Television Fundamentals and Transmitter -16Marks Unit-III Television Fundamentals and Transmitter -16Marks

Course Outcome Analyse the Composite Video signal used in TV signal Transmission

Aspect Ratio- The electron beam moves from left to right. When no picture the beam will trace white rectangle called Raster The width to height ratio of TV screen is called ASPECT RATIO. Aspect ratio= W/H=4/3 W(4) -------------------------------- H(3) D D= Viewing distance = 3 to 8 times height

Image Continuity Even though the picture information is transmitted line by line , We get impression of continuity due to a phenomenon called persistence of vision of human eye. persistence of vision is property of the retina of human eye that any impression produced on the retina will persist for 1/16th of a second. Definition- If within the short interval of persistence of vision , a series of images are presented to eye , eye can see all images without break is called Image continuity.

Pixels or Picture element

Resolution The scanning and reproduction of the finest details of the picture.

Vertical Resolution: The ability of a scanning to resolve vertical details in a scene depends upon the number of horizontal scanning lines used per frame. Horizontal Resolution: The ability of a scanning to resolve horizontal details in a scene .

Scanning Definition- Scanning is a process by means of which the optical image of the televised scene falling on the target plate of the camera is broken into series of horizontal lines by an electron beam shown in fig. e.g. reading information on page. scanning Sequential scanning Interlaced scanning

Scanning In the TV receiver the same process of scanning is repeated to form the picture on the florescent screen. Electron beam in the picture tube produced by horizontal and vertical scanning forms a uniformly lit rectangular area called the RASTER. ( i.e/ only lightened screen without picture is called raster) The scene is scanned rapidly both in horizontal and vertical directions simultaneously to provide sufficient number of complete picture or frames per second to give illusion of continuous motions.

Raster

Horizontal Scanning Definition- Electron beam moves from left to right line –by-line called horizontal scanning . Figure shows the trace and retrace.

Vertical scanning

Sequential Scanning Scanning is done line-by line from 1st to 625th line sequentially so called as sequential scanning. As electron beam sweeps across a line it falls over portions of different light intensities and is accordingly converted into electrical currents of different amplitudes. (Bright spot max.current, Dark spot min.current) In this way, current pulses are produced which corresponds in time sequence to bright and dark areas of the televised picture as they scanned by the electron beam. This electrical signal is called video signal.

Interlaced scanning In television pictures an effective rate of 50 vertical scans per second is utilized to reduce the flicker. This is accomplished by increasing the downward rate of travel of scanning of electron beam, so that every alternate line gets scanned instead of successive line. Then when the beam reaches the bottom of the picture frame it quickly returns to the top to scan those lines that were missed in the previous scanning.

Interlaced scanning Definition Interlaced scanning is the process in which the first 312.5 lines are scanned called odd field and then 312.5 even numbered lines are scanned called even field so that frame containing 625 lines is scanned twice.

Interlaced scanning

Advantages and Applications of Interlaced scanning- It reduces the video bandwidth because the total number of lines scanned /second remains unchanged. It also avoids the problem of flicker since scanning rate is doubled. i.e. 50 frames/ sec. Applications It is used in broadcast TV system.

Horizontal scanning Period Time required to complete one horizontal line with trace and retrace is called horizontal scanning period. Vertical Scanning period Time required to complete one vertical trace and retrace is called vertical scanning period.

Scanning period

Resolution Definition – The scanning and reproduction of finest details of the picture known as resolution.

Vertical Resolution Definition The ability of the scanning system to resolve picture details in vertical direction is called vertical resolution. Vr= Na x K Kell factor- It is the number of horizontal lines lost due to misalignment of electron beam. 40 lines lost as blank lines. Na= 625-40=585( Na=Active lines) Vr= NaxK=585x0.69=400lines.

Horizontal Resolution Definition The ability of the scanning system to resolve the picture details in horizontal direction is known as horizontal resolution. Alternate black and white segments(N) that get scanned in one horizontal line are- N=Nax Aspect ratio x k = 585x 4/3 x 0.69 N= 538

Vestigial Sideband Transmission AM is used for transmitting the video signal. AM has carrier frequency with two sidebands.

What is VSB transmission? In AM, only USB and part of LSB is transmitted, is called VSB transmission.

VSB Spectrum

Colour Characteristics Brightness- Brightness is the overall or average intensity of illumination and it determines background light level in the produced picture.

2. Contrast This is the difference in light intensity between black and white parts of the picture over average brightness . With contrast Contrast not adjusted

3.Viewing Distance The distance between viewer and TV screen is known as Viewing distance.

Luminance This is the amount of light intensity perceived by the eye regardless of colour.

Hue or Tint The colour itself is called hue and depends on dominant wavelength of the light. All colours e.g. red ,yellow, green are hues.

Saturation It represents the purity of colour. A fully saturated colour will have no white light present in it. E.g. pure red, green, blue.

Compatibility It is a phenomenon by which a colour TV system produces a normal B/W picture on B/W TV receiver without any modification in existing circuitry. Similarly, a B/W transmission should be able to produce a black and white picture on a colour TV screen. This is known as reverse compatibility.

Compatibility For complete compatibility between the colour TV system and the monochrome system, colour TV system must satisfy the following conditions. It must use the same 7MHz standard TV channel. It must have video BW of 5.5MHz. It must use the same line and frame synchronizing pulses. The picture and sound carrier frequencies should have same spacing of 5.5MHz.

Colour Theory Light ray are electromagnetic waves whose properties are governed by their frequency or wavelength. Light rays from a small part of spectrum of EM shown in fig.

Three colour Theory All light sensations to the eye are divided into three main groups. The optic nerve system separates different colour to perceive the actual colour of the object. This forms the basis of colour TV. For example- A yellow colour can be sense by eye when the red and green groups of the cones are excited at the same time.

Mixing of Colours Two types of colour mixing- Subtractive colour mixing Additive colour mixing.

Subtractive colour mixing In subtractive colour mixing, reflective properties of pigments are used, which absorb all wavelength but for their characteristic colour wavelength. For example- This type of mixing takes place in painting shop and colour printing

Subtractive colour mixing Primary colours are called subtractive primaries. Any of these colours can be produced by subtracting White light its coplementary colour through a colour filter as- = - = - CYA = - + + = WHITE BLUE YELLOW WHITE GREEN MAGENTA WHITE RED CYAN CYAN MAGENTA YELLOW BLACK

Additive Colour Mixing Additive mixture of the three primary colours red , green and blue with proper proportions can create any colour. Hence they are called additive primaries And used as basic colours in TV system. Primary colours GREEN BLUE RED

Complementary or Secondary colours + + = RED GREEN GREEN BLUE WHITE

Additive Colour Mixing

Additive Colour Mixing

Comparison

Grassman's Law The eye is not able to distinguish each of the colours that are mixed to form colour but instead perceives only resultant colour. For example- Yellow can produced by mixing 305 of red and 59% of green. Eye perceives new colour depending on the algebraic sum of red, green and blue light fluxes.This forms the basis of colour signal generation known as Grassman's Law.

Composition Video Signal The Video Signal containing the horizontal and vertical sync and blanking pulses is called as composite video signal.

Colour Burst Signal(Need) The subcarrier is suppressed in the modulated signal (at transmitter), it is necessary to generate it in the receiver for demodulation of the colour signal . This generated signal must be of exactly same frequency and same phase as that of the transmitter. To ensure this , short wave of 8 to 10 cycles called colour burst is sent to the receiver along with the sync. Pulse. The colour burst is placed onto the back porch of the blanking pulse of the composite video signal.

Colour burst signal (Need)---- the burst signal acts as pilot carrier. Burst signal in conjunction with the phase comparator circuit in the receiver is used to lock the local sub-carrier oscillator to frequency as well as the phase of the colour sub-carrier at the transmitter.

5.8µs 5.8µs

TV channel Allocation for Band I and Band III The carrier frequency should be chosen ten times of highest modulating frequency to get better selectivity At RF and IF tuned amplifier in the receiver. Highest modulating frequency for picture signal is 5MHz.Hence carrier frequency is always greater than 40MHz.TV transmission is generally in VHF and UHF bands. VHF Band= 30 MHz to 300MHz UHF Band= 300MHz to 3 GHz

Continued--- Lower band VHF channels(BAND I) Band I has three channels 2,3,4 from 47 to 68 GHz. VHF Band-I(47 to68MHz)Channel width=7MHz Channel No. Frequency band(MHz) 2 47 to 54 3 54 to 61 4 61 to 68

Continued--- Higher VHF Channels(Band III) Band III has 5 to 12 channels from 174 to 230MHz VHF Band-III(174 to 230 MHz)Channel width=7MHz Channel No. Frequency band(MHz) 5 174 to 181 6 181 to 188 7 188 to 195 8 195 to 202 9 202 to 209 10 209 to 216 11 216 to 223 12 223 to 230

Continued--- Band II- It is used for FM radio and for sound transmission in TV. Range-88 MHz to 108MHz

TV systems and standards CCIR ( Consultative committee for International Radio) made unique standard which were adopted all over TV transmission with 625- B CCIR standard.

CCIR-B common standards for Colour TV Parameter Standard Number of lines per picture 625 Interlace ratio 2:1 Frame frequency 25Hz Field frequency 50Hz Aspect Ratio 4:3 Line Frequency 15625Hz Channel bandwidth 7MHz Video bandwidth 5.5MHz Type of Sound modulation FM Pre emphasis 50µs Resolution 400lines Duration of horizontal line 64µs Line blanking interval 12µs Back Porch 5.8µs Front porch 1.5µs Synchronous pulse width 4.7µs Equalizing pulse width 2.3µs

CCIR-B Colour signal Transmission Standards Parameter Standard Camera output R,G and B video signal Luminance signal Y=0.30R +0.59 G +0.11 B Colour difference signal for transmission (B-Y) and (R-Y) Colour difference signal U = 0.493 (B-Y) V = 0.877 (R-Y) Colour subcarrier frequency 4.433185MHz Duration of Burst 10+ - 1 Croma encoding Phase and amplitude modulation Bandwidth for colour signal (U and V) Fsc-1.3MHz to Fsc+0.6MHz

Color TV Transmitter