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Sunday, 25 July 2021

Scanning Principles of Television

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Scanning is a technique for converting the charge images created within a television camera tube into a variable electrical signal. The TV image is scanned in a sequence of horizontal lines that are stacked one on top of the other. It's the same as reading a text page and covering all of the words in one line and all of the lines on the page. All of the picture elements are scanned one line at a time, from left to right and top to bottom.

The scene is scanned in both the horizontal and vertical directions at the same time to generate enough full images or frames per second to maintain continuous motion. Most television systems have a frame repetition rate of 25 frames per second. The following is the order in which all of the picture elements should be scanned. They are,

1. An electron beam passes over a horizontal line, transforming all of the image components in that line. Trace is the name given to this period. Horizontal scanning is used to accomplish this.

2. The beam immediately returns to the left side after each line to begin scanning the following horizontal line. Retrace or fly back are the terms used to describe the return lines. Because both the camera tube and the image tube are blanked out during retrace, no picture information is scanned. As a result, the retraces must be very quick, as they lose time in terms of image information.

3. When the beam returns to the left side, the vertical position of the beam is lowered so that it scans the next line down instead of repeating the same line. The vertical scanning motion of the beam achieves this.

Types of scanning:

a. Horizontal Scanning

b. Vertical Scanning

c. Flicker

d. Interlaced Scanning


The electron beam is deflected across the screen with a continuous, uniform motion for the trace from left to right due to the linear increase of current delivered to the horizontal deflection coils. The sawtooth wave direction swiftly reduces to its starting value at the top of the ascent. The retrace or fly back is the result of these quick reversals. At the left border of the raster, the horizontal trace begins. The finish is on the right edge, where the flyback causes a retrace to the left.

Figure 1:   Waveform of Horizontal Deflection Coils

Figure 1 shows the waveform of the horizontal deflection coil. The horizontal displacement to the right corresponds to ‘up' on the sawtooth wave. The usable scanning time is indicated by the dark line, while the retrace time is indicated by the dashed lines. To contain a higher number of image elements and hence more information, the number of scanning lines for a single full picture should be large. The horizontal scanning frequency is 15,625 Hz in the 625 line system.


While the electron beam is being deflected horizontally, the sawtooth current provided to the vertical deflection coils moves the electron beam at a consistent pace from top to bottom of the raster. As a result, when traveling from top to bottom, the beam creates one below the other. In Figure 2, we can see the vertical scanning waveforms.

Figure 2 Vertical Deflection Waveform

The vertical scanning beam is deflected to the bottom of the raster by the trace component of the sawtooth wave, as illustrated in Fig. After that, the beam is quickly vertically retraced back to the top. At the bottom of the raster, the vertical sweep current achieves its maximum amplitude, delivering the beam to the raster's bottom.

The horizontal scanning is ongoing during vertical retrace, and numerous lines are scanned during this time. The information is transformed into an electrical signal using the scanning beam. The vertical scanning frequency is 25Hz, that is every second 25 frames are scanned.

The scanning beams at the camera tube and picture tube are blanked and no picture information is picked up or reproduced during the horizontal and vertical retrace intervals.


The television picture's scanning rate of 25 frames per second is insufficient to allow the brightness of one picture or frame to merge seamlessly into the next. The screen is blanked in between each frame as a result of this effect. As the screen alternates between bright and dark, the effect is a distinct flicker of light. All the lines in the frame are scanned in a progressive sequence from top to bottom when progressive scanning is used. The Flicker effect is created because there are only 25 blank-outs each second. 50 blank-outs per the second result from scanning 50 full frames each second. Human eyes can no longer perceive this rapid fluctuation, therefore the flicker effect is no longer visible. The video frequencies associated with the image elements are multiplied in a line as a result of this effect. Interlaced Scanning is the approach used to tackle this sort of problem.


Flicker is reduced in these television pictures by using 50 vertical scans per second effective rate. This is accomplished by increasing the scanning electron beam's downward velocity of motion, causing each alternative line to be scanned instead of the next line. When the beam reaches the bottom of the picture frame, it quickly returns to the top to scan the lines missed during the previous scan. As a result, the total number of lines is divided into two groups known as "Fields." Each field can also be scanned separately. This scanning method is referred to as 'Interlaced Scanning.' Interlaced scanning is described in Figure 3 for the 625 line system.

Figure 3 Interlaced Scanning

Each Frame's total lines are split into two fields. They are:

a. Odd field and

b. Even field

Odd lines in the frames are contained in the first and subsequent odd fields. The even scanning lines appear in the second and all even fields. The frame repetition rate is 50 per second, with two fields each frame and 25 full frames scanned per second. In reality, increasing the vertical scanning frequency from 25 to 50 Hz scans every other line in the picture. There are 312.5 lines per field.

The beam originates at A and sweeps across the frame at a constant velocity, covering all of the image elements in a single horizontal line. The beam retraces quickly to the left side of the frame after this trace, ready to scan the following horizontal line. Because the vertical deflection signal creates a vertical scanning motion, which is slower than horizontal scanning, the horizontal line slopes downward in the scanning direction.

The beam is now at the left side, ready to scan line 3, omitting the second line, once line ‘1' has been scanned. The vertical scanning frequency is increased from 25 to 50 Hz to achieve this line skipping. The electron beam now scans all of the odd lines before arriving at a location towards the bottom of the frame, such as point B in the diagram. Because of the flyback on their vertical saw tooth defection signal, the vertical retrace starts at time B. The beam then returns to the frame's top, where the second or even fields begin.

The beam now moves several horizontal lines from B to C. 20 horizontal lines are drawn during this time. Because the scanning beam is turned off at this time, these 20 lines are known as inactive lines. As a result, the raster's second field begins in the center. The beam scans line 2 in the second field after scanning half of a line from point C.

The beam then scans the even lines that were left out of the initial field's scanning. In this field, the vertical scanning action is identical to that of the preceding field. As a consequence, the second field's even lines, down to point D, are scanned. Because the second field begins at a half-line point, points D and B are a half-line distance from each other.

In the second field, the vertical retrace starts at point D. The vertical flyback causes the beam to return to the top from here. The beam completes the second vertical retrace at A because there are so many vertical retrace lines. Because the number of vertical retrace lines in both fields is the same, the beam will always end the second vertical retrace where the first trace began. At point A, all odd fields begin. Point C is the starting point for all even fields. This technique is repeated 50 times each second, which effectively eliminates flicker.

Wednesday, 21 July 2021

PAL Colour TV Receiver Block Diagram with Explanation

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PAL Colour TV Receiver Block Diagram with Explanation:

A colour TV receiver is identical to a black-and-white receiver with the addition of a chroma section and a colour image tube. Figure 5.7 shows a block schematic of a PAL colour TV receiver. This receiver's many parts are detailed below.

1. VHF and UHF Tuner:

The TV receiver has a VHF and UHF tuner on the front. It has a tuned circuit that allows you to choose the channel you want. The antennal signal is amplified and transformed into an IF signal, which is then sent into the video IF amplifier. Tuners also include an automatic frequency tuning feature for accurate colour reproduction.

2. Video IF Amplifier:

Since the tuner's output signal isn't strong enough to drive the video detector, it's amplified to the appropriate level via cascaded IF amplifiers.

3. Video Detector:

From the modulated composite video stream, the video detector recovers the original video signal.

4. Sound Section:

Trap Circuit, Limiter, FM Detector, Audio Amplifier, Power Amplifier, and Loudspeaker are all included in the sound area. The original sound signal is initially recovered from the modified sound carrier signal at this step. The FM detector's output is suitably amplified and applied to the loudspeaker for sound reproduction.

5. Automatic Gain Control Circuit:

Despite the intensity of the input signal, the Automatic Gain Control (AGC) circuit keeps the output signal at a consistent amplitude. The sync separator recovers the H and V sync pulses in the deflection circuit. Proper oscillators and amplifiers are used to process them. Finally, applied to the V and H deflection coils to concurrently deflect electron beams in the V and H directions.

6. Luminance Signal:

From the video detector, the chrominance and luminance signals follow independent routes before rejoining in the matrix portion. From the composite video stream, the luminance signal processing network recovers the luminance (Y) signal. The cathodes of colour picture tubes generally receive a negative-going Y signal (-Y).

7.  Color Signal Processing:

The signal available at the output of the video detector is properly amplified before feeding it to the various sections.

a) Chrominance Band Pass Amplifier: The chrominance bandpass amplifier selects the chrominance signal while rejecting the composite signal's other undesirable components.

b) Burst Banking Circuit: During colour burst periods, this circuit blocks signal flow to the chrominance bandpass amplifier.

c) Burst Amplifier: The burst gate amplifier isolates the colour burst signal from the chrominance signal while also enhancing it to the appropriate level. This transmission has a frequency of 4.43 MHz.

d) Generation and Control of Subcarriers: The major goal of this section is to generate a subcarrier with the right frequency. The phase discriminator and variable reactance components combine to serve as an Automatic Phase Control circuit in the subcarrier oscillator, which is a crystal oscillator used for creating subcarrier signals with a frequency of 4.43 MHz. It picks up subcarrier and burst signals.

If the frequency of the subcarrier oscillator is exactly right, its phase is altered by 90o for the incoming burst signal. The APC circuit's output is fed into a 7.8 kHz tuned amplifier. The 7.8 kHz ac component was overlaid on the output signal in this circuit. The output of this circuit is sent into the colour killer and identification circuits.

Before applying the subcarrier output to the V demodulator, the identification circuit controls an electrical switch that alternately reverses the phase of the subcarrier output. The 7.8 kHz component is available at the APC circuit of the reference subcarrier oscillator when a colour signal is received. The chrominance bandpass amplifier now performs the usual operation.

There is no 7.8 kHz during monochrome signal reception. As a result, the chrominance bandpass amplifier is turned off by the colour killer circuits.

8. Separation of U and V Modulation Products:

Here, the PAL delay line circuit, adder, subtractor, V and U sync demodulators, and difference signal amplifiers with matrix network are considered. The chrominance signal generated by the chroma bandpass amplifier is sent into one of the adder and subtractor circuits' inputs. Using a PAL delay line, the same signal is delayed and applied to the other inputs of adder and subtractor circuits.

The U information is the adder's output. Similarly, the subtractor's output is V data. Two separate double sidebands, suppressed carrier RF signals emerge from the adder and subtractor's output. These signals are sent to synchronous demodulators in the U and V bands, respectively. The concern oscillator's colour subcarrier signal is applied straight to the U sync demodulator.

Similarly, the same signal is delivered to the V sync demodulator through an electrical switch to create a + or -90o line by line phase-shifted signal. The original B-Y signal is recovered by the U demodulator. Similarly, the R-Y signal is recovered by the V demodulator. The G-Y signal is produced by combining the two signals in a matrix network. These colour difference signals are applied to the colour picture tube's matching grids.

Friday, 16 July 2021

Compact Disc Recording System Block Diagram

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A compact disc (CD) may hold a wide range of data. Compact disc signals are stored in a high-density digital format. As a result, the signals captured are a replica of the original audio stream. Text, picture images, audio, video, and software are all stored on compact discs. The compact disc was formerly the standard for storing audio recordings. The typical CD measures 12 cm (5 inches) in diameter. It is made up of three layers.

1. Transparent Substrate:

A polycarbonate wafer [plastic disc] makes up this layer.

2. Thin metallic Layer:

A thin metallic coating of aluminium alloy is applied to the wafer base. This metallic layer has a lengthy series of pits with a diameter of around 0.5 micrometres. The CD drive generally reads the reflective aluminium film part of the disc.

3. Outer Layer of Protective Acrylic:

The underlying data is further protected by a plastic coating called a protective lacquer layer, which is applied to the aluminium sheet. The CD's layout is seen in Figure.

Fig: Layout CD-ROM Disc

When a CD is made, a glass disc is covered with a photoresist that is sensitive to laser light. The Laser light exposes a precise helical pattern on the photoresist material during recording. Typically, the track width is 0.5m. As each 1 happens, the Laser beam is switched ON and OFF alternately, recording the 1s in the digital data onto the photoresist surface.

No 0's are written to the disc; instead, they are regenerated by the CD player. The photo-resist surface of the disc is exposed during recording 1, resulting in a sequence of pits and lands. Pits appear on the surface of the CD as a result of recording the digital data ‘1'.

Similarly, there is no change in the surface of the digital data 0 when it is recorded. As a result, the pits have a data of 1 and the other areas have a data of 0. The period or distance between 1's determines the length of the pit or land.


Using a sample and hold circuit and an ADC, the signal to be recorded on CD is first amplified and then transformed into a digital signal. The output of the ADC is also used by the Laser Beam Generator. The control circuit and the servo system are both controlled by the signal from the crystal oscillator and Laser beam generator.

The servo system, which is controlled by a motor, regulates the disc rotation as well as the track and focus of the Laser beam generator. The picture depicts a block schematic of a CD recording system.

The unexposed photoresist material is chemically removed after recording, leaving a helical pattern across the glass disc's surface. This becomes the glass master for mass-production CDs.

Figure:  Block Diagram of CD Recording


The data retrieval system is made up of the phases listed below. 

1. A servomechanism, which spins the CD.

2. A laser head that moves in a radial pattern. The laser head can both emit and detect a 70nm laser beam.

When the disc spins, the laser beam is focused onto the playing surface, where it is reflected by the ‘lands' and scattered by the ‘pits,' resulting in a change in the quantity of light reflected whenever there is a pit-to-land or land-to-pit change. As a result, the pit borders are detected by a laser beam.

A pit border is a ‘1', whereas its absence is a ‘0.' No pit, i.e. land, is indicated by a strong reflection of the light. A light receptor receiver determines whether light is highly reflected, absent, or dispersed. The recorded holes on the CD produce a non-existent or diffused light reflection. The picture depicts the block diagram of a CD replication mechanism.

Figure: Block Diagram of CD Reproduction System

A crystal oscillator signal is compared to the clock signal acquired from the disc. The servo system is controlled by the control circuit's output. The servo system regulates the motor speed as well as the optical mirror and lens system's track correction and focuses adjustments.

The photodetector receives the reflected beam from the disc. The light fluctuations are converted into a digital signal. The digital output is then amplified and transformed to an analogue signal using DAC. As a result, the original modulating signal may be obtained.

The control circuit receives the clock signal from the disc as well as the signal from the crystal oscillator. The servo system is controlled by the control circuit utilizing the two signals. The servo system is in charge of controlling the motor's speed. The optical mirror and lens system, as well as the ray focusing, are all controlled by the servo system. There are two techniques for encoding data on CDs.

They are:

1. CIRC – Cross -Interleave Reed – Solomon Code

2. EFM – Eight to Fourteen Modulations

Advantages of Compact Disc:

1. Certain codes can be used to detect errors on the disc.

2. Because the CD is protected by transparent plastic, it is not damaged by dust, oil, or scratches.

3. The sound reproduction is of high quality, with minimal background noise.

4. There is a high signal-to-noise ratio.

5. The stereo effect is effectively retained.

6. Excellent Channel separation.

7. Small in size.

8. Broad frequency range.

9. There is very little distortion.

10. Flutter does not exist.

11. A compact disc allows for programme selection.

Monday, 12 July 2021

Working Principle of LED Display

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While both types of TVs use LCD technology and feature flat-panel designs, LED TV is a more sophisticated form of LCD TV. LED TVs use LED backlighting to illuminate LCDs, making them smaller, brighter, and more detailed than standard LCD TVs. However, not all LED televisions have the same backlighting.

LED television sets have redefined the watching experience in a whole new and improved way. LED displays are even smaller and lighter than LCDs. The operating mechanism of LED televisions is similar to that of LCD televisions. LED televisions may be thought of as a more advanced version of LCD televisions. LED TVs have reduced the disadvantages of LCD TVs.

The fluorescent bulbs in the LCD television took up a lot of space and were heavy, and they didn't always give good colour quality. These flaws have been overcome by LED televisions, which employ an array of LEDs to produce backlighting, as depicted in the figure. LED televisions take up less space than LCD televisions and are thus smaller and lighter. The LEDs may also display the most accurate black-and-white picture.

There are two types of LED TVs on the market right now. One is backlit, and it gets its backlighting from a slew of LEDs mounted on the back of the television display. The other is edge lighted, which gets its illumination from LEDs positioned around the borders of the television display. Edge-lit LED TVs are significantly slimmer than back-lit LED TVs. As a result, edge-lit LED TVs use less energy and are more efficient.

LED TVs use light-emitting diodes to illuminate their LCD panels. LEDs are made up of tiny semiconductors that light when exposed to electricity. The LED TV may change the alignment of the liquid crystal when an electric current is provided to a tiny specified location of the liquid crystal layer.

General concept of LED

This current travels between LED anodes and LED cathodes, which are positively and negatively charged electrodes, respectively. A conventional LCD TV, on the other hand, is backlit by fluorescent lights. These lamps work by emitting ultraviolet rays from mercury vapour, which light up the lamp's phosphor coating.

Unlike pure LED systems, LED-backlit LCDs are self-illuminating. There are several ways to backlight an LCD panel with LEDs, including using white or RGB (Red, Green, and Blue) LED arrays behind the panel, and edge LED lighting, which uses white LEDs arranged around the inside frame of the TV and a light diffusion panel to spread the light evenly behind the LCD panel.

LED Display Features:

1. Reduced energy usage

2. A more evenly distributed colour saturation

3. Creates a picture with a wider dynamic range.

4. a thinner panel

5. More efficient heat dissipation

6. A more vibrant display

7. Increased contrast

8. There are more options available.

9. Less pollution in the environment

Saturday, 10 July 2021

Working Principle of LCD Display

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Colour and monochrome liquid crystal displays (LCDs) of various sizes have been created for TV receivers. LCDs in video display devices are currently available in a variety of sizes and colours, ranging from a few inches to several inches. LCD TVs are television sets that create pictures using LCD technology.

When voltage is applied to an LCD panel, it shines a backlight through a layer of liquid crystals, twisting the changing amounts of light passing through colour filters to form a picture on the screen. By carefully filtering white light, LCD televisions generate a black and colourful image. A series of cold cathode fluorescent lamps (CCFLs) are generally used to give illumination at the back of the panel. A grid of millions of individual LCD shutters is used. It will open and close to enable a specific amount of white light to pass through.

Each shutter is coupled with a coloured filter that removes everything from the original white source except the red, green, and blue (RGB) components. A single sub-pixel is formed by each shutter filter combination. Because the sub-pixels are so small, the three sub-pixels with red, green, and blue colour filters are combined to create a single colour spot known as Pixel.

The relative strength of the light flowing through the sub-pixels is used to adjust the colour shade. The front and rear of a typical shutter assembly are formed by a sandwich of many layers placed on two thin glass sheets. The polarising film, glass sheet, active matrix components, and addressing electrodes appear first on the back sheet, followed by the director.

Fig: General concept of the LCD unit.

The active matrix components are replaced by patterned colour filters on the front sheet, which is comparable to the back sheet. A patterned plastic sheet is sandwiched between the two sheets, dividing the liquid crystal into separate shutters. The shutter assembly is coupled with control electronics and a backlight to form a full television.

A single bulb or a group of lights can be used to create a backlight. To disperse the light, a diffuser or frosted mirror is utilised. A column of liquid crystal molecules is suspended between two transparent electrodes and two polarising filters in each pixel. The polarizer's axes are perpendicular to one another. Light flowing through one would be blocked by the other if the liquid crystals were not there.

The polarization of light entering one filter is twisted by the liquid crystal, allowing it to pass through the other. The liquid crystal molecule is relaxed before an electrical charge is applied. The helical shape or twisting of these molecules is caused by charges on the molecules.

The light that passes through one polarised filter is rotated as it travels through the liquid crystal, allowing it to pass through the second polarised filter and making the assembly transparent. The molecules of the liquid crystal align themselves parallel to the electric field when an electric charge is given to the electrodes, thus restricting the rotation of incoming light. The light travelling through the liquid crystals will be polarised perpendicular to the second filter if they are entirely untwisted, and therefore completely blocked.

The pixel will appear to be dark. Light may be permitted to flow through in various amounts by regulating the twist of the liquid crystal in each pixel, lighting the pixel accordingly. The LCD panel provides excellent colour reproduction and contrast. CRT displays are limited in size, therefore they may be manufactured in bigger proportions. Despite their size, they are light in weight and maybe readily placed, especially if mounted on a wall.


Advantages of LCD are:

1. LCD's have great Compactness.

2. LCD's are thinner and lightweight devices.

3. It is inexpensive.

4. It uses a few microwatts for display, compared to a few milliwatts for LEDs.

5. When compared to CRT and LED, it uses less electricity.

Disadvantages of LCD:

1. Contrast and brightness are lacking.

2. Additional light sources are required.

3. The operating temperature range is restricted.

4. Lack of trustworthiness.

5. The speed is really slow.

6. AC drives are required for LCDs.

Applications of Liquid Crystal Display (LCD)

1. Thermometer with liquid crystals.

2. Optical imaging is a type of imaging that uses light to create images.

3. The liquid crystal display technology may also be used to visualise radiofrequency waves in waveguides.

4. It's used in medical settings.