Analog and Digital Signals


"NTSC Video is a 4:3 aspect ratio, interlaced analog signal with 525 total lines of resolution."

The last part of this definition we'll cover is "analog". What do we mean when we say analog versus digital? A strict definition of analog is, "of, relating to, or being a device in which data are represented by continuously variable, measurable, physical quantities, such as length, width, voltage, or pressure."

Simply put, an analog system sends information by encoding it as a continuous change in voltage or current. The receiving end then decodes these changes back into usable information. For example, an analog clock represents the sun circling the sky. An analog device converts a pattern, such as light or sound patterns, into electrical signals with similar patterns.

The definition of an analog signal can also reveal its shortcomings. Analog signals, because they rely on minute variations in voltage or current, are very susceptible to interference from outside sources. Even a small change in the analog signal from an outside source, such as electromagnetic interference, can cause dramatic changes in the signal and how the signal is interpreted.

Digital is defined as, "of, relating to, or using calculation by numerical methods or by discrete units." A better way to describe digital is to contrast it against an analog signal. In a digital signal, numerical values are finite. For example, if we were counting from zero to one, only the two values might be represented. In an analog system, everything between zero and one are represented as well.

If we were to simplify a digital signal even more, we would yield a binary digital system in which only two states are represented. We would have either a one or a zero, on or off, there or not there. Using this binary system to transmit data has a tremendous advantage over transmitting the same signal via analog. The signal is no longer susceptible to interference from outside sources and can be transmitted over very long distances.

This is because the receiving end of the transmission only needs to recognize if there is any signal or no signal transmitted over a given time period. For example, if we were transmitting a signal via a fiber optic cable, the receiving end would only need to tell if there was any light at all versus no light transmitted at any given moment, instead of trying to figure out how much light was transmitted. Obviously this is an overly-simplified example, but it does help give a basic understanding.


By now you should have a good grasp on our original statement, "NTSC Video is a 4:3 aspect ratio, interlaced analog signal with 525 total lines of resolution," so let's move on.