MLA Citation: Bloomfield, Louis A. "Question 1128"How Everything Works 15 Jul 2018. 15 Jul 2018 .
1128. I read a recent article about the FCC requiring all TV stations to switch to digital signals instead of analog ones by 2006. How are digital signals different from analog signals, and will they work with our current TV's? — JP
Current video signals use continuous physical quantities to represent the brightness and color of the spots on a television screen. For example, the current in a video cable can take any value and that value is used to represent the brightness and color of the spots. This use of a continuous physical quantity (such as current) to represent a continuous physical quantity (such as brightness) is called analog representation.

In a digital video signal, a physical quantity first represents numbers and then these numbers represent the brightness and color of the spots. The physical quantity representing the numbers doesn't have to be continuous. For example, a current that's on could represent the number 1 while a current that's off could represent the number 0. A certain pattern of on and off currents could represent larger numbers and these numbers could then represent brightness and color. This use of a continuous or non-continuous physical quantity (such as magnetization, charge, or current) to represent numbers and then these numbers to represent a continuous physical quantity (such as brightness) is called digital representation.

The other advantage to digital representation is that digital compression techniques become possible. A typical video signal contains lots of unnecessary and duplicated information. For example, when two people are standing in a room and the only things that are changing with time are the images of those two people, there is really no reason to keep sending an image of the room itself from the broadcast station to your home. Digital compression can identify redundant information and remove it from the transmission. In doing so, it can use the communication channel more efficiently.

By adopting a digital transmission scheme, the FCC has recognized that broadcasters will be able to send much clearer, more detailed images using digital representations than with the current analog representations, while still occupying the same portions of the electromagnetic spectrum. However, there is a cost—current televisions will not work directly with these new digital signals. To fix that shortcoming, there will be inexpensive converters that receive the new digital signals and recreate the analog signals needed for current televisions. This conversion will allow older televisions to keep working, but the new digital televisions will be designed to make better use of the enhanced details in the transmissions. The new transmissions will contain about 4 times the detail of current transmissions so that the images will be sharper as well as more immune to noise than the current transmissions.