Show What We Can See
Everyone knows there are sounds that a dog can hear but people can't. Radio, CD, and MP3 standards reproduce only the sounds we can hear. TV standards similarly concentrate on the picture elements we can see. To understand TV and computer monitors, it is important to first review the physics of light and the physiology of the human eye.
Color
Light is carried by particles called photons. Each photon has a wavelength. The range of wavelengths that the eye can see is called the visible spectrum. Within this range, light with a longer wavelength and therefore lower frequency is red, while light with shorter wavelengths and higher frequency is blue.
Light passes through the lens of the eye and is focused on the retina. There are two types of receptors on the retina called "rods" and "cones". There are a large number of rods that provide night vision and peripheral vision, but they cannot distinguish colors. Cones are in the center of the eye at the point of direct focus. They provide more precise vision and color discrimination.
There are three different groups of cone cells. Each is sensitive to a range of lightcentered at a slightly different wavelength. Historically they have been called "Red", "Green", and "Blue" cones although doctors who have actually measured sensitivity note that the "Red" cones are actually most sensitive to orange photons, while "Blue" cones are most sensitive to purplish photons.
Photons enter the eye, are focused by the lens, hit the cones in the retina, and trigger a signal to the brain. Yellow light will trigger some of the Green cones and some of the Red cones. The brain gets these signals, compares the amount of signals from each type of cone, and decides that the light must be Yellow.
Only a laser produces photons that have a single pure wavelength. Ordinary light has a mixture of photons of different colors. A prism can split the light into its component colors, but evolution was not sophisticated enough to give our eye a spectrometer. Instead, our eye and brain depends on differences in the response of the different types of cones to approximate the color.
This means that the eye therefore cannot distinguish between a pure yellow light that triggers some Green rods and some Red rods and a light that contains a mixture of some pure green photons and some pure red photons. Both cases produce the same response in the two types of cones, the brain gets the same signal, and it "sees" the same color.
White light contains approximately the same amount of all colors. Black is what we see when there is no light at all. When you are just distinguishing black and white (actually shades of grey), the rods of the eye can also participate. Put up two charts. One has increasingly thin lines of black and white. One has the same lines in colors. The eye can distinguish much finder detail in black and white than it can differences in color.
The old analog TV standard recognizes this. It starts with a strong black and white picture, then a much weaker color signal. Modern DVD and other digital TV technology applies a much higher level of compression to the color than to the black and white part of the picture. This saves storage space, since we simply can't see the very fine color detail. You can think of this as an algorithm that changes the brightness of every dot on the screen, but changes the color only every two dots.
Refresh
The eye is similarly imprecise about changes in the image over very short periods of time. The invention of the motion picture was made possible by the observation that when people see a continuous stream of still pictures they perceive it as continuous motion. Movies show 24 separate images every second. The TV did not have to show images faster than that, but 1/24th of a second turned out to be a difficult period of time for early electronic devices to support.
The US electric system of "alternating current" (AC) changes the direction of current 60 times a second. The early electronic equipment had a strong 60 cycle internal operation, and any attempt to use a competing rate produced problems. So the US TV signals adopted a standard where they would rewrite the content of the screen 60 times a second. Since the eye cannot distinguish changes that fast, pairs of two successive screen rewrites combine to form a single "frame" and the frames change 30 times a second.
In Europe, however, electricity changes direction 50 times a second. Since US and Europeans TVs operate on roughly the same range of radio frequencies, and each TV channel can carry the same amount of picture information, a European set that only has to display 25 frames per second can have 1/5 more detail on the screen than a US set that has to display 30 frames per second. So European TV has more lines and a sharper picture.
Today's monitors can refresh at any rate and are not slaved to the frequency of the electrical system. Analog computer TV cards from companies like Hauppauge can receive both US and European TV traditional broadcast and cable signals with just a change in configuration. HDTV has the same screen resolution and refresh rate throughout the world. However, for reasons that remain controversial, the US decided to adopt a different "modulation" technique for digital broadcast over the air to antennas (8VSB) than is used by other countries (DVB-T), and currently you have to buy different digital TV cards to operate in the US than in other parts of the world.
Both TV sets and CRT computer monitors generate their picture with a stream of electrons directed at the back of the TV screen. The electrons hit a phosphorescent coating that then glows for a while generating the light. Different coatings fade at different rates. This is called "persistence". If the glow fades away before the screen is rewritten, then the picture will flicker. Not everyone sees the flicker, but a small percentage of the population can detect it and finds it bothersome.
TV sets solve the problem by selecting phosphors with a persistence exactly tuned to the 50 or 60 cycle per second refresh rate. Computer monitors have a variable refresh rate. While they can be set to the same 60 Hz refresh rate of a TV set, CRT computer monitors operate better at rates of 72 to 85 Hz where no person can see a flicker. Flat panel LCD panels, however, operate on an entirely different flicker-free technique and are best operated at the old 60 Hz rate.
Interlace
The eye is also flexible in the way it handles detail. There are only 3000 cones at the focus of the eye. This contrasts to the millions of pixels in in a $200 digital camera. To see the entire picture, the eye has to move around focusing for a fraction of a second on specific points of interest. If someone makes a movie with a handheld camcorder, the picture bounces around and is difficult to watch. The eye is always bouncing around much more rapidly, but the brain filters out the movement. We see a fixed image even though the eye pieces the image together by rapidly focusing on different points.
TV standards take advantage of this with a technique called "interlacing". The TV camera captures one picture with twice as many lines of detail as the TV set can display. In one cycle it transmits every other (say "odd numbered") line. Then in the next cycle it transmits the lines skipped before (the "even numbered") lines. On the TV screen the contents of every line flickers, first showing one set of lines and then the other. The flicker happens 60 times a second in the US (50 times a second in Europe). Since the eye is not particularly good at distinguishing changes smaller than 1/24 th of a second, we don't really see the flicker. The brain sees some data from one set of lines and some data from the other. It attributes the difference to something like eye movement and merges the information from the two sets of lines together. Therefore, we "see" a picture with information from twice as many lines as the TV set can actually display.
However, the "interlace" trick only worked on a CRT (tube type) TV set. LCD and Plasma flat panel TV sets do not flicker fast enough to make the interlace work. However, modern flat panel sets have a lot more lines and dots than the old CRT TV sets. So if a TV source is generating interlaced data, the electronics in the set top box, tuner, or TV set has to convert it and display it properly on a modern, higher resolution screen.