Introduction to Analog Television Signals

In order to understand digital televisions and why they are better than analog TVs, it's best to know how analog televisions works, which is the main purpose of this page. The analog television signal format (like the NTSC standard in the US) takes its form from how analog CRT (cathode ray tube) televisions physically produce their pictures. Analog CRTs are just the good old TV tubes that for many years were the only television display technology in use.

A CRT works by shooting a focused beam of electrons onto a phosphor coated screen. Phosphor has the property of emitting light when it absorbs energy. In a CRT, the energy is provided by an electron beam. (A cathode is defined as an electron emitter, and hence the name cathode ray tube.)

To produce a picture, the electron beam, which is focused to only hit a single dot on the screen at a time, is rapidly moved horizontally across, and then down line by line, until it hits every point on the screen. Even tough the election beam only hits a single point at a time, a solid picture appears because the human brain assembles very rapid variations in what the eye sees into a single image.

Two things help smooth out the TV picture and reduce flicker. The phosphor material continues to emit light a bit after the electron beam passes, and also interlacing is used. Interlacing is the process of scanning the electron beam through every other horizontal screen line first, and then scanning the remaining lines, so that first, all the odd numbered lines emit light, and then all the even lines. In the NTSC standard the electron beam scans the screen 60 times a second in an interlaced fashion so that all the pixels are hit within a 1/30 of a second.

The electron beam forms a picture by varying its strength as it moves from point to point. A stronger beam produces a brighter light point. Colors are formed using combinations of red green and blue light dots. Each pixel, which is the smallest color producing area on a TV screen, is composed of three dots capable of emitting each of the three colors. Therefore, in a color television three electron beams scans the screen together, controlling each of the three color dots of every pixel as they pass by.

Once the electron beams finish scanning the last horizontal line, also called a scan line, they are moved back to the top without hitting any points on the screen, in a procedure called a retrace, to begin another picture frame.

The format of the analog TV signal closely matches what the electron beams must do to create a picture. This TV signal is also called a composite signal because it simultaneously carries signal information for forming the three colors, as well as some synchronization signals.

A simple way to represent colors in a composite signal is to have three signals, the strength of each in direct proportion to one of the three red, green, and blue colors. But, in order to easily accommodate black and white television, color is represented differently. Three signals are still used, but instead of encoding red, green, and blue, one signal is used to encode overall brightness, which is what black and white TV uses, and two more signals are used to represent phase, which is essentially directly translatable to the strength of the three colors relative to each other. Using some mathematics, this latter representation can be easily converted to the red, green, and blue representation.

Within the composite TV signal a synchronization signal is also transmitted to mark the moment of the electron beams retrace to the beginning of the frame and to the beginning of a scan line. The synchronization is used to match the location of the electron beams to the progress of the signal.

Finally to put all this together, a picture is created by the composite signal, which changes with time, and is synchronized to the motion of the electron beams.

Analog is defined by Merriam Webster dictionary as "relating to, or being a mechanism in which data is represented by continuously variable physical quantities." And therefore, true to it definition, the analog composite TV signal continuously varies each pixel's color.

For example, suppose the signal component responsible for each pixel's brightness has a range of 0 to 10, where 0 is darkest and 10 is brightest. The continuous nature of the signal means that a level 1.01 signal will produce a pixel that is brighter than a 1.00 signal, and a 1.001 level will also produce a brighter pixel than a 1.000 level, etc.

Therefore, the continuous nature of an analog signal is highly prone to errors. This is because even small interference with the signal shows up in the end result. To continue the example above, a +0.001 error will make a pixel a little brighter than it should be, and so will a +0.0001 error. When a TV signal is weak, the relative strength of any interfering signal is large and therefore noticibly distorts the picture.

Two types of unwanted artifacts commonly appear in analog TV pictures. The first is "snow" and the second is "ghost" images. Snow is fast randomly bouncing dots, and is caused by random electromagnetic noise originating from various sources such as electrical appliences, weak broadcasts from far away, and outer space. Ghost images appear as ugly shadows or duplicate images. These are caused by the boroadcast signal bouncing off of a tall building or a hill, and therefore arriving to the antenna again at a slightly different time than the main signal.

A digital signal eliminates vulnerability to interference because color levels are not represented continuously by signal strength. An explanation of how a digital signal does it's job can be found on the Introduction to Digital Television page.