Monitor

From Grokdoc

How do they work?

In one of the earliest computers, the Manchester Mark 1, the memory consisted of a cathode ray tube 'CRT' (like the tube in a television). If an area of screen was light, this was classed as a binary one (on) and if it was dark, this was a binary zero (off).

The light on the screen was caused by a stream of electrons which scans the screen, directed by electromagnets. When this stream hit the face of the monitor, it caused the chemical coating on the inside of the screen to phosfloresce (light up).

As the face of the screen was part of the mechanism for refreshing and reading the screen, it could not be seen by the operators. So an extra screen was set up to show the contents of the memory tube. In actuality there were several cathode ray tubes used for memory, and the "monitor" screen was switchable, so it could display the contents of any given cathode ray tube.

On this machine, the monitor was called the display tube.

Monitors were ideal for showing the state of the memory on a computer because they were quick and inexpensive.

Over the last twenty years, the cathode ray tube has been the mechanism of choice for showing the real time contents of a computers memory. After all, when you edit a document in your chosen word processor, all you are doing is manipulating the contents of the computer's memory.

CRT's have not changed in principle since their inception. They have gained another two electron guns (they now show three colours, red, green and blue). And the screen face has become flatter and squarer.

However, over the last five years or so, a new technology has become cheap enough to be sold with even a mid range computer... LCD.

LCD, Liquid Crystal Display, uses a lot less power and is much lighter than CRT. It consists of a grid of tiny little circuits "printed" onto a thin film. Each circuit consists of some funky electronics and a liquid crystal. There is also a light behind the grid. In front (usually) is a polarised grid.

When current is applied to a given crystal, it twists the light coming through it. If the twisted light is lined up with the polarised grid, then it gets through, if it is not then it does not.

If the current applied to the crystal is large, then the light is twisted a lot, and most of it will get through the polarised grid. If the current applied is small then not much light will get through. The upshot of this is that you can vary the intensity of light coming through a grid square by changing the current going through each tiny circuit.

Colours are created by putting a red/green/blue grid in front of the grid of circuits.

LCD and CRT are the two most common display technologies, there are others, Organic LCD, Multi mirror devices and Plasma, but they are not commonly used in "standard" computers.

What about all of those different plugs?

Until recently, there were usually only two plugs which you needed to worry about when pluging in a monitor. Power and Video.

The power is a cable plugged directly into a wall socket or into the back of the computer next the computer's power cable.

The video is a cable with 21 pins in three rows. This plugs straight into the back of the computer.

Now a new video cable has appeared. This has two groups of nine pins and a wide flat pin. The new cable is a digital cable.

Why did it appear?

Well, your computer created its pictures digitally. It then converts this digital signal to an analogue signal that the CRT monitor can deal with.

BUT, LCD monitors work digitally internally, so they have to take this signal and convert it back from analogue to digital.

The new cable simply skips the "convert to analogue" and "convert back from analogue" steps.

Other cables that you might need to plug into your monitor are sound cables, if it has speakers and a usb cable if it has an inbuilt usb hub.

--Tuxhead 13:38, 14 Jun 2004 (EDT)

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Television signals and computer video signals are inherently different and not directly compatible.

Televisions usually receive television signals through Copper Co-axal. The signals broadcast on these lines are:

  NTSC, Analog
     North America / South America / ..
  PAL, Analog
     Europe / ..
  SECAM, Analog
     Some Asian countries
  DVB
     Europe, ..
     Digital, Interlaced/Non-interlaced, Compressed
  ATSC
     North America / Some other countries
     Digital, Interlaced/Non-interlaced, Compressed, Advanced sound tech. (dolby digital)

The Computers of today's generation (early 2000's) generally use the VGA display standard which is analog. Another display format making ground is DVI. DVI is digtal. Both PC and DVI signals degrade in quality when sent over long distances. To a length of 15meters, one can start to notice display quality dropoff. This makes the integration of PC-type displays and TV's very unlikely for to-the-curb services. DVI displays are being built into Plasma and LCD Televisions these days. In those cases, the Television is usually a large format PC monitor with an NTSC adapter built in to read TV signals.