电视设计基础

Basic TV Course Video and SynchronizationProcessingList of content•Basic Function Block of TV system•Human vision in TV & Interlacing scan•Synchronization processing in TV–Phase-1 loop / Phase-2 loop / Sandcastle•CVBS signal & World conventional color TV standard–NTSC / PAL / SECAM•PAL / NTSC / SECAM Video Signal encoding•PAL/ NTSC/ SECAM System decoding•Luminance Signal processing as picture improvement –Peaking / LTI / CTI–Signal level stretch: Black / Blue / White level–Soft Clipping / Peak White Limiting–HistogramPrepared by: Wang Guo Ji Oct 2000Function Block of Video SystemFig. Basic TV system C prism R/G/B •Gammacorrection/VideoencoderScanningControl Synchro-nizationProcessorVideo Processing Sync Separation Deflection & EHT. GenerationCVBS ScanCurrent H/V Syncpulse Blanking pulse R/G/B Driver H/V pulse EHTScancurrent R/G/B CVBS IF&RFmodulatorTuner&IF Demodulator Source Selection Synchro-nization processor H/V drive TransmissionReceiver•Human visual perception to Red, Green ,BlueThe major light sensing element in the human eye is the retina. The retina consists of the major light receptors ---rods and cones which at the bottom of the retina.Image is formed by the light that reaching the main photoreceptors. Research shows that red, green and blue colors are the primary colors, as "If you mix light of these colors together you can make any color". They lie in the nature of the human eye.Some colors could have by following experiments :Projectorscreen•Understanding of visual persistence with human visionThe human eye retains an image for a fraction of a second after it views the image. This property (called persistence of vision) is essential to all visual display technologies. The still images are presented at a high enough frame repetition rate so that persistence of vision integrates these still images into motion. For some reason, The motion pictures was found to be unacceptable when image repetition rate is below 48 frames per second.Therefor to realize the repetition rate of 48 frame per second, in the television system one picture frame is splited into two fields, and the field frequency is defined as 50Hz or 60Hz (higher than 48Hz) determined by the video system. 50Hz/60Hz field repetition rate gives normal perception to the image on TV screen. The idea is that a single frame is scanned twice(see Fig. Interlacing lines). The first field scan includes only the odd lines, the next field scan includes only the even lines. With this method, the number of "flashes" per frame is two, and the field rate is just double the frame rate so that 50hz/60hz field rate has their frame rate at 25/30Hz in TV.The 50Hz field scan system contains 625 horizontal lines per frame and each field scans 312.5 lines. The 60Hz field scan system contains 525 lines per frame, each field scans 262.5 lines.•Interlaced Frame ScanScreen framefirst fieldstartsecond field startFig. Interlacing lines•Presentation of interlacing scanning In a TV, the electron beam is used to scan a phosphor screen. The scan is interlaced.For each horizontal line scan the information is always displayed from left to right, after each line is written, the signal is blanked, the beam starts returning back to the left. When the signal reached the bottom it is blanked until it returns to the top to write the first line in next field. the beam does not return directly to the top, but zig-zags route. See Fig. Demonstration of real scanning sequenceScreen framefirst fieldstartsecond field startFig. Demonstration of real scanning sequenceSynchronization components of video signal•Video signals during blanking period Timing of scanning is arranged by the sync pulses. Each field scan starts with a vertical sync, each line scan starts with a horizontal pulse. The horizontal and vertical blanking period are specified around the sync pulses. With blanking period the electron beam complete its fly-back from the right to the left for horizontal scan or from the bottom to the top of screen for field scan. During the period the electron beam is cutoff, so that no scan line is visible on the screen.The combination of horizontal blanking signal and synchronization pulse is defined as illustrated in Fig. Sync Signal during horizontal blanking period. For color signals, a so called …color burst‟ is inserted onto the "back porch”of the H sync, that carries the color system info.for use in color decoder(more on this later). The burst period is about 8-10 cycle time of the sub-carrier (info.)Fig. 50Hz Sync Signal duringhorizontal blanking periodSynchronization components of video signal•Video signal during one line period In time domain,the video signal may be diverted into two period --that is,blanking and scan period. Signal during blanking period is sent to the synchronization processor to re-generate the timing reference (horizontal and vertical drive signals) for picture scanning, the signal during scan period is sent to video processing function block (see Fig. Horizontal video signal),from which the primary color R,G,B are decoded and as the driver signal output topicture tuber. frontporchsyncpulsebackporchblankingimage signalscan periodBlack levelPeak white levelFig. Horizontal video signalSynchronization components of video signal•Video signal during field blanking period Signal during vertical blanking also has a number of synchronization pulses included. It is illustrated in Fig. Horizontal and vertical …mixed-syncs‟ signal. The pre-equalizing and post-equalizing are built to provide the horizontal synchronization during vertical retrace period, in order to have precise starting-point for each scanning field. (info.)For a picture re-production, the horizontal and vertical scan must be in synchronized with the input video signal. This is to re-establish the timing reference for picture scanning from the incoming video signal by the synchronization processor in receiver. It extracts the sync signal from the incomingvideo signal, re-generate the H drive and vertical drive signal with proper display timing incorporated.See Fig. Block diagram of sync processor .System synchronization in TVH.Sync separator 1st PhasecomparatorLow-pass filterH.V.C.OVertical sync separator -divider (625 or 525)-counterCoincidence detectorVertical SAW-tooth OSCCVBS input2nd Phase detectorH.drive outputSandcastle generationV.driveoutputFlyback in/sandcastle outH.driveV.drive Fig. Block diagram of sync processor .System synchronization in TV•Sync separationby applying a slicing level between the top sync and the black level the sync info may be separated from the video signal. The separated sync pulse stream consists of horizontal and vertical sync info. thatintended to control both timing of the horizontal and vertical scan.See Fig. Separation of the horizontal syncCVBS inputBASeparated syncH.Sync separatorSync slicing levelFig. Separation of the horizontal syncFig. A simple single-transistor sync separatorC1R1R2ReRcI_scanCVBSinputpulses output47u47u ABR3composite sync •A simple sync separation circuit -When sync pulses come, T is conducting and C1is charged through the emitter-base of transistor. As result the sync pulse stream are available across Rc at B .-During scan period, I-scan is discharge current from C1, transistor cuts off. C1sees high input-impedance. No signal output at B .-The slicing level may be adjusted by the biasing of Transistor, for which R2,R3,Re may be included.•Sync separation and Sync auto slicing CircuitR A1-+A2A3-+Clamping'low' to blackClampped CVBSabc absync slicing levelseperated syncblack levelc•Separation of Vertical syncThe sync separation outputs composite sync signal that a stream of mixed horizontal and vertical sync pulses.see Fig. Stream of composite sync pulses.Fig. Stream of composite sync pulsesThis sync stream may be used directly for horizontal synchronization after gating (more on later). But for vertical synchronization, It requires further process in order to have a real vertical sync pulse because during the vertical sync pulse period, it sees a double line rate sync pulses in the stream. The vertical sync separation can be carried out by using a simple mono-stable circuit in which the arrival of the broad pulses are detected. Such a circuit is shown in Fig. Vertical sync separated by simple mono-stable circuit,note that the output of separated vertical sync-pulses required to be further processed in order to form the real vertical sync signal.•Vertical sync seperator•Vertical sync pulse is to be separated from the composite syncpulse.•vertical separator circuit.ABSystem synchronization in TV+ Fig. Vertical sync separated by simple mono-stable circuitHorizontal and Vertical scan SynchronizationHorizontal synchronization processing includes two loops control that is, phase1 and phase2 loop control. Phase1 loop control consists of a voltage controlled saw tooth oscillator and a phase comparator. Phase1 loop control is to make the saw-tooth oscillator in synchronized with the incoming horizontal sync pulses, it generates the horizontal reference pulse for sync system usage. see Fig. Function diagram of phase1 loop. Phase2 loop is to control the shift of timing to horizontal drive pulse to adjust display position of picture on screen(more on later).•Operation of PH1 Loop In Fig. Function diagram of phase1 loop, the voltage controlled saw-tooth generator generates the horizontal saw-tooth signal. An error voltage that fed from the phase comparator (through LFP) may control the oscillating frequency higher or lower. Conversion of saw-tooth to horizontal reference pulse is done in the oscillator. In the phase comparator, the reference pulse are comparing to the composite sync pulses with their phase. As result, the error signal in form of current signal passing through the external low pass filter applied to the saw-tooth generator.Fig. Function diagram of phase1 loopRCVH/VL lev el Detector Acurrent sourceswitchabv arible VL input error v oltaget1t2t3t4error voltageVHVLV(a) or V(b)•Principle of voltage controlled horizontal saw-tooth generatorBefore t1, switch is closed and C is charged by the current source. When V(b)=V H is detected, the switch is open. C is discharging via R, till V(a)= V L is detected, the switch is closed, capacitor C is re-charged again. The time constant of RC may be defined in such a way that the charging period equals to the scan period and the discharging period equals to the fly-back period. The error voltage derived from the phase comparator through LPF offsets the V L which determined by the error detected in the phase comparator.•Generating of horizontal reference pulseGenerating of horizontal reference pulse is illustrated in Fig. Generating of horizontal reference pulse . The threshold level as in (b) is about (VH+VL)/2.System synchronization in TVhorizonta syncshorizontal saw-toothhorizontal reference pulses(a)(b)(c)Fig. Generating of horizontal reference pulsesync pulseshoriz. Ref. pulsesgating pulsesIpos.Ineg.ouput current from phase discriminaorDC error voltage outputformed by LPF(a)(b)(c)(d)(e)Fig. Function of Phase comparator System synchronization in TV•Operation of phase comparator In phase comparator, incoming sync pulse is comparing to the horizontal reference pulse, from which the error current is converted to error voltage by means of low-pass filter. the error voltage is fed to the voltage controlled saw-tooth generator to control the oscillator frequency. See Fig. Function of Phase comparator.Example of a phase lock process. See below: in case of fsync > fosc,the error current from phasecomparator outputs asymmetrically in the current waveform which resulting different error voltagelevel.sync pulseshoriz. Ref. pulsesgating pulsesIpos.Ineg.current ouput fromphase comparatorThe increased Ipos periodresults a higher errorvoltage at the LPF output, thenthe saw-tooth frequency willbe increased.fsync > fosc•the negative charging is less than the positive charging time.•The average output current is positive.•The DC voltage at the o/p of LPF increases.•The voltage is fed to the voltage control saw-tooth oscillator as result the frequency will increase correspondingly.VtVFig. Error voltage at LPF outputIn real signal condition the video signal can be easily disturbed by the other signals, especially in off-air. With the disturbance, additional pulses may be added onto the video signal. they also may be mixed in as part of the composite sync. It causes unstable horizontal synchronization, for which gating pulse is formed to get rid of these disturbance. See Fig. Timing of gating pulseVref. 1Vref. 2saw-tooth and ref.flyback pulse of the saw-tooth osc.pulse 1pulse 2gating pulseABCA+B+CFig. Timing diagram of the gating pulseSystem synchronization in TVThe gating pulse is generated around the H. fly-back period.It sets a timing window for the sync pulses. The phasecomparator works only when gating pulse is high.Fig. preventing disturbances from "illegal" pulseshorz. Refincoming sync with 'illegal pulses output w/o gating pulsegating pulseoutput with gatingpulseThe function of gating pulse is illustrated in Fig. Preventing disturbance from “illegal pulses”The illegal pulses mixed in composite sync.Result in …false‟ error current output to LPF.“illegal” pulses is outside the window of gating.The “illegal pulses are rejected by window gating, no influence from disturbance.HH/22.5HH/22.5H 2.35us4.7us4.7ustop s yncblack leveleven field25HCan be removed by gating.Disturbance to the phase 1 control loop•The disturbance due to the vertical sync pulse will upset the oscillator.•The effect on screen if w/o gating/horizontal sync during Vertical blanking period:flutterTop flutter or top bendingTop bendingsync processor Hor.driv estorage delayTdHorizontal f ly -back pulseFig. Horizontal .drive and Horizontal .output•Principle of Phase2 loopthe phase 1 loop synchronizes the horizontal oscillator with the incoming video signal. The phase 2 is to provide a right timing and stable picture position. This is necessary because due to beam current variations that the storage time of the line output transistor varies See Fig. Horizontal drive and Horizontal output.,and the horizontal fly-back pulse required for the blanking purpose. In-correct timing of fly-back results in missing of picture contents.See Fig. Missing picture content by wrong blanking timing.System synchronization in TV•If directly use horizontal drive generated during phase1 loop (no timingcompensation to horizontal drive pulse). In worse case, fly-back pulse from LOT will goes into the picture scanning period.•As the Hor. Fly-back pulse is use to be a horizontal blanking pulse in the signal processing, at the beginning of each line some a small portion of picture content will be blanked.System synchronization in TVon offphase 2 comparatorline output stageHor. drive phase compensator+Vloop filterflyback from LOTPhase 2 Hor. ref. pulseH.dirve pulseHor.saw-toothFig. Function diagram of Phase 2To avoid wrong blanking timing. Phase 2 loop is to coordinate the timing adjustment between the H-drive and sync info. See Fig. Function diagram of phase 2 loop The phase 2 loop compares the horizontal reference with the horizontal fly-back pulse, which intend to compensate the time delay by re-generating the horizontal drive with the delay compensated. the error signal from phase 2 comparator is fed to timing compensator of horizontal drive for proper timing to horizontal drive.System synchronization in TVSystem synchronization in TVH.saw-tooth/ PH2 referenceVideo InputH. fly-backfrom LOTH-Drive pulse timingshifting by resultfrom PH2Vertical synchDirect synchronization--the vertical oscillator is triggered (synchronization) by the vertical syncpulses. The vertical sync pulse starts the vertical fly-back. When no sync, the vertical oscillator is in free running mode, at a lower frequency than the actual vertical frequency. see below for example of vertical direct synchronization.Where no vertical sync pulse, the vertical saw-tooth frequency f0is lower than the vertical frequency fv . When vertical sync pulse comes, it turns to start the vertical fly-back and bring f0 = fv .T1T2V+C T3vertical syncscan vertical syncA BA Bfree runningSystem synchronization in TV•The sandcastle pulse and its applicationduring the process of video processing, some operation such as clamping, burst gating, blanking...etc need timing reference signal, for which a so called …sandcastle‟ signal is generated in the synchronization processor. The sandcastle signal gets its name from the shape of the pulse. Its upper portion is taken from the incoming sync, lower portion from the horizontal fly-back pulse. See Fig.application of the sandcastle pulse andFig. Sandcastle pulseSystem synchronization in TVSystem synchronization in TVFig. Sandcastle pulseSystem synchronization in TV•Vertical dividerThe vertical divider system has a full integrated functionality of the vertical synchronization. It can handle both 50Hz or 60Hz standard system, or can be forced to desired system by IIC bus control. When in the searching mode the divider can catch between about 45 and 65HZ, in case of RGB input mode, the catching range is enlarged and ranges from 44 to 72Hz.Formation of CVBS video signal•Gamma Correction Because of the receiver CRT does not emit the light in direct proportion to the voltage applied between its grid and cathode,which may be expressed:L is the luminance on screen, V g is the grid/cathode voltage.To compensate for the non-linear CRT characteristic, an opposite law must be introduced at the studio source. If E is the camera voltage that resulting from a given light input L , then the gammacorrection may be expressed:The gamma standard used is 2.8 , which is close to a modern color display beam current/grid voltage characteristic and the drive circuit arrangement.()γgV L ∝()EE V L g ∝⎪⎪⎭⎫ ⎝⎛∝∝γγγ1•In studio the R,G,B signals are corrected with characteristics of γ=1/2.8.•Gamma correction in Studio and ReceiverGrey scale patternVin VinVin 0Bd BdBd+•On screen the CRT will emit the linear light to the eyes( γ= 1).•In TV receiver the gamma corrected R,G,B signalsare applied to drive the cathode of CRT by the grid/cathode modulation. γ==2.8•Normal view on screenFormation of CVBS video signalFormation of CVBS video signal•Color-Difference signalThe signal Y which represents the brightness information, was used in the monochrome television transmission service before color TV. But in color TV the RGB primary signals are required to drive CRT. In color TV, additional color-difference signals (R-Y) and (B-Y) were created to provide the color information. It is added to the Y signal after its modulation to the sub-carrier. In the receiver the (R-Y) and (B-Y) will be first demodulated from sub-carrier and sent to the matrix network with Y signal, thus recovers the primary R, G and B. This method of coding reduces redundancy in the combined signal and also remained compatible with black and white television. In a non-colored scene, color-difference signals are zero.As can seen:R=( R-Y ) + YB =( B-Y ) + Yfor G signal, may be recovered from these two color-difference signals by applying expression below: Y=0.299R + 0.587G + 0.114BWeighting factor for Color-Difference Signal To avoid over-modulation in RF modulation. The full-amplitude color difference component must be scaled down by weighting factors in coding and scale up in the decoding. Thus we have:V = 0.877( R-Y )U = 0.493( B-Y )V and U are the weighted color difference signal used in the encoder as a standard.•Formation of CVBS signalFormation of CVBS video signalFormation of CVBS video signal•chrominance phasor position expression as known,the U and V signal are created to carry the chrominance information. They may can be expressed in a coordinate U-V. See Fig. 100% color bar phasor expressionFig. 100% color bar phasor expression The dotted phasor position iscorresponding to the PAL line.In the coordinate, magnitude of vector may be expressed:that mainly determines the saturation of the color.Angle of the vector may be expressed as:that mainly determine the tone of a color.()22V U C +=U V arc tg =θ•Color television world standards There are three major TV standards used in the world today. These are the American NTSC(National Television Systems Committee) color television system, the European PAL (PhaseAlternation Line rate) and the French-Former Soviet Union SECAM (Sequential Couleur avecMemoire). All three systems use the same definition for luminance. The largest differencebetween the three systems is the vertical lines. NTSC uses 525 lines (interlaced) while both PALand SECAM use 625 lines. NTSC frame rates are slightly less than 1/2 the 60 Hz power linefrequency, while PAL and SECAM frame rates are exactly 1/2 the 50 Hz power line frequency.lines active vertical aspect horizontal frame ratelines resolution ratio resolutionNTSC 525 484 242 4/3 427 29.94PAL 625 575 290 4/3 425 25SECAM 625 575 290 4/3 465 25 •Chrominance signal of Color television world standardsin PAL and NTSC standards, U and V color difference signals are using amplitude modulation to the sub-carrier(AM) ,in SECAM U and V signal are using frequency modulation to the sub-carrier. After modulation, U and V are added to form chrominance signal. The modulation method differentials the difference of system between NTSC , PAL and SECAM system. The chrominance signal defines the difference between a color area and a neutral Grey of the same luminance, and the chromiance information embraces that part of the video signal which enables a color receiver to describe an area in color rather than in monochrome.NTSC chrominace signal:()()())33cos()33sin()(cos sin )(cos )(sin )()(︒++︒+=+=-+-=t t I t t Q t t V t t U t e tY R t Y B t e sc sc sc sc c sc sc c ωωωωωωMhzf s c 57954506.3=s c f πω2=In following fig. The positive polarity of Q is purple, the negative is green. The positive polarity of I isorange, the negative is cyan. Thus, Q is often called the "green-purple" or "purple-green" axisinformation and I is often called the "orange-cyan" or "cyan-orange" axis information.:Formation of CVBS video signalIt turns out that the human eye is more sensitive to spatial variations in the "orange-cyan" than it is for the "green purple". Thus, the "orange-cyan" or I signal has a maximum bandwidth of 1.5 MHz and the "green purple" only has a maximum bandwidth of 0.5 MHz. The Q and I signals are both modulated by a 3.58 MHz carrier wave. These two signals are then summed together to make the C or chrominance signal. this new chrominance signal (formed by I and Q) has the interesting property that the magnitude of the signal represents the color saturation, and the phase of the signal represents the hue.In the color decoder, the burst signal that carrys the phase reference information of chroma must besupplied with the chrominance signal. The burst is transmitted at “back porch”of each horizontal sync pulse during field scan.•Phase error of NTSC System the phase of the chrominance signal may be shifting according to the luminance level, if non-linear transmission characteristics happen to the system, it is so called …differential phase error ‟.As in the color decoder the reference sub-carrier is lock to the burst, a phase shifted chrominance signal after decoding will have phase error. Phase Shifted color difference signal resulting in a different color tone(hue ). In NTSC receiver a user controlled …TINT ‟ is required to correct hue manually.•Spectrum characteristics of NTSC signalChrominance signal of PAL may be expressed:()()()()()[]t t t C t ec tt V t t t U t e t Y R t K t Y B t e s c s c K s c c s c s c c θωωωωω+=Φ+=-Φ+-=sin co s )(sin )(co s ))((sin )()(s c f πω2=Mhzf sc 4296875.4=t3T H 2T H T H 0+1-1ΦK(t)•PAL chrominance signalIn PAL system, phase of the V signal is reversed by 180 degrees line by line. It is to reduce …differential phase error ‟ that causes distorted color tone.The V signal shifts its phase with 180 degree alternative line. Bandwidth for both U and V are approximately 1.3MHz.•Color Burst-Chrominance Sync The burst of sub-carrier is modulated on the back porch of each line-blanking period. It contains both the reference of frequency and phase information for usage in color decoder. In each burst, 10±1cycles of sub-carrier are transmitted.•Burst for NTSC system :()180)()(+=t Sin t K t e sc b ωThe burst in NTSC has fixed phase 180°.Formation of CVBS video signalNTSC linePAL lineNTSC line•Burst for PAL system :⎪⎭⎫ ⎝⎛Φ-︒+=4)(180)()(πωt K t Sin t K t e s c b The burst in PAL has phase shifting in alternative line. Magnitude of bust is 300mV .NTSC line: 135°,PAL line 225°.The difference of phase shifting is 90°line from line.•CVBS signal encoder for NTSC and PAL systemCVBS signal carries full information for a color television needs. It includes sync pulses and horizontal and vertical blanking information, luminance signaland chrominance signal(including burst and system information).•NTSC video encodersee Fig. Function diagram of NTSC encoderFormation of CVBS video signalColor system encoding•NTSC system encodingAB C D EF Gburst Hx2。