Conventional Analog Television - An Introduction

I. The physiology of vision

A. Trireceptor theory of vision -- or why we use RGB monitors

If you ask someone why red, green and blue are used in computer monitors -- the immediate answer is "Because these are the primary colors". If you then ask, "But why are these the primary colors?" -- the answer you get is that "If you mix light of these colors together you can make any color". However, if you then ask "Why is this so?" -- usually the answer is dead silence. (No doubt the person being asks the question thinks you are too weird for words ...)

However, there are some very fascinating reasons why RGB are the primary colors -- and they have nothing to do with either light ... or Fourier series. They lie in the nature of the human eye.

The major light sensing element in the human eye is the retina[1].

The retina consists of a number of different and important layers of cells. Notice however, that from an engineering viewpoint, the retina is constructed upside down. The major light receptors (the rods and cones) are at the bottom of the retina -- not at the top. Thus, light must pass through nerves, blood, and tissue before reaching the main photoreceptors. This means that some serious signal processing is required by the retina and the brain to edit out these structures in your visual perception!

There are two types of photoreceptors, rods and cones. Rods are responsible for low level light detection and are most sensitive in the blue/green. They are very light sensitive and motion sensitive, but at the cost of resolution. Rods are virtually missing in the fovea (the center of the visual field) but are scattered elsewhere throughout the eye -- thus governing peripheral vision as well as nighttime vision. At night, the fovea is very insensitive and most of the visual information is being carried by rods in the periphery of your eye. Cones carry the color information and provide higher resolution -- but at the cost of sensitivity. Cones are concentrated in the fovea, providing high resolution central daytime vision.

Rods contain a blue/green pigment called rhodopsin. Cones contain three pigments, a blue-sensitive pigment[2] (447 nm), a green sensitive pigment called chlorolabe (540 nm), and a red sensitive pigment called erythrolabe (577 nm) . These three pigments are the pigments responsible for "primary colors". Individuals missing one or two of these pigments will have various forms of color blindness. (Notice that three pigments is by no means standard. There is a species of shrimp that has 20 different photoreceptor pigments!)

The graph below indicates the wavelength dependence of the three cone pigments in the human eye[3].

The idea of trireceptor vision was worked out far before the physical mechanism of retinal pigments was understood. A common diagram for describing human color perception was developed by the International Commission on Illumination (CIE). The CIE diagram is an attempt to precisely quantify the trireceptor nature of human vision.

Color perceptions were measured by giving subjects various combinations of the three standard CIE primary colors (435.8 nm, 546.1 nm and 700 nm) and measuring their perceptions. These perceptions are plotted on an x-y diagram called the CIE Chromaticity diagram[4].

The "pure" colors lie along the outer locus of the diagram, and the center of the diagram (CIE Illuminant C) is the CIE defined "white". Thus, any point on this diagram can be uniquely indicated by an x,y value. Therefore, in some fundamental way, only two basis functions are really require to describe any particular color or hue. This fact will become very important to us in color TV.

Jumping ahead just a bit, it is interesting to compare the CIE Chromaticity diagram against commonly observed colors. In the follow graph, the CIE Chromaticity diagram is overlaid against surface colors of common paints and dyes (the gray blob) as well as the primary color triangle of the American NTSC (National Television Systems Committee) color television system and the European PAL (Phase Alternation Line rate) and SECAM (Sequential Couleur avec Memoire) systems[5].

B. Visual persistence

The human eye retains an image for a fraction of a second after it views the image. This property (called persistence of vision) is essential to all visual display technologies. The basic idea is quite simple, single still frames are presented at a high enough rate so that persistence of vision integrates these still frames into motion.

Motion pictures originally set the frame rate at 16 frames per second. This was rapidly found to be unacceptable and the frame rate was increased to 24 frames per second. In Europe, this was changed to 25 frames per second, as the European power line frequency is 50 Hz. (Just as an aside, 24 frame/second American movies are routinely broadcast at 25 Hz in Europe ... the 4% difference does not seem to bother anyone!)

When NTSC television standards were introduced, the frame rate was set at 30 Hz (1/2 the 60 Hz line frequency). Then, the rate was moved to 29.97 Hz to maintain 4.5 MHz between the visual and audio carriers. (As we will see -- this decision has lead to some problems in developing an HDTV standard.) Movies filmed at 24 frames per second are simply converted to 29.97 frames per second on television broadcasting.

Now, there is a glitch. For some reason, the brighter the still image presented to the viewer ... the shorter the persistence of vision. So, bright pictures require more frequent repetition. If the space between pictures is longer than the period of persistence of vision -- then the image flickers. Large bright theater projectors avoid this problem by placing rotating shutters in front of the image in order to increase the repetition rate by a factor of 2 (to 48) or three (to 72) without changing the actual images.

Unfortunately, there is no easy way to "put a shutter" in front of a television broadcast! Therefore, to arrange for two "flashes" per frame, the flashes are created by interlacing.

The basic idea here is that a single frame is scanned twice. The first scan includes only the odd lines, the next scan includes only the even lines. With this method, the number of "flashes" per frame is two, and the field rate is double the frame rate. Thus, NTSC systems have a field rate of 59.94 Hz and PAL/SECAM systems a field rate of 50 Hz.

Although interlacing sounds like a great idea -- a number of aberrations appear due to the fact that you really do not have a frame rate of 50/60 Hz. For example, vertically adjacent picture elements do not appear at the same time. If the scene is moving, then this creates a series of serrations on the edge of moving objects. Other aberrations include such things as misalignment (where the horizontal edges of one scan do not match with the next), and interline flicker (where slight mismatches between subsequent lines cause a shimmering effect).

The other situation that must be considered is rapid motion. If the still frame images are presented at too low a rate, rapid motion becomes jerky and odd looking. This is especially a problem in action movies -- where high speed chase scenes are common. However, as of yet, there has been little interest in converting movie projectors to either 29.97 or 30 Hz due to the large investments in such equipment.