01_Funda of TV

1FUNDAMENTALS OF

MONOCHROME AND COLOUR TV SYSTEM

Picture formation

A picture can be considered to contain a number of small elementary areas of light or shade which are called PICTURE ELEMENTS. The elements thus contain the visual image of the scene.

In the case of a TV camera the scene is focused on the photosensitive surface of pick up device and a optical image is formed. The photoelectric properties of the pick up device convert the optical image to a electric charge image depending on the light and shade of the scene (picture elements). Now it is necessary to pick up this information and transmit it. For this purpose scanning is employed. Electron beam scans the charge image and produces optical image. The electron beam scans the image line by line and field by field to provide signal variations in a successive order.

The scanning is both in horizontal and vertical direction simultaneously.

The horizontal scanning frequency is 15,625 Hertz.

The vertical scanning frequency is 50 Hz.

The frame is divided in two fields. Odd lines are scanned first and then the even lines. The odd and even lines are interlaced. Since the frame is divided into 2 fields the flicker reduces. The field rate is 50 Hertz. The frame rate is 25 Hertz (Field rate is the same as power supply frequency).

Number of TV Lines per Frame

If the number of TV lines is high larger bandwidth of video and hence larger R.F. channel width is required. If we go for larger RF channel width the number of channels in the R.F. spectrum will be reduced. However, with more no. of TV lines on the screen the clarity of the picture i.e. resolution improves. With lesser number of TV lines per frame the clarity (quality) is poor.

Induction Course(TV)

A compromise between quality and conservation of r.f. spectrum led to the selection of 625 lines in CCIR system B. Odd number is preferred for ease of sync pulse generator (SPG) circuitary to enable interlace of fields.

Resolution

The scanning spot (beam) scans from left to right. The beam starts at the left hand edge of the screen and goes to right hand edge in a slightly slanty way as the beam is progressively pulled down due to vertical deflection of beam (as top to bottom scanning is to take place simultaneously). When the beam reach the right hand edge of the screen the direction of beam is reversed and goes at a faster rate to the left hand edge (below the line scanned). Once again the beam direction is reversed and scanning of next line starts. This goes on till the beam completes scanning 312 and half lines reaching the bottom of the screen. At this moment the beam flies back to top and starts scanning starting from half line to complete the next 312 and half lines of the frame.

To avoid distortions in the picture whenever the beam changes its direction, it is blanked out for a certain duration.

The horizontal blanking period is 12 microseconds. Since each line takes 64 micro seconds the active period of line is 64 -12 = 52 micro seconds.

(Since 625 lines are scanned at the rate of 25 Hz i.e. 25 cycles per second, the number of lines scanned in one second is 625 multiplied by 25 which yields 15,625. So the horizontal frequency is 15,625 hertz and hence each line takes 64 micro seconds).

Similarly there is vertical blanking period and 25 TV lines are blanked out during the period. So in one frame 50 TV lines are blanked out. Hence effective lines are 625 minus 50 i.e. 575.

The vertical resolution depends on the number of scanning lines and the resolution factor (also known as Kell factor). Assuming a reasonable value of Kell factor as 0.69. The vertical resolution is 575 multiplied by 0.69 which gives nearly 400 lines.

The capability of the system to resolve maximum number of picture elements along scanning lines determines the horizontal resolution. It means how many alternate black and white elements can be there in a line. Let us also take another factor. It is realistic to aim at equal vertical and horizontal resolution. We have seen earlier that the vertical resolution is limited by the number of active lines. We have already seen that the number of active lines are 575. so for getting the same resolution in both vertical and horizontal directions the number of alternate black and white elements on a line can be 575 multiplied by Kell factor and aspect ratio. Therefore, the number of alternate black and white dots on line can be 575 x 0.69 x 4/3 which is equal to 528.

STI(T) Publication 2 003/IC(TV)/2001

Fundamentals of Monochrome & Colour TV System

It means there are 528 divided by 2 cyclic changes i.e. 264 cycles. These 264 cycles are there during 52 micro seconds. Hence the highest frequency is 5 MHz.

Therefore the horizontal resolution of the system is 5 MHz.

A similar calculation for 525 lines system limits the highest frequency to 4 MHz and hence the horizontal resolution of same value.

In view of the above the horizontal bandwidth of signal in 625 lines system is 5 MHz.

Grey Scale

In black and white (monochrome) TV system all the colours appear as gray on a 10-step gray scale chart.

TV white corresponds to a reflectance of 60% and TV black 3 % giving rise to a Contrast Ratio of 20:1 (Film can handle more than 30:1 and eye’s capability is much more).

In black and white TV the concept of gray scale is adopted for costumes, scenery etc. If the foreground and back ground are identical in gray scale, they may merge and the separation may not be noticed clearly on the screen.

Brightness

Brightness reveals the average illumination of the reproduced image on the TV screen.

Brightness control in a TV set adjusts the voltage between grid and cathode of the picture tube (Bias voltage).

Contrast

Contrast is the relative difference between black and white parts of the reproduced picture.

In a TV set the contrast control adjusts the level of video signal fed to the picture tube.



Brightness and contrast controls are to be adjusted in a TV set to reproduce faithfully as many gray scale steps as possible. Ultimately the adjustment depends on individual viewing habit.

Viewing Distance

Optimum viewing distance from TV set is about 4 to 8 times the height of the TV screen. While viewing TV screen one has to ensure that no direct light falls on the TV screen.

Composite Video Signal (CVS)

Composite Video Signal is formed with Video, sync and blanking signals. The level is standardized to 1.0 V peak to peak (0.7 volts of Video and 0.3 volts of sync pulse). The composite video signal (CVS) has been shown in figure 1.

Fig. 1 Composite Video Signal (CVS)



Fig. 2 Separation of H and V sync pulses from CVS

Frequency Content of TV Signal

The TV signals have varying frequency content. The lowest frequency is zero. (when we are transmitting a white window in the entire active period of 52 micro seconds the frequency is Zero). In CCIR system B the highest frequency that can be transmitted is 5 MHz even though the TV signal can contain much higher frequency components. (In film the reproduction of frequencies is much higher than 5 MHz and hence clarity is superior to TV system.) long shots carry higher frequency components than mid close ups and close ups. Hence in TV productions long shots are kept to a minimum. In fact TV is a medium of close ups and mid close ups.

DC Component of video signal and DC restoration

A TV signal is a continuously varying amplitude signal as the picture elements give rise to varying level which depends on how much of incident light the picture elements can reflect and transmit the light signal to the TV camera. Hence the video signal has an average value i.e. a DC component corresponding to the average brightness of the scene to scene.

Let us examine the following wave forms of figure 3.



Fig. 3 Waveforms showing DC Restoration.

The blanking level is the reference black level and the signals are asymmetrical. While the DC component carries the information about scene brightness (mean value) the AC component carries information regarding scene detail. For correct tonal reproduction both the components should be present at the input of picture tube.

We have the average levels or DC levels vary in all the three cases. If such signals pass through video amplifiers (AC coupled amplifiers) the signals will have different zero axis (the DC level becomes the zero axis) and if such signals are fed to the picture tube of a TV set the reproduction of the picture is not faithful and the concept of maintaining the original scene brightness is lost. A method is to be devised to maintain black level as always the reference level. The reference level (or blanking level) is always the same irrespective of the average brightness of the scene. This method of restoring the DC level (with respect to black level) is called DC restoration. DC restoration circuits such as clamping circuits restores original wave form i.e. with the average value as it was in the signal scene. Clamping of signal can be done at back porch level which is reference black level. This is called back porch clamping. It means at the end of each TV line the level is always brought to the reference black level before the next line starts. In this way DC is restored from the signal which has passed through AC circuits. TV receivers as well as TV monitors employ the back porch clamping. Clamping at sync tip level is also possible.



Gamma Correction

The overall transfer characteristic of a television system relates brightness levels in the final displayed image to the brightness levels in the televised scene. Hence it is the combination of individual transfer characteristics of pickup device (camera tube or CCD), Video amplifiers, transmitter, receiver and finally the picture tube of the TV receiver.

Fig. 4 Gamma Correction

If Gamma is less than unity whites are compressed (crushed) and blacks are expanded (stretched). If Gamma is more than unity whites are stretched and blacks are crushed.

A Gamma of slightly more than unity is preferred to compensate for the loss of contrast in the system due to optical flare etc.

For example if the scene has a contrast of 10:1 and is transmitted through a system whose overall Gamma is 2, the displayed image will have a contrast ratio of 100:1 (10 raised to the power of 2 is 100). This is too much as there will be intolerable white stretching and black compression. A Gamma around 1.2 is tolerable.

Gamma of picture tube is around 2.8. Let us assume Gamma of pick up device is unity. We need to have overall Gamma say 1.2. Then the Gamma correction required can be calculated, as follow

Overall Gamma = Gamma of pick up device x Gamma of the corrector Circuit

Picture tube, video, transmitting and receiving equipment is assumed to have unity Gamma (Linear operation)



From the above figure we can now find out the Gamma Correction need.

1.2 = 1 x G x 2.8

G =

Gamma correction needed is 0.43 and can be achieved by video circuits preceding the transmitter input.

RF Transmission of Vision and Sound Signals

TV Transmission takes place in VHF Bands I and III and UHF Bands IV and V.

Picture is amplitude modulated and sound is frequency modulated on different carriers separated by 5.5 MHz.

Also for video amplitude modulation negative modulation is employed because of the following main advantages.

Pictures contain more information towards white than black and hence the average power is lower resulting in energy saving. (Bright picture points correspond to a low carrier amplitude and sync pulse to maximum carrier amplitude).

Interference such as car ignition interfering signals appear as black which is less objectionable.

Picture information is in linear portion of modulation characteristic and hence does not suffer compression. Any compression that may take place is confined to sync pulse only.

The design of AGC circuit for TV Receiver is simpler.

AM produces double side bands. The information is the same in both side bands. It is enough to transmit single side band only. Carrier also need not be transmitted in full and a pilot carrier can help. However, suppressing the carrier and one complete side band and transmitting a pilot carrier leads to costly TV sets. A compromise to save RF channel capacity is to resort to vestigial side band system in which one side band in full, carrier and a part of other side band are transmitted.



Fig. 5 Theoretical representation of the side bands in VSB transmission.

Fig. 6 IF, curve of TV set (Theoretical)

Fig. 7 Modulation of video signal



Sound Signal Transmission

In CCIR system B sound carrier is 5.5 MHz above the vision carrier and is frequency modulated. The maximum frequency deviation is 50 KHz.

Also the ratio of vision and sound carriers is 10:1 (20:1 is also employed in some countries)

If we assume maximum audio signal is 15 KHz the band width is 130 KHz.

According to Carson’s Rule the bandwidth is 2 x (Maximum frequency deviation + highest modulating frequency).

However, calculated value(using Bessel’s function) of Bandwidth is 150 KHz i.e. 75 KHz on either side of sound carrier.

In CCIR system picture IF is 38.9 MHz and sound. IF is 33.4 MHz.

At the receiver end it is necessary to ensure that signal frequencies in the region of the vestigial side band do not appear with double amplitude after detection. For this purpose the IF curve employs NYQUIIST slope as shown in the figure 8.



The Colour Television

It is possible to obtain any desired colour by mixing three primary colours i.e. Red, Blue and green in a suitable proportion.

The retina of human eye consists of very large number of light- sensitive cells. These are of two types, rods and cones. Rods are sensitive only to the intensity of the incident light and cones are responsible for normal colour vision.

The small range of frequencies to which the human eye is responsive is known as visible spectrum. This visible spectrum is from 780 mm (Red) to 380 mm(Violet). (Fig.9)

Fig. 9 Approximate relative sensitivity of the average human eye to different wave lengths

Additive Colour Mixing

The figure 10 shows the effect of projecting red, green, blue beams of light so that they overlap on screen.

Y= 0.3 Red + 0.59 Green + 0.11 Blue



Fig. 10 Additive Colour Mixing

The detail discussion on PAL colour Television System is discussed in Chapter # 2.


01_Funda of TV

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