its widthdivided by it height and is often expressed as the ratio of twointegers. The original aspect ratio of the 405-line standard was5:4, but it was later changed to 4:3 to be the same as theso-called 'Academy Ratio' of 35mm cinema films. More recently, withthe advent of digital transmissions, a second change has been made,to 16:9, and this 'widescreen' format is running side-by-side with4:3 in many countries. Both 4:3 and 16:9 pictures use exactly thesame portion of the transmitted signal. The latter format issometimes called 'anamorphic' by analogy with the cinema format inwhich a cylindrical lens is used to squash a wide picture'anamorphically' into a standard Academy Ratio frame at the expenseof reduced horizontal resolution.Other aspect ratios may be created within 4:3 or 16:9 frames byeffectively widening the horizontal or vertical blanking periods inorder to matte down the visible picture to the required shape.British broadcasters frequently use this technique to present a14:9 version of a 16:9 picture on analogue transmissions. Sincethese extra pseudo-blanking periods are really part of the activepicture it is possible to include graphics in them. In particularsports and light entertainment producers like to add swirlingcoloured 'curtains' to 4:3 or 14:9 segments of their widescreenshows.There is more about aspect ratios in 'More thanjust a pretty face...'|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Resolution  ESOLUTION, OR 'definition' is a measure ofthe fineness of detail that can be seen in a picture. Inphotography, the resolution is generally the same horizontally andvertically, but in television the two are separate, though interdependent. Verticalresolution is determined by the number of scanning lines in thepicture, and horizontal resolution by the video bandwidthavailable. In most line standards the two are made to appear equalto the eye, though there is some disagreement about whatconstitutes equality. One solution has been to apply a 'Kellfactor' in calculations to determine resolution.There is more about resolution on the page 'More than justa pretty face...' in the Test Cards section of this web site.In colour television systems the chrominance (hue and saturation) resolution is generally much lower than that of the luminance (black and white information).This is discussed in the chapter on Colour below.|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Gamma  ISPLAY TUBES are not linear devices (thoughcamera pick-up tubes tend to be). Typical cathode ray tubes producea light output that is proportional to the driving voltage raisedto the power of 2.8 ±0.3, known as the 'transfercharacteristic' or 'gamma factor'. This is due mainly to the triodetransfer characteristics of the display tube.To correct for this non-linearity, the video signal shouldrequire an exponent of 1/2.8 or 0.357, but it has been found thatan overall system gamma of unity renders monochrome pictures whichappear flat and lacking in contrast. A value for gamma correctionof 0.45, giving an overall system gamma of 1.26, has been chosen innon-NTSC countries for monochrome transmissions.However, the various equations used to matrix the colour signalsrequire an overall system gamma of unity in order to yield correctcolorimetry, so when using these equations a precise transfercharacteristic of 1/0.4545 = 2.2 is assumed, and a gamma correctionvalue 0.4545 is appled to all colour standards, despite the factthat a display transfer characteristic of 2.8 is still assumed for625-line colour standards.Values of 2.2 for transfer characteristic and 0.4545 for gammacorrection are used in the NTSC countries, leading to a systemgamma of unity. For computer displays, Windows assumes a transfercharacteristic of 2.2 while Macintosh uses 1.8, leading to a lowoverall gamma of 0.82, in which low luminance levels are renderedbrighter.It has always been the practice to perform this gamma correctionin the camera (or the colour encoder) in order to reduce thecomplexity of receiver video circuitry, and to reduce the effectson dark parts of the picture of noise accumulated in thetransmission system.Signals that have been gamma corrected should properly bewritten with a prime mark ('), for example Y', R'G'B'(or, when referring to the voltages: E'Y,E'RE'GE'B). However,since most signals are gamma-corrected I have left out the primemarks in general to avoid cluttering up the text. They are includedin some of the equations in order to clarify which values aregamma-corrected and which are not.|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Colour  AVING ESTABLISHED workable monochrometelevision systems the designers turned their minds to colour.Original colour scene RGBGamma correctionGamma correction in colour television theory is a thorny subject.It leads to errors in the decoded signal depending on where it is applied,and muddies the waters where monochrome compatibility is concerned.Generally the R, G and B colour separation signals have gamma correction applied at an early stageand the gamma corrected luminance signal Y' is derived from them.But other ways of doing it are possible.In this discussion I have mainly ignored gamma and left the prime marks(the ticks - ' - that indicate that a signal has been gamma corrected) out of the equations.To reproduce a colour scene requires the image to be sampledseparately in the three additive primary colours red, green andblue (R, G and B). In colour photography, printing and computing,it is usually these three colour separations, or their subtractivecounterparts (C - cyan, M - magenta, Y - yellow and K - black, orkey), which are stored, manipulated and displayed. However, thelegacy of the monochrome transmitters and receivers all over theworld, together with the huge amount of frequency spectrum thatwould have been required, meant that a different approach wasneeded for colour television.At this stage of the process the levels are normalised such thatfor peak white, R = G = B = 1 [1],and gamma correction is applied to the three colour separationsignals as it is these that will be used to drive the cathodes ofthe three colour display tube guns.[1] See the section on Component video levels below for the actualvoltages used.Colour separation signals   RGBIt was recognised that any colour system should be compatible inboth directions - ie no change should have to be made to monochromereceivers, and colour sets should display monochrome transmissionscorrectly and automatically. The black-and-white picture wastherefore redefined as 'luminance' (Y - not to be confused with theyellow component of CMYK colour space) and is synthesised by addingtogether the three separate colour separationsignals in the proportionsY = 0.299R + 0.587G + 0.114B, thesevalues having been determined to produce a compatible'panchromatic' display on a monochrome receiver. Again, for peakwhite, where R = G = B = 1,Y = 0.299 + 0.587 + 0.114 = 1.Luminance signal YThis luminance signal is transmitted in exactly the same way asthe old black-and-white signal.Now that some of the colour information is effectively coded inthe luminance signal, it is only necessary to transmit two furthersignals in order to be able to obtain the separate R, G and Bsignals in the receiver. The method that has been universallyadopted is to matrix the R and B signals with the Y signal andtransmit (R-Y) and (B-Y), where(R-Y) = 0.701R - 0.587G - 0.114B, and(B-Y) = - 0.299R - 0.587G + 0.886B.This has the huge advantage that in the case of a monochromepicture, or areas of grey in a colour picture, the colour valuesare such that Y = R = G = B andtherefore (R-Y) = (B-Y) = (G-Y) = 0.In other words these 'colour difference' signals vanish when thereis no colour information, improving compatibility and reversecompatibility (since a colour receiver, seeing a monochrometransmission with no colour-difference signal present, willautomatically display a black-and-white picture).Colour difference signals_Grey.jpg) _Grey.jpg) _Grey.jpg) (R-Y)(G-Y)(B-Y)Being 'difference' signals, the (R-Y), (B-Y) and (G-Y) voltages,unlike other video signals, can be negative as well as positive.For the purpose of these illustrations I have added a mid-greypedestal so that the 'negative' excursions are visible. The greyareas represent zero colour difference voltage (ie colourless areasof the picture) and because the eye is sensitive to very smallchanges in the 'colour temperature' of neutral shades, the threesignals have to be very accurately clamped to 0V in the coder anddecoder circuitry.The (R-Y) and (B-Y) signals were chosen for transmission becausethey have larger maximum voltage excursions than the (G-Y) signal,which is therefore recoverable in the receiver by attenuating,rather than amplifying, the other two. This has advantages in termsof signal-to-noise ratio as well as decoder complexity. The greencolour difference signal is given by(G-Y) = - 0.509(R-Y) - 0.194(B-Y), and inearly colour receivers the addition of the Y signal ('matrixing')to recreate the R, G and B signals, was done within the crt displaytube, thus economising further on valves.Colour difference displays_Colour.jpg) _Colour.jpg) _Colour.jpg)  (R-Y)(G-Y)(B-Y)(R-Y), (G-Y), (B-Y)The four pictures above show the displays obtained when thecolour difference signals are applied individually and incombination, in the absence of the luminance signal, to the matrixcircuitry from which the final R, G and B signals are extracted.When the luminance signal is also applied to the matrix theoriginal colour separation signals are obtained as shown below.Colour separation displays   RGBRGBThese final four pictures show the displays obtained when thecolour separation signals are applied individually and incombination to the display device.The above process is essentially the same for all analogue anddigital colour television systems.Encoding the colour signalThe next problem was to accommodate this three-fold increase ininformation without increasing the bandwidth of the transmittedsignal. Two phenomena - one physical and one physiological -allowed this to happen.Colour vision acuity, it was discovered, is different from thatof brightness-only vision. In fact, if sufficient detail isavailable in the brightness of a scene, the detail in the colourscan be reduced considerably with no apparent reduction in thesharpness of the scene. This allows the bandwidth of the colourdifference signals to be reduced to half, or less, of that requiredfor the luminance channel. This fact is further exploited in theNTSC system, where thecolours to which the eye is least sensitive in terms of detail areassigned a narrower bandwidth, by transmitting I (orange-cyan) andQ (green-magenta) signals instead of (R-Y) and (B-Y) and allowingthe Q signal only half the bandwidth of the I signal.Secondly, the video waveform does not have a continuousfrequency spectrum, like that of sound. Because of the way thepicture is scanned, the energy in the spectrum is bunched aroundmultiples of the line and field scanning frequencies, with littleenergy in the gaps. By shifting the colour-difference signals infrequency (by modulating them onto a subcarrier) it is possible tomake the peaks in colour energy fall in the gaps in the luminanceenergy, thus allowing the signals to be separated at the receiverby means of a 'comb filter'. The subcarrier frequency must be highenough that the dot-matrix pattern created on a black-and-whitereceiver is not too coarse, yet must be low enough that the upperchrominance sideband fits within the vision bandwidth of thetransmitted signal without attenuation or distortion. The precisevalue of the subcarrier frequency is then determined by addingfractions of the line and field scanning frequencies in order tocreate a dot pattern that is not distracting.Thus the three channels of colour information may be fitted intothe same bandwidth as existing black-and-white video signals,whilst maintaining both forward and reverse compatibility. Thedifferences between the three main colour systems occur in the waythe colour difference signals are modulated on the subcarrier, andthe precise frequency of the subcarrier (which depends to a largeextent on the line and field frequencies and the bandwidth of thetransmitted signal). These are detailed in the colour standards section of this page.Causes of loss of resolution in the decoded colour signalAlthough for most picture content the reduction in chrominance bandwidth is perfectly acceptable,in certain circumstances it can lead to unwanted visible effects,especially at the boundaries between saturated colours or where brightness detail occurs in areas of saturated colour.Captions, for example, in red or blue appear smeared and fuzzy.Also scenes illuminated by light of a primary colour appear noisy and blurred.  Here is a waveform of a multiburst test signal.It ranges from black (0%) to white (100%) with a mean level of 50%.In a digital transmission all the gratings up to 6MHz will be visible on the screen.In an analogue 625-line signal the 6MHz grating will appear plain grey,as will the 5MHz grating in a 525-line signal,because they are beyond the frequency response of the luminance channel.However, the response of the chrominance channels is much less - around 1MHz for PAL, SECAM and NTSC, and around 3MHz for digital. Let us see what happens if we transmit a multiburst entirely in one primary colour.The diagrams that follow show one grating that is within the chrominance passband followed by one that is outside it.The signal is applied to the red channel only, so the green and blue channels are at black level (0%). The luminance and the three colour-difference channels are shown here.Adding together the luminance and each of the colour difference signals in turn will give the same R, G and B signals as above. However, the chrominance channels are low-pass filtered.The lower frequency grating remains unchanged,but the higher one becomes a straight line having the same mean level as the grating. When the luminance signal is added to each colour-difference signal,the amplitude of the high-frequency grating is reduced in the active channel,and phantom gratings appear in the other two channels,reducing the saturation (and fortuitously increasing the brightness slightly) of the details.The red and green channels contain negative-going information which will either be clipped by the display circuitry or cut off in the cathode ray tube.The loss of luminance resolution is less pronounced in areas of saturated green and more pronounced in saturated blue because of the relative contributions made by the colour separation signals to the luminance signal. This sequence of screenshots shows the two sections of multiburst as a black-and-white signal... ..as a signal in the red channel only as displayed on an RGB monitor before encoding... ..and as displayed after decoding.This effect is increased by the way that gamma correction is applied in generating the luminance signal.As discussed above, the luminance signal is derived from the three colour separation signals using the following relationship:Y = 0.299R + 0.587G + 0.114Bin which the uncorrected luminance signal is obtained by summing the uncorrected colour separation signals.The gamma corrected luminance signal Y' is then obtained by applying gamma correction to Y.However, in terms of practical circuitry, it is less complicated to derive Y' by summing R', B' and B' as follows:Y' = 0.299R' + 0.587G' + 0.114B'which unfortunately has the result that more of the high luminance frequencies are transferred from the Y' signal to the (R'-Y') and (B'-Y') signals after matrixing.These are then filtered out by the coder and lost, resulting in yet more reduction of the resolution.An additional effect is produced because many television cameras and telecine units employ a technique called 'contours out of green' in which the the waveforms used for horizontal and vertical aperture correction are derived not from the luminance signal obtained by matrixing the three colour channels, but from the green channel alone.Aperture correction is a technique used to compensate for the fact that when scanning is employed, the sampling device - the electron beam in a camera tube, the spot of light in flying-spot telecine or the 'pixel' in a charge coupled device - is of finite size, which limits the resolution of the device in a predictable and correctable manner.The addition of a 'crispening' signal to the three colour separation signals can overcome this deficiency, and deriving it from the green channel alone improves the sharpness of the picture, since fine detail in all three channels might not be perfectly coincident due to poor registration.Since most scenes contain high levels of green, there are usually no unwanted effects,but when the pictures contain large amounts of saturated red or blue detail,as in the case of red captions on a film, or 'disco' scenes,the pictures appear blurred because the green channel is contributing little to the aperture correction circuitry- other than a high level of undesired noise -and so the colour separation signals contain only the 'unsharpened' video information.Unlike the effect described above, this effect is also seen on monochrome receivers, where pictures with low green content lack high-frequency definition.Not all cameras use 'contours out of green'.Indeed Sony developed one model in the late 1970s that used 'contours out of red' specifically for photographing surgical operations.A further problem is that with scenes illuminated by a primary colour limiting can occur in the channel of that colour, resulting in no detail at all being visible in the clipped areas.Again, this also affects monochrome displays and leads to odd looking pictures because the clipping occurs at low luminance values rather than peak white.Developments in technology, especially the area of home videorecording, have led to improved, though non-compatible (with oldB&W receivers), ways of delivering colour signals at basebandrather than radio frequency. Where the standard coded colour signal('CVBS' - Colour, Video, Blanking, Syncs) carries luminance andchrominance within a single circuit 'S-Video' ('Separate video' -not to be confused with the tape format 'SVHS' - Super VHS, whererecorders often incorporate S-Video inputs and outputs) carries theluminance in one circuit and the coded chrominance in a second,improving the bandwidth available to each whilst eliminatingcrosstalk. With digital systems the luminance and two colourdifference signals are encoded separately, and so digital decoderscan be made to generate RGB, YPbPr (a versionof Y(B-Y)(R-Y) - see Standard Video Levelsbelow), S-Video or CVBS depending on the capabilities of thedisplay device.The colorimetry - that is the precise colours used for theprimaries red, green and blue, and also 'white' - differs fromstandard to standard, and has also changed over the years. Theseare described and discussed in the colour standards section of this page.Monochrome compatibility and reverse compatibilityIt has been stated that analogue colour television systems should exhibit both compatibility and reverse compatibility- that is a monochrome transmission received on a colour set should be displayed in black and white,and a colour transmission received on a monochrome set should display a picture that is indistinguishable from a monochrome one.The reason for this is that both have to share the same transmitted signals.The same is not true with digital transmissions,because no monochrome analogue receiver will be able to display them directly,but a standard or high definition digital decodershould still be able to send a compatible analogue signal to a monochrome receiver or display.But how far does compatibility go? The word 'COLOUR' may appear real enough in the caption on the left,but when viewed on a greyscale computer monitor,or a colour telly with the colour turned all the way down,it will disappear,since it has been arranged that all the information pertaining to it appears in the colour difference channels and none in the luminance channel. It is a nice little party trick,but when the same caption is displayed as a coded PAL or NTSC video signal on a monochrome crt displaythe word 'COLOUR' is clearly visible.Why is this, and is it a bad thing?The effect is purely to do with the colour subcarrier,which is present in coloured, but not neutral, areas,on the coded signal.Areas of saturated colour are not displayed on a monochrome screen as lines of constant grey,as areas of neutral colour are.Instead the peaks and troughs of the subcarrier appear as tiny light and dark dots- rather like the dots that make up a newspaper photograph -and the brightness of these dots is related to the luminance.The eye sees the mean value of the dots and interprets it as solid grey.However, for several reasons the grey the eye sees is brighter than the electrical mean level of the subcarrier.Here, gamma correction raises its head.The crt has a non-linear transfer characteristic that means that as the video signal increases,the light emitted by the screen increases to an enhanced degree(to the power of gamma - around 2.2 - in fact).In the camera,the video signal is corrected in the opposite direction to compensate for this,but the subcarrier is added after this so-called gamma correction.The light and dark dots therefore appear brighter than they should, and the eye sees a brighter mean level.Moreover, the darker saturated colours (magenta, red and blue in colour bars)have subcarrier that descends below black level.On a crt the light dots are displayed as normal,but the associated dark dots appear as a uniform black.The optical mean level of the dots is therefore even higher for these darker colours.So, is this a bad thing?Is it a breakdown of the compatibilty requirement?Well, no and yes.In fact that pesky gamma correction has already contributed to an unwanted darkeningof the luminance in areas of saturated colour that would be displayed if the subcarrier were to be filtered out.In a colour decoder this effect is corrected when the luminance and colour difference signalsare matrixed to recover the red, green and blue signals,but in a monochrome receiver no such action can take place,so the optical averaging effect restores some brightness to saturated colours(but it does not quite bring them up to the correct values).Conversely the subcarrier must be filtered out in a colour display,otherwise saturated colours would appear too bright and desaturated.|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Standard component video levels NTSCThe 525-line standard composite waveform is subtly differentfrom all others. P-P voltage is still 1V, and blanking level is at0V, but the blanking level to peak white amplitude (+714mV) isdivided into 100 so-called IRE units and the sync tip amplitude is-40 IRE units (-286mV). Black level may either be at 0 IRE (0V) oron an optional pedestal of +7.5 IRE (+53.55mV). The above appliesonly to the composite NTSC signal and not to the component signals(including composite luminance).The NTSC subcarriers are not modulated with the U and Vcomponents directly. Instead they are projected onto the I and Qaxes which lead U and V by 33°.This complication is in order to reduce the chroma bandwidth separately for the two signals.The accuity of the human eye is much worse along the magenta-green Q-axis than the orange-cyan I-axis.It is physically impossible tomodulate 100% saturated bars onto the NTSC System M vision carrierwithout severe distortion, so test signals having 75% of theamplitude of 100% bars are used. It is necessary to ensure thathigh saturation values of certain colours are not included inprogramme material. The same is true of most 625-line systems,though System I is theoretically capable of carrying 100% bars,with the minimum carrier excursion not quite reaching zeromodulation.SECAM also uses the U and V components, though the resultingsignals, after much processing, are frequency modulated onto twosubcarriers inserted on alternate lines. E HAVE already seen that the standardmonochrome composite video signal has a peak to peak amplitude of1V into 75ohm. With blanking level at 0V, sync tips are -300mV andpeak white is +700mV. This is the same for the luminance componentof a colour signal as well, though the levels of the colourcomponents depend upon the use to which the signal is to beput.100% colour bars (for further details of colour bars and othertest patterns see the Colour Barssection of the Test Cards page) provide a source of the extremes ofvoltage excursions allowed by the colour tv system (its 'gamut').In the diagrams below, the colour of the trace indicates the signalbeing considered and the background colour indicates the bar beingdisplayed. Note that the waveforms below represent actual voltages,while the values indicated in the Colour Bars section arenormalised - that is, black = 0 and white = 1.   RG compositeBThe gamma corrected non-composite red, green and blue colourseparation signals each have a maximum amplitude of 700mV.Synchronisation is either by separate wire(s),carrying mixed syncs or separate H and V sync pulses,or by incorporatingmixed syncs onto one (usually green) or all of the separationsignals. The domestic SCART system uses non-composite 700mV RGBsignals with the display being synchronised by the accompanyingencoded composite signal, which has to be carried for compatibilitywith non-RGB input capable displays.   Y compositeB-YR-YThese values of RGB yield luminance (Y) and colour difference(B-Y and R-Y) signals of amplitudes 0-700mV, ±620mV and±491mV respectively. These are the (relative) values thatmust be matrixed to recover the correct RGB signals in the decoder,but the colour difference signals are unsuitable for distributionat these levels.   PAL compositeUVTo produce an encoded PAL signal the colour difference signals,after scaling, are amplitude modulated onto two supressedquadrature subcarriers. In order to achieve levels of subcarrierthat can be further amplitude modulated onto a vision carrier,whilst maintaining reasonable signal-to-noise ratios for the twosignals, the weighting factors are as follows:E'U = 0.493(E'B - E'Y)E'V = 0.877(E'R - E'Y)These particular weighting factors ensure that the maximum subcarrier excursions are around33% above white level for saturated yellow and cyan colour bars and 33% below black level for red and blue bars.The p-p subcarrier amplitude (indicated on the compositewaveform by blocks of saturated colour) is twice the vector sum ofthe amplitudes of the U and V signals for each bar.However, because the frequency response of the system is not always perfect,the recovered subcarrier amplitude,and hence the amplitudes of the demodulated colour difference signals,may be higher or lower than this.In the PAL and NTSC standards that would affect the saturation of displayed colours,so the colour burst signal (shown here in grey) inserted into the front porch of the line blanking periodin order to synchronise the reinserted local subcarrier has an amplitude of 300mV p-pwhich is used as the reference level for the automatic chroma gain control to ensure that the subcarrier is demodulated with the correct amplitude.The three purple plimsoll lines at the top left of the compositewaveform represent zero carrier level in Systems I, B/G/D/K and M,for which the modulation depths for peak white are respectively20%, 15% and 10% (100% modulation is sync tip level in all threecases). It is clear that none of the systems can safely carry thefull gamut of 100% saturated colours without severely distortingthe transmitted signal.  Y compositePbPrAlthough four-wire (with additional control wires) RGB signalscarried by the French SCART (Peritel) interconnection system havebeen in use in Europe since the early 1980s, the preferred methodin NTSC countries is three-wire component, calledYPbPr. The colour difference signals areindividually scaled in order to obtain a maximum amplitude of±350mV for them both, the same swing as the black-to-white portion of the luminancesignal. The scaling factors are as follows:E'Pb = 0.564(E'B - E'Y)E'Pr = 0.713(E'R - E'Y)  Y compositeCbCrWhen component signals are digitised a 350mV pedestal is addedto the scaled colour difference signals in order to bring them intothe same 0-700mV range as the luminance signal. The trio is thentermed YCbCr. The luminance signals aresampled at 13.5MHz and the chrominance signals at 6.75MHz for both525- and 625-line standards, as specified in the document ITU-RBT.601. The eight-bit quantisation levels (range 0-255 decimal)corresponding to the 0, 700mV luminance signal are 016 and 235decimal. For the 0, 350, 700mV colour difference signals they are016, 128 and 240 decimal.The negative-going synchronisation pulses are not digitised since they are outside the sampling period (720 samples @ 13.5MHz over 576 scanning lines in the 625-line system and 480 lines in the 525-line system).|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Transmission HE FINAL leg of the journey is to get thevideo, and associated audio signals, into the home. The art ofradio transmission was quite mature at the time of the introductionof television, but to transmit the vast bandwidth of a video signal(one thousand times wider than an audio signal) required carrierfrequencies of an order of magnitude higher than those that hadbeen used previously for broadcast purposes.Also, the theory of modulating a varying signal onto a carrierwave had to be rethought. Because audio signals are symmetricalabout a mean value, it is only necessary to ensure that that valueequates to 50% of the carrier level of the output of thetransmitter. As the modulation level increases, the positive andnegative peaks move towards 100% and 0% carrier respectively. However, a vision signal is not like that. The negative part ofthe signal between 0 and -300mV is constant, and carries thesynchronisation pulses. The positive portion between 0 and +700mVcarries the vision proper, and depending on the amount of white inthe picture the mean level of the signal can vary between around-30mV and +950mV. Such a signal would be useless for a modulatorthat expected a constant 50% mean level, because the positive andnegative peaks could float above 100% and/or below 0% carrierlevels according to picture content. Alan Blumlein introduced theconcept of DC restoration, whereby the video signal presented tothe modulator is 'clamped' so that blanking level represents afixed modulation level. Oscilloscope trace of a System L carrier modulated with two linesof monochrome video using positive modulation Oscilloscope trace of a System I carrier modulated with two linesof monochrome video using negative modulationThe first frequencies chosen for television transmission werearound 50MHz in the hitherto unexplored vhf band I (AlexandraPalace had vision at 45.0MHz and sound at 41.5MHz). Amplitudemodulation was used for the vision signal and although the originalAlexandra Palace transmitter radiated the full double-sidebandsignal, all later transmissions have been vestigialsideband, with one of the sidebands filtered out beyond thefirst few hundred kilohertz. The sound carrier was placed justbeyond the radiated vision sideband, where the video modulationenergy was relatively low.The possible options of vision modulation sense, soundmodulation mode and vision sideband supression, in addition to thevarious line standards, has led to a multitude of differenttransmission standards around the world.Vision modulation may be either positive or negative. Withpositive modulation, the sync pulse tips are held at thezero-modulation level, whilst peak white is 100% and black levelaround 30%. With negative modulation, the sync tips are at 100%,black and blanking levels around 75% and peak white 10-20%,depending on the precise transmission system used. This method hasthe advantage that there is a portion of the waveform that isalways at 100% modulation, so that the receiver can measure thecarrier strength and adjust its automatic gain controlaccordingly.The sound carrier can be amplitude or frequency modulated. Theconvention is that amplitude modulated sound is used with positivevision modulation because in the intercarrier sound methodof frequency modulation detection the sound carrier and visioncarriers are mixed together to give an accurate intermediatefrequency set by the transmitter, allowing them to share a commonintermediate frequency amplifier chain in the receiver. In positivevision modulation the carrier level falls to zero during sync tips,making it unsuitable for this purpose and so AM sound, which is notso sensitive to local oscillator drift in the receiver, is used,with a separate intermediate frequency amplifier from that used forthe vision signal. However, AM sound requires a larger amplitude ofcarrier than FM for the sound and vision service areas tomatch.The choice of which vision sideband to supress is immaterial formost purposes, except that it affects the position of the soundcarrier relative to the vision carrier. Indeed, the French System E819-line network had a mixture of upper and lower sidebandtransmissions shoehorned into bands I and III in order to providemore useable channels.Analogue direct-to-home satellite broadcasts by comparison usefrequency modulated video with a vision bandwidth of just over 5MHzand several frequency modulated sound carriers (used in pairs forstereo) between just below 6MHz and 8MHz. The channel width isabout 27MHz compared with the 8MHz of terrrestrial systems B,G, I Dand K.Stereo and multilingual soundtracks have been added to analogueterrestrial transmissions in many countries. These have beenincorporated either by multiplexing the existing sound carrier orby adding further analogue or digital carriers. These are detailedin the table of CCIR transmission systems.|Top | Contents |Timeline | Scanning| Interlace | Aspect Ratio | Resolution | Gamma |Colour | Levels |Transmission | Bookmarks |Vestigial Sideband HEN Acarrier wave is amplitude modulated, its amplitude varies in sympathy with the modulating waveform.This is shown for a video transmission in the diagrams above, where the envelope of the waveform is shown as a full line.In reality the envelope is simply defined by the peak tips of the carrier waveand the modulating waveform itself is not sent.These diagrams show what is happening in the time domain. Frequency spectrum of an amplitude modulated telephony signalThe frequencies marked are relative to thecarrier. The levels and slopes of the curves are stylised forclarity.It is not obvious what happens in the frequency domain.In fact when two sinusoidal frequencies (pure tones) are mixed together in a non-linear way (as happens in amplitude modulation)the result comprises the two original frequencies as well as their sum and their difference.In a radio signal, one of these frequencies is called the carrier - this is the frequency to which you tune your receiver.The original baseband modulating frequencies are filtered out of the transmission as they are not required.In an audio am transmission for example, the rf portion that is transmitted comprises the carrier wave itself,plus the sum of it and every audio frequency in the modulating signal gathered together in what is called a sideband.A mirror image of this sideband comprises the difference between the carrier frequency and all the contributing audio frequencies.These are called the upper and lower sidebands.Note that the amplitude of each sideband is half that of the carrier, and that the bandwidth of the transmission is twice that of the highest modulating audio signal, which in this case is around 3kHz(it is a telephony communications, rather than entertainment broadcast, signal).In the simplest form of demodulator, called an envelope detector these three signals are used to recover the original audio signal, which was filtered out of the transmission.Now, the information contained in each sideband is exactly the same,and to send it twice is wasteful of bandwidth and power.The carrier wave, once it has been used to generate the two sidebands,carries no information whatsoever - either sideband would fly just as far without it.In telephony communications therefore,both the carrier wave and one sideband are often filtered out,and the receiver regenerates the original audio - or a close enough approximation -by inserting a locally generated carrier and using that to demodulate the surviving sideband.Because the amplitude, frequency and phase of the original carrier cannot be known exactly,some skill on the part of the operator is required to resolve intelligible speech as opposed to garbled Donald Duck noises.This method of transmission, called single sideband, supressed carrier(the supressed carrier part of the description is usually, ahem, supressed)is clearly unsuitable for music or entertainment, but it is possible to transmit a smidgeon of the original carrier wavein what is called single sideband, reduced carrier to which the locally generated carrier signal may be synchronised in phase.Double sideband amplitude modulation however, survives on long, medium and short wave radio broadcasts (and in some television systems) for the sake of simplicity in the receiver.But to use it for video would be wasteful of both bandwidth and power, and would have made the design of suitable high-bandwidth, high gain receivers difficult in the early years.Equally, it would be difficult to filter out the carrier and the whole of one sideband at the transmitter without introducing distortion into the other sideband (unlike telephony audio, which contains no energy below about 300Hz, a video signal contains a dc component - 0Hz - resulting in sidebands that converge upon the carrier frequency), and it would not have been easy to design a simple ssb video demodulator for the receiver.For these reasons the first 405-line station at Alexandra Palace in London radiated dsb vision, and the early receivers had dsb detectors. Frequency spectrum of a System I vestigial sideband transmission showing ideal receiver frequency responseThe frequencies marked are relative to the visioncarrier. The levels and slopes of the curves are stylised forclarity.But in time, in true British style the boffins came up with a compromise.The whole of the carrier and one sideband would be transmitted, together with a bit (a vestige) of the other.This has several advantages.The remaining sideband suffers no distortion in the transmitter and an unmodified dsb envelope detector can be used in the receiver.All that is required is an rf (or if - intermediate frequency - in the case of the new-fangled superheterodyne receivers)response that is tailored to suit the incoming vestigial sideband transmission.In this, the response near the carrier frequency is reduced such that the carrier itself is received at half strengthand the response tails off as it penetrates the vestigial sideband.In this way the lower video frequencies are received in both sidebands and the upper frequencies come from the full sideband alone, albeit a little distorted by the action of the envelope detector in the presence of only one sideband.More modern receivers with synchronous detectors do not suffer from this distortion, and the whole video spectrum is recovered from the full sideband alone.Vestigial sideband receivers of either vintage may be used to receive double sideband transmissions.This is just as well, since the cheap uhf modulators incorporated into video games, vcrs, satellite receivers etc all operate on dsb.Vestigial sideband is also used in NTSC and PAL colour signals, though it is seldom mentioned as such.In these two standards the two colour difference signals are amplitude modulated onto two subcarriers of the same frequency with a 90° phase difference.Because of the 90° phase shift, one subcarrier is at a maximum amplitude excursion when the other is crossing zero, and this enables the two colour signals to be completely separated in the demodulator.The actual subcarriers are supressed before multiplexing with the video (luminance) signal, and about ten cycles of subcarrier,called the colour burst, are added to the horizontal blanking interval as a reference for the demodulator in the receiver.If the full dsb signal comprising subcarrier and sidebands were to be added to the video signal it would produce a strong interference pattern all over the picture.We have seen that the amplitude of the carrier must be twice that of the highest modulating signal,and so the whole signal would have to be attentuated severely to enable it to be transmitted at all,and then more to reduce the interference level,resulting in a very poor signal-to-noise ratio for the colour information.The dsb supressed carrier signal can be inserted at a higher level, since only very saturated colours have significant sideband amplitudes - in fact the amplitude for greys is zero.This mode is often called QAM (quadrature amplitude modulation), particularly in digital communications.The bandwidth of the colour signals is restricted to 1.3MHz and with a subcarrier frequency of 4.43361875MHz in the PAL 625-line standard that would give an upper sideband excursion of about 5.7MHz.In Systems B/G, I and D/K the video cut-off frequencies are respectively 5.0, 5.5 and 6.0 MHz.These are nominal, built into the specifications for each standard, but often the video is cut off around 5MHz for all standards.So the colour information is effectively vsb, but since there is little energy in the removed portion of the sideband no account of this is usually taken in the PAL receiver.In the case of NTSC however, things are much tighter.With a 3.57954545MHz subcarrier and a 4.2MHz video bandwidth in System M, only 600kHz is available for the usb of the colour signal.Some fiddling is done to ensure that one colour signal corresponds to the colours to which the human eye has least accuity - it sees them blurred in other words.This (called the Q signal) is given a 500kHz bandwidth while the other (I) is afforded the 'full' 1.3MHz.However, the reduction of the usb of the I signal to 500kHz in transmission means that either the receiver must filter the whole chrominance signal to 500kHz or perfom ssb demodulation on the 1.3MHz lower sideband of the I signal in order to avoid severe distortion caused by demodulating the vsb colour signal as a dsb one.Related sections: | E-mailme | Home Page | 405-Line Standard | Test Cards | Teletext |World TV section: | Overview | Line Standards | Colour Standards | CCIR Systems | Radio Channels |This page: |Top | Contents | Timeline |Scanning | Interlace | AspectRatio | Resolution | Gamma | Colour | Levels | Transmission| Bookmarks | Television Website Bookmarks MikeBrown/MB21/Ether.netAndrew Emmerson/PaulStenning/405 Alive/British Vintage Wireless SocietyKeithHamerDavidLaine/Vintage BroadcastingRichardLogue/irishtvDarren MeldrumRichard RussellPeterVince/Barney WolAndrew Wiseman/625 RoomBill Wright Back ToTop Pembers' PonderingsCompiled by Alan Pemberton Sheffield, South Yorkshire, EnglandEmail me |
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