Jumaat, Oktober 18, 2013

The Scanning Process. How a Monitor and Camera Work:



Experimental Television Center, Binghamton, NY (1976)



Raster refers to the scanned cathode ray tube (CRT); it appears as a rectangular block of light. The picture tube of a television receiver or monitor is a CRT and the display of an oscilloscope is also a CRT. The raster is produced by the electron beam inside the tube moving horizontally and slightly downward to create 525 blank lines when there is no video input. The horizontal and vertical motion is called scanning.


The following description refers to the standard 625 line system used in the many cuntry. The process is similar for most video equipment but the numbers indicated may be different for different machines and in different countries.

Video is a series of still information which is displayed so rapidly that, due to persistence of vision, the eye and brain perceive motion.

The scanning process produces a raster by drawing 625 horizontal lines across and down the screen. The drawing is done by a beam of electrons. The beam first scans each of the 312 1/2 odd lines across and slightly down the screen (one field) and then returns to the top and scans each of the 312 1/2 even lines between the odd lines (one field) to compose one Frame of 625 lines or the raster.

The beam begins to scan at the top left on the first odd line and moves horizontally and slightly downward to the right edge. This one line is called a trace. The beam then returns from the right to the left at the next odd line to begin a new trace. The return portion of the beam is not seen because it is blanked out by a blanking pulse. The return portion of the beam is called the retrace. The scanning of each of the 312 1/2 odd lines continues in this manner to the bottom of the rectangle. It takes 1/54th of one second to complete the scanning of all the odd lines, when the beam reaches the bottom of the screen it must return to the top to begin the scanning process for all the even lines. The return of the beam from the bottom to the top is also blanked out so it is not seen. The 312 1/2 odd lines which are scanned make up one field.

When the beam has returned to the top, scanning of the even lines begins. The process is the same as the one for odd lines except that only even lines are scanned. The alternation of odd and even lines is called, interlaced scanning. For each horizontal line the beam moves across from left to right and slightly down then back to the left side again (the retrace) to begin the next even line. At the bottom the beam again returns to the top while the screen is blanked out and the beam is ready to scan odd lines. The 312 1/2 even lines which are scanned also make up one field and take 1/54th of one second.

The odd field and the even field together comprise one frame of 625 horizontal lines. There are 25 frames displayed every second (frame frequency) or 50 fields displayed every second (field frequency). In each second there are a total of 15,620 lines: 1 frame=625 lines, 25 frames per second -  25 x 625 = 15,620 lines per second. This is called line frequency.

The scanning process occurs in both the camera and the monitor and must be regulated so that the process happens at exactly the same time and at the same rate for both. This timing is accomplished by sync pulses (see below).

odd field

trace begins at A
trace moves A to 2 left to right
retrace moves 2 to 3 right to left
when trace reaches B, beam moves from B to C and is blanked out

solid lines are traces
broken lines are retraces

even field

trace begins at C
retrace moves C to 1 left to right
retrace moves 1 to 2 right to left
when trace reaches D, beam moves back to A (blanked out)

one frame consists of 1 odd and 1 even field

 solid lines are odd traces
broken lines are even traces
retraces are not shown


Monitor: video signal goes directly into this to display the image

Receiver: video signal must be converted to RF (radio frequency) before it can be displayed. A cable from RF output of the camera or deck is attached to the antenna leads of a television set (receiver).

The cathode produces a beam of electrons which are emitted in the direction of the fluorescent screen. The control grid has a tiny hole which opens and allows more electrons to pass through, or closes and lets fewer electrons pass. A positive voltage opens the grid and a negative voltage closes the grid. A negative voltage is kept on the grid, but this is variable by the brightness control on the monitor or receiver. By turning the brightness up, you reduce the negative voltage, open the control grid and allow more electrons to pass brightening the screen. By turning the brightness down, the negative voltage is increased, the control grid is closed and fewer electrons pass, darkening the screen. By varying the brightness you change the average background illumination. If the brightness is very high when the video signal is fed in, even the peak video voltages may not darken the screen; if brightness is down, the peak video voltages may not open the grid enough to allow my electrons to pass and the screen does not get lighter.

The scanning process is the same as previously described. The motion of scanning is controlled by the horizontal and vertical deflection plates. The electron beam is moved from left to right by the horizontal deflection plates to produce the traces. The beam is moved slightly downward as it scans horizontally and back to the top from the bottom by the vertical deflection plates.

The screen is coated with a fluorescent material which releases light when hit by electrons; the more electrons which hit the material, the more light is released. The video signal sent from the camera is composed of fluctuations in voltage. A positive voltage produced by the camera by the presence of light will open the control grid and allow more electrons to pass, brightening the screen. A negative voltage produced by the camera in the absence of light will close the control grid, allowing fewer electrons to pass and. darkening the screen.

The contrast control on a monitor increases the difference between the positive and negative peaks of a video signal. Since the positive peak lightens the screen and a negative peak darkens the screen, an increased difference between the two will cause an increase in the difference between light and dark.

Thus in one trace there can be many variations of relative light and dark. 525 traces, each with its variations, compose a mosaic of lights and darks which can be perceived as an image


 Light is reflected by an object and enters the camera through the aperture opening of the lens. Light falls on the camera tube (usually a vidicon tube). The target is that section of the tube sensitive to light (photosensitive). The light causes electrons to be emitted producing areas of positive charge; the more light, the more electrons emitted and the greater the number of positively charged areas.

The cathode emits a beam of electrons in the direction of the target. This beam sweeps the target from left to right horizontally and slightly down in the same manner as the monitor scans and at the same rate. The deflection coils move the deflection beam horizontally across the target and vertically from top to bottom and back up during the entire scanning process. As the beam scans the target the areas of negative charge produced by light hitting these areas attract the negatively charged electrons while those areas of negatively charged electrons while those areas of negative charge produced by less light do not attract the electrons. Thus the number of returning electrons varies with the number of positive charges on the target. If th target has many areas of positive charge (indicating the presence of light), fewer electrons return. If the target has few positive charges (indicating the absence of light), more electrons return. The beam electrons then produce a fluctuating voltage which is called the current of video frequency. Positive voltage is created by brighter areas and negative voltage is created by darker areas. Within each trace there can be many variations of light and dark, thus many areas of positive and negative charge. This voltage is then sent from the camera to the deck to be recorded or to the monitor to be displayed.

The camera converts variations in light to fluctuations in voltage. This signal is sent to the monitor to be converted back to variations of light and dark.


An electrical impulse noted in terms of:
Frequency in cycles per second or hertz (Hz)
Strength in terms of volts (V)

Non-composite video signal: Composed of video signal only; no sync is supplied

Composite Video signal: Composed of video signal and horizontal sync and vertical sync

Composite Sync: Composed of vertical and horizontal sync

Representative of one trace. Horizontal sync to begin trace. Video signal fluctuations to compose the trace


Representative of one field

Horizontal sync pulses to begin 262 ½ traces
Vertical sync pulse to begin next field
Video signal fluctuations to compose each trace

Blanking pulse: prevents visibility of retrace, when beam returns from right to left to begin scanning of next line and when beam returns to top after field is scanned.


Synchronization: to be contemporary with.

Refers to the process which maintains the timing and the rate of the scanning process. It insures that both the camera and the monitor begin to scan simultaneously, begin each line and field at the same time and that the rate for both is the same. If sync were not present, the image displayed on the monitor would not be readable as an image. In any video system there must be a sync source. In a single camera system, sync may be generated by the camera or by the deck, but not by both at the same time. In a multiple camera system, the cameras must have one common sync source; usually the sync is generated by a sync generator within the video mixing machine. A separate sync generator may also be used. Sync pulses are produced by oscillators oscillating at specific frequencies.

There are two elements to sync:
horizontal sync maintains the side to side orientation. Loss produces diagonal lines.

vertical sync maintains top to bottom orientaion. Loss produces rolling.

 There are two types of sync:

random sync where only horizontal sync is supplied; this results in traces on the monitor which are not evenly spaced
2: 1 sync or interlace wherein the traces are evenly spaced

 Sync pulses are also recorded on video tape along with video information so that on playback the sync track is read by the monitor and the image displayed approximated the image the camera recorded.

A horizontal sync pulse is generated at the beginning of each horizontal trace.

A vertical sync pulse is generated between fields


LCD TV is a television display technology based on a liquid crystal display. LCD TVs consume much less power than plasma displays because they work on the principle of blocking light rather than emitting it.

An LCD display uses either a passive matrix or an active matrix display grid. The active matrix LCD is also known as a thin film transistor (TFT) display. The passive matrix LCD has a grid of conductors with pixels located at each intersection in the grid. A current is sent across two conductors on the grid to control the light for any pixel. An active matrix has a transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more frequently, which improves the refresh rate.

Some passive matrix LCDs have dual scanning, meaning that they scan the grid twice with current in the same time that it took for one scan in the original technology. However, active matrix is considered a superior technology.

An LED TV is a type of LCD TV that uses LEDs to backlight the display, rather than the cold cathode fluorescent lights (CCFLs) used in most LCD TVs.

Vendors of LCD TVs include Aquos, LG, Samsung and Sony.

Pros: Not prone to burn-in. Available in smaller sizes than plasma, so may be a better option depending on the available space.

Cons: Can suffer from slower response, which can create a ghosting effect. Some models are also prone to the screen door effect, which means that a faint mesh pattern may be visible.

Learn more:

See our Flat-panel TV Guide to learn more about the various types of flat-panels as well as important features to look for.

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Plasma vs LCD Television

Plasma and LCD displays work in two very different ways. A plasma TV is sometimes called an "emissive" display while an LCD panel has a "transmissive" display. Let's explore the differences in these two technologies.

How a Plasma TV works

A plasma display consists of two transparent glass panels with a thin layer of pixels sandwiched in between. Each pixel is composed of three gas-filled cells or sub-pixels (one each for red, green and blue). A grid of tiny electrodes applies an electric current to the individual cells, causing the neon and xenon gas in the cells to ionize. This ionized gas (plasma) emits high-frequency UV rays, which stimulate the cells' phosphors, to glow the desired colour.

Because a plasma panel is illuminated at the sub-pixel level, images are extremely accurate, and the panel's light output is both high and consistent across the entire screen area. Plasma TVs also provide very wide horizontal and vertical viewing angles. Picture quality looks sharp and bright from virtually anywhere in the room.

Most flat-panel TVs are progressive displays - they draw the entire image at once. Panasonic's 1080p plasma displays illuminate over two million pixels for true high-definition clarity, deeper blacks and warmer colours.

How an LCD TV works

Light in an LCD panel isn't created by the liquid crystals themselves; instead, a light source behind the panel shines light through the display, while a white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform image.

The display consists of two polarizing transparent panels and a liquid crystal solution sandwiched in between. The screen's front layer of glass is etched on the inside surface in a grid pattern to form a template for the layer of liquid crystals. Liquid crystals are rod-shaped molecules that bend light in response to an electric current - the crystals align so that light cannot pass through them. Each crystal acts like a shutter, either allowing light to pass through or blocking the light. This pattern of transparent and dark crystals forms the image.

LCD TVs use the most advanced type of LCD, known as an "active-matrix" LCD. This design is based on thin film transistors (TFT). Their job is to rapidly switch the LCD's pixels on and off. In a colour LCD TV, each colour pixel is created by three sub-pixels with red, green and blue color filters.

One of the biggest challenges for LCD TV manufacturers has been speeding up the pixel response time (how fast an individual pixel switches from fully off to fully on) to ensure that fast-moving objects don't exhibit "motion lag" or ghosting. It's especially critical for larger-screen LCD TVs where much of the viewing will be DVD movies and/or HDTV.

An important difference between plasma and LCD technology is that an LCD screen doesn't have a coating of phosphor dots (colours are created through the use of filters). That means you'll never have to worry about image burn-in, which is great news, especially for anyone planning to connect a PC or video game system. LCD TVs are extremely energy-efficient, typically consuming 60% less power than comparably-sized tube-type TVs.

Which type is right for you?

Plasma TVs have been around longer than LCD TVs, and their technology is a little further along. Plasma screens use a phosphor coating like tube TVs, so they have the natural colour we're used to with tube models.

Plasmas have better contrast and black level performance than LCDs, and offer slightly wider viewing angles. People often describe plasma's picture quality as richer or more "cinematic," so it's a great choice for a home theatre, or your main TV.

A plasma TV might be for you if:
You want really rich, warm colors and deep blacks
You like to watch sports and other fast-action TV
You'll be sitting off-axis when you watch TV or movies
Your viewing room doesn't have a lot of ambient light, or you can easily reduce the light by closing blinds.

If you're looking at screen sizes over 42", Plasma would be the better choice.

A flat-panel LCD TV might be for you if:
You watch a lot of TV shows or play lots of video games with static images on the screen for extended periods of time, multiple days a week
Your TV room is relatively bright, or you do a lot of daytime viewing

Plasma and LCD Research
In Australia, Great Britain and several other European markets, independent surveys have been conducted on the perception of Plasma versus LCD technology.

Initially 56% of those surveyed thought LCD would offer better picture quality than Plasma with 42% thinking Plasma was better.

These viewers were then shown both Plasma and LCD TVs in normal home lighting conditions and the swing to Plasma was very marked - up to 69%, with only 31% believing that LCD looked better.

Panasonic recognise the relative strength of both technologies.

VIERA Plasma's are the choice for living rooms and larger spaces. With their wide viewing angle, deep rich colour, VIERA Plasma sets also stand up to the rigour of family life with their full glass front.

Panasonic LCD sets, manufactured in a range from 32" to 42", have two key technologies of Motion Picture Pro and Intelligent Scene Controller.

Motion Picture Pro takes a 50Hz signal and doubles it by creating an intermediate frame between each original frame, thus making the signal 100Hz and greatly reducing motion blur, a previously common problem with LCD.

Intelligent Scene Controller adjusts the level of backlight on a frame by frame basis. It will bring up the backlight for a bright scene and lower it for a darker scene, bringing a massive 8,500:1 contrast ratio.

These sets stand up extremely well in daytime lighting and with their smaller sizes (32"-42") suit smaller rooms.

LCD and Plasma

Because plasma TVs are self-illuminating, the images are beautiful when viewed from any angle. Black areas do not fade, and colours remain almost identical regardless of where the viewer is positioned.

Take a look at the Panasonic Viera range of Plasma & LED LCD TVs to find which best suits your viewing needs.

- See more at: panasonic.co.nz

Related Links

Khamis, September 26, 2013

555 Burglar Alarm

The circuit illustrated here is used as an Burglar alarm. LDR is kept at such a place that when thief enters our house then a shadow will fall on the LDR. A small beam of light source is also needed to supply continuous signal to LDR. For best Light source we can use Laser diode which will work for few KMs. For home use Infra Red LED’s will be good and will be tricky to thief and works with same efficiency at night.
This circuit uses a popular timer I.C which is 555. I.C 555 is connected as comparator with pin 6 connected with positive supply, the output goes high-1 when the trigger pin 2 is at lower than 1/3 level of the supply voltage. Conversely the output goes low-0 when it is above 1/3. So small change in voltage of pin 2 is enough to change the output state of pin 3 from 1 to 0 and 0 to 1. The output has only two states high and low and can not remain in any intermediate stage. It is power by 9V battery for portable use. The circuit is economic in power consumption. Pin 4,6& 8 is connected to the positive supply and pin 1 is grounded.
To detect the present robber we have used LDR and a source of light.
LDR is a special type of resistance whose value depends on the brightness of the light which is falling on it. It has a resistance of about 1 megaohms when in total darkness,but a resistance of only about 2-5 k ohms when brightly illuminated. It responds to a large part of the light spectrum.
The source of light and LDR is so adjusted with a reflector that light will directly fall on the LDR but when robber enters inside then it will block the beam of light and LDR will be under darkness.
We have made a potential divider circuit with LDR and 100 K variable resistance connected in series. Voltage is directly proportional to conductance so more voltage we will get by this divider when LDR is getting light and low voltage in darkness. Sensitiveness can be adjusted by variable resistance. Divided voltage is given to pin 2nd of 555. As soon as LDR gets dark the voltage of the pin 2 drops 1/3 of the supply voltage and pin 3 gets high and Buzzer Beeps.
For Demo we have used simple LED for LED1 may be Red or White Color

Circuit Diagram of Burglar Alarm

LED = Light Emitting Diode
LDR = Light Dependent Resistance
IC = Integrated Circuit

  1. 9V battery with snap
  2. LDR
  3. Variable resistance 100K ohms
  4. Resistance 470 ohms
  5. LED
  6. IC 555
  7. Switch

Water or Rain Alarm

Circuit Diagram of Rain Alarm

Water is a conductor of electricity. When water is in contact with the probe then there is a flow of current which reaches to the base of Q1. Transistor Q1 is a NPN transistor which conducts. With the conduction of Q1 electron reaches to Q2 which is a PNP transistor .Q2 also conducts and current flows through the speaker. In a speaker there is inductive coil which causes motion in one direction and also produce induce current which is in opposite direction to the flow of current this induce current in the form of pulse flows through a capacitor, resistance and switches off Q1 and relax .this process repeats again and again till probe is in contact with water or we can say there is a oscillation in the circuit thus speaker diaphragm vibrates and gives a tone. Frequency of the circuit depends on the value of Speaker Coil impendence, Capacitor and Resistance Value.


Rain Alarm Project

Automatic Street Light

An introduction:
Needs no manual operation for switching ON and OFF. When there is need of light it automatically switches ON. When darkness rises to a certain value then sensor circuit gets activated and switches ON and when there is other source of light i.e. day time, the street light gets OFF. The sensitiveness of the street light can also be adjusted. In our project we have used four L.E.D for indication of bulb but for high power switching one can connect Relay (electromagnetic switch) at the output of pin 3 of I.C 555. Then it will be possible to turn ON/OFF any electrical appliances connected all the way through relay.

Principle :
This circuit uses a popular timer I.C 555. I.C 555 is connected as comparator with pin-6 connected with positive rail, the output goes high(1) when the trigger pin 2 is at lower then 1/3rd level of the supply voltage. Conversely the output goes low (0) when it is above 1/3rd level. So small change in the voltage of pin-2 is enough to change the level of output (pin-3) from 1 to 0 and 0 to 1. The output has only two states high and low and can not remain in any intermediate stage. It is powered by a 6V battery for portable use. The circuit is economic in power consumption. Pin 4, 6 and 8 is connected to the positive supply and pin 1 is grounded. To detect the present of an object we have used LDR and a source of light.

LDR is a special type of resistance whose value depends on the brightness of the light which is falling on it. It has resistance of about 1 mega ohm when in total darkness, but a resistance of only about 5k ohms when brightness illuminated. It responds to a large part of light spectrum. We have made a potential divider circuit with LDR and 100K variable resistance connected in series. We know that voltage is directly proportional to conductance so more voltage we will get from this divider when LDR is getting light and low voltage in darkness. This divided voltage is given to pin 2 of IC 555. Variable resistance is so adjusted that it crosses potential of 1/3rd in brightness and fall below 1/3rd in darkness.
Sensitiveness can be adjusted by this variable resistance. As soon as LDR gets dark the voltage of pin 2 drops 1/3rd of the supply voltage and pin 3 gets high and LED or buzzer which is connected to the output gets activated.

Circuit Diagram of Automatic Street Light

Component used :
  1. 9v Battery with strip
  2. Switch
  3. L.D.R (Light Depending Resistance)
  4. I.C NE555 with Base
  5. L.E.D (Light Emitting Diode) 3 to 6 pieces.
  6. Variable Resistance of 47 KΩ
  7. P.C.B (Printed Circuit Board of 555 or Vero board.

  1. Battery: For 9v power supply we can use 6pcs dry cell or 6F22 9v single piece battery.
  2. Switch:  Any general purpose switch can be used. Switch is used as circuit breaker.
  3. L.D.R: (Light Depending Resistance)  It is a special type of resistance whose value depends on the brightness of light which is falling on it. It has resistance of about 1mega ohm when in total darkness, but a resistance of only about 5k ohms when brightness illuminated. It responds to a large part of light spectrum.
  4. L.E.D: (Light Emitting Diode)    A diode is a component that only allows electricity to flow one way. It can be thought as a sort of one way street for electrons. Because of this characteristic, diode are used to transform or rectify AC voltage into a DC voltage. Diodes have two connections, an anode and a cathode. The cathode is the end on the schematic with the point of the triangle pointing towards a line. In other words, the triangle points toward that cathode. The anode is, of course, the opposite end. Current flows from the anode to the cathode. Light emitting diodes, or LEDs, differ from regular diodes in that when a voltage is applied, they emit light. This light can be red (most common), green, yellow, orange, blue (not very common), or infa red. LEDs are used as indicators, transmitters, etc. Most likely, a LED will never burn out like a regular lamp will and requires many times less current. Because LEDs act like regular diodes and will form a short if connected between + and -, a current limiting resistor is used to prevent that very thing. LEDs may or may not be drawn with the circle surrounding them.
  5. Potentiometer    Resistors are one of the most common electronic components. A resistor is a device that limits, or resists current. The current limiting ability or resistance is measured in ohms, represented by the Greek symbol Omega. Variable resistors (also called potentiometers or just “pots”) are resistors that have a variable resistance. You adjust the resistance by turning a shaft. This shaft moves a wiper across the actual resistor element. By changing the amounts of resistor between the wiper connection and the connection (s) to the resistor element, you can change the resistance. You will often see the resistance of resistors written with K (kilohms) after the number value. This means that there are that many thousands of ohms. For example, 1K is 1000 ohm,2K is 2000 ohm, 3.3K is 3300 ohm, etc. You may also see the suffix M (mega ohms). This simply means million. Resistors are also rated by their power handling capability. This is the amount of heat the resistor can take before it is destroyed. The power capability is measured in W (watts) Common wattages for variable resistors are 1/8W, 1/4W, 1/2W and 1W. Anything of a higher wattage is referred to as a rheostat
  6. PCB (Printed Circuit Board)   with the help of P.C.B it is easy to assemble circuit with neat and clean end products. P.C.B is made of Bakelite with surface pasted with copper track-layout. For each components leg, hole is made. 
Connection pin is passed through the hole and is soldered.

When light falls on the LDR then its resistance decreases which results in increase of the voltage at pin 2 of the IC 555. IC 555 has got comparator inbuilt, which compares between the input voltage from pin2 and 1/3rd of the power supply voltage. When input falls below 1/3rd then output is set high otherwise it is set low. Since in brightness, input voltage rises so we obtain no positive voltage at output of pin 3 to drive relay or LED, besides in poor light condition we get output to energize.

  1. Use a Sensitive LDR. You can test it using a multimeter.
  2. I.C should not be heated too much while soldering, excess heat can destroy it. For safety and easy to replace, use of I.C base is suggested. While placing the I.C pin no 1 should be made sure at right hole.
  3. Opposite polarity of battery can destroy I.C so please check the polarity before switching ON the circuit. One should use diode in series with switch for safety since diode allows flowing current in one direction only.
  4. L.E.D glows in forward bias only so incorrect polarity of L.E.D will not glow. Out put voltage of our project is 7.3 volt therefore 4 LED in series can be easily used with out resistance.
  5. Each component should be soldered neat and clean. We should check for any dry soldered.
  6. LDR should be so adjusted that it should not get light from streetlight itself.

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<script>Defines a client-side script
<section>NewDefines a section in a document
<select>Defines a drop-down list
<small>Defines smaller text
<source>NewDefines multiple media resources for media elements (<video> and <audio>)
<span>Defines a section in a document
<strike>Not supported in HTML5. Deprecated in HTML 4.01. Defines strikethrough text
<strong>Defines important text
<style>Defines style information for a document
<sub>Defines subscripted text
<summary>NewDefines a visible heading for a <details> element
<sup>Defines superscripted text
<table>Defines a table
<tbody>Groups the body content in a table
<td>Defines a cell in a table
<textarea>Defines a multiline input control (text area)
<tfoot>Groups the footer content in a table
<th>Defines a header cell in a table
<thead>Groups the header content in a table
<time>NewDefines a date/time
<title>Defines a title for the document
<tr>Defines a row in a table
<track>NewDefines text tracks for media elements (<video> and <audio>)
<tt>Not supported in HTML5. Defines teletype text
<u>Defines text that should be stylistically different from normal text
<ul>Defines an unordered list
<var>Defines a variable
<video>NewDefines a video or movie
<wbr>NewDefines a possible line-break

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