Technology: Plasma v LCD v Rear Projection

Monitor v TV

Resolution

Contrast

Colour depth

Inputs

Scart, composite, component, RGB, DVI, HDMI

Mounting Options

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

- Technology: Plasma v LCD v Rear Projection back to menu

Three technologies, none of them particularly new, each with strengths and weaknesses.

Plasma, developed in the 60's works in a similar (though flatter) method to that of conventional picture tubes in a CRT or Cathode Ray Tube. Instead of an electron gun firing a stream of electrons at the phosphors on the inner surface of the tube, a plasma works by using a very high charge to tear apart atoms of an inert gas like Xenon. The plasma thus created gives off energy radiation in the form of Ultra Violet light which in turn excites the phosphors on the inside surface of the panel. So like a CRT it generates light at the surface, and like CRT it is very bright and responds very quickly to changes in energy input.

LCD is a very different kind of flat panel technology. Here the brightness of the panel is provided by a very bright white light source (usually arrayed around the periphery of the display) being reflected through a Liquid Crystal array. The LCD works by passing a charge to cells on the surface of the display which then physically change their orientation (Polarization) either allowing or blocking the passage of the reflected backlight.

LCD requires less power to operate than does a plasma, making them ideal for use in cars and boats. The downside of LCD is that their is a measurable time required for a pixel to physically rotate from one orientation to the other. This delay is most noticeable when objects in the image are moving very quickly against the scan of the image. LCD pictures tend to appear brighter than plasma though they typically have a significantly lower contrast ratio. LCD panels run cooler than plasma. They are also typically smaller.

Rear projection technology works in precisely the same way that a front projector works. Using single or multiple projection "guns" the image is reflected off a large first surface mirror onto the back of a fresnel lens. It is this lens that gives the rear projection TV it's characteristic lines across the image. The fresnel lens is designed to concentrate the light of the image into a narrower field of view, making the image appear brighter and more contrasty. Off-axis viewing of rear projection displays is almost impossible and decidedly ugly as far as brightness and contrast goes. Typical rearpro's have a viewing cone of approx 35-55 degrees, while an LCD is typically 120-150 degrees and a plasma 170 + degrees.  A rearpro TV often has a depth associated with it that is more comparable to old CRT TV's than modern flat panel displays, although new technologies such as LCOS etc are reducing the cabinet depth somewhat.

- Monitor v TV back to menu

When looking at purchasing a widescreen display it's worth a quick note about comparing apples with apples.

There are numerous configurations in which to purchase a wide screen display. The most common one that people relate best to is the TV. By TV we mean a device that contains the same 3 major subsystems as the big box you've had in your lounge for the last 30 years. That is, it contains a display, an audio system and a tuner.

Buying a plasma or LCD display as a full TV is generally the most expensive way to purchase a display - although it is by far the easiest to convert users to, as the functions are as obvious as well...a TV.

If you are more confident with the operation of multiple separate devices, or you have an existing sound system you wish to integrate, then some of the reasons for purchasing a full TV wide screen display are diminished. (note: the TV's speakers are quite useful for watching the news or late at night for instance, as they don't create anything like the sound pressure or the low frequencies that can travel through the home.)

Wide screen displays, including projectors are much more commonly sold as display-only devices. In this configuration there is no tuner or sound system, these tasks expected to be covered by outboard devices, like Home Theatre Receivers and Digital Set Top Boxes. While this complicates the cabling and the control somewhat (unless you look at control solutions), it does provides for the simple upgrade of individual components. With analog TV scheduled to be switched off around 2008, the long term purpose of an analog tuner is also a little unclear, as is the picture from analog. The bigger the display, the more critical it becomes to feed it higher quality digital signals.

- Resolution back to menu

The spatial resolution of a plasma panel, projector or LCD display is measure by the number of pixels (picture elements - dots of colour). This is given as a figure like 853x480 or 1280x720 and describes the actual number of discrete pixels horizontally x vertically.

As a general rule of thumb - the higher the resolution the better, and the longer it will stay current with the existing and next-gen High Definition DVD players and Digital TV broadcasts.

Any device with a vertical resolution of about 576 lines or less is known as a Standard Definition display device. Anything higher is obviously therefore a High Definition device. This is to say a 720p (720 line vertical resolution progressive signal) from a Digital STB displayed on a Standard Def would down-convert the information to the lower resolution of the panel. Send the same signal to a HD panel display and hey presto - you have all the additional detail that was broadcast.

Conversely, sending a Standard Def (or worse - analog TV) to a HD panel display does not mean necessarily a better picture. The quality of the image will depend upon the quality of the video scaler used in the display device.

- Video Scalers back to menu

Video scalers are devices built into panels and projectors which tell the panel, line by line, how to take a signal at one resolution and display it on the native resolution of the hardware available i.e. the panel or projector chip.

Let's look at an example. Take a SD 576 line signal and feed it to 720 line projector. The video scaler determines how to create the extra 144 lines (720 minus 576) available on the panel. The data for those lines has to be interpolated from the available SD 576 lines. So effectively one quarter (144/576 = 0.25) of the image you see on the 720 line display is "made up".

The situation is even worse where you have an interlaced SD signal going into a very high resolution projector - you just don't do it!. Say a 576i signal is fed into a 1920x1080 native resolution projector. Since you're really only effectively getting half the 576 lines (288 lines) at any given instant the projector's video scaler needs to create 1080-288 or 792 lines of the 1080 it displays! It mean's the three quarters of what's being displayed at any given instant is video scalerwizardry.

The importance of good algorithms used by good video scalers can't be overestimated, and the higher the resolution of the display device - the more critical the role of the video scaler becomes. Companies like Faroudja specialize in the design and manufacture of high quality video scalers for use in panels and projectors.

- Contrast back to menu

Nothing too complicated here, this is simply a question of "How black are the blacks and how bright are the whites". Normally the black areas on screen are created by either turning off the pixel (in the case of plasma) or by blocking the light from passing through the pixel (in the case with LCD). Well might you imagine that the absence of glow from a plasma is darker than a bright light being blocked by a thin liquid crystal. Some panels  use gloss black lines on the walls between pixels to increase the apparent "blackness" of an off pixel.

Depending on the colour depth of the device, there is also a difference in the number of "steps" between white and black. The greater the colour depth, the more steps, therefore the more subtlety in contrast is available.

It's also worth noting that contrast for front projection depends entirely on the amount of ambient light in the room. Using brighter projectors will never render proper blacks in a daylight or otherwise lit environment. This is why home theatre projection should always be set up in a room that can be properly blacked out.

- Colour depth back to menu

Is a measure of the number of "bits" of data used to describe a colour to the device. All digital images are made from 0's and 1's. Using them in strings can describe colours. Red, Green and Blue (RGB) each contribute a certain amount to any colour you wish to create. So a colour might be described as containing:

R: 01110110

G: 10111101

B: 01000110

In this example there are 8 "bits" of data for each of RGB. If we changed the Red component from 01110110 to say, 00010111 we would have a totally different colour. The short calculation is this, with 8 bits of data in 3 channels, there are a total of 16.7 million possible combinations of 0's and 1's making 16.7 million possible colours from white to black. This happens to be enough to convince the human eye that everything looks ok.

Most consumer digital cameras use 8 bit per channel (24 bit colour), whereas professional products tend to higher colour depth.

Some display devices use 10 or 12 bit per channel colour handling internally to ensure subtlety and smoothness. Some displays downscale the colour depth at the point of display.

- Inputs back to menu

Display devices usually have multiple inputs. These can be of any of the basic Scart, composite, component, RGB, DVI, HDMI types of inputs, but the positioning of the inputs may become an issue for installation and ease of control.

Typically display only panels have their inputs along one edge of the panel. This means that for wall mounting of the panel there must be provision made to conduit the cabling down through the wall to attach the other source media devices.

Some panels come with an external media box. This simplifies the cabling to wall mounted panels (usually requiring a digital HDMI or DVI linkage from the media box to the panel. In this way source media devices can plug into the media box conveniently located near the rest of the home entertainment equipment. Often these media boxes allow for the retransmission of IR control signal from the panel back down to the media box where it can be re-emitted. This is particularly useful where the source devices and other electronics resides in a closed cupboard.

- Scart, composite, component, RGB, DVI, HDMI back to menu

Composite is the lowest standard of video connection. Normally this is a yellow RCA plug.

S-Video is the next higher quality video connection having much greater bandwidth with the additional conductor cores. S-Video separates colour from brightness and contrast.

Component Video is the highest standard of consumer analog video connection, containing a much greater bandwidth again than S-video, this schema separates colours into Red Green and Blue, much the same way as your eyes do.

As discussed elsewhere in the articles pages, SCART connectors contain all the necessary bi-directional wiring (SCART-SCART cables) to carry left and right audio, composite, S-Video, RGB and control signals. It was designed to be a simple modular system. Also known as the Euro connector for it's prevalence in European devices and markets.

DVI appears to be an interim standard until HDMI really gets off the ground.

HDMI is an all digital end-to-end cable that is designed with security in mind not just quality. It will eventually be the ONLY way to get true high and ultra high definition digital video from the next generation of HD devices. These are modular plug cables and much more difficult to route easily through walls in existing houses.

- Aspect Ratios back to menu

There is probably nothing more confusing than the aspect ratios 4:3 versus 16:9 for projectors and screens. Video comes in many different aspect ratio formats. Material made for regular TV is 4:3 (1.33 / 4 divided by 3 = 1.33), HDTV broadcast is 16:9 (1.78 / 16 divided by 9 = 1.78), movies, music videos, and other content on DVD comes in a variety of formats like 1.33, 1.78, 1.85, 2.00, 2.35, 2.4, 2.5 etc. There is no universal standard for the shape of a video picture.

For general use, there is no perfect solution and both have limitations for which one must compromise.

- Mounting Options back to menu

Wall brackets come in all manner of shapes and sizes to fit all types of LCD and Plasma  panels. Some feature tilt or swing adjustments

Panel lifts enable your display panel to be safely stored out of sight when not in use

Table mounting your display on a stand is probably the most common way of sitting a panel. The disadvantages are the ability to hide the cables and leads as well as the relative ease with which a thief could escape with your lovely display.