The camera tube has: Has a mosaic screen, onto which the scene is focused through the lens system of the television camera. An electron gun forms a beam which is accelerated toward this photoelectric screen.
B ecause certain amplitude levels in the composite video signal must correspond to specific percentage modulation values this amplifier uses clamping to establish the precise values of various levels of the signal which it receives.
The beam in the camera or picture tube moves at a constant velocity across the screen and, when it reaches the end of the screen on the right-hand side , it “whips back” to the left hand edge of the screen and starts again.
525 lines per frame 30 frames per second 15,750 lines scanned in 1 second It takes 1/30 seconds to scan the entire picture frame
63.5 μ s -total time taken from the beginning of one line to the instant when the next line begins to be scanned. -this also includes the time of retrace. Retrace-rapid return from right to left -10.2 μ s or 16% of the time allocated to scanning one line. Retrace time= 0.16H Active time=0.84H H=63.5 μ s
REQUIREMENTS AND STANDARDSBLACK AND WHITE TRANSMISSIONBLACK AND WHITE RECEPRION
The TV set The TV set is the central element of any audio/visualsetup. It allows us to watch programs received by antenna,cable or satellite and movies via our VCR. We can also usethe TV as a monitor to play games on game computers andCD-I players. The development of television system hasmade a revolutionary change in the field of electronics.Television has become the most popular, powerful, mostmedia for communication and entertainment. Tele-Visionmeans “to see at a distance”. The word television is derivedfrom a combination of two words “Tele” – a Greek worddenoting “far” a “vision” is taken from the Latin word “see”.
Development of Color Television Several system of color television have beendeveloped in the fist color system approved by theFederal Communications Commission (FCC), a motor-driven disc with segments in three primary colors –red, blue, and green rotated behind the cameralens, filtering the light from the subject so thatthe colors could post through in succession. Thereceiving unit of this system formed monochrome(black and white) images through the usual cathode– ray tube, but a color white, identical with thataffixed to the common and synchronized with it,transformed the images book to their original
This method is said to be “field –sequential” because the monochrome image is“painted” first in one color, then another, andfinally in the third, in rapid enough succession sothat the individual colors are blended by theretentive capacities of the eye, giving theviewer the impression of a full colored image.This system, developed by the ColumbiaBroadcasting System (CBS), was established bin1950as standard for the United States by theFCC. However it was not “compatible” i.e. fromthe same signal a good picture could not beobtained on standard black and white sets, so itfound scant public acceptance.
Another system, a simultaneous compatiblesystem, was developed by the Radio Corporation ofAmerica (RCA). In 1953 the FCC reversed its 1950ruling and revised the standard for acceptablecolor television system. The RCA system meet thenew standards (the CBS system did not) and waswell received by the public. This system based on an“element – sequential” system. Light from thesubject is broken up into its three colorcomponents, which are simultaneously scanned bythree pick ups. However, the signals correspondingto the red, green, and blue portions of the scannedelements are combined electronically so that therequired 4.1.MHz bandwidth can be used.
In the receiver the three color signals areseparated for display. The elements, or dots, onthe picture tube screen are each subdivided intoareas of red, green and blue phosphor. Beams fromthe three electron guns, modulated by the threecolor signals, scan the elements together in such away that the beam from the gun using a given colorsignal strikes the phosphor of the same color.Provision is made electronically for forming propergray tones in black – and white receivers. The FCCallowed stereo audio for television in 1984.
A television system may be required to produce:1. The shape of each object, or structuralcontent2.The relative brightness of each object, or tonalcontent3.Motion, or kinematic content4.Sound5.Color, or chromatic content6.Perspective, or stereoscopic content
5 different Television System:1.Federal Communication Commission (FCC)system for monochrome.2. NTSC ( National Television StandardCommittee)3.System for color (American Standard)4.CCIR (comite Consultatif International deRadio)System for monochrome5.PAL (Phase Alternation by Lire) system forcolour.6.SECAM (Sequential Technique and MemoryStorage) system for colour.
Standard American system European SystemNumber of lines per frame 525 625Number of frames per second 30 25Field frequency, HZ 60 50Line frequency, Hz 15750 15625Channel width, MHz 6 7Video bandwidth, MHz 4.2 5Color subcarrier 3.58* 4.43*Sound system FM FMMaximum sound deviation, kHz 25 50Intercarrier frequency, MHz 4.5 5.5
Apart from the difference, the two majorsystems have the following standards in common.1.Vestigial sideband amplitude modulation forvideo, with most of the lower sideband removed.This is done to save bandwidth.2.Negative video modulation polarity. In bothsystems, black corresponds to a highermodulation percentage than white.
3.2:1 interlace ratio. This is can be seen from4.4:3 aspect ratio. This is the ratio of the
Crystal RF Power Combining oscillator amplifier amplifier networkCamera tube Video AM Sound amplifier modulating transmitter amplifier microphone Scanning and FM Audio synchronizing modulating amplifier circuits amplifier Basic monochrome television (transmitter)
Picture tube Common Video Video tuner IF amplifiers detector amplifierSound IF Sound Audio Scanningamplifiers demodulator amplifiers and synchronizin loudspeaker g circuits Basic monochrome television (receiver)
• The camera tube has a mosaic screen, onto which the scene is focused through the lens system of the television camera.• It has an electron gun which forms a beam which is accelerated toward this photoelectric screen.• The beam scans the screen, from left to right and top to bottom, covering the entire screen 30 times per second.
Electronic Scan (Camera Pickup Tube)Object Lens Target Electron Beam Video Signal
Video stages• The output of the camera is fed to a video switcher which may also receive videotape or outside broadcast video at other inputs.• The function of the switching system is to provide the many video controls required.• It is at this point that mixing or switching of the various inputs, such as fading in of one signal and fading out of another , will takes place.
• The output of this mixing and switching amplifiers goes to more amplifiers, whose function it is to raise the signal level until it is sufficient for modulation.• Vertical and horizontal blanking and synchronizing pulses are required by receivers to control their scanning processes.
• The final video amplifier is the power amplifier which grid modulates the output RF amplifier.• Because certain amplitude levels in the composite video signal must correspond to specific percentage modulation this amplifier uses clamping to establish the precise values of various levels of the signal which it receives.
RF and sound circuitry• The sound transmitter is a frequency modulated transmitter the only difference is that maximum deviation is limited to 25kHz instead of the 75 kHz limit for FM broadcast transmitter.• The RF aspects of the video transmitter must be broadband , in view of the large bandwidth of the transmitted video modulated signals.
• The output stage is followed by a vestigial sideband filter .• The output of the sound and picture transmitter is fed to the antenna via a combining network.• Although both the picture and sound transmitters are connected to the antenna with a minimum of loss, neither is connected to the other.
• The beam in the camera or picture tube moves at a constant velocity across the screen and, when it reaches the end of the screen on the right-hand side , it “whips back” to the left hand edge of the screen and starts again.• Meanwhile, it has descended down the screen so that the next line traced out is somewhat below the first one.• The process continues until the bottom of the screen is reached and “whips back” to the top of the screen.
HORIZONTAL SCANNING• The total time taken from the beginning of one line to the instant when the next line begins to be scanned is 63.5μs.• This time includes not only the scan of the picture but also the rapid return, or retrace, from right to left.• A period of 10.2μs is allocated to the retrace which is 16% of the time allocated to scanning one line. That is, the retrace time is 0.16H, and the active time is 0.84H, where H=63.5μs.
• Blanking is a process in which it reduces the scanning bean current to zero, from just before the beginning of the retrace until just after its end. This consists in adding a pulse to the video waveform, at the right time and for the correct period, to ensure that the signal level has been raise to that corresponding to black.
Horizontal scanning:1. A s t h e a c t i v e b e a m
VERTICAL SCANNING• Basically, vertical scanning is similar to horizontal scanning except for the obvious difference in the direction of movement and the fact that everything happens much more slow which is 60 times per second.• The field rate of 60 Hz is the vertical scanning frequency.• This is the rate at which the electron beam completes its cycles of vertical motion, from top to bottom and back to the top again.• Therefore the time of one field is 1/60 seconds.
• The picture repetition rate is 30 frames per second.• A frame consist of two fields: first (odd) field and second (even) field.• Every field contains 262 ½ lines or 525 lines in a frame.• Therefore the number of lines per second is: 262 ½ lines ÷ 1/60 seconds = 15750 lines /second Or (30 frames/second)(525 lines/frames) = 15750 lines /second
Interlaced scanning:• Here all odd line are scanned first from top to bottom, and the even lines are skipped.• After this vertical scanning cycle a rapid vertical retrace causes the electron scanning beam to return to the top of the frame.• Then all even lines that were omitted in the first scanning are scanned from top to bottom.
1. Starting at the upper left corner of the frame at point A. for this line 1, the beam sweeps across the frame with uniform velocity to cover all the picture elements in one horizontal line. At the end of this trace, the beam retraces rapidly to the left side of the frame, to begin scanning the next horizontal line.2. After line a is scanned the beam is at the left side , ready to scan line 3, omitting the second line. The electron beam scans all the odd lines, then, finally reaching a position, such as point B, at the bottom of the frame.3. At time B the vertical retrace begins. Then the beam returns to the top of the frame to begin the second, or even field. The beam moves from point B up to C, traversing a whole number of horizontal lines.
4. Horizontal scanning of the second field begins with the beam at point C. after scanning a half-line from point C, the beam scans line 2 in the second field. Then the beam scans between the odd lines----it scans the even lines that were omitted during the scanning of the first field. All the even lines of the second field are scanned down to point D.5. The vertical retrace in the second field starts at point B. from here, vertical flyback causes the beam to return to the top. The beam finishes the second vertical retrace at A where the first trace started because the number of vertical retrace lines id the same in both fields.6. And so when it reaches point a 2 fields were completed and is ready to scan the third field.
•Three parts of a composite videosignal:1. the camera signal corresponding tothe variations of light in he scene2.The synchronizing pulses, or sync, tosynchronize the scanning.3.The blanking pulses to make theretraces invincible.
Blanking •The composite video signal contained blankingpulses to make the retrace lines invincible bychanging the signal amplitude to black when thescanning circuits produce retraces. •All picture information is cut off duringblanking time. Normally the retraces areproduced within the time of blanking. •When the picture is blanked out, before thevertical or horizontal retrace, a pulse of suitableamplitude and duration is added to the videovoltage , at the correct instant of time.
Horizontal Blanking• Front porch is a part of the blanking signal which is located before the sync pulse which is 0.02H wide or 1.27 μs.• Back porch is a part of the blanking signal which is located after the sync pulse which is 0.06H wide or 3.81 μs.• After the front porch of blanking , horizontal retrace can begin when the sync pulse starts, the flyback (retrace) is definitely blanked out because the sync level is blacker than black.• After the retrace to the left edge blanking is still in effect , as a result, the first part of trace at the left is blanked. After the blanked traced time at the left edge, the blanking pulse is removed.• The effect of the horizontal blanking is illustrated by the black bars at the left and right sides of the picture.
Vertical Blanking• The vertical blanking pulses change the video signal amplitude to black , so that the scanning beam is blanked out during vertical retraces.• The width of the vertical blanking pulses is 0.05V to 0.08V. If we take 8 percent as maximum and since V is 1/60 seconds. The vertical blanking time is: (0.08)(1/60 seconds) = 1333 μs• When we divide the vertical blanking time by the total line period: 1333 μs ÷ 63.5 μs = 21 linesTherefore, 21 lines are blanked out in each field and 42 lines are blanked out in one frame.
Synchronizing pulses in Vertical Blanking• The sync pulses inserted in the composite video signal during the wide vertical blanking pulse includes equalizing pulses, vertical sync pulses and some horizontal sync pulses
• The last-four horizontal scanning lines at the bottom of the raster are shown with the required horizontal blanking and sync pulses.• Immediately following the last visible line the video signal is brought to black by the vertical blanking pulse in preparation for vertical retrace.• The vertical blanking period begins with a group of six equalizing pulse, spaced at half-line intervals.• Next is the serrated vertical sync pulse that actually produces vertically flyback in the scanning circuits. The serrations also occur half-line intervals. Therefore, the complete vertical sync pulse is three line wide.
• Following the vertical sync is another group of six equalizing pulses and a train of horizontal pulses.• During the entire vertical blanking period, no picture information is produced, because the signal is blacker than black so that vertical retrace can be blanked out.
• The serrated vertical sync pulse forces the vertical deflection circuits to start the flyback. However, the flyback generally does not begin with the start of vertical sync because the sync pulse must build up charge in a capacitor to trigger the scanning.• If we assume that a vertical flyback starts with the leading edge of the third serration, then the time of one line passes during vertical sync before vertical flyback starts. Also, six equalizing pulses equal to three lines occur before vertical sync. Then 3 + 1 = 4 lines are blanked at the bottom of the picture, just before vertical retrace starts.
• As the scanning beam retraces from bottom to top of the raster, five complete horizontal lines are produced. This vertical retrace can be completed easily within the vertical blanking time. With 4 lines blanked at the bottom before flyback and 5 lines blanked during flyback, 12 lines remain of the total 21 during vertical blanking. These 12 lines are at the top of the raster at the start of the vertical trace downward. Therefore in the total of 2 fields, 8 lines are blanked at the bottom and 24 lines at the top.
The scanning lines that are producedduring vertical trace, but made black byvertical blanking, form black bars at the topand bottom of the picture. The height ofthe picture is slightly reduced with blanking.
The television bandwidth is 6 MHz. The sub-carrier for thecolor is 3.58 MHz off the carrier for the monochromeinformation. The sound carrier is 4.5 MHz off the carrier forthe monochrome information. There is a gap of 1.25 MHz onthe low end, and 0.25 MHz on the high end to avoid cross-talkwith other channels. Television has a maximum frequencybandwidth of 6 MHz. This says that the highest resolutionsignal is something like 1/6MHz or 166.7 nS. This isconsistent with a 330 element scan line with a 8.7 uSblanking time.
The lower visual sideband extends only 1.25 MHzbelow its carrier with the remainder filtered out, but theupper sideband is transmitted in full. The audio carrier is 4.5MHz above the picture carrier with FM sidebands ascreated by its ±25-kHz deviation. The lower sideband is mostly removed by filters thatoccur near the transmitter output. While only one sidebandis necessary, it would be impossible to filter out the entirelower sideband without affecting the amplitude and phase ofthe lower frequencies of the upper sideband and the carrier.Thus, part of the 6 MHz bandwidth is occupied by a“vestige” of the lower sideband (about 0.75 MHz out of 4MHz). It is therefore commonly referred to as vestigial-sideband operation. It offers the added advantage thatcarrier reinsertion at the receiver is not necessary as in SSBsince the carrier is not attenuated in vestigial-sidebandsystem.
Picture tube Deflection V coils4.5-MHz 4.5-MHz AF andsound sound IF Sound (FM) powertakeoff takeoff detector amplifiers loudspeakerVHF and Picture Video Video VideoUHF tuners (common) IF output detector amplifier amplifiers amplifier AGC stageSync Vertical Vertical Verticalseparator sync deflection output sync separator oscillator amplifier V Damper AGC diodeHorizontal Horizontal Horizontal Horizontal syncsync deflection output High-voltage AFC circuit Hseparator oscillator amplifier power supply AGC H+V sync antennas
UHF antenna 45.75 and UHF RF tuned VHF RF tuned circuits 700.75 MHz circuits 175.25MHz (45.75MHz) (609.25 MHz) VHF RF amplifier UHF diode mixer degna GGanged VHF mixer tuned circuits 175.25 MHz (45.75MHz) UHF L.O tuned 45.75 and circuits 266.75 MHz (655MHz) (45.75 MHz) VHF mixer UHF local Picyure 1st IF oscillator (L.O) VHF L.O. tuned circuits amplifier 45.75 221MHz MHz VHF local oscillator (L.O.) IF out to 1st picture IF amplifier, 45.75 MHz
Fundamentals TV receivers use the superheterodyneprinciple. In addition, however, there isextensive pulse circuitry to ensure that thedemodulated video is displayed correctly. Tothat extent the TV receiver is quite similar to aradar receiver, but radar scan is generallysimpler, nor are sound and color normallyrequired for radar. It is also worth making thegeneral comment that TV receivers of currentmanufacture are likely to be either solid-stateor hybrid.
TUNERVHF tuner•Must cover the frequency range from 54 to216MHz Band.•Antenna most frequently used for reception is theYagi-uda.•Often used a turret principle
UHF tuner•Must cover the frequency range from 470 to 890 MHz Band.•The antenna used is quiet likely to be a log-periodic, with theone antenna covering the whole band.•Active stages are a diode mixer and a bipolar or FET localoscillator.•Used coaxial transmission lines instead of coils, and they aretuned by means of variable capacitors.•Alternative means of UHF tuning consists of having varactordiodes to which fixed DC increments are applied to changecapacitance, instead of variable capacitors. One of theadvantages of this arrangement is that it facilitates remote-control channel changing.
Three things happen when the VHF tuner is set tothe UHF tuner position:•The UHF local oscillator is enabled•The VHF local oscillator is disabled•The VHF tuner RF and mixer tuned circuits areswitched to 45.75MHz
Picture IF amplifiers The picture IF amplifiers are almost invariably double-tuned, because of the high percentage bandwidth required. As inother receivers, the IF amplifiers provide the majority of thesensivity and gain before demodulation, consequently, three orfour stages of amplification are normally used. The IF stagesprovide amplification for the luminance, chrominance and soundinformation. IF a TV receiver is misaligned or purposely mistuned, thesound carrier may correspond to a point higher on the IF responsecurve. If this happens, the extra gain at this frequency willcounteract the subsequent 4.5MHz filtering, and some of thesound signal will appear in the output of the video amplifiers. Thiswill result in the appearance of distracting horizontal sound barsacross the picture, moving in tune width sound frequency changes.
Video stages It will be seen that the last picture IF amplifieris followed by the video detector and two videoamplifiers, whose output drives the picture tube, atvarious points in this sequence , signals are taken offfor sound IF, AGC and SYNC separation.Two functions of transformer in the emitter of thefirst video amplifier:1.To provide the sound IF takeoff point2.The sound IF transformer thus acts as a trap, toattenuate 4.5MHz signals in the video output.Preventing the appearance of the sound bars.
The video amplifiers of the TV receiver havean overall frequency response. The second stagedrives the picture tube, adjusting the instantaneousvoltage between its cathode and grid in proportion tothe video voltage. This modulates the beam currentand results in the correct degree of whitenessappearing at the correct point of the screen, whichin turn is determined by the deflection circuits. Theblanking pulses of the composite video signal drivethe picture tube beyond cutoff, correctly blankingout the retraces. Although the sync pulses are stillpresent, their only effect is to drive the picturetube even further beyond cutoff. This is quiteharness, so that the removal of the sync pulses fromthe composite video signal is not warranted.
The contrast and brightness controls arelocated in the circuitry of the output video amplifier.The contrast control is in fact the direct videoequivalent of the volume control in a radio receiver.When contrast is varied, the size of the outputvoltage is adjusted, either directly or through avariation in the gain of the video output sage. Thebrightness control varies the grid- cathode DC biason the picture tube, compensating for the averageroom brightness.
The sound section The sound section of a television receiver isidentical to an FM receiver. Note that the ratiodetector is used for demodulation far more oftenthan not. Note further that the intercarriersystem for obtaining the FM sound information isalways used, although it is slightly modified incolor receivers.
SYNCHRONIZING CIRCUITS The task of the synchronizing circuits in atelevision received information, in such a way asto ensure that the vertical and horizontaloscillators in the receiver work at the correctfrequencies.Three specific functions of this task:1.Extraction of sync information from itscomposite waveform2.Provision of vertical sync pulses3.Provision of horizontal sync pulse
Sync separation The “clipper” portion of the circuit shows the normalmethod of removing the sync information from thecomposite waveform received. The clipper uses leak-typebias and a low drain supply voltage to perform a functionthat is rather similar to amplitude limiting. It is seen from the waveforms that video voltagehas been applied to an amplifier biased beyond cutoff sothat only the tips of the sync pulses cause output currentto flow. It would not be practicable to use fixed bias forthe sync clipper, because of possible signal voltage variationat the clipper input. If this happened, the fixed bias couldalternate between being too high to pass any sync, or so lowthat blanking and even video voltages would be present inthe output for strong signals.
Horizontal sync separation The output of the sync clipper is split, aportion of it going to the combination of C3and R2.A positive pulse is obtained for each sync pulseleading edge, and a negative pulse for each trailingedge. When the input sync waveform has constantamplitude, no output results from thedifferentiating circuit. The time constant of thedifferentiating circuit is chosen to ensure that, bythe time a trailing edge arrives, the pulse due tothe leading edge has just about decayed.
Vertical Sync separation The coupling capacitor Cc is taken to acircuit consisting of C1, R1, and C2, whichshould be recognized as a standardintegrating circuit. Its time constant ismade long compared with the duration ofhorizontal pulses but not with respect tothe width of the vertical sync pulse.
ecessary to drive the electron beam horizontally across the face of the tube 15,750 times a second and at the same time, move down the screen relatively slowly Vertical-Deflection Circuits To develop a TV picture on the screen it is necessary to drive the electron beam horizontally across the face of the tube 15,750 times a second and at the same time, move down the screen relatively slowly and then back up rapidly 60 times a second.
The 2-MΩ rheostat in the triode grip circuit withthe 0.003-mμF capacitor determines the frequency ofoscillation and enables the oscillator to be synchronizedby the received vertical sync pulses to produce thecorrect vertical sweep frequency. This is called THEVERTICAL HOLD CONTROL. For vertical centering of the picture on thescreen, a DC bias can be introduced into the yokecoils by using a center.-tapped potentiometer.Centering can also be accomplished mechanicallyby positioning of the yoke coils.
Transistorized vertical-sweep circuitsmay use a multivibrator or a blocking-typeoscillator that develops narrow pulses at thesweep frequency. The pulses are amplifiedby a driver stage, and the required sawtooth waveform is develop by an RC circuitat the input of the output amplifier stag. AnIC may be used for all this circuitry exceptfor the larger capacitors and the vertical-linearity hold and height- controlpotentiometers.
ecessary to drive the electron beam horizontally across the face of the tube 15,750 times a second and at the same time, move down the screen relatively slowly Horizontal-Deflection Circuits The horizontal deflection section of a TV receiver has many functions. It generates a 15,750 Hz saw tooth current that sweeps the beam across the screen.
The same AC that accomplishes the horizontalsweep is also stepped up and rectified to produce therequired high aquadag voltages. A small portion of thehorizontal fly back voltage is used to key automaticgain control (AGC) or automatic frequency control(ATC) circuits into operation. The multivibrator could be synchronized byfeeding a positive or negative pulse (from the syncseparator) to one of its grid circuits. However anyreceived static or noise pulses could upset the propersynchronization and result in the tearing cut ofportions of the picture.