Jumaat, Oktober 18, 2013

The Scanning Process. How a Monitor and Camera Work:

 

Source:

Experimental Television Center, Binghamton, NY (1976)

 



RASTER

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.


SCANNING PROCESS

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





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










CAMERA





 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.







SIGNAL

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.



SYNC

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


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.


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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

 
 
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