Broadcasting is a mass communication method. The basic idea of broadcasting is to make information sent from one source can be received by multiple receivers.
Digital broadcasting system is a telecommunication system of sending and receiving information by means of digital signals. This information can be sounds, moving pictures, or data.
Before digital broadcasting systems emerged, information is broadcasted using analog signal. Analog signal is signal that is continuous both in time and amplitude. Analog signal can be used to convey information by making variations of its properties. This variation is equivalent or analogous to information it conveys. For example, electrical analog signal is used to convey information by varying the voltage or frequency of the signal. By this property, small fluctuations in analog signal are meaningful. Any small different in the signal means different information.
In contrast to analog signal, digital signal is discrete in time. The values of the signal are noted at fixed intervals rather than continuously (for example, every millisecond). In addition to that, the value, instead of being measured to the exact value, is approximated to certain precision. This will allow the value to be represented by limited number of digits. In other words, digital signal’s values are quantized.
Generally, digital signal is superior to analog signal. There are several reasons to this. The first reason is the easiness of digital signal to be manipulated. In digital systems, the value or the shape of digital signal doesn’t directly represent the information it conveys. It only represents codes which later have to be translated to acquire the real information. For example, the value “100” means “A” while the value “1001” means “B”. This code can be easily changed according to needs. For example, in order to reduce the size of transmission, the code can be shortened. The value “00” means “A” while the value “001” means “B”. Using this kind of digital signal’s property, information in digital form can be easily compressed, edited, superimposed, scrambled and de-scrambled (for security purpose), etc.
Analog signal, on the other hand, represents the information by its value / shape. Modifying the value of the signal is equivalent to modifying the information the signal conveys. So unless information change is expected (which is purposely done in some applications), analog signal is expected not to change in value / shape.
The next reason, digital signal takes up less bandwidth compare to analog signal to convey the same information. Because digital signal can be compressed easily, given the same bandwidth needed to deliver one analog channel, several digital channels can be delivered simultaneously.
Another reason, digital signal is much more resistant to noise than analog signal. As signal copied or transmitted over long distance, random variation / noise (caused by weather, other signals, etc.) will interfere and get into the signal. This interference will cause changes in signal value / shape.
In digital signal, all values are quantized. There is no possibility that the signal will have value between the quantification levels. When the signal degraded after noise interference, slight variations on value, as long they are small compared to the quantization range, do not matter since they are ignored when the signal is received.
Analog signal however, cannot be easily restored from noise interference. In analog signal, even the slightly different values represent different information. Once a value is altered by noise, it is difficult to restore the original value. When the signal degraded after noise interference, the quality of information it conveys will also degrade.
Because of the indirect relation between signal value and information it conveys, digital broadcasting system is more complex than analog broadcasting system. A system has to have encoder to encode information on transmitter side and decoder to decode the signal on receiver side. But the rapid drop in cost and the rapid expansion of power of digital devices (such as processor, flash memory, etc.) has made the implementation of digital broadcasting system became very feasible.
These are the reasons why recently digital broadcasting system is preferred to analog broadcasting system. Digital television (HDTV and SDTV), digital radio, video broadcast and radio broadcast via internet are examples of popular applications that utilize digital broadcasting system.
To be written: Digital Television
