ATV HandbookATV Handbook, HamRadio, misc-ham2
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//-->AmateurTelevisionHandbookBy John L. Wood, G3YQC and Trevor Brown, G8CJSAcknowledgementsThe British Amateur Television Club expresses its gratitude to the following companies, societiesand individuals who have provided material and assistance for this publication.Blean Video Systems, Canterbury, KentPlessey Semiconductors Ltd., Swindon, WiltshireRadio Ref. ParisRadio Society of Great Britain, Doughty Street, LondonWasco Electronics, Queens Street, LeicesterC. BrowabridgeM.ChamleyM. CramptonC. G. DixonA.EmmersonJ. GoodeA.F.WoodF3YXG8DLXG8CGKG8PTHG3RDCD.E.JonesJ.LawrenceT. MitchellR. S. RobertsR.T.RussellN. WalkerGW8PBXGW3JGAG3LMXG6NRG4BAUG8AYC© British Amateur Television Club, 1981This special A4 sized edition edited by Ian Pawson, August 1998Editors note:This book was originally printed A5 size. This version has the same content, but hasbeen rearranged to A4 size. The quality of some of the diagrams and pictures is not up to our usualstandard as they have been scanned in from an original paper copy.Amateur Television HandbookPage 1ContentsContentsAcknowledgements............................................................................................................................................................................... 1Contents ................................................................................................................................................................................................. 2Preface.................................................................................................................................................................................................... 3Principles ............................................................................................................................................................................................... 3Scanning ..................................................................................................................................................................3Television Standards................................................................................................................................................4The Modulating Waveform......................................................................................................................................4Bandwidth and Channel Space ................................................................................................................................5The Station...............................................................................................................................................................6Aerials .................................................................................................................................................................................................... 7Feeder Systems ........................................................................................................................................................7Receivers................................................................................................................................................................................................ 9The ELC1043 Series Tuners....................................................................................................................................9A High Performance Wideband Tuner ..................................................................................................................10An Amateur Television Receiver...........................................................................................................................12Transmission........................................................................................................................................................................................ 16Video Modulators ..................................................................................................................................................16A Modular Linear Amplifier..................................................................................................................................16Vision Sources..................................................................................................................................................................................... 20An Electronic Character Generator........................................................................................................................20Page 2Amateur Television HandbookPrefacePrefaceAmateur television is a large and complicated subjectand to do justice to its many facets a book of severalvolumes would be required.Amateur television handbooks in the past have tried tocover as many subjects as possible and consequently,due to size limitations, each subject has been treated inrather less detail that one could have wished. Theoriginal conception of this handbook was to deal ingreater depth with the more complex and in some casesless-well publicised techniques used in the modernamateur television station. You will therefore find lessinformation on basic principles, aerials, operatingtechniques,licensingrequirementsandeventransmitters, all of which are adequately covered inbooks and frequently found in magazines. Instead,emphasis has been placed on subjects such as modernreceiving systems, electronic video sources, vision-processing techniques and of course colour television.These subjects are particularly susceptible to changes inmodern techniques and innovations and therefore thedesigns need to be periodically up-dated.The newcomer to ATV has not been forgotten howeverand a chapter explaining such things as the compositionof the modern TV signal and the organisation of anamateur station has been included. There is alsoguidance on aerials, feeders, simple receivingequipment and colour television principles and it ishoped that this information will adequately augment thelarge amount of already published data in other booksand periodicals.Almost all of the projects in this volume have neverbefore been published and indeed some were designedespecially for this book. Printed circuit boards will bemade available to the constructor in order that the morecomplex circuitry may be successfully built by lessexperienced constructors. The video projects are allcompatible with each other and the PC boards havebeen made to a standard size and use standard edgeconnectors that enable them to be installed in acard4rame cabinet system if required. This ensurescomplete flexibility and permits the use of only thoseunits that are required.The British Amateur Television club is pleased topresent this book in the hope that it will encourage andstimulate television amateurs throughout the world tostrive for technical improvement and will helpnewcomers to enjoy this fascinating hobby.PrinciplesThere are several methods of picture transmission; highand low definition television, slow scan television andfacsimile (FAX) for still pictures, pictures built up usingradio teletype, and so on. This book is mainly concernedwith high definition television and sets out to describethe equipment necessary to build a modern amateurtelevision station.The broad principles involved in television transmissionare well known, and this brief review is intended tohighlight many of the important features of a modernsystem which are dealt with in full detail in thefollowing chapters.All forms of picture transmissionand reception differ from normal‘seeing’ with a human eye in oneimportant respect, the human eyeusesabout150millionsimultaneous channels of visualcommunication, but an electronicsystem uses only one channel atany instant in time, consequently aprocess termed ‘scanning’ has tobe used whereby the visualinformation to be transmitted andreceived is explored bit by bit andtranslated into electrical terms formodulation of a transmitter. Thereceived signal is demodulated andused to build up a reconstitutedpicture on the screen of a cathode-Amateur Television HandbookPage 3ray tube.ScanningTo simplify the explanation we will consider a picturemade up of only eight lines and displaying a blacksquare in the centre of the screen.Scanning requires, firstly, that the picture to betransmitted is framed in a field of view having an‘aspect ratio’. The standard aspect ratio for television is4 x 3 units, as shown in Fig 1(a). It is seen that theactual picture size is of no importance so long as theaspect ratio is correct.PrinciplesFig 1(a) shows a scanning spot that traverses the fieldline by line, (similar to the manner in which we read abook page), translating the variations of light and shade(and possibly colour) into voltage variations which areused to amplitude-modulate the transmitter. The camera,with its optics and electronics carries out this operation.At the receiver, a CRT beam is swept across the face ofthe tube in synchronism with the camera scan, and thedemodulated signal is used to modulate the beamcurrent, thus writing a reproduction of the picturescanned at the transmitter.the field. There are many reasons why amateurtelevision should follow existing broadcast standards,not least of which is the availability of receivers. Thereare two UK broadcast standards, the original 405 line‘black and white’ system A, and the later 625 linesystem I which includes colour. Both use an aspect ratioof 4 x 3, and both transmit 25 pictures per second. Thehighest modulation frequency generated duringscanning in system A is about 3MHz, whilst 5 to5.5MHz can be generated in system I.Any TV system can include a sub-carrier with colourinformation, the normal 625 line system uses a coloursub-carrier frequency of 4.43MHz. A black and whitesystem does not of course require a colour sub-carrier.The Modulating WaveformThe scanning system output will be used to amplitude-modulate the transmitter, and it is necessary for thereceiver tube beam to be in the same two-dimensionalposition as the scanning beam in the transmitter camera.As stated earlier, a single communication channel canonly handle one bit of information at any instant of timebut, in addition to the video information, it is necessaryfor the transmitter to send synchronising information tothe receiver indicating the precise position of thescanning spot in both horizontal and vertical planes.Fig 1(b) shows the voltage obtained by scanning (say)line four of the picture. Because electronic circuitrycannot respond instantly the changes from white toblack and from black to white at the edges are notsharply defined. To improveresolution the spot is madesmaller and the number oflines increased.Television, in dealing withmoving pictures, requires acomplete scan of the field tobe so fast that, compared withany movement taking place inthe scene, each complete scanis of a virtually still picture.Standard broadcast televisionin the UK scans 25 picturesper second.Allbroadcasttelevisionsystems use a techniquecalled ‘interlaced scanning’,this means that the screen isscanned and every other lineis displayed onto the screen,during the next scan the in-between lines are displayedthus completing the picture.Referring to the eight linepicture in Fig 1(a), interlacedscanning would require thatthe complete field would bescanned by lines 1, 3, 5 and 7and then the gaps would befilled by re-scanning the fieldwith lines 2,4, 6 and 8.Television StandardsPicture quality is determined by the scanning spot sizeand, therefore, the number of lines required to fully scanPage 4Fig 2(a) shows the modulating waveform during a one-line scan. The video signal varies the transmitter outputaccording to its amplitude. Time is taken from thecomplete video line scan by ‘blanking’ the video signalAmateur Television HandbookPrinciplesfor a fraction of the total line period. During theblanking period a line-synchronising pulse is insertedwhich takes the transmitter output from 30% to nearzero. This pulse is processed in the receiver to ‘tell’ thesweep circuits when to start the line scan across theCRT face. When the line scan reaches the bottom of thefield, a field blanking pulse blanks several lines and atrain of broad pulses are inserted during the blankinginterval (Fig 2(b)). The receiver processes this train ofpulses to return the CRT beam to the top of the displaytube to retrace its vertical sweep.Fig 2(a) shows what is termed ‘positive’ modulation asused in system A, in which peak white corresponds tomaximum transmitter output. The 625-line system I usesthe same principle but an inverted waveform (‘negative’modulation) is used in which sync tips drive thetransmitter to maximum output, and peak white is nearzero.cycles) is inserted on the back porch, together with thetiming associated with the line blanking pulse.Bandwidth and Channel SpaceTelevision is characterised by the need to handle veryhigh video frequencies throughout the system from thecamera to the receiver, and this includes the aerialsystem Amplitude modulation of the transmitter wouldproduce the normal double sidebands which, for systemA, would require a channel space of about 6MHz, andup to 11MHz for system I. Including a sound channel toeither system would increase the channel width by aboutanother 1MHz.It was realised very early in the history of broadcasttelevision that the heavy demands for channel spacewould limit the number of available channels, and a newsystem for saving channel space was evolved and called‘vestigial sideband’ (VSB), or ‘asymmetric sideband’For a black and white system, a complete picturerequires two cycles of video and synchronisinginformation, as shown in Fig 2(b). Colour requires, inaddition, further information in the form of a ‘burst’ ofabout ten cycles of sub-carrier on the back porch. Thisburst experiences a phase change on every line and,although interlacing is completed in two scans, thecomplete cycle of blanking, pulses and colour-burstphase requires four fields, as shown in Fig 2(c). Fig 2(d)shows how the burst of colour sub-carrier (about tenAmateur Television Handbook(ASB). Fig 3 shows the channel spectrum for onesystem I channel. VSB involves filtering off a large partof the lower sideband, leaving only about 1 MHz or so,and an overall channel width (with guard bands) of 8MHz. With suitable tuning of the receiver IF circuitsdistortion of the vision signal by the loss of part of asideband can be reduced to negligible proportions.The bandwidth required for a single system I channel is8 MHz. The 2-meter band is thus quite unsuitable fortelevision transmissions. The 430 MHz band can onlyPage 5 [ Pobierz całość w formacie PDF ] |
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