Introduction to Digital Television Signals

Why is Digital Television Better Than Analog TV?

Digital television and particularly its high quality version, high definition television (HDTV), is the greatest advance in TV technology since television's mass market standarization in the 1950s. High definition televisions offer a much greater improvement in quality than color television has over black and white. High definition television picture quality is up to ten times better than that of the older analog television technology. Furthermore, because of its digital nature, digital TV picture is always perfect until it can't be received anymore.

Digital television signals retain perfect picture quality up until the point when they can no longer be received.

Unlike analog television signals, which directly represent what color every pixel should have, digital television signals encode pictures using numbers. A TV picture can be easily represented numerically. A television picture is made of a grid of color elements called pixels. Each pixel is composed of red, green, and blue (RGB) color producing sub-elements that combine to create nearly every humanly visible color. A picture can therefore be represented as a sequence of numbers representing the brightness or each of the RGB colors in every pixel. In the electronic and computer world, numbers are represented using the binary numerical system with only two digits, zero and one, because it is the simplest system to work with. Therefore the sequence of numbers will be nothing more than a sequence of 0s and 1s.

Zeros and ones can be encoded electromagnetically using two types of signals one for a 0 digit and another for 1 digit. This is the reason why digital signals can made to retain their perfect quality. If the continuous analog signal changes just a bit, so does the represented data. But the digital signal is discrete, i.e. it can have only a finite (two in our specific discussion) number of valid signal types. If a receiver gets a distorted signal for a 0, it's still fine, as long as it's recognizable as a 0. A 0 remains a 0, and a 1 remains a 1, as long as the signal is not so badly damages as to make the distinction impossible. Therefore, a weakening digital television signal will still produce a perfect picture until the point where the quality falls below the level that makes the distinction between the digits possible. Furthermore, if a signal needs to be amplified and retransmitted, it can be done so perfectly, eliminating all signal errors.

Another advantage of digital television signals is the ability to use compression to make the television picture sequence take less room, or bandwidth, to transmit. Compression means taking data and changing its representation so that it takes less space to encode, while still retaining the quality of the data. In the digital realm the space means the number of digits. It is much easier to compress digital data because it is not tied to the physical nature of the signal. A compressed digital picture is still a sequence of 1s and 0s, just like an uncompressed picture.

The advantage that compression brings is the ability to transmit high quality picture formats, such as HDTV, that have a large number of pixels. The compression also allows digital TV stations to take up a smaller slice of the radio wave spectrum, freeing up some bandwidth for other uses.