How CRT TV's Work
Despite being the mainstay of the average living room for over 50 years, relatively few people understand how the CRT works. Basically, a beam of electrons is fired from the back of the picture tube and then steered by an electrically charged metal coil to paint horizontal lines on the screen.
CRT TV's are named for their Cathode Ray Tube, the technology they use to display an image. A CRT TV is easily recognised by its bulky form, as it has to be very deep to house the large conical CRT tube. Some CRT TV's are referred to as 'flat screens' (i.e. the glass at the front of the TV is a flat sheet), but this is not to be confused with flat panel displays, which refer to LCD or Plasma TV's.
Unlike LCD and Plasma TV's, CRT TV do not produce an image composed of individual pixels, rather the image is composed of a series of horizontal lines drawn in sequence across the display. The more horizontal lines a CRT TV has in its display the higher the resolution and clearer the picture.
The CRT picture tube is basically a large cone containing a vacuum, with the neck of the cone at the back of the TV. In the neck of the cone is a negatively charged filament (a 'cathode'). When electricity is supplied, the filament heats up and produces a stream or 'ray' of electrons which are attracted to positively charged anodes which focus the electrons into three narrow beams (known as a triad), accelerating them to strike the phosphor-coated screen at the front of the TV. The phosphors in the screen glow when exposed to the electron beam, emitting visible light. Phosphors that emit red, green, and blue light are used in a colour monitor, arranged in horizontal stripes that are traced by the electron beam during each cycle. The three electron beams are used to excite the three colours in the combinations needed to create the various hues that form the picture visible on screen.
To precisely direct the beams, copper steering coils are used to create magnetic fields inside the tube. The fields move the electron beams vertically or horizontally. By applying varying voltages to the steering coils, a beam can be positioned at any point on the screen. Each horizontal line making up the image image is painted across the horizontal phosphor lines on the screen between 25 and 60 times (known as the refresh rate) each second depending on the TV format used (NTSC, PAL, SECAM, and interlaced or progressive scan). The scanning must be done even when the picture being displayed is static, as the phosphor only glows for a very short time. If the phosphor glowed for longer, moving images would produce a smearing effect across the screen.
Refresh Rate
The refresh rate indicates how many times per second the screen is repainted. Though TV's differ in their capabilities, lower resolutions normally have higher refresh rates because it takes less time to paint a lower resolution. Therefore a setting of 800 x 600 might have a refresh rate of 85Hz, (the screen will be repainted or refresh 85 times per second), while a resolution setting of 1024 x 768 may have a refresh rate of 72Hz. Still higher resolutions usually have refresh rates closer to 60Hz. Anything less than 60Hz is generally considered inadequate, and some people will detect "flicker" even with acceptable refresh rates. Generally speaking, high-end TV's are capable of higher refresh rates overall than lower-end models.
Dot Pitch
The dot pitch of a CRT refers to the distance (measured in millimetres) between phosphor lines of the same colour (i.e. the height of the triad, plus the distance between triads) which is a measure of the sharpness of the picture. The lower dot pitch the better. Dot pitch is a bigger issue for CRT computer monitors than CRT TV's, due to the shorter viewing distance making the dot pitch more apparent. Some older CRT monitors have a .32 dot pitch, but today virtually all CRT TV's are .28 or better.
Size
An outer border of approximately 2.5 cm (1 inch) of the CRT display does not display an image. CRT TV's are measured diagonally from corner to corner, but the size rating takes into account the border, and only refers to the viewable area of the display.